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Symbols and Acronyms

hardness   A characteristic of water imparted primarily by salts of calcium and magnesium, such as bicarbonates, carbonates, sulfates, chlorides, and nitrates, that causes curdling and increased consumption of soap, deposition of scale in boilers, damage in some industrial processes, and sometimes objectionable taste. It may be determined by a standard laboratory titration procedure or computed from the amounts of calcium and magnesium expressed as equivalent calcium carbonate. See also carbonate hardness.

hazardous waste  Any waste that is potentially damaging to environmental health because of toxicity, ignitability, corrosivity, chemical reactivity, or other reasons.

head  (1) The height of the free surface of fluid above any point in a hydraulic system; a measure of the pressure or force exerted by the fluid. (2) The energy, either kinetic or potential, possessed by each unit weight of a liquid, expressed as the vertical height through which a unit weight would have to fall to release the average energy possessed. It is used in various compound terms such as pressure head, velocity head, and loss of head. (3) The upper end of anything, such as a headworks. (4) The source of anything, such as a head-water. (5) A comparatively high promontory with either a cliff or steep face extending into a large body of water, such as a sea or lake. An unnamed head is usually called a headland.

header  (1) A structure installed at the head or upper end of a gully to prevent overfall cutting. (2) A supply ditch for the irrigation of a field. (3) A large pipe installed to intercept the ends of a series of pipes; a manifold. (4) The closing plate on the end of a sewer lateral that will not be used immediately.

head gate  A gate at the entrance to a conduit such as a pipeline, penstock, or canal.

head loss  Energy losses resulting from the resistance of flow of fluids; may be classified into conduit surface and conduit form losses.

headworks  (1) All the structures and devices located at the head or diversion point of a conduit or canal. The term as used is practically synonymous with diversion works; an intake heading. (2) The initial structures and devices of a water or wastewater treatment plant.

heat exchanger  A device providing for the transfer of heat between two fluids.

heat treatment  A sludge conditioning process combining high temperature, time, and pressure to improve the dewaterability of organic sludge.

heavy metals  Metals that can be precipitated by hydrogen sulfide in acid solution, for example, lead, silver, gold, mercury, bismuth, and copper.

high-purity oxygen  A modification of the activated-sludge process using relatively pure oxygen and covered aeration tanks in a conventional flow arrangement.

high-rate aeration  A modification of the activated-sludge process whereby the mixed liquor suspended solids loadings are kept high, allowing high food-to-microorganism (F;M) ratios and shorter detention times.

humus sludge  Sloughed particles of biomass from trickling media that are removed in the secondary clarifier.

hydrated lime  Limestone that has been “burned” and treated with water under controlled conditions until the calcium oxide portion has been converted to calcium hydroxide.

hydraulic loading  The amount of water applied to a given treatment process, usually expressed as volume per unit time, or volume per unit time per unit surface area.

hydraulic radius  The cross-sectional area of a stream of water divided by the length of that part of its periphery in contact with its containing conduit; the ratio of area to wetted perimeter. Also called hydraulic mean depth.

hydrocarbon  Any of the class of compounds consisting solely of carbon and hydrogen. Usually derived from petroleum.

hydrogen-ion concentration  The concentration of hydrogen ions in moles per liter of solution (moles/L). Commonly expressed as the pH value, which is the logarithm of the reciprocal of the hydrogen-ion concentration. See also pH.

hydrogen sulfide (H2S)  A toxic and lethal gas produced in sewers and digesters by anaerobic decomposition. Detectable in low concentrations (%) by its characteristic “rotten egg” odor. It deadens the sense of smell in higher concentrations or after prolonged exposure. Respiratory paralysis and death may occur quickly at concentrations as low as 0.07% by volume in air.

hydrostatic level  The level or elevation to which the top of a column of water would rise from an artesian aquifer or basin, or from a conduit under pressure.

hypochlorination  The use of sodium hypochlorite (NaOCl2) for disinfection.

hypochlorite  Calcium, sodium, or lithium hypochlorite.