drinking water disinfection


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Ozonation / Ozone Water Treatment

Description: Ozone is a form of oxygen (O2) with the molecular formula O3. It forms when oxygen in the air is exposed to the discharge of a powerful electric current through air. In nature, it forms in the upper atmosphere when lightning passes through the air. The pungent odor often associated with passage of a thunder storm which leads some to exclaim how "clean" the atmosphere smells, is attributed to naturally formed ozone. Ozone is unstable and changes to O2 shortly after its formation. It is a powerful oxidant and one of the most powerful disinfectants available in water treatment.

Drinking Water Disinfection. Although ozone is significantly more effective than chlorine at inactivating and / or killing viruses, bacteria and cysts (e.g., Cryptosporidium and Giardia) and has been widely used in Europe for many years to treat municipal drinking water, it has not had similar acceptance in the US. Reasons include its higher cost and the fact it does not remain present long in water. US regulatory authorities have specified less expensive disinfectants such as free chlorine, chlorine dioxide or chloramines to maintain a residual capable of continuing to kill organisms throughout the distribution system. Legislation over the past two decades, such as the Safe Drinking Water Act Amendments, Surface Water Treatment Rule, and other regulations place stricter rules on both the range and amount of disinfection needed and the concentrations of disinfection-by-products (e.g., trihalomethanes) allowed in drinking water. This is making the use of free chlorine and its derivative disinfectants less cost-effective. Security concerns since 9/11 about the presence of large tanks of compressed, liquefied chlorine located in water plants in densely populated areas may also play a role in the utilization of ozone in the future.

Since it quickly inactivates or kills virtually all bacteria, cysts and viruses but leaves no long-lasting residual, ozone is the disinfectant of choice for most bottled water bottlers.

Ozone Waste Water Treatment. Since ozone quickly converts to oxygen and leaves no toxic residual, it may be more advantageous than chlorine to treat wastewater prior to discharge. Since dissolved ozone reverts to oxygen, the effluent will exert less biological oxygen demand (BOD) on the receiving stream. Ozone’s effectiveness as an oxidant often makes it the method of choice for removing color, organic chemicals and odor-causing contaminants in wastewater. In many cases, depending on the ozone contact time and concentration, it is capable of oxidizing these contaminants to water and carbon dioxide.

As examples, toxic herbicides and pesticides may be reduced to more environmentally friendly components; non-biodegradable organic compounds may be reduced to smaller biodegradable parts; proteins and carbohydrates may be lysed at double-bonded carbons to damage and destroy critical components of organisms found in the water.. Since the free hydroxyl is so highly reactive, the contact time necessary is minimal, as compared to other disinfectants.

Ozone may be combined with other oxidation processes, such as ultraviolet irradiation, hydrogen peroxide (another powerful oxidant) and proprietary catalysts to speed these oxidation process. This combination of oxidation steps are referred to as Advanced Oxidation Processes or AOP.

Ozone Generation. Ozone is a highly unstable molecule with a relatively short half-life preventing it from being stored or transported, thus requiring that it be generated on-site. It can be generated from any source of gas which contains oxygen molecules. The most common sources for ozone generation are commercially prepared liquefied compressed oxygen or air in the atmosphere. Use of pure oxygen gas results in a higher efficiency of ozone generation but it increases the production cost. Using air as an oxygen source requires that the air be compressed and cleaned and dried (i.e., dehumidified). Compression of the air serves to increase the concentration of oxygen. The removal of foreign particulate such as dirt and dust is accomplished through the use of filters. Dehumidification is accomplished by lowering the dew point by refrigeration. Most ozone generators require clean, dry gas for optimal production.

Clean dry gas is passed through a chamber where the continuous discharge (arcing) from a high voltage electric current disperses electrons into the air. The electrons convert oxygen molecules to ozone molecules and oxygen atoms. The highly unstable oxygen atoms bond with hydrogen atoms in the air to form hydroxyl radicals.

It is these hydroxyl radicals that give ozone its oxidation characteristic. The gas, now containing ozone and hydroxyl radicals, is introduced into water. Ozone is not very soluble in water and is typically dissolved using a venture which relies on intimate air / water contact under very high turbulence or a diffuser which breaks the gas into very tiny bubbles which are allowed to be in contact with the water for an extended period of time.

The latter arrangement may use large aerator stones (similar in substance to those used in a small aquarium) to bubble the ozone / air mixture into a very tall column of water in a tank ("contact tank"). Fresh water is introduced at the top of the contact tank and ozonated water flows out the bottom of the tank.

In all cases, the concentration of ozone imparted to the water depends on the amount of ozone present in the air after it passes through the ozone generator, the surface area presented at the gas-water interface (e.g., bubble surface) and the contact time between the gas and water.

Typically the water flow is held constant and the amount of ozone in the system is regulated by adjusting the voltage of the current producing the electrical discharge in the generator or by adjusting the flowrate of the gas. The ozone concentration produced by a generator is typically expressed as pounds, grams or kilograms per day.

Because ozone solubility in water rapidly decreases with increasing temperature, the seasonal range in feedwater temperature is a critical design parameter.

Toxicity. Because it is such an aggressive oxidizing agent, ozone has a deleterious effect on all living organisms. It is a severe upper respiratory irritant at very low concentrations and workplace limits have been established in the US by OSHA. Discharge limits for receiving streams and the ambient air are established by the US EPA. Ozone degradation can be accelerated by the addition of hydrogen peroxide or passing the system through an ultraviolet ozone destruct system. Activated carbon may also be effective in catalyzing ozone destruction in air or water. Also, contact time with the water or wastewater can also be lengthened to allow for further destruction.

Assistance: The WaterProfessionals® are experienced in combining technologies, including ozonation, to achieve water treatment objectives.

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