BIOSOLIDS: Nutrient-rich, organic material generated from household waste, removed during the treatment process. Once digested and dewatered, the material, called biosolids, can be beneficially recycled.

BOD: (Biological Oxygen Demand) Rate at which organisms use oxygen in water and wastewater.

COGENERATION: Co-production of electrical and thermal energy, also called combined heat and power (CHP).

DIGESTION: Process by which organisms break down sludge, creating as byproducts methane gas, carbon dioxide, solid organic material, and water. (Aerobic digestion takes place in the presence of oxygen and anaerobic digestion takes place with the absence of oxygen.)

DISINFECTION: Final step in the tertiary wastewater treatment process, when chlorine or sodium hypochlorite is added to the treated wastewater to kill disease-causing organisms. Ultraviolet light is another means of disinfection.

EFFLUENT: “Cleaned” wastewater, which flows out of a treatment plant.

GRIT CHAMBER: Tank in which the flow of wastewater is slowed, allowing heavy solid materials, such as pebbles, sand, coffee grounds, and eggshells, to sink to the bottom.

HEADWORKS: Area of treatment plant where influent begins treatment.

INFLUENT: Untreated wastewater when it flows into a treatment plant.

INFLUENT SCREENS: First step in the wastewater treatment process. The screen consists of bars between ¾” and three inches apart. Wastewater is allowed to pass through the bars, but large solid objects, such as rocks, cans, plastic bags, and sticks, are stopped.

MGD: Million Gallons per Day – 694.4 gallons per minute.

RECLAIMED WATER: Product produced by tertiary treatment of wastewater.

RUNOFF: Rainfall or other water that is not absorbed into the soil and drains off the street or land. This runoff moves through the Storm Drain system, not the sewerage system.

SCUM: Floatable material in wastewater made up of mainly fats, cooking oil, and grease that are skimmed off during the treatment process.

SECONDARY TREATMENT: Second stage of wastewater treatment that uses a biological process in which bacteria consume organic matter, then settle out as sludge.

SLUDGE: The solid waste material that settles out in the wastewater treatment process.

TERTIARY TREATMENT: The use of filtration to remove microscopic particles from wastewater that has already been treated to a Secondary Level. Anthracite coal is the filter medium used by the Public Utilities Department Wastewater Branch.

TSS: (Total Suspended Solids) Solids in water that can be trapped by a filter. TSS can include a wide variety of material, such as silt, decaying plant and animal matter, industrial wastes, and sewage.

The terms and definitions for this glossary were compiled from a resource on the City of San Diego’s Public Utilities Department Wastewater Branch.