8.0  Pollution Prevention and Wastewater Treatment Technologies 8.0 POLLUTION PREVENTION PRACTICES AND WASTEWATER TREATMENT TECHNOLOGIES In general, MP& M facilities generate process wastewater containing metals, cyanide, oil and grease, and suspended solids. Pollution prevention practices and wastewater treatment technologies currently used by facilities evaluated for the final rule (  MP& M facilities ) are designed to remove these pollutants before they are discharged to either a receiving stream ( direct discharge) or public owned treatment works ( indirect discharge). The type of pollution prevention practice and wastewater treatment technology a MP& M facility selects depends on the manufacturing operations generating the wastewater. Many facilities have implemented process modifications for waste reduction. Some of those modifications include prolonging process bath life by removing contaminants, redesigning part racks to reduce dragout, installing spray or fog nozzle rinse systems, and installing dragout recovery tanks ( 1).

Most MP& M facilities rely on chemical precipitation and gravity or membrane clarification to remove metals; however, certain pretreatment techniques may be necessary when chelated metals or hexavalent chromium are present. Facilities that generate oily wastewater from operations such as machining and grinding typically use chemical emulsion breaking followed by gravity or membrane clarification. If cyanide is present, facilities typically use oxidation techniques such as alkaline chlorination.

This section describes the pollution prevention practices and wastewater treatment technologies that are used by MP& M facilities, in the first instance, to prevent the generation of wastewater pollutants or, secondarily, to reduce the discharge of wastewater pollutants. Section 8.1 describes flow reduction practices, Section 8.2 describes in process pollution prevention technologies, Section 8.3 describes management practices for pollution prevention, Section 8.4 describes technologies used for the preliminary treatment of waste streams, and Section 8.5 describes end of pipe wastewater treatment and sludge dewatering technologies. This section discusses the most prevalent treatment technologies, as determined by survey responses and site visits, in place at facilities evaluated for the final rule. This section includes descriptions of all the technologies evaluated for the final rule and used as a basis for the MP& M effluent guidelines ( see Section 9.0). Additional technologies may be applicable for some MP& M facilities, depending on the waste streams generated. Additionally, not all technologies discussed in this section are applicable to all MP& M facilities; the applicability of a technology is driven by the unit operations performed and waste streams generated on site. EPA presents pollution prevention practices and wastewater treatment information potentially applicable to all facilities evaluated for the final rule (  MP& M facilities ).

8.1 Flow Reduction Practices MP& M facilities applies flow reduction practices to process baths or rinses to reduce the volume of wastewater discharged. Flow reduction practices consist of optimizing rinse tank design and configuration, and installing flow reduction technologies such as flow restrictors or timers. Table 8 1 lists various flow reduction practices and the number 8 1 8.0  Pollution Prevention and Wastewater Treatment Technologies observations at EPA MP& M site visits and surveys ( see Section 3.0). This table also provides EPA s estimate of the number of MP& M facilities employing the various flow reduction practices based on occurrence at surveyed facilities and their respective survey weights. The following subsections discuss these flow reduction practices in greater detail.

8.1.1 Rinse Tank Design and Innovative Configurations Rinsing follows many proposed MP& M operations1 to remove dirt, oil, or chemicals remaining on parts or racks from a previous unit operation ( i. e., drag out). Rinsing improves the quality of the surface finishing process and prevents the contamination of subsequent process baths. Rinse tank design and rinsing configuration greatly influence water usage. The key objectives of optimal rinse tank design are to quickly remove drag out solution from the part and to disperse the drag out throughout the rinse tank. MP& M facilities uses various rinsing configurations. The most common are countercurrent cascade rinsing, drag out rinsing, and spray rinsing. EPA estimates that over 5,000 MP& M facilities use at least one of these rinse schemes to reduce wastewater flow. The use of single overflow rinse tanks following each process tank is the most inefficient use of rinse water. Multiple rinse tanks connected in series ( i. e., cascade rinsing) reduce the water needs of a given rinsing operation by one or more orders of magnitude ( i. e., less water is needed to achieve the same rinsing quality). Spray rinsing, where the part is suspended over a tank and rinsed with water applied by spray nozzles, also may be used to reduce water use requirements, although less than countercurrent cascade rinses. Below are descriptions of some of the common rinse types.

Cascade Rinsing Cascade rinsing is a method of reusing water from one rinsing operation to another, less critical rinsing operation before being discharged to treatment. Some rinse waters acquire chemical properties, such as low pH, that make them desirable for reuse in other rinse systems. For example, water from an acid treatment rinse may be reused in an alkaline treatment rinse. In this case, the rinse water both removes drag out from the work piece and neutralizes the drag out.

1Note: EPA evaluated a number of unit operations for the May 1995 proposal, January 2001 proposal, and June 2002 NODA ( see Tables 4 3 and 4 4). However, EPA selected a subset of these unit operations for regulation in the final rule ( see Section 1.0). For this Section, the term   proposed MP& M operations  means those operations evaluated for the two proposals, NODA, and final rule. The term   final MP& M operations  means those operations defined as   oily operations  ( see Section 1.0, 40 CFR 438.2( f), and Appendix B to Part 438) and regulated by the final rule.

8 2 8.0  Pollution Prevention and Wastewater Treatment Technologies 8 3 Table 8 1 MP& M Flow Reduction Technologies Technology Technology Description Demonstration Status Number of Facilities Visited Using the Technologya Number of Survey Facilities Using the Technologyb Estimated Number of MP& M Facilities Using the Technologyc Countercurrent Cascade Rinsing Series of consecutive rinse tanks that are plumbed to cause water to flow from one tank to another in the direction opposite of the work flow. Water is introduced into the last tank of the series, making it the cleanest, and is discharged from the first tank, which has the highest concentration of pollutants. 110 130 1,569 Drag Out Rinsing Stagnant rinse, initially of fresh water, positioned immediately after process tanks. The drag out rinse collects most of the drag out from the process tank, preventing it from entering the subsequent flowing rinses. Drag out rinse is commonly reused as make up for heated process bath to replace evaporative loss. 62 139 1,737 Spray Rinsing Water sprayed on parts above a process tank or drip/ drag out tank;

uses considerably less water than immersion for certain part configurations. This technology can also be performed as countercurrent cascade rinsing with spray rinses instead of overflow immersion rinses. 75 187 1,767 Flow Restrictors Equipment that prevents the flow in a pipe from exceeding a predetermined flow rate. Flow restrictors can be used to limit the flow into a rinse system. For continuously flowing rinses, a flow restrictor controls the flow into the system, ensuring a consistent,

optimum flow rate. 50 127 1,581 8.0  Pollution Prevention and Wastewater Treatment Technologies Table 8 1 ( Continued)

Technology Technology Description Demonstration Status Number of Facilities Visited Using the Technologya Number of Survey Facilities Using the Technologyb Estimated Number of MP& M Facilities Using the Technologyc Conductivity Probes Equipment that measures the conductivity of water in a rinse tank to regulate the flow of fresh rinse water into the rinse system. A solenoid valve on the rinse system fresh water supply is connected to the controller, which opens the valve when a preset conductivity level is exceeded and closes the valve when conductivity is below that level. 40 29 320 8 4 Source: MP& M site visits, MP& M sampling episodes, MP& M surveys and technical literature. Statistics specific to wastewater discharging facilities.

a Indicates the number of MP& M facilities visited by EPA that use the listed technology. EPA visited a total of 221 facilities.

b Number of survey facilities based on data collected in 1996 detailed survey only. The 1989 survey did not request this information. EPA sent the 1996 detailed survey to 311 facilities.

c Indicates the estimated number of MP& M facilities currently performing this technology based on the 1996 detailed survey. EPA s national estimate of the 1996 detailed survey includes approximately 4,900 facilities. EPA estimated numbers in this column using statistical weighting factors for the 1996 detailed survey respondents. See Section 3.0 for a discussion of the development of national estimates and statistical survey weights. 8.0  Pollution Prevention and Wastewater Treatment Technologies Countercurrent Cascade Rinsing Countercurrent cascade rinsing refers to a series of consecutive rinse tanks that are plumbed to cause water to flow from one tank to another in the direction opposite of the work flow. Fresh water flows into the rinse tank located farthest from the process tank and overflows ( i. e., cascades) into the rinse tank that is closest to the process tank. This is called countercurrent rinsing because the work piece and the rinse water move in opposite directions. Over time, the first rinse becomes contaminated with drag out solutions and reaches a stable concentration of process bath constituents that is lower than the concentration in the process bath. The second rinse stabilizes at a lower concentration, which enables less rinse water to be used than if only one rinse tank were in place. The more countercurrent cascade rinse tanks ( three stage, four stage, etc.), the less rinse water is needed to adequately remove the process solution. This differs from a single, overflow rinse tank where the rinse water is composed of fresh water that is discharged after use without any recycle or reuse. Figure 8 1 illustrates countercurrent cascade rinsing.

Figure 8 1. Countercurrent Cascade Rinsing The rinse rate needed to adequately dilute drag out depends on the concentration of process chemicals in the initial process bath, the concentration of chemicals that can be tolerated in the final rinse tank to meet product specifications, the amount of drag out solution carried into each rinse stage, and the number of countercurrent cascade rinse tanks. These factors are expressed in Equation 8 1 ( 2):

( 8 1)

8 5 8.0  Pollution Prevention and Wastewater Treatment Technologies where:

gal/ min. Vr = the flow rate through each rinse stage, gal/ min; Co = the concentration of the contaminant( s) in the initial process bath, mg/ L; Cf = the tolerable concentration of the contaminant( s) in the final rinse to give acceptable product cleanliness, mg/ L; n = the number of rinse stages used; and VD = the drag out carried into each rinse stage, expressed as a flow rate,

This mathematical rinsing model is based on complete rinsing ( i. e., removal of all contaminants from the work piece) and complete mixing ( i. e., homogeneous rinse water in each rinse stage). Under these conditions, each additional rinse stage can reduce rinse water use by 90 percent. However, each rinse stage needs to have sufficient residence time and agitation for complete mixing to occur in each rinse tank to achieve these conditions. For less efficient rinse systems, each added rinse stage reduces rinse water use by 50 to 75 percent.

Countercurrent cascade rinsing systems have higher capital costs than do overflow rinses and require more space to accommodate the additional rinse tanks. Also, when countercurrent cascade rinsing is used, the low flow rate through the rinse tanks may not provide the needed agitation for drag out removal. In such cases, air or mechanical agitation may be added to increase rinsing efficiency.

Drag out Rinsing Drag out rinse is a stagnant rinse, initially filled with fresh water, positioned immediately after the process tank. Work pieces are rinsed in drag out tanks directly after exiting the process bath. The drag out rinse collects most of the drag out from the process tank, thus preventing it from entering the subsequent flowing rinses and reducing pollutant loadings in those rinses. Gradually, the concentration of process chemicals in the drag out tank rises. In the most efficient configuration, a drag out tank follows a heated process tank that has a moderate to high evaporation rate. A portion of the fluid in the drag out tank returns to the process tank to replace the evaporative loss. The level of fluid in the drag out tank is maintained by adding fresh water. Electrolytic recovery, discussed in Section 8.2.6, is commonly used to remove dissolved metals from drag out tanks.

Spray Rinsing For certain work piece configurations, spray rinsing uses considerably less water than does immersion rinsing. During spray rinsing, the parts are held over a catch tank and are sprayed with water. Water then drips from the part into the catch tank, and is then either recycled to the next stage or discharged to treatment. Spray rinsing can occur in a countercurrent cascade configuration, further reducing water use. Spray rinsing can enhance draining over a process 8 6 8.0  Pollution Prevention and Wastewater Treatment Technologies bath by diluting and lowering the viscosity of the process fluid film clinging to the work piece. Using spray rinsing can control rinse water flow.

8.1.2 Additional Design Elements In addition to rinse configuration, unit operations can be modified in other ways to reduce drag out of process bath chemicals. For example, air knives and drip tanks reduce the pollutant loading and volume of rinse water requiring treatment. Other aspects of good rinse tank design include positioning the water inlet and discharge points of the tank at opposite locations in the tank to avoid short circuiting, using air agitation for better mixing, using a flow distributor, and using the minimum tank size possible ( 3). Four rinse design elements are described in more detail below.

Air Knives Air knives are high pressure air blowers installed over a process tank or drip shield and are designed to remove drag out by blowing the liquid off the surface of work pieces and racks and into a catch tank. Liquid from the catch tank is pumped back to the process tank. Air knives are most effective with flat parts and cannot be used to dry surfaces that passivate or stain due to oxidation.

Drip Shields Drip shields are inclined sheets installed between process tanks and rinse tanks to recover, and drain to the process tank, process fluid that drips from racks and barrels and would otherwise fall into rinse tanks or onto the floor. Often, drip shields are composed of polypropylene or another inert material.

Drip Tanks Drip tanks are installed immediately after the process tank. Work pieces exiting a process bath are held over the drip tank and the process fluid that drips from the work pieces collects in the drip tank. When enough fluid is collected in the drip tank, the fluid flows back to the process tank.

Long Dwell Time Automatic finishing lines can be programmed to include optimum drip times. Long dwell times over the process tank reduce the volume of drag out reaching the rinsing system. On manual lines, racks can be hung on bars over process baths to allow the fluid drip. Barrels can be rotated over the process bath to enhance drainage. Increases in drip time may be unsuitable for surfaces that can be oxidized or stained by exposure to air.

8 7 8.0  Pollution Prevention and Wastewater Treatment Technologies 8.1.3 Rinse Water Use Control Facilities can reduce water use by coordinating and closely monitoring rinse water requirements ( e. g., rinse water use is optimized based on drag out rates so that the rinse quality is consistent). Matching water use to rinse water requirements optimizes the quantity of rinse water used for a given work load and tank arrangement ( 3). Inadequate controlling water use negates the benefits of using multiple rinse tanks or other water conservation practices and results in a high water usage.

Many facilities use some form of rinse water control. The four most common methods are flow restrictors ( these can be used with other methods to regulate the rate at which water is dispensed), manual control ( i. e., turning water valves on and off as needed), conductivity controls, and timer rinse controls. Using data from the 1996 MP& M industry survey, EPA estimates there are over 1,900 MP& M facilities using this equipment to control rinse water flow. These are discussed below.

Flow Restrictors A flow restrictor prevents the flow in a pipe from exceeding a predetermined flow rate. Flow restrictors are commonly installed on a rinse tank s water inlet. These devices contain an elastomer washer that flexes under pressure to maintain a constant water flow regardless of pressure. Flow restrictors can maintain a wide range of flow rates, from less than 0.1 gal/ min to more than 10 gal/ min. As a stand alone device, a flow restrictor provides a constant water flow and is therefore best suited for continuous rinsing. For intermittent rinsing operations, a flow restrictor does not coordinate the rinse flow with drag out introduction. Precise control with intermittent operations typically requires a combination of flow restrictors and rinse timers. However, for continuous rinsing ( e. g., continuous electroplating machines), flow restrictors may be adequate for good water use control.

Conductivity Controllers Conductivity controllers use conductivity probes to measure the conductivity ( total dissolved solids ( TDS)) of water in a rinse tank to regulate the flow of fresh rinse water into the rinse system. Conductivity controllers consist of a controller, a meter with adjustable set points, a probe that is placed in the rinse tank, and a solenoid valve. As parts are rinsed, dissolved solids enter the water in the rinse tank, raising the conductivity of the water. When conductivity reaches a set point where the water can no longer provide effective rinsing, the solenoid valve opens to allow fresh water to enter the tank. When the conductivity falls below the set point, the valve closes to discontinue the fresh water flow.

In theory, conductivity control of rinse flow is a precise method of maintaining optimum rinsing conditions in intermittent rinsing operations. In practice, conductivity controllers work best with deionized rinse water. Incoming fresh water conductivity may vary day to day and season to season, which forces frequent set point adjustments. In addition,

8 8 8.0  Pollution Prevention and Wastewater Treatment Technologies suspended solids and nonionic contaminants ( e. g., oil) can cause inadequate rinsing and are not measured by the conductivity probe.

Rinse Timers Rinse timers are electronic devices that control a solenoid valve. The timer usually consists of a button that, when pressed, opens the valve for a predetermined time period, usually from 1 to 99 minutes. After the time period has expired, the valve automatically closes. The timer may be activated either manually by the operator or automatically by the action of racks or hoists. Automatic rinse timers are generally preferred for intermittent rinses because they eliminate operator error. Rinse timers installed in conjunction with flow restrictors can provide precise control when the incoming water pressure may rise and fall. Rinse timers are less effective in continuous or nearly continuous rinse operations ( e. g., continuous electroplating machines) because the rinse operates nearly continuously.

8.1.4 Pollution Prevention for Process Baths Facilities also can implement measures that will reduce or prevent pollution in process baths to reduce the drag out pollutant loadings and therefore the amount of drag out solution produced. Examples of these technologies are increasing bath temperature, operating at lower batch concentration, and using wetting agents, discussed below:

Temperature and viscosity are inversely related; therefore, operating a bath at the highest possible temperature will lower process bath viscosity and reduce drag out.

Operating at the lowest possible concentration reduces the mass of chemicals in a given volume of drag out. Also, viscosity and concentration are directly related; therefore, lower process bath concentration will result in lower process bath viscosity and less drag out volume. Contaminants and other process bath impurities should be minimized, if possible, to extend the usefulness of the bath, reducing the frequency of treatment or disposal.

Adding wetting agents or surfactants to some process baths reduces viscosity and surface tension, thereby significantly reducing drag out.

8.2 In Process Pollution Prevention Technologies This section describes in process pollution prevention technologies used at MP& M facilities to reduce pollutant loadings to the wastewater treatment system. Table 8 2 lists a number of in process pollutant prevention technologies. This table also provides EPA s estimate of the number of MP& M facilities employing the various in process pollutant 8 9 8.0  Pollution Prevention and Wastewater Treatment Technologies 8 10 Table 8 2 MP& M In Process Pollution Prevention Technologies Technology Technology Description Demonstration Status Number of Facilities Visited Using the Technologya Number of Survey Facilities Using the Technologyb Estimated Number of MP& M Facilities Using the Technologyc Evaporation with Condensate Recovery Removes water by evaporation, leaving a concentrated residue for disposal and water vapor for condensation and reuse. 7 15 147 Ion Exchange ( in

process) Removes metal salts from electroplating rinse water using combined cation and anion exchange. Effluent ( permeate) from the ion exchange flows back to the electroplating rinse system. Ion exchange regenerants are either discharged to the end of pipe chemical precipitation unit for metals removal or to electrolytic recovery for metals recovery. 35 33 437 Reverse Osmosis Forces wastewater through a membrane at high pressure, leaving a concentrated stream of pollutants for disposal. Reverse osmosis may provide an effluent clean enough for reuse. 3 1 3 Centrifugation of Painting Water Curtains Removes the heavier solids from the water curtain by centrifugation,

allowing the water to be reused. The solids are collected as a cake in the basket of the centrifuge. This technology can achieve closed loop reuse of water curtains. 3 1 12 Filtration of Painting Water Curtains Removes solids by filtration ( cloth, sand, diatomaceous earth, etc.)

followed by reuse. This technology can achieve closed loop reuse of water curtains. 2 3 20 Settling of Painting Water Curtains Removes the heavier solids from the water curtains by gravity separation. This technology can be used in conjunction with other removal technologies to lessen the solids loading. 5 5 23 Biocide Addition to Lengthen Coolant Life Can impede the growth of microorganisms that cause rancidity.

Machining coolant is often discarded as it becomes rancid. 9 27 216 Centrifugation of Machinery Coolant Removes the solids from the coolant by centrifugation to extend its usable life. Some high speed centrifuges can also perform liquid liquid separation to remove tramp oils and further extend coolant life. 18 10 78 8.0  Pollution Prevention and Wastewater Treatment Technologies Table 8 2 ( Continued)

Technology Technology Description Demonstration Status Number of Facilities Visited Using the Technologya Number of Survey Facilities Using the Technologyb Estimated Number of MP& M Facilities Using the Technologyc Filtration of Machinery Coolant Removes the solids from the coolant using filters such as cloth, sand, or carbon to extend its usable life. 18 18 142 Skimming of Tramp Oils in Machinery Coolants Removes tramp oils using mechanical skimming to extend coolant life. Tramp oil buildup often makes machining coolant unusable. 8 9 82 Pasteurization of Machinery Coolants Kills the microorganisms that cause rancidity using heat. Machining coolant is often discarded as it becomes rancid. 2 2 18 General Filtration of Baths and Solutions Removes metals and other impurities from process tanks, including electrolytic plating solutions and acid/ alkaline cleaning tanks.

Increases bath longevity. Technologies include paper filters, carbon adsorption, and magnetic separators. 6 Electrolytic Recovery ( Electrowinning) Recovers dissolved metals from concentrated sources using an electrochemical process. For rinses, electrolytic recovery is typically restricted to drag out rinses. Flowing rinses are generally too dilute for efficient electrolytic recovery. This technology effectively recovers metals from ion exchange regenerants. 22 23 142 8 11 Source: MP& M site visits, MP& M sampling episodes, MP& M surveys and technical literature. Statistics specific to wastewater discharging facilities.

a Indicates the number of MP& M facilities visited by EPA that use the listed technology. EPA visited a total of 221 facilities.

b Number of survey facilities based on data collected in 1996 detailed survey only. The 1989 survey did not request this information. EPA sent the 1996 detailed survey to 311 facilities.

c Indicates the estimated number of MP& M facilities currently performing this technology based on the 1996 detailed survey. EPA s national estimate of the 1996 detailed survey includes approximately 4,900 facilities. EPA estimated numbers in this column using statistical weighting factors for the 1996 detailed survey respondents. See Section 3.0 for a discussion of the development of national estimates and statistical survey weights. 8.0  Pollution Prevention and Wastewater Treatment Technologies prevention technologies based on occurrence at surveyed facilities and their respective survey weights. In process pollution prevention technologies can be applied to process baths or rinses. Not all technologies discussed in this subsection are applicable to all MP& M facilities.

Process baths become contaminated with impurities that affect their performance. The sources of process bath contamination include: ( 1) breakdown of process chemicals; ( 2) buildup of by products ( e. g., carbonates); ( 3) contamination from impurities in make up water, chemicals, or anodes; ( 4) corrosion of parts, racks, tanks, heating coils, etc.; ( 5) drag in of chemicals; ( 6) errors in bath additions; and ( 7) airborne particles entering the tank. If not properly maintained, process baths become prematurely unusable and require disposal. Regeneration and maintenance techniques help keep baths in good operating condition, thereby extending the useful lives of process solutions. Using these technologies reduces the frequency of process bath discharges, and therefore reduces pollutant loadings to the wastewater treatment system. This, in turn, reduces wastewater treatment requirements and sludge disposal costs.

Rinsing removes residual process chemicals from the surface of a work piece. As more and more work pieces are rinsed, the concentration of process chemicals ( contaminants) in the rinse water increases. At some point, the concentration of process chemicals in the rinse water becomes so high that an unacceptable amount of process chemicals remain on the surface of the work piece. When this occurs, clean water is added to the rinse solution to lower the concentration of process chemicals to a level that will not impact the quality of the work piece. Overflow from the rinsing operation goes to treatment for removal of the residual process chemicals. For continuous processing operations, clean water may continuously flow into the rinse process to ensure that the concentration of contaminants will not exceed the quality limit for the work piece.

This section describes the following technologies used to treat and reuse process solutions:

  Activated carbon adsorption;

  Carbonate freezing;

  Centrifugation and pasteurization of machining coolants;

  Centrifugation and recycling of painting water curtains;

  Electrodialysis;

  Electrolytic recovery;

  Evaporation;

  Filtration;

  Ion exchange; and   Reverse osmosis.

8.2.1 Activated Carbon Adsorption Activated carbon adsorption is a common method of removing organic contaminants from electroplating baths. Process solution flows through a filter where the carbon 8 12 8.0  Pollution Prevention and Wastewater Treatment Technologies adsorbs organic impurities that result from the breakdown of bath constituents. Carbon adsorption can be either a continuous or batch operation, depending on the site s preference. Carbon treatment is most commonly applied to nickel, copper, zinc, and cadmium electroplating baths but also can be used to remove organic contaminants from paint curtains.

8.2.2 Carbonate   Freezing

Carbonate   freezing  removes excessive carbonate buildup by forming carbonate salt crystals at a low temperature that are then removed. MP& M facilities most often apply this process to electroplating baths formulated with sodium cyanide. Carbonates build up in the process bath by the breakdown of cyanide ( especially at high temperatures) and the adsorption of carbon dioxide from the air. An excessive carbonate concentration reduces the product quality of many metal finishing operations. Carbonate   freezing  takes advantage of the low solubility of carbonate salts in the sodium cyanide bath. The method lowers the bath temperature to approximately 26 ° F ( 3 ° C), at which point hydrated salt ( Na2 CO3   10H2 O) crystallizes out of solution. The crystallized carbonate can be removed by decanting the fluid into another tank or by filtration.

8.2.3 Centrifugation and Pasteurization of Machining Coolants Most machining coolants contain water soluble oil in water. The water soluble coolant typically is pumped from a sump, over the machining tool and work piece during machining, and back to the sump. Over a period of time, recycled coolant becomes ineffective, or spent, for one or more of the following reasons:

  The concentration of suspended solids in the coolant begins to inhibit performance;

  Nonemulsified, or   tramp,  oil collects on the surface of the coolant, inhibiting performance;

  The coolant becomes rancid due to microbial growth; or   Coolant additives are consumed by drag out and organic breakdown, thus reducing corrosion prevention and lubrication properties.

As shown in Table 8 2, EPA estimates that nearly 300 MP& M facilities use centrifugation and biocide/ pasteurization processes to extend the life of their water soluble coolants.

Coolant recycling is most effective when facilities minimize the number of different coolants used on site and use a centralized coolant recycling system. However, some facilities may not be able to use a single recycling system because of multiple coolant types required by product or customer specifications. In this case, facilities may need to purchase dedicated coolant recycling systems for each type of coolant used.

8 13 8.0  Pollution Prevention and Wastewater Treatment Technologies Using a centrifugal separator and pasteurization unit can extend the useful life of machining coolants. The separator is a rotating chamber that uses centrifugal force to push the coolant through a mesh chamber, leaving behind solid contaminants of sludge. Sludge is scraped from the centrifuge and collected in a sludge hopper. Some high speed centrifuges also can perform liquid liquid separation to remove tramp oils. The coolant undergoes pasteurization after separation to kill the microorganisms that cause bacterial growth. Adding a biocide can also control bacterial growth. Figure 8 2 shows a diagram of a typical machine coolant recycling system.

Figure 8 2. Machine Coolant Recycling System Centrifugal separators are very reliable and require only routine maintenance, such as periodic cleaning and removal of accumulated solids. Flow rate is the primary operating factor to control. The sludge generated from this technology is commonly classified as a hazardous waste, based on the metal type processed and the amount of metal that dissolves into the coolant. Facilities typically haul the sludge off site for treatment and disposal.

Centrifugation and pasteurization can be used in conjunction with oil skimming and biocide addition to reduce coolant discharge and pollutant generation at the source. Oil skimming using a vertical belt system ( described in Section <IP_ADDRESS>) removes large amounts of tramp hydraulic oils floating on the surface of the machine coolant. Oil skimming and biocide addition can further extend the life of water soluble coolant, thereby reducing the amount of coolant and wastewater requiring treatment and disposal, and minimizing fresh coolant requirements.

8.2.4 Centrifugation and Recycling of Painting Water Curtains Water curtains are a continuous flow of water behind the work piece being spray painted in a paint booth. The water traps paint overspray and is continuously recirculated in the paint curtain until the solids content in the wastewater necessitates either in process treatment and recycling or discharge. Based on data from the 1996 MP& M detailed survey, approximately 12 MP& M facilities centrifuge and recycle water from their paint curtains.

8 14 8.0  Pollution Prevention and Wastewater Treatment Technologies Wastewater from painting water curtains commonly contains organic pollutants as well as certain metals. Eliminating the discharge of wastewater from painting water curtains may eliminate the need for an end of pipe treatment step for organic pollutants at certain facilities. Moreover, if a facility uses only painting water curtains and continuously recycles the water, the facility would not need end of pipe wastewater treatment.

Figure 8 3 shows a diagram of a typical in process centrifugation and recycling treatment system for a paint curtain. Centrifugal separators remove the solids and recycle the water curtain, eliminating the need for discharge. This system can recycle, the paint curtain water continuously. The system pumps the water curtain from the paint curtain sump to a holding tank, then through the centrifugal separator, which separates the solids from the wastewater ( see section 8.2.3). Solids from the centrifuge are hauled for off site disposal, while the treated wastewater is returned to the paint booth. Centrifugation of the paint curtain proceeds until all wastewater is treated and only sludge remains in the paint curtain sump. Operators must remove the sludge in the paint curtain sump either manually, with a sludge pump, or by vacuum truck. The facility may add detactifiers before centrifugation to increase the solid separation efficiency. Detactifiers make the paint solids less sticky, allowing them to be more easily removed from the centrifuge. Make up water is added to the system to compensate for evaporation.

Figure 8 3. Centrifugation and Recycling of Painting Water Curtains As discussed in Section 8.2.3, centrifugal separators are very reliable and require only routine maintenance. Flow rate is the primary operating factor to control. One disadvantage of this technology is that it may not be economically feasible for facilities generating only a small amount of paint curtain wastewater. Facilities that have multiple sumps can use portable centrifuges.

8 15 8.0  Pollution Prevention and Wastewater Treatment Technologies The sludge generated from painting water curtains is commonly classified as a hazardous waste, based on the type of paint used, and typically is hauled off site for treatment and disposal. See Appendix D for more information on pollution prevention practices with painting operations.

8.2.5 Electrodialysis Electrodialysis is a process in which dissolved colloidal species are exchanged between two liquids through selective semipermeable membranes ( 11). The technology applies a direct current across a series of alternating anion and cation exchange membranes to remove dissolved metal salts and other ionic constituents from solutions.

An electrodialysis unit consists of a rectifier and a membrane stack. The rectifier converts alternating current to direct current. The stack consists of alternating anion and cation specific membranes that form compartments. As the feed stream enters the unit, ions move across the electrodialysis membranes, forming a concentrated stream and a deionized stream. When the compartments are filled, a direct current is applied across each membrane in the stack. Cations traverse one cation specific membrane in the direction of the cathode and are trapped in that concentrate compartment by the next membrane, which is anion specific. Anions from the neighboring compartment traverse the anion specific membrane in the direction of the anode, joining the cations, and are likewise trapped in the concentrate compartment by the next cation specific membrane. In this way, the technology depletes the feed stream of ions, and traps anions and cations in each concentrate compartment. Facilities typically use electrodialysis to remove metal ions from electroplating wastewater. Figure 8 4 shows a diagram of an electrodialysis cell.

8 16 8.0  Pollution Prevention and Wastewater Treatment Technologies Figure 8 4. Electrodialysis Cell By using the electrodialysis cell, facilities remove impurities from the process bath, extending its life. Facilities can treat the removed concentrate stream on site, or haul it off site for disposal, treatment, or metals reclamation.

8.2.6 Electrolytic Recovery Electrolytic recovery is an electrochemical process used to recover metal contaminants from many types of process solutions and rinses, such as electroplating rinse waters and baths. Electrolytic recovery removes metal ions from a waste stream by processing the stream in an electrolytic cell, which consists of a closely spaced anode and cathode. Equipment consists of one or more cells, a transfer pump, and a rectifier. Current is applied across the cell and metal cations are deposited on the cathodes. The waste stream is usually recirculated through the cell from a separate tank, such as a drag out recovery rinse.

Facilities typically apply electrolytic recovery to solutions containing either nickel, copper, precious metals, or cadmium. Chromium cannot be electrolytically recovered because it exists primarily in anionic forms such as dichromate. Drag out rinses and ion exchange regenerant are solutions that commonly are processed using electrolytic recovery. Some solutions require pH adjustment prior to electrolytic recovery. Acidic, metal rich, cation regenerant is an excellent candidate stream for electrolytic recovery and is often electrolytically 8 17 8.0  Pollution Prevention and Wastewater Treatment Technologies recovered without pH adjustment. In some cases, when the target metal concentration is reached, the waste stream can act as cation regenerant.

The capacity of electrolytic recovery equipment depends on the total cathode area and the maximum rated output of the rectifier. Units are available with a cathode area ranging from 1 ft2 to 100 ft2 or larger, and an output of 10 to 1,000 amperes or more. Faraday s law, which states the amount of chemical change produced by an electric current is proportional to the quantity of electricity used, determines theoretical electrolytic recovery rates. Theoretical recovery rates range from 1.09 grams/ amp hour for nickel to 7.35 grams/ amp hour for monovalent gold. Actual rates are usually much lower and depend on the metal concentration in the waste stream. At concentrations under 100 mg/ L, electrolytic recovery rates may be below 10 percent of the theoretical maximum.

Electrolytic recovery units use various types of cations, depending mainly on the concentration of metal in the waste stream. Cathodes are often classified by their surface area. Flat plate cathodes have the lowest surface area and are used only for recovering metal from metal rich waste streams ( usually 1,000 to 20,000 mg/ L of metal). Reticulate cathodes, which have a metallized woven fiber design, have a surface area 10 times greater than their apparent area. These cathodes are effective over a wide range of metal concentrations but typically are used where the dissolved metal concentration is below 100 mg/ L. Carbon and graphite cathodes have the highest surface area per unit of apparent area. Their use is usually restricted to metal concentrations below 1,000 mg/ L.

Reticulate or carbon cathodes can recover metals in electrolytes to concentrations as low as 5 mg/ L. Electrolytes are substances that dissociate into ions in solution ( i. e., water), thereby becoming electrically conducting ( 4). In practice, however, the target effluent concentration for most applications is 50 to 250 mg/ L or higher because of the time and energy required to achieve concentrations less than 100 mg/ L. With flat plate cathodes, the target effluent concentration is usually above 500 mg/ L, because plating efficiency drops as concentration falls. Plating time required to lower the concentration of a pollutant from 100 to 10 mg/ L can be several times longer than that required to lower the concentration from 10,000 mg/ L to 100 mg/ L. Also, unit energy costs ( measured in dollars per pound of metal recovered) increase substantially at lower metal concentrations.

Electrolytic recovery units have relatively low labor requirements. Units recovering dissolved metal from drag out rinse tanks only may require occasional cleaning and maintenance. Units treating batch discharges from ion exchange units ( see Section <IP_ADDRESS>) require more labor due to the higher metal content of the solution and the resultant increase in cathode loading frequency. Energy costs for this technology can be high, and, in some cases, exceed the recovery value of the metal. Energy requirements depend on several factors, including required voltage, rectifier efficiency, and current efficiency. In addition, from an energy standpoint, electrolytic recovery removes metals from concentrated solutions more efficiently than from dilute solutions. Electrode replacement costs may be significant for units 8 18 8.0  Pollution Prevention and Wastewater Treatment Technologies using disposable cathodes, especially for high metal recovery rates. However, if electrodes are constructed properly, cathodes and anodes may last more than five years for most applications.

Numerous vendors offer electrolytic recovery technology. The technology is applicable to a wide range of processes, drag out rinses, and ion exchange regenerants due to the diversity of materials and configurations available for anodes and cathodes. Electrolytic recovery is not applicable to flowing rinses due to the lower metal concentrations and the extended time required for metal recovery. In most cases, this technology cannot cost effectively remove dissolved metals to concentrations required for discharge to POTWs or surface waters.

8.2.7 Evaporation Evaporation is a volume reduction and water recovery technology applicable when raw water costs are high or discharge to either a receiving stream or the local sewerage district is not permitted. EPA estimates there are 147 MP& M facilities using evaporation to reduce the volume of their waste and to recover and reuse their water. Evaporators have the potential to recover 95 percent of the water in a waste stream for reuse in the process. MP& M facilities use two basic types of evaporators: atmospheric and vacuum. Atmospheric evaporators are more prevalent and are relatively inexpensive to purchase and easy to operate. Vacuum evaporators are mechanically more sophisticated and are more energy efficient. Facilities typically use vacuum evaporators when evaporation rates greater than 50 to 70 gallons per hour are required. MP& M facilities use evaporators to recover metals from ion exchange regenerates, to reduce the volume of oily wastes that require off site transfer, and to recover and reuse rinse water from plating operations.

Equipment required for evaporation systems include ( 12):

  Basket strainers in lift stations and sumps to prevent items like shop rags from reaching the evaporator;

  Equalization tanks to handle batch dumps of process water;

  An oil skimmer in the equalization tank to remove floatable oil;

  Evaporators ( either vacuum or atmospheric);

  Residue holding tanks;

  Air pollution control equipment;

  A condenser to capture water vapor for return to the manufacturing process; and   Natural gas or propane tanks for evaporator fuel storage.

8 19 8.0  Pollution Prevention and Wastewater Treatment Technologies Residue from evaporators can be recycled if sufficiently pure, disposed of off site, or used for energy recovery if the material has a sufficient BTU content.

8.2.8 Filtration Filtration removes suspended solids from surface finishing operations. EPA estimates there are nearly 150 MP& M facilities that use filters on their machining and grinding operations to remove solids, debris, or swarf from machining coolants. If solids are not removed from machining coolants, they may cause a rough or burred surface on the work piece. Filtered coolants return to the manufacturing process. In process filtration extends the life of the coolant and reduces the amount of oil and grease sent to treatment. Filtration equipment includes cartridge filters, precoat diatomaceous earth filters, sand, and multimedia filters.

Cartridge filters are available with either in tank or external configurations. The in tank units are used mostly for small tanks and the external units for larger tanks. Most cartridges are disposable; however, washable and reusable filters are available, which further reduce waste generation. Precoat, sand, and multimedia filters are used mostly for large tanks. The filter media used depends on the chemical and physical characteristics of the bath, which determine the filter material type, density, nominal micron retention, wet strength, mullen burst, and air permeability. Material type is important to ensure the media is compatible with the liquid being filtered. Media density is how close or dense the media fibers are laid, laminated, or woven. Nominal micron retention indicates the smallest particle size the media will retain to develop a filter cake. Flux rate through the filter is determined by the air permeability characteristics. All filtration systems are sized based on solids loading and the required flow rate.

Membrane filtration also can remove oils and metals from process baths or rinses, and remove solids from paint curtains or tramp oils from machine coolants to extend usable life. They are also commonly used to recover and recycle electrophoretic painting (  e coat ) solutions. Membrane filtration is a pressure driven process that separates solution components based on molecular size and shape. Solvent and small solutes can pass through the membrane while the membrane retains and collects larger compounds as a concentrated waste stream. The cleaner permeate can be reused in the process while the concentrated waste stream is discharged to treatment. Figure 8 5 shows a typical membrane filtration unit.

8 20 8.0  Pollution Prevention and Wastewater Treatment Technologies Figure 8 5. Membrane Filtration Unit <IP_ADDRESS> Ion Exchange ( in process)

Ion exchange is a commonly used technology within MP& M facilities. In addition to water recycling and chemical recovery applications, ion exchange is used to soften or deionize raw water for process solutions. Figure 8 6 shows a typical ion exchange system.

Ion exchange is a reversible chemical reaction that exchanges ions in a feed stream for ions of like charge on the surface of an ion exchange resin. Resins are broadly divided into cationic or anionic types. Typical cation resins exchange H+ for other cations, while anion resins exchange OH for other anions ( 10).

8 21 8.0  Pollution Prevention and Wastewater Treatment Technologies Figure 8 6. Ion Exchange A feed stream passes through a column, which holds the resin. The feed stream is usually either dilute rinse water ( in process ion exchange) or treated wastewater ( end of pipe ion exchange). Often, prior to ion exchange treatment, the feed stream passes through a cartridge filter and a carbon filter to remove suspended solids and organic pollutants that foul the resin bed. The exchange process continues until the capacity of the resin is reached ( i. e., an exchange has occurred at all the resin sites). A regenerant solution then passes through the column. For cation resins, the regenerant is an acid, and the H+ ions replace the cations captured from the feed stream. For anion resins, the regenerant is a base, and OH ions replace the anions captured from the feed stream. The metals concentration is much higher in the regenerant than in the feed stream; therefore, the ion exchange process not only separates the metals from the waste stream but also results in a more concentrated waste stream.

MP& M facilities use ion exchange for water recycling and metal recovery. For water recycling, cation and anion columns are placed in series. The feed stream is deionized and the product water is reused for rinsing. Often, the system can achieve closed loop rinsing. The regenerant from the cation column contains metal ions, which are recoverable in elemental form via electrolytic recovery ( see Section 8.2.6). The anion regenerant typically flows to wastewater treatment. Facilities use this type of ion exchange to recycle relatively dilute rinse streams. Generally, the TDS concentration of such streams must be below 500 mg/ L to maintain an 8 22 8.0  Pollution Prevention and Wastewater Treatment Technologies efficient regeneration frequency. Reducing drag out can enhance the efficiency of the recovery process. Effluent TDS concentrations of 2 mg/ L or less are typical.

When facilities are seeking only metal recovery, they use a single or double cation column unit containing selective resin. These resins attract divalent cations while allowing monovalent cations to pass, a process usually called metal scavenging. This technology is efficient if the metal ions being scavenged are the primary source of ions in the stream. Ion exchange provides effective metals recovery even when the metal content of the stream is only a small fraction of the TDS present in the stream, making scavenging suitable over a wider range of TDS than water recycling. Scavenging also provides a highly concentrated regenerant, particularly suitable for electrolytic recovery ( see Section 8.2.6). Water recycling using this ion exchange configuration is not possible because only some of the cations and none of the anions are removed. Standard units typically achieve effluent metal concentrations of under 0.5 mg/ L.

Many process wastewaters are excellent candidates for ion exchange, including the rinse water from plating processes of chromium, copper, cadmium, gold, lead, nickel, tin, tin lead, and zinc. Ion exchange resins usually are regenerated using inexpensive chemicals such as sulfuric acid and sodium hydroxide. Gold bearing resins are difficult to regenerate and frequently require incineration to recover the gold content. Lead also is difficult to recover from ion exchange resins. Methane sulfonic acid and fluoboric acid ( usually not suitable for electrolytic recovery) are effective regenerants for lead ion exchange but are very expensive. Cyanide rinse waters are amenable to ion exchange; cation resins can break the metal cyanide complex and the cyanide is removed in the anion column. The metals in the cation regenerant can be recovered electrolytically and the cyanide present in the anion regenerant can be returned to the process or discharged to treatment.

Ion exchange equipment ranges from small, manual, single column units to multi column, highly automated units. Two sets of columns are necessary for continuous treatment; one set receives the wastewater flow while the other set is being regenerated. Thus, two column metal scavenging and four column deionizing systems are common. Automatic systems direct the wastewater flow and initiate regeneration with little or no operator involvement.

The labor requirements for ion exchange depend on the automation level of the equipment. Manual systems can have significant labor costs associated with preparing, transporting, and disposing of regenerants. Automatic systems require far less labor. Resins need to be replaced periodically due to organic contamination, resin oxidation, and fouling from suspended solids. This process can be hastened by misuse, accidents, or poor engineering.

Equipment size is based on flow rate and concentration. Resin capacity varies but often ranges from 1 to 2 lbs/ ft3 . Flow rates may range from 1 to 20 or more gpm. Columns typically are sized to handle wastewater flow for at least a period of time equal to that required for regeneration. Automatic systems are sized to provide continuous treatment. Regeneration volume typically ranges from 2 to 4 resin bed volumes of dilute acid or caustic. Concentrations of feed stream contaminants generally range from 10 to 20 g/ L.

8 23 8.0  Pollution Prevention and Wastewater Treatment Technologies <IP_ADDRESS> Reverse Osmosis Reverse osmosis is a membrane separation technology used by MP& M facilities for chemical recovery and water recycling. The system pumps dilute rinse water to the surface of the reverse osmosis membrane at pressures of 400 to 1,000 pounds per square inch gauge ( psig). The membrane separates the feed stream into a reject stream and a permeate. The reject stream, containing most of the dissolved solids in the feed stream, is retained by the membrane while the permeate passes through. Reverse osmosis membranes reject more than 99 percent of multivalent ions and 90 to 96 percent of monovalent ions, in addition to organic pollutants and nonionic dissolved solids. The permeate stream usually is of sufficient quality to be recycled as rinse water, despite the small percentage of monovalent ions ( commonly potassium, sodium and chloride) that pass through the membrane. Reverse osmosis equipment is similar to the equipment shown in Figure 8 5.

A sufficiently concentrated reject stream can be returned directly to the process bath. Recycling the stream through the unit more than once or by increasing the feed pressure can increase the reject stream concentration. In multiple stage units containing more than one membrane chamber, the reject stream from the first chamber is routed to the second, and so on. The combined reject streams from multistage units may, in some cases, have high enough concentrations to go directly back to the bath.

The capacity of reverse osmosis equipment generally is measured in flow volume, and is determined by the membrane surface area and operating pressure. Increasing the surface area of the membrane usually increases the membrane capacity. Operating at higher pressures increases the permeate flow volume per unit membrane area ( also called the flux). Reject stream concentration increases with pressure and decreases as flow volume increases.

Facilities may need to prefilter and pretreat the feed stream to lengthen membrane life or reduce the frequency of fouling; filtration to remove suspended solids is usually necessary. Adjusting pH may prevent precipitation as the feed stream is concentrated, but it may make the concentrate unfit to return to the process bath.

Reverse osmosis is most applicable to electroplating rinse waters, including electroplating of Watts nickel, bright nickel, brass cyanide, copper cyanide, and zinc cyanide. This technology can treat TDS concentrations of up to 1,000 mg/ L. Permeate TDS concentrations of 250 mg/ L or less are typical, and the dissolved solids are mostly commonly monovalent ions, allowing the permeate stream to be reused in many rinsing operations.

The maximum achievable reject stream concentration for basic reverse osmosis equipment is approximately 20,000 mg/ L TDS. Multipass and multistage units achieve concentrations of 30,000 mg/ L TDS or higher. If the reject stream is acceptable to return directly to the process bath and the permeate is recycled as rinse water, a closed loop is created. However, returning the reject stream directly to the bath is uncommon because the concentration is often too low. When the reject stream concentration is not high enough to return it to the bath,

8 24 8.0  Pollution Prevention and Wastewater Treatment Technologies it can be concentrated with an evaporator, electrolytically recovered, or discharged to wastewater treatment. When evaporators are used, however, reverse osmosis loses its low energy advantage over other in process reuse and recovery technologies.

Reverse osmosis often has a higher capital cost than does ion exchange when both technologies include an electrolytic recovery unit. When used for water recycling, reverse osmosis and ion exchange both remove similar quantities of metals; however, reverse osmosis may allow for more water recycling.. During reverse osmosis, only the pumps use energy. In most cases, water is recycled; in some cases, a closed loop is possible. Compared to ion exchange, reverse osmosis can treat somewhat higher feed stream concentrations. The concentration of reverse osmosis reject streams are near or higher than that of ion exchange regenerants. Both are less effective in handling oxidizing chemistries or feed streams high in organic compounds and total suspended solids. Ion exchange effluent generally has a lower TDS concentration than does reverse osmosis permeate and can be recycled in most rinses.

For most applications, reverse osmosis membranes last for one to five years, although they are susceptible to fouling from organic pollutants, suspended solids, or misuse. Reverse osmosis units may be able to track the condition of the membrane by measuring the flux. If the membrane fouls or clogs, the flux rate drops, indicating that the membrane should be cleaned. Labor associated with operating reverse osmosis equipment is for periodic membrane cleaning. Membrane and pump replacement are the primary maintenance items.

Best Management Practices and Environmental Management Systems for Pollution Prevention EPA encourages the wide spread use of Best Management Practices ( BMPs), and Environmental Management Systems ( EMS), to achieve improved environmental performance and compliance, pollution prevention through source reduction, and continual improvement ( see EPA Position Statement on Environmental Management Systems, May 15, 2002, DCN 17848, Section 24.4). However, as described in the Section IV of the preamble to the final rule, EPA is not requiring the use of BMPs or EMSs for compliance with the MP& M effluent guidelines.

Best Management Practices ( BMPs) are inherently pollution prevention practices. BMPs may include the universe of pollution prevention encompassing production modifications, operational changes, material substitution, materials and water conservation, and other such measures ( 17). BMPs include methods to prevent the discharge of toxic and hazardous pollutants. BMPs are most effective when organized into a comprehensive facility EMS.

MP& M facilities employ many types of pollution prevention measures including the following: training and supervision; production planning; process or equipment modification; raw material and product substitution or elimination; loss prevention and housekeeping; waste segregation and separation; and closed loop recycling. These practices are discussed in further detail below ( 1).

8 25 8.3 8.0  Pollution Prevention and Wastewater Treatment Technologies   Training and Supervision Training and supervision ensure that employees are aware of, understand, and support the company s pollution prevention goals. Effective training programs translate these goals into practical information that enables employees to minimize waste generation by properly and efficiently using tools, supplies, equipment, and materials.

  Production Planning Production planning can minimize the number of process operation steps and eliminate unnecessary procedures ( e. g., production planning can eliminate additional cleaning steps between process operations).

  Process or Equipment Modification Facilities can modify processes and equipment to minimize the amount of waste generated ( e. g., changing rack configuration to reduce drag out).

  Raw Material and Product Substitution or Elimination Where possible, facilities should replace toxic or hazardous raw materials or products with other materials that produce less waste and less toxic waste ( e. g., replacing chromium bearing solutions with non chromium bearing and less toxic solutions, or consolidating types of cleaning solutions and machining coolants).

  Loss Prevention and Housekeeping Loss prevention and housekeeping includes performing preventive maintenance and managing equipment and materials to minimize leaks, spills, evaporative losses, and other releases ( e. g., inspecting the integrity of tanks on a regular basis; using chemical analyses instead of elapsed time or number of parts processed as the basis for disposal of a solution).

  Waste Segregation and Separation Facilities should avoid mixing different types of wastes or mixing hazardous wastes with nonhazardous wastes. Similarly, facilities should not mix recyclable materials with noncompatible materials or wastes. For example, facilities can segregate scrap metal by metal type, separate cyanide bearing wastewater for preliminary treatment, and segregate coolants for recycling or treatment.

  Closed Loop Recycling Facilities can recover and reuse some process streams. For example, some facilities can use ion exchange to recover metal from electroplating rinse water, reuse the rinse water, and reuse the regenerant solution as process solution make up.

8 26 8.0  Pollution Prevention and Wastewater Treatment Technologies The following sections describe pollution prevention opportunities for a few MP& M facilities.

8.3.1 Pollution Prevention for Cleaning and Degreasing Operations The majority of facilities in the Oily Wastes Subcategory perform cleaning and degreasing operations to remove residual oil and coolants from metal parts following machining and grinding operations. These facilities also perform cleaning and degreasing on equipment undergoing maintenance. Opportunities to reduce waste from these operations include process elimination, material substitution, in process recycling, waste segregation, maintenance/ housekeeping, procedures/ scheduling, and equipment layout/ piping/ automation. Examples of these opportunities are presented below ( 15).

Process Elimination   Determine whether parts need to be cleaned;

  Use easy to clean or no clean rust inhibitors and lubricants;

  Review the parts handling process to determine why parts are getting dirty, and take action to prevent it from happening in the future; and   Purchase clean input stock.

Material Substitution   Clean by brushing and wiping where possible;

  Use aqueous based cleaners;

  Use solvents with low vapor pressure and high flash point; and   Use citrus or terpene cleaners.

In Process Recycling   Use countercurrent rinsing;

  Skim/ filter and reuse aqueous cleaners;

  Reuse solvents by installing filtration or distillation units; and   Install a bioremediation parts washer that uses enzymes to remove oil and grease.

8 27 8.0  Pollution Prevention and Wastewater Treatment Technologies Waste Segregation   Segregate solvents to allow recycling;

  Keep solvents out of waste oil;

  Keep fuel, brake fluid and other fluids out of solvents to prevent the mixture from becoming hazardous; and   Keep solvents out of aqueous cleaners.

Maintenance/ Housekeeping   Use secondary containment for solvent storage; and   Implement a maintenance program to fix and prevent leaks.

Procedures/ Scheduling   Reduce dragout by increasing drain time; and   When dripping parts, lift them such that it reduces dragout.

Equipment Layout/ Piping/ Automation   Install sliding lids on solvent tanks;

  Increase the freeboard height to significantly reduce solvent evaporation;

  Install automatic parts lift on vapor degreasers;

  Use drain racks to reduce dragout; and   Drain parts using a rotating rack.

8.3.2 Pollution Prevention for Machining Operations Many machining operations use metal working fluids to cool and lubricate parts and machining tools during cutting, drilling, milling, and other machining operations. These fluids become contaminated and begin to lose their working characteristics. If neglected, the fluids become unusable and require treatment and disposal. Through proper care, the life span of the fluids can be extended indefinitely. For most machining operations, prolonging metal working fluid life reduces the cost of treatment and disposal, as well as the cost of fresh coolant.

Many MP& M facilities use some type of pollution prevention and water conservation practices for machining wastewaters. Some facilities have implemented numerous pollution prevention and water conservation methods and technologies that result in very low machining wastewater discharge rates and in some cases eliminate the discharge of machining fluids. Pollution prevention and water conservation practices are applicable to all machining 8 28 8.0  Pollution Prevention and Wastewater Treatment Technologies operations; however, process related factors and site specific conditions may restrict the utility of certain methods.

The Agency has identified two categories of pollution prevention and water conservation practices and technologies that can be used to reduce metal working fluid discharge: those used to prevent metal working fluid contamination and those used to extend the life of machining fluids, including recovering and recycling metal working fluids. Within each of these categories are several specific practices and technologies. See Appendix D for more information on these pollution prevention practices.

8.3.3 Painting Operations Paint is applied to a base material for protective and decorative reasons in various forms, including dry powder, solvent diluted formulations, and water borne formulations. There are various methods of application, the most common being immersion and spraying. Water is used in painting operations in paint booth water wash systems ( water curtains), in water borne formulations, in electrophoretic painting solutions and rinses, and in clean up operations. This discussion is directed at water use in spray painting booths; however, Appendix D also provides some information on rinsing following electrophoretic painting and water clean up. EPA has identified three categories of pollution prevention and water conservation practices that, if implemented, can reduce or eliminate wastewater discharges from painting operations: practices to reduce the quantity of paint entering the water system; recycling technologies for paint booth water; and conversion of water wash booths to dry filter booths. These are discussed in this subsection and summarized in Appendix D. It is possible, however, that facilities can reduce or eliminate wastewater discharges using different practices than those described here.

8.3.4 Pollution Prevention for Printed Wiring Board Manufacturing Printed wiring board manufacturers use a large amount of water each day, mostly for rinsing and electroplating processes. The following BMP s developed specifically for printed wiring board manufacturing outline water saving process changes and controls that can be inexpensively incorporated in the production process. A number of these pollution prevention processes are described in more detail in Section 8.1.

  Use dry film photoresist instead of wet applications.

  Examine the pre plating rinse processes:

 Based on monitoring data, eliminate unnecessary cycles and rinse only until desired cleanliness is reached.

 Switch from continuous to on demand rinsing, and from once through to closed loop use.

8 29 8.0  Pollution Prevention and Wastewater Treatment Technologies  Use counter current rinsing.

 Use air or workpiece agitation to increase rinsing efficiency.

 Spray rinse with high pressure, low flow nozzles. This can reduce rinse water use up to 60 percent.

 Link flow controls to conductivity meters that measure the total dissolved solids in the rinses.

  Examine the electroplating process. Extending bath life will reduce both water consumption and toxics in the effluent.

 Reduce drag in through efficient rinsing.

 Use deionized or distilled water for makeup.

 Reduce drag out through the following methods:

a)

b)

c) Minimize bath chemical concentrations.

Use nonionic wetting agents to reduce surface tension in the process baths.

Prior to rinsing, maximize water returned to the process bath through several measures   withdraw pieces from the baths slowly, install drainage boards between process baths and rinses to return drag out back to the process bath, install rails above process baths to hang workpiece/ racks for drainage and/ or use air knives or spray rinses above process baths to rinse excess solution into the process bath.

 Restore barrel holes.

 Maintain bath solution quality through monitoring, replacement of reagents and stabilizers, and impurity removal.

  Install multiple baths after the process bath for using counter current rinsing wherever possible.

8.4 Preliminary Treatment of Segregated Wastewater Streams Preliminary treatment systems reduce pollutant loadings in segregated waste streams prior to combined end of pipe treatment. Wastewater containing pollutants such as 8 30 8.0  Pollution Prevention and Wastewater Treatment Technologies cyanide, hexavalent chromium, oil and grease, or chelated metals may not be treated effectively by chemical precipitation and gravity settling without preliminary treatment. Proper segregation and treatment of these streams is critical for the successful treatment of process wastewater. Highly concentrated metal bearing wastewater also may require pretreatment to reduce metal concentrations before end of pipe treatment. This subsection describes the following wastewater streams that typically undergo preliminary treatment at MP& M facilities:

  Chromium bearing wastewater;

  Concentrated metal bearing wastewater;

  Cyanide bearing wastewater;

  Chelated metal bearing wastewater; and   Oil bearing wastewater.

Table 8 3 summarizes these preliminary treatment operations.

8.4.1 Chromium Bearing Wastewater MP& M facilities generate hexavalent chromium bearing wastewater from acid treatment, anodizing, conversion coating, and electroplating operations and rinses. Hexavalent chromium exists in an ionic form and does not form a metal hydroxide; therefore, hexavalent chromium cannot be treated by chemical precipitation and sedimentation ( discussed in Section 8.5.1). The wastewater requires preliminary chemical treatment to reduce the hexavalent chromium to trivalent chromium, which can be removed by chemical precipitation and sedimentation. As shown in Table 8 3, EPA estimates there are over 1,800 MP& M facilities that perform hexavalent chromium reduction. The chemical reduction process is discussed below. Figure 8 7 presents a diagram of a continuous chromium reduction system.

8 31 8.0  Pollution Prevention and Wastewater Treatment Technologies 8 32 Table 8 3 MP& M Preliminary and End of Pipe Treatment Technologies Technology Technology Description Demonstration Status Number of Facilities Visited Using the Technology a Number of Survey Facilities Using the Technologyb Estimated Number of MP& M Facilities Using the Technologyc Chemical Emulsion Breaking Followed by Gravity Oil/ Water Separation Adds acids ( typically sulfuric), polymer, and sometimes alum to oil

bearing wastewater to break oil/ water emulsions for subsequent gravity separation. Separated oil is skimmed and hauled by a contractor. A facility may purchase the recycled oil for reuse. 13 56 958 Chemical Emulsion Breaking Followed by Dissolved Air Flotation Adds acids ( typically sulfuric), polymer, and sometimes alum to oil

bearing wastewater to break oil/ water emulsions for subsequent gravity separation. Introduces gas bubbles into the wastewater, bringing oils and solids to the surface for subsequent removal. 85 25 244 Chemical Reduction of Hexavalent Chromium Reduces hexavalent chromium to trivalent chromium using a reducing agent such as sulfur dioxide, sodium bisulfite, or sodium metabisulfite. 56 103 1,839 Cyanide Destruction by Alkaline Chlorination Destroys cyanide by adding chlorine ( usually sodium hypochlorite or chlorine gas) to high pH wastewater to first oxidize cyanide to cyanate,

then cyanate to carbon dioxide and nitrogen gas. 14 53 1,136 Oil Skimming of Oily Wastewater Streams Removes free floating oil by gravity separation and mechanical skimming. This technology does not remove emulsified oils. 45 89 2,087 Cyanide Oxidation by Ozone Ozone oxidizes cyanide to ammonia, carbon dioxide and oxygen. 0 1 4 Chelation Breaking/

Precipitation to Remove Complexed Metals Wastewater from electroless plating and some cleaning operations contains chelated metals that cannot be removed by chemical precipitation. Strong reducing agents such as dithiocarbamate are added to break the metal organic chelate bond and precipitate the metal. 15 49 555 Ultrafiltration Removes emulsified or free floating oils. This technology also removes other solids. Uses a membrane of very small pore size. 19 23 351 Activated Carbon Adsorption Removes dissolved organic pollutants by filtration through and adsorption on activated carbon. This technology requires preliminary treatment to remove suspended solids and oil and grease. 9 21 165 Aerobic Biological Treatment Biochemically decomposes organic materials in the presence of oxygen using microorganisms. 1 ( used to treat nonprocess wastewater) 4 130 Air Stripping Removes dissolved volatile organic pollutants by contacting the organics in the wastewater with a continuous stream of air bubbles. Volatile organic pollutants are transferred from the wastewater to the air. 0 2 14 8.0  Pollution Prevention and Wastewater Treatment Technologies 8 33 Table 8 3 ( Continued)

Technology Technology Description Demonstration Status Number of Facilities Visited Using the Technology a Number of Survey Facilities Using the Technologyb Estimated Number of MP& M Facilities Using the Technologyc Neutralization Neutralizes high or low pH wastewater to within an acceptable range using acidic or alkaline chemicals. Common acids include sulfuric and hydrochloric. Common alkaline chemicals include lime and sodium hydroxide. 63 233 3,713 Chemical Precipitation and Gravity Sedimentation Removes metals by precipitating insoluble compounds such as hydroxides, sulfides, or carbonates. Precipitation as metal hydroxides using lime or sodium hydroxide is the most common. Precipitated and flocculated solids are removed by gravity sedimentation in a clarifier. 149 203 2,981 Chemical Precipitation and Microfiltration Removes metals by precipitating insoluble compounds such as hydroxides, sulfides, or carbonates. Precipitation as metal hydroxides using lime or sodium hydroxide is the most common. Precipitated and flocculated solids are removed by microfiltration through a porous membrane. 6 5 36 Atmospheric Evaporation Includes both natural solar evaporation and forced atmospheric evaporation by which the evaporation rate is accelerated by increased temperature, air flow, and surface area. 4 12 142 Ion Exchange ( end of pipe) Polishing technique after metals precipitation to scavenge low concentrations of residual metals ( cations) using combined cation and anion exchange. Anions remain in solution and are discharged.

Concentrated metal containing regenerants are typically returned to the metals precipitation system. 17 39 251 Multimedia Filtration Removes solids from wastewater using filter media of different grain size. Coarser media remove larger particles and finer media remove smaller particles. Media include garnet, sand, and anthracite coal. The filter is periodically backwashed to remove solids. 12 16 354 Sand Filtration Removes solids from wastewater using a sand filter. The filter is periodically backwashed to remove solids. 46 41 830 Gravity Settling Physically removes suspended particles by gravity. This technology does not include the addition of any chemicals. 7 46 1,679 Centrifugation of Sludge Separates water from solids using centrifugal force. Centrifugation dewaters sludges, reducing the volume and creating a semisolid cake.

Centrifugation of sludge can typically achieve a sludge of 20 35 percent solids. 7 9 127 Gravity Thickening of Sludge Physically separates solids and water by gravity. Gravity thickening can typically thicken sludge to 5 percent solids. 83 85 1,161 8.0  Pollution Prevention and Wastewater Treatment Technologies Table 8 3 ( Continued)

Technology Technology Description Demonstration Status Number of Facilities Visited Using the Technology a Number of Survey Facilities Using the Technologyb Estimated Number of MP& M Facilities Using the Technologyc Pressure Filtration of Sludge Physically separates solids and water by pressure filtration. Most commonly performed in a plate and frame filter press where the sludge builds up between the filter plates and water is filtered through a cloth.

Pressure filtration can produce a sludge cake with greater than 40 percent solids. 140 189 3,106 Sludge Drying Dries sludge by heating, which causes the water in the sludge to evaporate. 28 48 835 Vacuum Filtration of Sludge Physically separates solids and water by vacuum filtration. Most commonly performed in a cylindrical drum vacuum filter, where water is pulled by vacuum through the filter and dewatered sludge is retained and subsequently scraped from the filter surface. Vacuum filtration can produce a sludge cake with 20  30 percent solids. 11 9 193 8 34 Source: MP& M site visits, MP& M sampling episodes, MP& M surveys and technical literature. Statistics specific to wastewater discharging facilities.

a Indicates the number of MP& M facilities visited by EPA using the listed technology. EPA visited a total of 221 facilities.

b Indicates the number of water discharging survey facilities that reported using this technology. Based on 874 MP& M survey respondents for the 1996 detailed survey and the 1989 survey.

c Indicates the estimated number of MP& M facilities currently performing this technology based on the 1989 and 1996 detailed surveys. EPA s national estimate of the 1996 detailed survey and the 1989 survey includes approximately 44,000 water discharging facilities. EPA estimated numbers in this column using statistical weighting factors for the MP& M survey respondents. See Section 3.0 for a discussion of the development of national estimates and statistical survey weights. 8.0  Pollution Prevention and Wastewater Treatment Technologies Figure 8 7. Chemical Reduction of Hexavalent Chrome Reduction is a chemical reaction in which electrons are transferred from one chemical ( the reducing agent) to the chemical being reduced. Sulfur dioxide, sodium bisulfite, sodium metabisulfite, peroxide, and ferrous sulfate form strong reducing agents in water. MP& M facilities use these agents to reduce hexavalent chromium to the trivalent form, which allows the metal to be removed from solution by subsequent chemical precipitation.

Sodium metabisulfite, sodium bisulfite, and sulfur dioxide are the most widely used reducing agents at MP& M facilities ( 14). Below is an equation showing the sulfur dioxide reaction ( reduction using other reagents is similar chemically):

( 8 2)

An operating pH of between 2 and 3 is normal for chromium reduction. At pH levels above 5, the reduction rate is slow, and oxidizing agents such as dissolved oxygen and ferric iron interfere with the reduction process by consuming the reducing agent.

Typically, the chemicals are retained in a reaction tank for 45 minutes. The tank is equipped with pH and oxidation reduction potential ( ORP) controls. Sulfuric acid is added to maintain a pH of approximately 2, and a reducing agent is metered to the reaction tank to maintain the target ORP.

Chemical reduction of hexavalent chromium is a proven technology that is widely used at MP& M facilities. Operation at ambient conditions requires little energy, and the process 8 35 8.0  Pollution Prevention and Wastewater Treatment Technologies is well suited to automatic control. For high concentrations of chromium, treatment chemical costs may be significant.

Maintenance of chemical reduction systems consists of sludge removal, the frequency of which depends on the concentration of contaminants. There also may be small amounts of sludge generated due to minor shifts in the solubility of the contaminants ( e. g., iron hydroxides). This sludge can be removed by the sludge handling equipment associated with subsequent end of pipe chemical precipitation and sedimentation.

8.4.2 Concentrated Metal Bearing Wastewater Facilities use several methods to manage concentrated metal bearing wastewater from spent process solutions. Facilities may:

  Meter the concentrated metal bearing wastewater slowly to the end of pipe chemical precipitation system and commingle it with other facility wastewater;

  Treat the concentrated metal bearing wastewater in a batch pretreatment system; or   Send concentrated metal bearing wastewater for off site treatment.

Batch pretreatment allows better control of the treatment system ( e. g., the treatment chemicals can be better tailored to the specific solution being treated), better treatment of difficult to treat materials ( e. g., photo resist bearing wastewater), and potential recovery of metals from the sludge. With batch treatment, facilities typically discharge effluent from the batch treatment tank to the end of pipe treatment system for additional polishing.

Batch chemical precipitation of concentrated metal bearing wastewater typically occurs in a single stirred tank, where a precipitating agent ( e. g., sodium hydroxide, lime, sodium sulfide) is added to create an insoluble metal hydroxide or sulfide complex. Following precipitate formation, a polyelectrolyte is added to flocculate the metal hydroxide or metal sulfide particles into larger clumps that will settle to the bottom of the reaction tank following mixing. Clarified effluent from the batch tank is discharged to the end of pipe treatment system and the settled sludge, typically containing only one type of metal, is transferred off site for metals recovery.

8.4.3 Cyanide Bearing Wastewater Plating and cleaning wastewater may contain significant amounts of cyanide, which should be removed through preliminary treatment. In addition to its toxicity, cyanide forms complexes with metals that prohibit subsequent removal in chemical precipitation systems.

8 36 8.0  Pollution Prevention and Wastewater Treatment Technologies 8 37 Figure 8 8. ide Destruction Through Alkaline Chlorination Cyanide typically is treated using alkaline chlorination with sodium hypochlorite or chlorine gas or by ozone oxidation. o processes are described below.

<IP_ADDRESS>Alkaline Chlorination Alkaline chlorination is in wide use in industrial wastewater treatment to destroy cyanide. MP& M facilities using alkaline chlorination to remove cyanide. pically used as either chlorine gas or sodium hypochlorite ( i. e., bleach). idizes cyanides to carbon dioxide and nitrogen by the following two step chemical reaction ( 10):

( 8 3)

( 8 4)

Figure 8 8 presents a diagram of an alkaline chlorination system.

Treatment equipment often consists of an equalization tank followed by two continuous reaction tanks, although the batch reaction can occur in a single tank. an electronic controller to monitor and maintain the required pH and ORP. idize cyanides to cyanates, chlorine or sodium hypochlorite is metered to the first reaction tank as necessary to maintain the ORP at 350 to 400 millivolts, and aqueous sodium hydroxide is added to maintain a pH of approximately 11. that most of the cyanide exists in the CN form, rather than as the highly toxic hydrogen cyanide ( HCN) form. n the second reaction tank, the ORP and the pH level typically are maintained at 600 millivolts and 8 to 9, respectively, to oxidize cyanate to carbon dioxide and nitrogen. tank has a chemical mixer designed to provide approximately one turnover per minute.

The batch process typically occurs in two tanks, one to collect water over a specified time period and one to treat an accumulated batch. f concentrated wastes are Cyan These tw Table 8 3 shows there are over 1,100 Chlorine is ty The alkaline chlorination process ox Each tank has To ox This pH dictates I Each reaction I 8.0  Pollution Prevention and Wastewater Treatment Technologies frequently dumped, another tank may be required to equalize the flow to the treatment tank. When the holding tank is full, the liquid is transferred to the reaction tank for treatment.

Alkaline chlorination can take place at ambient temperature, can be automatically controlled at relatively low cost, and can achieve effluent concentrations of free cyanide that are below the detection limit. Disadvantages include the need for careful pH control, possible chemical interference in treating mixed wastes, and the potential hazard of storing and handling chlorine gas ( if sodium hypochlorite is not used). If organic compounds are present, chlorinated organic compounds may be generated. Additionally, there are several safety concerns associated with handling chlorine gas and with the gas feed system. This technology is not effective in treating metallocyanide complexes, such as ferrocyanide.

<IP_ADDRESS> Ozone Oxidation A less common cyanide treatment method is ozone oxidation. Ozone, generated as a gas, is bubbled through a wastewater solution containing free cyanide. The ozone reacts with cyanide, converting it to cyanate. Additional ozone reacts with the cyanate to convert it to nitrogen gas, ammonia, and bicarbonate, as shown by the reactions below.

CN + O3 > CNO + O2 ( 8 5)

3CNO + 2O3 + 2OH + 2H2 O > 3HCO3  + NH3 + N2 + 2O2 ( 8 6)

The reaction rate is limited by mass transfer of ozone to the solution, the cyanide concentration, and temperature. Literature data show that oxidation can reduce amenable cyanide in electroplating wastewaters to below detection ( 5). Ozone is not effective in treating metallocyanide complexes, such as ferrocyanide, unless ultraviolet light is added to the reaction tank ( 6).

One advantage ozone has over chlorine is the type of residuals formed. Chlorine oxidation of organic compounds has the potential to form trihalomethanes. Ozone oxidizes organic compounds to form relatively less toxic, short chain organic acids, ketones, and aldehydes. Equipment required for ozone oxidation of cyanides includes an ozone generator, gas diffusion system, a mixed reaction tank, and off gas controls to prevent the release of unreacted ozone.

The major disadvantage of the ozone oxidation process is the capital and operating cost ( 12). Ozone must be manufactured on site and delivered directly to the reaction tank. Ozone generation equipment is expensive, and facilities also must purchase closed reaction tanks and ozone off gas treatment equipment.

8 38 8.0  Pollution Prevention and Wastewater Treatment Technologies 8.4.4 Chelated Metal Bearing Wastewater Certain process wastewaters evaluated for the final rule contain chelating agents that form metal complexes and interfere with conventional chemical precipitation treatment. This wastewater is often associated with electroless plating and requires specific treatment for the chelated metals. In general, there are three methods of treating these wastewaters:

  Reduction to elemental metal;

  Precipitation as an insoluble compound; and   Physical separation.

<IP_ADDRESS> Reduction to Elemental Metal Reduction to elemental metal can be done using one of two methods. One method is electrolytic recovery ( see Section 8.2.6), in which the dissolved metal is deposited on a cathode for reclamation or disposal. The electric current provides the electrons to reduce the metal ion to its elemental form. The reaction rate and achievable concentration for this technology depend on the volume of wastewater per unit surface area of cathode. This method typically does not lower metal concentrations to levels sufficient for wastewater discharge.

The second method uses a reducing agent to provide the electrons to reduce the metal. Possible reducing agents for treating chelated wastewater streams include:

  Dithiocarbamate ( DTC);

  Sodium borohydride;

  Hydrazine; and   Sodium hydrosulfite.

Upon reduction, the metal forms a particulate in solution, which a solids removal technique, such as gravity clarification, can remove. For effective use, these reducing agents sometimes require the use of other chemicals ( e. g., lime or sodium hydroxide) for pH adjustment. Figure 8 9 presents a diagram showing this method of chemical reduction of chelated metals.

8 39 8.0  Pollution Prevention and Wastewater Treatment Technologies Figure 8 9. Chemical Reduction / Precipitation of Chelated Metals <IP_ADDRESS> Precipitation as an Insoluble Compound Chelating agents hinder the formation of hydroxides, making hydroxide precipitation ineffective for treating chelated metal bearing wastewaters. Other precipitation methods that are less affected by chelating agents include sulfide precipitation, DTC precipitation, and carbonate precipitation. Section 8.5.1 discusses sulfide precipitation and carbonate precipitation.

DTC is added to solution in stoichiometric ratio to the metals present. Equation 8 7 shows the reduction of nickel using DTC:

Ni2+ ( aq) + DTC2 ( aq)   Ni0 ( s) ( 8 7)

DTC is effective in treating wastewater containing chelated metals. Based on information provided in the MP& M Detailed Surveys, approximately 53 percent of MP& M facilities with chelated metals use DTC for treatment. DTC compounds are a class of pesticides and, if used incorrectly, may cause process upsets in the biological treatment system used at the POTW and can potentially be harmful to the environment ( e. g., lead to fish kills if it passes through the POTW and reaches surface waters). Another disadvantage is that DTC precipitation generates large amounts of sludge.

Other treatment chemicals used by MP& M industries for treatment of chelated metals include:

  Borohydride;

  Sodium hydrosulfite;

8 40 8.0  Pollution Prevention and Wastewater Treatment Technologies   Sodium metabisulfite;

  Polysulfide polymer;

  Sodium hydroxide;

  Ferrous sulfate;

  Ferris chloride; and   Formaldehyde.

EPA evaluated the treatment performance of polysulfide polymer ( Sampling Episode 6462) and determined this compound effectively treated chelated copper and nickel to metal finishing effluent limits ( 40 CFR 433). Further concentration reductions may have been achievable if additional jar testing was conducted. Iron or calcium salts and pH adjustment may also provide acceptable methods for chelated metals treatment; however, no data are available for evaluation.

The Orange County Sanitation District ( OCSD) compile a study of a NDMA and found that the highest concentrations of a probable human carcinogen, n nitrosodimethylamine ( NDMA), at a printed circuit board manufacturer were observed at effluent from batch treatment ( 18).   Overdosing  of DTC in batch treatment systems may be common and may lead to the formation of NDMA. During its evaluation OCSD encouraged facilities and treatment chemical vendors to develop non NDMA forming treatments. EPA compiled information on DTC alternative treatments for the record ( see   DTC Alternatives for Treatment of Chelated Metals,  Section 24.6.1 of the rulemaking record, DCN 17962).

<IP_ADDRESS> Physical Separation Ion exchange and reverse osmosis can separate metals from solution. These technologies are not affected by chelating agents in the wastewater, making them effective in treating wastewater from electroless plating. Sections <IP_ADDRESS> and <IP_ADDRESS>, respectively, discuss these technologies.

8.4.5 Oil Bearing Wastewater Some MP& M wastewater ( e. g., alkaline cleaning wastewater and water based metal working fluids) contains significant amounts of oil and grease. This wastewater sometimes requires preliminary treatment to remove oil and grease and organic pollutants. Oil/ water separation includes breaking oil/ water emulsions ( oil dispersed in water, stabilized by electrical charges and emulsifying agents) as well as gravity separation of oil. When only free oil ( i. e., nonemulsified oil) is present, oil skimming is enough for effective treatment. Techniques available to remove oil include chemical emulsion breaking followed by oil/ water separation or dissolved air flotation ( DAF), oil skimming, and ultrafiltration. These technologies are described in more detail below.

Oil/ water separation not only removes oil but also removes organic compounds that are more soluble in oil than in water. Subsequent clarification removes organic solids 8 41 8.0  Pollution Prevention and Wastewater Treatment Technologies directly and may also remove dissolved organic compounds by adsorption on inorganic solids. In MP& M operations, sources of these organic compounds mainly are process coolants and lubricants, additives to formulations of cleaners, paint formulations, or leaching from plastic lines and other materials.

<IP_ADDRESS> Chemical Emulsion Breaking Chemical emulsion breaking is used to break stable oil/ water emulsions. A stable emulsion will not separate or break down without chemical and or physical treatment. Chemical emulsion breaking is applicable to wastewater containing emulsified coolants and lubricants such as machining and grinding coolants and impact and pressure deformation lubricants. This technology also is applicable to cleaning solutions that contain emulsified oils. Figure 8 10 shows a diagram of a type of continuous chemical emulsion breaking system.

Figure 8 10. Continuous Chemical Emulsion Breaking Unit with Coalescing Plates Treatment of spent oil/ water emulsions involves adding chemicals to break the emulsion followed by oil/ water separation. The major equipment required for chemical emulsion breaking includes reaction chambers with agitators, chemical storage tanks, chemical feed systems, pumps, and piping. Factors to be considered for breaking emulsions are type of chemicals, dosage and sequence of addition, pH, mixing, heating requirements, and retention time.

Chemicals ( e. g., polymers, alum, ferric chloride, and organic emulsion breakers) break emulsions and allow coagulation ( 13) by neutralizing repulsive charges between particles, precipitating or salting out emulsifying agents, or weakening the interfacial film between the oil 8 42 8.0  Pollution Prevention and Wastewater Treatment Technologies and water so it is readily broken. Reactive cations ( e. g., H+ , Al+ 3 , Fe+ 3 ) and cationic polymers are particularly effective in breaking dilute oil/ water emulsions. Once the charges are neutralized or the interfacial film broken, the small oil droplets and suspended solids either adsorb on the surface of the floc that is formed, or break out and float to the top. Different types of emulsion breaking chemicals are used for different types of oils. If more than one chemical is required, the sequence of adding the chemicals can affect both breaking efficiency and chemical dosages.

Another important consideration in emulsion breaking is pH, especially if cationic inorganic chemicals, such as alum, serve as coagulants. For example, a pH of between 2 and 4 keeps the aluminum ion in its most positive state where it most effectively neutralizes charges. After some of the oil is broken free and skimmed, raising the pH into the 6 to 8 range with lime or caustic causes the aluminum to hydrolyze and precipitate as aluminum hydroxide. This floc entraps or adsorbs destabilized oil droplets, which can then be separated from the water. Cationic polymers can break emulsions over a wider pH range and thus avoid acid corrosion and the additional sludge generated from neutralization; however, this process usually requires adding an inorganic flocculent to supplement the adsorptive properties of the polymer emulsion breaker.

Mixing is important in effectively breaking oil/ water emulsions because it provides proper chemical feed and dispersion. Mixing also causes droplets to collide and break the emulsion and promotes subsequent agglomeration into larger droplets. Heating also improves chemical emulsion breaking by lowering the viscosity and increasing the apparent specific gravity differential between oil and water. In addition, heating increases the frequency of droplet collisions, which helps to rupture the interfacial film.

Once an emulsion is broken, the oil floats to the surface of the water because of the difference in specific gravity between oil and water. Solids usually form a layer between the oil and water because some solids become suspended in the oil. The longer the retention time, the more complete the separation between the oil, solids, and water. Oils and solids typically are skimmed from the surface of the water after chemical emulsion breaking. Often, other techniques such as air flotation or rotational separation ( e. g., centrifugation) enhance separation after chemical emulsion breaking.

The advantages of chemical emulsion breaking are the high removal efficiency potential and the possibility of reclaiming the oily waste. Disadvantages include corrosion problems associated with acid alum systems, operator training requirements for batch treatment, chemical sludges produced, and poor efficiency for low oil concentrations.

Chemical emulsion breaking is a very reliable process. The main control parameters are pH and temperature. Some MP& M facilities may achieve effective emulsion breaking by lowering the pH with acid, by heating the wastewater, or both. Maintenance is required on pumps, mixers, instrumentation and valves, as is periodic cleaning of the treatment tank to remove any accumulated solids. Energy use typically is limited to mixers and pumps, but 8 43 8.0  Pollution Prevention and Wastewater Treatment Technologies also can include heating. Solid wastes generated by chemical emulsion breaking include surface oil and oily sludge, which are usually contract hauled for disposal by a licensed contractor. If the recovered oil contains a low enough percentage of water, it may be burned for its fuel value or processed and reused.

<IP_ADDRESS> Oil Skimming Oil skimming is a physical separation technology that removes free or floating oil from wastewater using the difference in specific gravity between oil and water. Common separation devices include belts, rotating drums, disks, and weir oil skimmers and coalescers. These devices are not suited to remove emulsified oil, which requires chemical treatment, ultrafiltration, or other treatment. Figures 8 11a and 8 11b show diagrams of disk and belt oil skimming units, respectively, that are applicable for small systems or on process tanks. The oil removal system shown in Figure 8 10 is a coalescing separator used for large systems.

Figure 8 11a. Disk Oil Skimming Unit 8 44 8.0  Pollution Prevention and Wastewater Treatment Technologies Figure 8 11b. Belt Oil Skimming Unit To separate oil from process solutions, oil skimming devices typically mount onto the side of a tank and operate on a continuous basis. The disk skimmer is a vertically rotating disk that is partially submerged in the solution ( see Figure 8 11a). The disk continuously revolves between spring loaded wiper blades that are located above the liquid surface. The disk s adhesive characteristics cause the floating oil to remain on the disk. As the disk s surface passes under the wiper blades, the blades scrape off the oil, which is diverted to a run off spout for collection. Belt and drum skimmers operate in a similar manner, with either a continuous belt or drum rotating partially submerged in a tank. As the surface of the belt or drum emerges from the liquid, the oil that adheres to the surface is scraped off ( drum) or squeezed off ( belt) and diverted to a collection vessel. The oil typically is hauled off site for disposal.

Gravity separators use overflow and underflow weirs to skim a floating oil layer from the surface of the wastewater. The oil layer flows over the weir into a trough for disposal or reuse while most of the water flows underneath the weir. A diffusion device, such as a vertical slot weir, helps create a uniform flow through the system and increase oil removal efficiency.

An oil skimmer s removal efficiency depends on the composition of the waste stream and the retention time of the water in the tank. Larger, more buoyant particles require less retention time than do smaller particles. The retention time necessary for phase separation and subsequent skimming varies from 1 to 15 minutes, depending on the wastewater characteristics. Gravity type separators tend to be more effective for wastewater streams with consistently large 8 45 8.0  Pollution Prevention and Wastewater Treatment Technologies amounts of surface oil. Drum and belt type skimmers are more applicable to waste streams containing smaller amounts of floating oil. A gravity separator in conjunction with a drum type skimmer effectively removes floating contaminants from nonemulsified oily waste streams.

Coalescers remove oil droplets too finely dispersed for conventional gravity separation skimming technology. Coalescing also reduces the residence times ( and therefore separator sizes) required to separate oil from some wastes. The basic principle of coalescence involves the attraction of oil droplets to the coalescing medium ( typically plates). The oil droplets accumulate on the medium and then rise to the surface of the solution as they combine to form larger particles. The most important requirements for coalescing media are attraction for oil and large surface area. Coalescing media include polypropylene, ceramic, and glass.

Coalescing stages may be integrated with a wide variety of gravity oil separators, and some systems may incorporate several coalescing stages. A preliminary oil skimming step avoids overloading the coalescer.

<IP_ADDRESS> Flotation of Oils or Solids Air flotation combined with chemical emulsion breaking is an effective way to treat oily wastewater containing low concentrations of metals. Flotation separates oil and grease from the wastewater, and entrainment or adsorption will remove small amounts of metal. In DAF, air is injected into a fluid under pressure. The amount of air that can dissolve in a fluid increases with increasing pressure. When the pressure is released, the air comes out of solution as bubbles, which attach to oil and grease molecules and   float  the oil and grease to the surface. Induced air flotation uses the same separation principles as DAF systems but the gas is self induced by a rotor disperser mechanism.

Figure 8 12 shows a diagram of a DAF unit. A DAF system consists of a pressurizing pump, air injection equipment, pressurizing tank, a pressure release valve, and a flotation tank. DAF systems operate in two modes: full flow pressurization and recycle pressurization. In full flow pressurization, all influent wastewater is pressurized and injected with air. The wastewater then enters the flotation unit where the pressure is relieved and bubbles form, causing the oil and grease to rise to the surface with the air bubbles. In recycle pressurization, part of the clarified effluent is recycled back to the influent of the DAF unit, then pressurized and supersaturated with air. The recycled effluent then flows through a pressure release valve into the flotation unit. Pressurizing only the recycle reduces the amount of energy required to pressurize the entire influent. DAF is the most common method of air flotation.

8 46 8.0  Pollution Prevention and Wastewater Treatment Technologies Figure 8 12. Dissolved Air Flotation Unit <IP_ADDRESS> Ultrafiltration Ultrafiltration is a membrane based process used to separate solution components based on molecular size and shape. Under pressure, solvent and small solute species pass through the membrane and are collected as permeate while the membrane retains larger compounds, which are recovered as concentrate. Figure 8 5 shows a typical membrane filtration unit.

Ultrafiltration typically removes materials ranging from 0.002 to 0.2 microns or molecular weights from 500 to 300,000. It can be used to treat oily wastewater. Filtering the ultrafiltration influent removes large particles and free oil to prevent membrane damage and fouling. Most ultrafiltration membranes consist of homogeneous polymer or copolymer material. The transmembrane pressure required for ultrafiltration depends on membrane pore size, and typically ranges between 15 to 200 psi.

Ultrafiltration typically produces a concentrated oil phase that is two to five percent of the influent volume. Oily concentrates typically are hauled off site or incinerated, and the permeate ( water phase) can be either treated further to remove water soluble metals and organic compounds or discharged, depending on local and state requirements.

An ultrafiltration system includes: pumps and feed vessels, piping or tubing, monitoring and control units for temperature, pressure, and flow rate; process and cleaning tanks; and membranes. Membranes are designed specifically to handle various waste stream 8 47 8.0  Pollution Prevention and Wastewater Treatment Technologies parameters, including temperature, pH, and chemical compatibility. There are different types of membranes, including hollow fiber, tubular, flat plate, and spiral wound. The type selected depends on the application. For example, tubular membranes commonly separate suspended solids, whereas spiral wound membranes separate oil from water. Ultrafiltration systems designed to remove oil typically are more expensive than are DAF systems. Membranes must be cleaned periodically to ensure effective treatment.

End of Pipe Wastewater Treatment and Sludge Handling Technologies This subsection describes end of pipe technologies that MP& M facilities use for wastewater treatment and sludge handling. Table 8 3 describes each technology and lists the number of MP& M facilities that use the technology. Section 8.5.1 discusses metal removal by chemical precipitation, Section 8.5.2 discusses oil removal technologies, Section 8.5.3 discusses wastewater polishing technologies, and Section 8.5.4 discusses sludge handling technologies.

8.5.1 Chemical Precipitation for Metals Removal The most common end of pipe treatment technology used at MP& M facilities to remove dissolved metals is chemical precipitation and flocculation followed by gravity clarification. The data in Table 8 3 show there are nearly 3,000 MP& M facilities that use chemical precipitation and gravity settling to treat their metals bearing wastewater. Some MP& M facilities use microfiltration, filter press operations, centrifuge operations, DAF, and American Petroleum Institute ( API) separation in place of clarification, but this subsection discusses only clarification and microfiltration. The types of equipment used for chemical precipitation vary widely. Small batch operations can take place in a single tank that typically has a conical bottom to permit removal of settled solids. Continuous processes usually occur in a series of tanks, including an equalization tank, a rapid mix tank for dispersing the precipitating chemicals, and a slow mix tank for adding coagulants and flocculants and for floc formation.

For continuous flow systems, the first tank in the treatment train typically is the equalization tank. The flow equalization tank prevents upsets in processing operations from exceeding the hydraulic design capacity of the treatment system, improves chemical feed control, and allows wastewater neutralization.

Commingled wastewater from the equalization tank enters the rapid mix tank, along with various types of precipitation chemicals added to convert the soluble metals into insoluble compounds. Following precipitation, the wastewater flows into a flocculation tank where polyelectrolytes ( polymers) are added, causing the precipitated solids to coagulate into larger particles that gravity settling or other separation techniques can remove.

Chemical precipitation is a highly reliable technology when properly monitored and controlled. The effectiveness of this technology depends on the types of equipment used and numerous operating factors, such as the characteristics of the raw wastewater, types of treatment reagents used, and operating pH. In some cases, subtle changes in operating factors ( e. g., varying 8 48 8.5 8.0  Pollution Prevention and Wastewater Treatment Technologies the pH, altering chemical dosage, or extending the process reaction time) may sufficiently improve the system s efficiency. In other cases, modifications to the treatment system are necessary. For example, some raw wastewater contains chemicals that may interfere with metals precipitation, and may require additional, specialized treatment reagents such as ferrous sulfate, sodium hydrosulfate, aluminum sulfate, or calcium chloride. These chemicals may be added prior to or during the precipitation process.

Chemical precipitation systems require routine maintenance for proper operation. This includes: calibrating instrumentation and cleaning probes; maintaining chemical pumps and mixers ( inspection, cleaning, lubrication, replacing seals and packing, replacing check valves, cleaning strainers); and monitoring tanks and sumps ( inspection, cleaning, corrosion prevention).

There are several basic methods of performing chemical precipitation and flocculation and many variations of each method. The four most common methods are described below. Figure 8 13 shows a typical continuous chemical precipitation system.

Figure 8 13. Continuous Chemical Precipitation System with Lamella Clarifier Removing precipitated metals typically involves adding flocculating agents or polymers to destabilize the hydrodynamic forces that hold the particles in suspension. For a continuous treatment system, polymer is either added in line between the reaction tank and the flocculation tank, or in a small rapid mix tank between the reaction tank and flocculation tank. In the flocculation tank, the mixer is slowed to promote agglomeration of the particles until their density is greater than water and they settle from solution in the clarifier.

8 49 8.0  Pollution Prevention and Wastewater Treatment Technologies Hydroxide Precipitation Hydroxide precipitation is the most common method of removing metals from MP& M wastewater. This process typically consists of several stages. In an initial tank, which is mechanically agitated, alkaline treatment reagents such as lime ( calcium hydroxide or hydrated lime), sodium hydroxide, or magnesium hydroxide are added to the wastewater to precipitate metal ions as metal hydroxides. The reaction for precipitation of a divalent metal using sodium hydroxide is shown in the following equation:

( 8 8)

The precipitation process usually operates at a pH of between 8.5 and 11, depending on the types of metals in the wastewater. The pH set point for each hydroxide precipitation system is determined by jar testing. Jar testing results determine the optimum pH, flocculent type and dosage to maximize the removal of target metals. Figure 8 14 shows the effect of pH on hydroxide precipitation. Figure 8 14 was developed based on empirical studies using single metal solutions in reagent free water. However, metal solubilities in complex wastewater may differ from those shown in the figure, and therefore facilities must test their actual wastewater to define the minimum solubility for all metals.

Iron Coprecipitation Iron coprecipitation is one method that has proven effective at reducing the concentration of metals such as arsenic, beryllium, cadmium, copper, lead, nickel and zinc to less than could be achieved with hydroxide precipitation alone ( 7). Iron coprecipitation involves adding an iron source such as ferric sulfate or ferric chloride to the pH adjustment tank in the chemical precipitation treatment system. Iron is then precipitated as iron oxyhydroxide ( 7). During this process, other metal hydroxides ( e. g., nickel hydroxide, copper hydroxide) may be incorporated as an impurity within the iron oxyhydroxide matrix or physically entrapped within its pore spaces. Metal hydroxides may also be adsorbed to the surface of the iron oxyhydroxide precipitate. Factors affecting the iron coprecipitation process include iron dose and iron oxidation state, pH, the target metals oxidation state, the initial concentration of the target metal, and competition for adsorbent sites from other species. Facilities should conduct jar testing using their actual wastewater to optimize the operating conditions for this process.

Sulfide Precipitation The sulfide precipitation process uses equipment similar to that used for hydroxide precipitation. The major difference between the two processes is the treatment reagents used. Sulfide precipitation uses either soluble sulfides ( e. g., hydrogen sulfide or sodium sulfide) or insoluble sulfides ( e. g., ferrous sulfide) in place of alkali reagents used in hydroxide precipitation. The sulfide reagents precipitate dissolved metals as metal sulfides, which often have lower solubility limits than metal hydroxides. Therefore, the sulfide precipitation process 8 50 8.0  Pollution Prevention and Wastewater Treatment Technologies can ( for many metals) achieve lower levels of residual dissolved metal in the effluent than hydroxide precipitation treatment ( see Figure 8 14). The sulfide precipitation reaction is shown in the following equation:

( 8 9)

Figure 8 14. Effect of pH on Hydroxide and Sulfide Precipitation ( 10)

Unlike hydroxides, sulfide can precipitate most chelated metals and can remove hexavalent chromium without first reducing the chromium to its trivalent state.

The major disadvantages of sulfide precipitation as compared to hydroxide precipitation are higher capital and operating costs. Additional disadvantages of sulfide 8 51 8.0  Pollution Prevention and Wastewater Treatment Technologies precipitation are the potential for toxic hydrogen sulfide gas generation and excessive sulfide releases in the effluent, and the generation of sulfide odors.

Carbonate Precipitation Carbonate precipitation typically uses sodium carbonate ( soda ash), sodium bicarbonate, or calcium carbonate to form insoluble metal carbonates. The reaction is shown in the following equation:

( 8 10)

Carbonate precipitation is similar in operation to hydroxide precipitation, and its purpose is to remove metals such as cadmium or lead. For these metals, carbonate precipitation operates at a lower pH to achieve effluent concentrations similar to those achieved by hydroxide precipitation. Facilities sometimes operate carbonate precipitation in conjunction with hydroxide precipitation, which may improve the overall performance of certain systems.

Carbonate precipitation is less common than hydroxide precipitation due to the higher cost of treatment reagents and certain operational problems, such as the release of carbon dioxide, which can result in foaming and floating sludge. Also, because many metal carbonates are more soluble than are sulfides or hydroxides, this process does not effectively precipitate all target metals.

Chemical Precipitation Performance Factors Ionic strength of the wastewater is another factor that can negatively affect the performance of the chemical precipitation system ( 8). As MP& M facilities lower water usage by implementing technologies such as flow restrictors, countercurrent cascade rinsing, and timed rinses, the ionic strength of the wastewater reaching the treatment system will increase. In process chemistry, a precipitate always forms or dissolves in the presence of indifferent electrolytes. Although ions from such species do not participate directly in the solubility equilibrium reaction, they do affect the solubility behavior of the precipitate. The following chemical equilibrium equations show the impact of ionic strength on the precipitation process:

CA( s)   C( aq) + A( aq) ( 8 11)

The equilibrium constant expression for this reaction is given by ( Ka ) eq = ( C)( A) ( 8 12)

or ( Ka ) eq = gm [ C] gm [ A] ( 8 13)

8 52 8.0  Pollution Prevention and Wastewater Treatment Technologies This equation can be rewritten as ( Ke ) eq = ( Ka ) eq / ( gm ) 2 ( 8 14)

The greater the concentration of indifferent electrolytes, the greater the ionic strength of the solution and the smaller the value of the activity coefficient. In process chemistry, the value of g is normally less than 1.0. Therefore, the smaller the value of g, the larger the value of ( Ke ) eq , indicating the solubility of the solid phase ( metal hydroxide precipitate) will increase. This means that the solubility of a precipitate will increase if the concentration of indifferent electrolytes in solution increases ( 8). MP& M facilities that reduce process water usage should be aware of these equilibria changes that will occur within their treatment system. Facilities should conduct additional jar testing to determine if they can mitigate the negative impacts with new treatment chemistry or add process water to improve treatment efficiency.

One issue raised during the MP& M public comment period was that treatment system performance is fixed ( i. e., percent removal) and therefore the effluent concentration is a direct function of influent concentration. The MP& M sampling episode data, however, indicate the effluent concentration is a function of the minimum solubility of the metal, regardless of the influent concentration. As explained in the June 2002 NODA ( 67 FR 38779), EPA reviewed graphical displays of the paired influent and effluent values and other data analyses. Because the results were inconclusive and sometimes inconsistent, EPA was unable to reach a conclusion about the effect of influent concentrations on the effluent concentrations. If a facility finds that influent concentrations appear to affect its effluent concentrations, it may be useful to perform jar testing on a representative sample of wastewater to optimize the treatment conditions for both high and low influent concentrations.

After precipitation, the metal hydroxide particles are very fine and resistant to settling. To increase their particle size and improve their settling characteristics, coagulating and flocculating agents are added, usually in a second tank, and slowly mixed. Coagulating and flocculating agents include inorganic chemicals such as alum and ferric sulfate, and a highly diverse range of organic polyelectrolytes with varying characteristics suitable for different wastewaters. The type and dosage of flocculent and coagulant are based on the results of jar testing done using the actual facility wastewater.

Flocculated particles with densities greater than water settle in a separate clarification tank ( e. g., a lamella clarifier), under quiescent conditions. Operators remove the solids from the bottom of the settling tank or clarifier, then transfer them to a thickener or other dewatering process ( see Section 8.5.4). Clarifier effluent either undergoes further processing in a polishing unit such as a multimedia filter or discharges.

8 53 8.0  Pollution Prevention and Wastewater Treatment Technologies <IP_ADDRESS> Gravity Clarification for Solids Removal Gravity sedimentation to remove precipitated metal hydroxides is the most common method of clarification ( solids removal) used by MP& M facilities. Typically, two types of sedimentation devices are used: inclined plate clarifiers ( e. g., lamella clarifiers) and circular center feed rim flow clarifiers.

Lamella clarifiers contain inclined plates oriented at angles varying between 45 and 60 degrees from horizontal. As the water rises through the clarifier, the solids settle on the plates. Clarified effluent continues to the top of the clarifier, passes over a weir, and collects in a holding tank. The solids collect on the inclined plates and slide downward and into the bottom of the clarifier. When sufficient solids collect in the bottom of the clarifier, they are scraped into a sludge hopper and then discharged, usually to a thickener. Figure 8 13 presents a lamella clarifier.

Overflow rates for lamella clarifiers ( i. e., between 1,000 and 1,500 gpd/ ft2 for metal hydroxide sludges) are two to four times higher than the overflow rates for clarifiers not equipped with inclined plates. Clarifier inlets must be designed to distribute flow uniformly through the tank and plate settlers. In addition, because solids can build up on plate surfaces and adversely affect flow distribution, the clarifier should be cleaned periodically.

Lamella clarifiers are more common at MP& M facilities than other types of clarifiers because of the smaller area required. They typically require only 65 to 80 percent of the area required for clarifiers without inclined plates. Their design promotes laminar flow through the clarifier, even when the water throughput is relatively high.

In a center feed rim flow clarifier, wastewater flows into the bottom of a center feed well and then up into a circular tank. Heavy particles settle to the bottom of the tank where they are raked to a discharge pipe and removed. Materials with a density less than the density of water float to the top of the water and are skimmed from the water surface and discharged to a scum pit through a scum trough. Scum is removed from the scum pit periodically and then disposed of. Clarified effluent flows over the top of the clarifier and is collected in an effluent channel and discharged. Figure 8 15 shows a center feed rim flow clarifier.

8 54 8.0  Pollution Prevention and Wastewater Treatment Technologies Figure 8 15. Center Feed Rim Flow Clarifier <IP_ADDRESS> Microfiltration for Solids Removal Microfiltration is an alternative to conventional gravity clarification after chemical precipitation. Microfiltration is a membrane based process used to separate small suspended particles based on size and shape. Water and small solute species pass under pressure through a membrane and are collected as permeate while larger particles such as precipitated and flocculated metal hydroxides are retained by the membrane and are recovered as concentrate. Microfiltration is similar to ultrafiltration ( Section <IP_ADDRESS>) but has a larger pore size.

Microfiltration removes materials ranging from 0.1 to 1.0 microns ( e. g., colloidal particles, heavy metal particulates and their hydroxides). Most microfiltration membranes consist of homogeneous polymer material. The transmembrane pressure required for microfiltration typically ranges between 3 to 50 psi, depending on membrane pore size.

Microfiltration produces a concentrated suspended solid slurry that typically goes to dewatering equipment such as a sludge thickener or a filter press. The permeate can either be treated further to adjust the pH or be discharged, depending on local and state requirements. Figure 8 5 shows a typical membrane filtration system.

The microfiltration system includes: pumps and feed vessels; piping or tubing; monitoring and control units for temperature, pressure, and flow rate; process and cleaning tanks; and membranes. Membranes are designed specifically to handle various waste stream parameters, including temperature, pH, and chemical compatibility. Different types of membranes are available, including hollow fiber, tubular, flat plate, and spiral wound. The configuration selected for a particular facility depends on the type of application. For example, tubular membranes commonly separate suspended solids, whereas spiral wound membranes 8 55 8.0  Pollution Prevention and Wastewater Treatment Technologies separate oils from water. Microfiltration is more expensive than conventional gravity clarification. Membranes must be cleaned periodically to prevent fouling and ensure effective treatment.

<IP_ADDRESS> Optimization of Existing Chemical Precipitation Treatment System Facilities can optimize the performance of an existing chemical precipitation and clarification system using a variety of techniques such as adding equalization prior to treatment, conducting jar testing to optimize treatment chemistry, upgrading control systems, and providing operator training.

Equalization Equalization is simply the damping of flow and concentration variations to achieve a constant or nearly constant wastewater treatment system loading ( 8). Equalization improves treatment performance by providing a uniform hydraulic loading to clarification equipment, and by damping mass loadings, which improves chemical feed control and process reliability. MP& M facilities implement equalization by placing a large collection tank ahead of the treatment system. All process water and rinse water entering this tank are mixed mechanically and then pumped or allowed to gravity flow to the treatment system at a constant rate. The size ( volume) of the tank depends on the facility flow variations throughout the day. Operating data collected during MP& M sampling episodes indicate hydraulic residence times for equalization tanks average 4 to 6 hours.