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Glossary of Terms

40 CFR Part 503 (503 Regs)

Code of Federal Regulations that addresses use and disposal of biosolids, including land application, landfill disposal and incineration. Stipulates permitting practices, pollutant limits, stabilization, disposal, monitoring and reporting requirements.

Activated Sludge

Biological treatment process used to convert organic material in wastewater into carbon dioxide.

Aerate

A process used to expose biosolids to circulating air, creating aerobic systems.

Aerobic System

Process occurring in an environment containing oxygen.

Anaerobic System

Process occurring in an environment without oxygen.

Biosolids

Wastewater solids that have been treated for safe use as a fertilizer or soil amendment.

Biosolids Recycling

Beneficial reuse of biosolids as a soil conditioner or by applying them to land in order to improve the soil and boost plant growth.

Centrifuge

Process of thickening or dewatering solids using equipment that applies centrifugal force to separate solids and liquids.

Class A Biosolids

Stabilized solids meeting stringent quality standards.

Class B Biosolids

Stabilized solids with less stringent pathogen treatment standards and more handling and disposal restrictions.

Composting

Treatment process that converts dewatered biosolids to a humus-like material for use as a soil amendment. Requires the addition of bulking agents such as yard waste.

Dewatering

Standard method of water removal in biosolids production.

Digestion

A process used to stabilize solids that reduces organic content and pathogen concentration.

Aerobic

Most suited for waste activated sludge (WAS), also called secondary solids; occurs in an environment containing oxygen that must be provided using aerating mixers or diffusers.

Anaerobic

Typically used to convert a combination of primary and secondary solids to methane and carbon dioxide.

Dissolved Air Flotation (DAF)

Process of thickening WAS that bubbles air into a tank to float WAS solids. Thickened solids are skimmed off the surface of the liquid and collected for additional treatment.

Effluent

The liquid portion of the flow, after the solids have settled to the bottom of the basin, that is returned to the wastewater plant’s liquid treatment processes.

Gravity Belt Thickener (GBT)

Process for thickening WAS that uses a fine mesh belt to separate solids and liquids by gravity. Liquid drops through the belt and is returned to the wastewater plant’s liquid treatment processes, and thickened solids are collected for additional treatment or disposal.

Grease

Usually resulting from food at restaurants that is not disposed of properly, grease collects with other insoluble materials and causes problems in biosolids disposal.

Grit

Sand, gravel and heavy inorganic material from wastewater removed to prevent accumulation in basins and tanks and damage to downstream pumping equipment. After removal, grit is dewatered and landfilled.

Heat Drying

Stabilization process that uses heat to remove virtually all water from and to pasteurize biosolids. 

Hydrogen Sulfide

Gas produced during the anaerobic decomposition of organic material in wastewater. Associated with “rotten egg” smell, can cause corrosion of metal equipment.

Incineration

An on-site method of untreated sludge disposal where the materials are destroyed.

Land Application

A method of biosolids disposal where the organic material is spread on the land or injected below the land surface.  It can be used beneficially on different types of land, including agricultural land, forests and reclaimed land.

Landfilling

A method of biosolids disposal where the material is layered with solid waste in landfills and covered in soil.

Lime Stabilization

Stabilization proves that uses lime to raise the temperature and pH of biosolids.  Used to meet either Class A or Class B pathogen requirements, depending on the quantity of lime added.

Polymer

A chemical compound that is added to wastewater that causes the solids suspended in the water to adhere, preparing the wastewater for the dewatering and thickening processes.

Primary Clarification

Treatment process used to remove primary solids from wastewater. Clarified wastewater typically undergoes further treatment.

Primary Solids

Solids removed during the initial wastewater treatment process.

Screenings

Solids removed from wastewater influent during treatment through bar racks and screening equipment. 

Scum

Grease, light plastics and other floatable material that is skimmed from the surfaces of primary and secondary clarifiers.

Secondary Solids

Solids generated during the treatment of the biodegradable portion of wastewater. Also known as Waste Activated Sludge (WAS).

Sludge

The untreated byproduct of wastewater treatment consisting of primary and secondary solids.

Stabilization

Treatment process that reduces pathogens, reduces offensive odors, and reduces additional biological decay. Can be achieved by biological or chemical processes to reduce volatile content of solids or by chemical or heat sterilization.

Thickening

Process that concentrates primary solids by removing free liquid.  Thickened primary solids are usually 5 to 8 percent solid and can still be pumped.  Gravity belt thickeners, dissolved air filtration thickeners, rotary drum thickeners and centrifuges are the more common types of thickeners.

Volatility

Describes the biological stability of biosolids and their ability to degrade further.

Waste Activated Sludge (WAS)

Diluted secondary solids removed from the activated sludge process; not usually associated with offensive odors.