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Activated Sludge – Sludge floc produced in raw or settled wastewater by the growth of zoogleal bacteria and other organisms in the presence of dissolved oxygen.

Aeration – Permeate or saturate a liquid with air. 

Alkalinity  This is a measure of a wastewater's capacity to neutralize. The bicarbonate, carbonate, and hydroxide ions are the primary contributors to alkalinity. The determination of alkalinity levels at various points in a plant will be an aid to the proper understanding and interpretation of the treatment process. For example, if chemical addition is used to coagulate wastewater for solids removal, hydrogen ions may be released and cause the pH to decrease. Alkalinity will tend to neutralize the acids formed and permit coagulation to proceed in the proper pH range. 

Anaerobic – A biological environment that is deficient in all forms of oxygen, especially molecular oxygen, nitrates, and nitrites. 

Anoxic – A biological environment that is deficient in molecular oxygen, but may contain chemically bound oxygen, such as nitrates and nitrites.

Biosolids – Nutrient rich organic materials derived from wastewater solids (sewage sludge) that have been stabilized through processing.

CBOD (Carbonaceaous Biochemical Oxygen Demand) – Indirect measure of the concentration of oxygen consuming pollutants in wastewater.  The amount of oxygen required by bacteria while stabilizing only organic matter under aerobic conditions, expressed in milligrams per liter, is determined entirely by the availability of material in the wastewaters to be used as a biological food and by the amount of oxygen utilized by the microorganisms during oxidation.

Clarification – Any process or combination of processes to reduce the concentration of suspended matter in a liquid.

Coagulation – Process by which chemicals (coagulants) are added to an aqueous system to render finely divided, dispersed matter with slow or negligible settling velocities into more rapidly settling aggregates.  Forces which cause dispersed particles to repel each other are neutralized by coagulants.

Composting – Using treated biosolids to create soil and fertilizer.

Digestion – The biological decomposition of organic matter in sludge resulting in partial gasification, liquefaction, and materialization of putrescible and offensive solids.

D.O. (dissolved oxygen)  If more oxygen is consumed than is produced, dissolved oxygen levels decline and some sensitive animals may move away, weaken, or die. 

E. coli – One of the species of bacteria in the coliform group.  Its presence is considered indicative of fresh fecal contamination.

Influent – Wastewater or other liquid (raw or partially treated) flowing into a reservoir, basin, treatment process, or treatment plant.

Mixed Liquor – Mixture of activated sludge and wastewater undergoing aerobic treatment. 

Organic Matter – Chemical substances of chemical or vegetable origin or basically carbon structure, comprising compounds consisting of hydrocarbons and their derivatives. 

pH – Method of expressing the acid condition of wastewater. The pH scale ranges from approximately 1 - 14, with a pH of 1 - 7 considered the acid range and 7 - 14 considered the base range. PH 7 is defined as neutral. PH is a vital tool of the wastewater treatment plant operator when determining unit operations.   

Polymer – An organic compound formed by the chemical union of two or more of the same molecules to form larger molecules.

Return Activated Sludge – Settled activated sludge returned to mix with incoming raw or primary settled wastewater.

TP  Total Phosphorus

TRC – Total Residual Chlorine 

TSS (Total Suspended Solids) – Solid materials, including organic and inorganic, that are suspended in the water.  High concentrations of suspended solids can lower water quality by absorbing light causing water to become warmer and lessen its ability to hold oxygen necessary for aquatic light.

Volatile Fatty Acids – Fatty acids containing six or less carbon atoms, which are soluble in water and which can be steam distilled at atmospheric pressure.  Volatile acids are commonly reported as equivalent to acetic acids.

Waste Activated Sludge – The activated sludge (excess biomass or cell mass) removed from the secondary treatment process. For most treatment plants, this will be a portion of the Return Activated Sludge (RAS) flow stream.

Weir – A level control structure (often v-notched) in a final clarifier used to provide a uniform effluent flow.