General Information and Definitions Related to Wastewater and Wastewater Treatment

PCWRA's Biological Nutrient Removal

Because our industry is filled with scientific terms, some of our reports and postings may contain unfamiliar wording. The following terminology may be helpful for commercial business owners and other interested citizens or organizations. This is a partial listing.

•Aerobic treatment unit (ATU)
A mechanical wastewater treatment unit that provides secondary wastewater treatment for a single home, a cluster of homes, or a commercial establishment by mixing air (oxygen) and aerobic and facultative microbes with the wastewater. ATUs typically use a suspended growth process (such as activated sludge-extended aeration and batch reactors), a fixed-film process (similar to a trickling filter), or a combination of the two treatment processes.

Policies which ensure protection of water quality for a particular water body where the water quality exceeds levels necessary to protect fish and wildlife propagation and recreation on and in the water. This also includes special protection of waters designated as outstanding natural resource waters. Antidegradation plans are adopted by each state to minimize adverse effects on water.

•Best Available Technology Economically Achievable (BAT)
Technology-based standard established by the Clean Water Act (CWA) as the most appropriate means available on a national basis for controlling the direct discharge of toxic and nonconventional pollutants to navigable waters. BAT effluent limitations guidelines, in general, represent the best existing performance of treatment technologies that are economically achievable within an industrial point source category or subcategory.

•Best Conventional Pollutant Control Technology (BCT)
Technology-based standard for the discharge from existing industrial point sources of conventional pollutants including BOD, TSS, fecal coliform, pH, oil and grease. The BCT is established in light of a two-part "cost reasonableness" test which compares the cost for an industry to reduce its pollutant discharge with the cost to a POTW for similar levels of reduction of a pollutant loading. The second test examines the cost-effectiveness of additional industrial treatment beyond BPT. EPA must find limits which are reasonable under both tests before establishing them as BCT.

•Best Management Practices (BMPs)
Schedules of activities, prohibitions of practices, maintenance procedures, and other management practices to prevent or reduce the discharge of pollutants to waters of the United States. BMPs also include treatment requirements, operating procedures, and practice to control plant site runoff, spillage or leaks, sludge or waste disposal, or drainage from raw material storage.

•Best Practicable Control Technology Currently Available (BPT)
The first level of technology-based standards established by the CWA to control pollutants discharged to waters of the U.S. BPT effluent limitations guidelines are generally based on the average of the best existing performance by plants within an industrial category or subcategory.

A test used to evaluate the relative potency of a chemical or a mixture of chemicals by comparing its effect on a living organism with the effect of a standard preparation on the same type of organism.

•Biochemical Oxygen Demand (BOD)

A measurement of the amount of oxygen utilized by the decomposition of organic material, over a specified time period (usually 5 days) in a wastewater sample; it is used as a measurement of the readily decomposable organic content of a wastewater.


Sewage sludge that is used or disposed through land application, surface disposal, incineration, or disposal in a municipal solid waste landfill. Sewage sludge is defined as solid, semi-solid, or liquid untreated residue generated during the treatment of domestic sewage in a treatment facility.

•Categorical Pretreatment Standards

Limitations on pollutant discharges to publicly owned treatment works promulgated by EPA in accordance with Section 307 of the Clean Water Act that apply to specified process wastewaters of particular industrial categories [40 CFR 403.6 and Parts 405-471].

•Centralized Wastewater Treatment System

A managed system consisting of collection sewers and a single treatment plant used to collect and treat wastewater from an entire service area. Traditionally, such a system has been called a publicly owned treatment works (POTW) as defined at 40 CFR 122.2.

•Chemical Oxygen Demand (COD)

A measure of the oxygen-consuming capacity of inorganic and organic matter present in wastewater. COD is expressed as the amount of oxygen consumed in mg/l. Results do not necessarily correlate to the biochemical oxygen demand (BOD) because the chemical oxidant may react with substances that bacteria do not stabilize.

•Combined Sewer Overflow (CSO)

A discharge of untreated wastewater from a combined sewer system at a point prior to the headworks of a publicly owned treatment works. CSOs generally occur during wet weather (rainfall or snowmelt). During periods of wet weather, these systems become overloaded, bypass treatment works, and discharge directly to receiving waters.

•Combined Sewer System (CSS)

A wastewater collection system which conveys sanitary wastewaters (domestic, commercial and industrial wastewaters) and stormwater through a single pipe to a publicly owned treatment works for treatment prior to discharge to surface waters.

•Compliance Schedule

A schedule of remedial measures included in a permit or an enforcement order, including a sequence of interim requirements (for example, actions, operations, or milestone events) that lead to compliance with the CWA and regulations.

•Conventional Pollutants

Pollutants typical of municipal sewage, and for which municipal secondary treatment plants are typically designed; defined by Federal Regulation [40 CFR 401.16] as BOD, TSS, fecal coliform bacteria, oil and grease, and pH.

•Daily Discharge

The discharge of a pollutant measured during any 24-hour period that reasonably represents a calendar day for purposes of sampling. For pollutants with limitations expressed in units of mass, the daily discharge is calculated as the total mass of the pollutant discharged during the day. For pollutants with limitations expressed in other units of measurement (e.g., concentration) the daily discharge is calculated as the average measurement of the pollutant throughout the day (40 CFR 122.2).

•Daily Maximum Limit
The maximum allowable discharge of pollutant during a calendar day. Where daily maximum limitations are expressed in units of mass, the daily discharge is the total mass discharged over the course of the day. Where daily maximum limitations are expressed in terms of a concentration, the daily discharge is the arithmetic average measurement of the pollutant concentration derived from all measurements taken that day.

•Discharge Monitoring Report (DMR)
The form used (including any subsequent additions, revisions, or modifications) to report self-monitoring results by NPDES permittees. DMRs must be used by approved states as well as by EPA.

•Hazardous Substance

Any substance, other than oil, which, when discharged in any quantities into waters of the U.S., presents an imminent and substantial danger to the public health or welfare, including but not limited to fish, shellfish, wildlife, shorelines and beaches (Section 311 of the CWA); identified by EPA as the pollutants listed under 40 CFR Part 116. Any substance, other than oil, which, when discharged in any quantities into waters of the U.S., presents an imminent and substantial danger to the public health or welfare, including but not limited to fish, shellfish, wildlife, shorelines and beaches (Section 311 of the CWA); identified by EPA as the pollutants listed under 40 CFR Part 116.

•Million Gallons per Day (mgd)

A unit of flow commonly used for wastewater discharges. One mgd is equivalent to 1.547 cubic feet per second.

•Municipal Separate Storm Sewer System (MS4)

A conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, or storm drains). StormOwned and operated by a state, city, town, borough, county, parish, district, association, or other public body (created by or pursuant to state law) having jurisdiction over disposal of sewage, industrial wastes, stormwater, or other wastes, including special districts under state law such as a sewer district, flood control district or drainage district, or similar entity, or an Indian tribe or an authorized Indian tribal organization, or a designated and approved management agency under section 208 of the Clean Water Act (CWA) that discharges to waters of the United States; 2.Designed or used for collecting or conveying stormwater; 3.Which is not a combined sewer; and 4.Which is not part of a publicly owned treatment works (POTW). [40 CFR 122.26(b)(8)].

•Municipal Sources

Publicly owned treatment works (POTW) collect domestic sewage from houses, other sanitary wastewater, and wastes from commercial and industrial facilities. POTWs discharge conventional pollutants, and are covered by secondary treatment standards and state water quality standards. POTWs also produce biosolids during the treatment process.


A measure of the hydrogen ion concentration of water or wastewater; expressed as the negative log of the hydrogen ion concentration in mg/l. A pH of 7 is neutral. A pH less than 7 is acidic, and a pH greater than 7 is basic.

•Point Source

Any discernible, confined, and discrete conveyance, including but not limited to, any pipe, ditch, channel, tunnel, conduit, well, discrete fissure, container, rolling stock concentrated animal feeding operation (CAFO), landfill leachate collection system, vessel or other floating craft from which pollutants are or may be discharged. This term does not include return flows from irrigated agriculture or agricultural stormwater runoff. Basically a point source is when it is known as to where the wastewater has come from.


Dredged spoil, solid waste, incinerator residue, filter backwash, sewage, garbage, sewage sludge, munitions, chemical wastes, biological materials, radioactive materials (except those regulated under the Atomic Energy Act of 1954, as amended (42 U.S.C. 2011 et seq.)), heat, wrecked or discarded equipment, rock, sand, cellar dirt, and industrial, municipal, and agricultural waste discharged into water [40 CFR 122.2].

The reduction of the amount of pollutants, the elimination of pollutants, or the alteration of the nature of pollutant properties in wastewater prior to or in lieu of discharging or otherwise introducing such pollutants into a publicly owned treatment works [40 CFR 403.3(q)].

•Process Wastewater

Any water which, during manufacturing or processing, comes into direct contact with, or results from the production or use of any raw material, intermediate product, finished product, byproduct, or waste product.

•Publicly Owned Treatment Works (POTW)

A public water treatment entity, like Plum Creek Water Reclamation Authority, as defined by state regulations, and is owned by the state or municipality. This definition includes any devices and systems used in the storage, treatment, recycling, and reclamation of municipal sewage or industrial wastes of a liquid nature. It also includes sewers, pipes, and other conveyances only if they convey wastewater to a POTW treatment plant.


Radionuclides are used in two major ways: for their chemical properties and as sources of radiation. Radionuclides of familiar elements such as carbon can serve as tracers because they are chemically very similar to the non-radioactive nuclides, so most chemical, biological, and ecological processes treat them in a near identical way. •Sanitary Sewer A pipe or conduit (sewer) intended to carry wastewater or water-borne wastes from homes, businesses, and industries to the public wastewater treatment facility.

•Sanitary Sewer Overflows (SSO)

Untreated or partially treated sewage overflows from a sanitary sewer collection system.


Stormwater runoff, snow melt runoff, and surface runoff and drainage.

•Total Maximum Daily Load (TMDL)
The sum of the individual wasteload allocations (WLAs) for point sources and load allocations (LAs) for nonpoint sources and natural background. Additional information on TMDLs can be found at To determine if there is an approved or established TMDL on your project's receiving water, refer to for state and regional specific TMDL information related to the construction general permit. You may also have to contact your EPA regional office or state agency.

•Total Suspended Solids (TSS)
A measure of the filterable solids present in a sample.


An exceptional incident in which there is unintentional and temporary noncompliance with the permit limit because of factors beyond the reasonable control of the permittee. An upset does not include noncompliance to the extent caused by operational error, improperly designed treatment facilities, inadequate treatment facilities, lack of preventive maintenance, or careless or improper operation.


The term "wastewater" is a broad, descriptive term. Generally it includes liquids and waterborne solids from domestic, industrial or commercial uses as well as other waters that have been used (or "fouled") in man's activities, whose quality has been degraded, and which are discharged to a sewage system. The term "sewage" has been used for many years and generally refers to waters containing only sanitary wastes. However, "sewage" technically denotes any wastewaters, which pass through a sewer.

•Wastewater Categories

Two general categories of wastewaters, not entirely separable, are recognized: domestic wastewaters and industrial wastewaters. Domestic wastewaters originate principally from domestic, household activities but will usually include waters discharged from commercial and business buildings and institutions as well as ground water. Surface and storm waters may also be present. Domestic wastewaters are usually of a predictable quality and quantity. Industrial wastewaters, on the other hand, originate from manufacturing processes, are usually of a more variable character, and are often more difficult to treat than domestic wastes. While domestic wastewaters can be dealt with in general terms with respect to character and treatment, industrial wastewaters must be examined on an industry-by-industry basis.

•Wastewater Treatment

Outside of the laboratory, there is no such thing as "pure" water. Even rainwater, for example, will take on gases, solid particles and other "impurities" as it falls through the air to the earth. As water strikes the ground and flows across or through the surface of the earth, as would be expected, it takes on the characteristics of the materials it has encountered. For example, minerals are dissolved and contribute to the dissolved salts that are normally found in waters. At the same time organic matter from decomposed vegetation or from the soil, will also dissolve or be carried along suspended within the water. Thus, waters will have many natural impurities. Generally these impurities do not detract from utilization of the water, e.g., drinking, washing, etc. Our main concern will be with waters that have been used by man and then discarded. While such waters have in the past been commonly referred to as sewage, over the more recent years they have come to be called "Wastewater."

•Water Quality Criteria
Comprised of numeric and narrative criteria. Numeric criteria are scientifically derived ambient concentrations developed by EPA or states for various pollutants of concern to protect human health and aquatic life. Narrative criteria are statements that describe the desired water quality goal.

•Water Quality-Based Effluent Limit (WQBEL)
A value determined by selecting the most stringent of the effluent limits calculated using all applicable water quality criteria (e.g., aquatic life, human health, and wildlife) for a specific point source to a specific receiving water for a given pollutant.

A complete list of terms related to wastewater management can be found at: