Technical Resources

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Symbols and Acronyms

backflow connection   In plumbing, any arrangement whereby backflow can occur. Also called interconnection, cross connection.

backflow preventer  A device on a water supply pipe to prevent the backflow of water into the water supply system from the connections on its outlet end. See also vacuum breaker and air gap.

backflushing  The action of reversing the flow through a conduit for the purpose of cleaning the conduit of deposits.

back-pressure valve  A valve provided with a disk hinged on the edge so that it opens in the direction of normal flow and closes with reverse flow; a check valve.

backwashing  The operation of cleaning a filter by reversing the flow of liquid through it and washing out matter previously captured in it. Filters include true filters such as sand and diatomaceous earth-types, but not other treatment units such as trickling filters.

bacteria  A group of universally distributed, rigid, essentially unicellular microscopic organisms lacking chlorophyll. They perform a variety of biological treatment processes including biological oxidation, sludge digestion, nitrification, and denitrification.

bacterial analysis  The examination of water and wastewater to determine the presence, number, and identity of bacteria; more commonly called bacterial examination.

bacterial examination  The examination of water and wastewater to determine the presence, number, and identity of bacteria. Also called bacterial analysis. See also bacteriological count.

bacteriological count  A means for quantifying numbers of organisms. See also most probable number.

baffles  Deflector vanes, guides, grids, gratings, or similar devices constructed or placed in flowing water, wastewater, or slurry systems as a check or to produce a more uniform distribution of velocities; absorb energy; divert, guide, or agitate the liquids; and check eddies.

barminutor  A bar screen of standard design fitted with an electrically operated shredding device that sweeps vertically up and down the screen cutting up material retained on the screen.

bar screen  A screen composed of parallel bars, either vertical or inclined, placed in a waterway to catch debris. The screenings are raked from it either manually or automatically. Also called bar rack, rack.

base  A compound that dissociates in aqueous solution to yield hydroxyl ions.

basic data  Records of observations and measurements of physical facts, occurrences, and conditions, as they have occurred, excluding any material or information developed by means of computation or estimate. In the strictest sense, basic data include only the recorded notes of observations and measurements, although in general use it is taken to include computations or estimates necessary to present a clear statement of facts, occurrences, and conditions.

Beggiatoa  A filamentous organism whose growth is stimulated by H2S.

belt screen  A continuous band or belt of wire mesh, bars, plates, or other screening medium that passes around upper and lower rollers and from which the material caught on the screen is usually removed by gravity, brushes, or other means.

bicarbonate alkalinity  Alkalinity caused by bicarbonate ions.

bioassay  (1) An assay method using a change in biological activity as a qualitative or quantitative means of analyzing a material’s response to biological treatment. (2) A method of determining the toxic effects of industrial wastes and other wastewaters by using viable organisms; exposure of fish to various levels of a chemical under controlled conditions to determine safe and toxic levels of that chemical.

biochemical  (1) Pertaining to chemical change resulting from biological action. (2) A chemical compound resulting from fermentation. (3) Pertaining to the chemistry of plant and animal life.

biochemical oxidation  Oxidation brought about by biological activity resulting in the chemical combination of oxygen with organic matter. See oxidized wastewater.

biochemical oxygen demand (BOD)  A measure of the quantity of oxygen used in the biochemical oxidation of organic matter in a specified time, at a specific temperature, and under specified conditions.

biochemical oxygen demand (BOD) load  The BOD content (usually expressed in mass per unit of time) of wastewater passing into a waste treatment system or to a body of water.

biodegradation  The destruction of organic materials by microorganisms, soils, natural bodies of water, or wastewater treatment systems.

biofilm  Accumulation of microbial growth on the surface of a support material.

biological contactors  Inert surfaces engineered to provide a high specific surface area on which a biofilm can develop; usually designed so that the surface is cyclically moved through the medium to be biologically oxidized and through the open air so that oxygen transfer occurs.

biological denitrification  The transformation of nitrate nitrogen to inert nitrogen gas by microorganisms in an anoxic environment in the presence of an electron donor to drive the reaction.

biological filter  A bed of sand, gravel, broken stone, or other medium through which wastewater flows or trickles. It depends on biological action for its effectiveness.

biological filtration  The process of passing a liquid through a biological filter containing fixed media on the surfaces of which develop zoogleal films that absorb and adsorb fine suspended, colloidal, and dissolved solids and release end products of biochemical action.

biological oxidation  The process by which living organisms in the presence of oxygen convert organic matter into a more stable or mineral form.

biological process  (1) The process by which metabolic activities of bacteria and other microorganisms break down complex organic materials into simple, more stable substances. Self-purification of polluted streams, sludge digestion, and all the so-called secondary wastewater treatments depend on this process. (2) Process involving living organisms and their life activities. Also called biochemical process.

biomass  The mass of biological material contained in a system.

biosolids  The organic product of municipal wastewater treatment that can be beneficially used.

bleed  (1) To drain a liquid or gas, as to vent accumulated air from a water line or to drain a trap or a container of accumulated water. (2) The exuding, percolation, or seeping of a liquid through a surface.

blinding  (1) Clogging of the filter cloth of a vacuum filter, belt press, belt thickener, or pressure filter.
(2) Obstruction of the fine media of a sand filter.

blowdown  (1) The removal of a portion of any process flow to maintain the constituents of the flow within desired levels. The process may be intermittent or continuous. (2) The water discharged from a boiler or cooling tower to dispose of accumulated dissolved solids.

bottom contraction  The reduction in the area of overflowing water caused by the crest of a weir contracting the nappe.

bottom ventilation  Movement of air through the medium of a wastewater filter facilitated by vent stacks or provisions for the entrance or exit of air at the base of the filter.

bound water  (1) Water held strongly on the surface or in the interior of colloidal particles. (2) Water associated with the hydration of crystalline compounds.

branch circuit  That portion of the wiring system between the final overcurrent device that protects the circuit and the outlet.

breakpoint chlorination  Addition of chlorine to water or wastewater until the chlorine demand has been satisfied, with further additions resulting in a residual that is directly proportional to the amount added beyond the breakpoint.

brush aerator  A surface aerator that rotates about a horizontal shaft with metal blades attached to it; commonly used in oxidation ditches.

buffer  A substance that resists a change in pH.

bulking  Inability of activated-sludge solids to separate from the liquid under quiescent conditions; may be associated with the growth of filamentous organisms, low DO, or high sludge loading rates. Bulking sludge typically has an SVI . 150 mL/g.

bus  An electrical conductor that serves as a common connection for two or more electrical circuits. A bus may be in the form of rigid bars, either circular or rectangular in cross section, or in the form of stranded conductor cables held under tension.

butterfly valve  A valve in which the disk, as it opens or closes, rotates about a spindle supported by the frame of the valve. The valve is opened at a stem. At full opening, the disk is in a position parallel to the axis of the conduit.
bypass  An arrangement of pipes, conduits, gates, and valves by which the flow may be passed around a hydraulic structure appurtenance or treatment process; a controlled diversion.