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Publicly Owned Treatment Works: Operations
Table of Contents
Background and Overview
P2 Opportunities
Reasons for Change
Complete List of Links

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Water Environment Federation
An interactive schematic diagram of a typical wastewater treatment plant.

It is estimated that in 1972 only 30 to 40 percent of assessed waters of the U.S. met water quality goals and were safe for fishing and swimming. Today, monitoring data indicate that between 60 to 70 percent of assessed waters meet state water quality goals. Thirty years ago, sewage treatment plants served only 85 million people in the U.S. Today, the number of people who have access to adequate wastewater treatment facilities has more than doubled, to 173 million people.

In the U.S. wastewater is discharged from a source such as a home or industry and is transported via a collection system consisting of sewers; lift stations, and other pumping facilities to a wastewater treatment facility. In many cases there is treatment of wastewater before it is discharged into the collection system. This is because in many larger wastewater treatment systems, regulations require that industrial facilities pre-treat their wastewater to remove many hazardous contaminants before it is sent to a wastewater treatment plant. Wastewater treatment facilities monitor incoming wastewater streams to ensure their compliance with pretreatment requirements and compatibility with the treatment plant process.

Once the wastewater reaches the plant, domestic sewage goes through physical, chemical and biological processes that treat the domestic sewage and eventually remove the unwanted solids. Initially the wastewater undergoes preliminary treatment consisting of screening debris from the influent followed by grit removal. Next, is primary treatment, which typically uses clarification to remove a large percentage of biochemical oxygen demand (BOD) and solids. Following primary treatment is the core element of wastewater treatment, the biological treatment process (secondary treatment). The biological process most often used today is the activated sludge process, which utilizes microorganisms in an aerobic environment to remove a majority of the primary pollutants found in wastewater.

At the conclusion of this type of treatment a certain percentage of the solids produced are recycled to maintain the plant's microorganism population. The rest of the solids are removed from the process and often treated for recycling as fertilizer. If necessary, the "wasted" or removed solids are treated with lime to raise the pH level to eliminate objectionable odors. Pathogen reduction (disease-causing organisms, such as bacteria, viruses and parasites) for these solids is typically carried out a separate process known as digestion. If further treatment of the wastewater is required, such as nutrient removal, tertiary treatment processes such as biological or chemical treatment processes can be used. The final effluent of the wastewater treatment process is always disinfected before being released into receiving water. The processes most commonly used for disinfection are chlorination/dechlorination, ultraviolet radiation, or ozone.

Industry Profile
Publicly owned treatment works receive waterborne wastes, generally from a mix of household and industrial sources, and treat the wastes so that they can be discharged into appropriate receptors with concentrations of all pollutants within prescribed limits.
Treatment plants generally work in two stages. "Primary" treatment mechanically separates the coarser solids from the water, generally by screening and settling. "Secondary" treatment uses oxygen from the air to drive aerobic digestion of the organic wastes by microorganisms. The products of secondary treatment are a volatilized fraction that escapes into the air (generally uncontrolled), a liquid with much lower oxidizable organic content than the incoming waste stream, and a solid fraction that is separated out from the liquid (often by pressure filtration).

The products of the settling tanks used in primary treatment, in addition to the water that is sent on to secondary treatment, include material that has settled to the bottom of the tanks ("primary sludge"), as well as material such as oil and grease that is skimmed off the surface of the tanks. These solids, together with the unrecycled fraction of sludge produced in the secondary treatment tanks ("activated sludge"), are typically digested using an anaerobic process that produces methane, along with lesser amounts of other components such as hydrogen sulfide. In many plants, this offgas is burned to help supply process energy needs, such as process heat for the digesters. The digested solids, termed "biosolids", are often used as fertilizer, or are landfilled. In some plants, the solids are incinerated.

After secondary treatment the water may be released to a natural receiving body, or may be used for irrigation. Depending on the situation, it may be disinfected before release, often by chlorination, or, less frequently, by ultraviolet irradiation. It may also undergo additional filtration, such as through a sand filter, before release.



Click here for an interactive diagram of wastewater flow in a POTW:

Industrial Wastes
Industry production processes create a variety of wastewater pollutants, some that are difficult and/or costly to treat. Wastewater characteristics and levels of pollutants vary significantly by industry. Additionally, fluctuations may occur in the wastewater's daily and hourly flow, temperature, and composition. In some instances, several waste streams from different processes at an industry location may discharge at the same location. Discharges from industrial facilities can be direct or indirect. Direct discharges are made directly to a waterway or water body. Indirect discharges are usually made to Publicly Owned Treatment Plants (POTWs), and the discharge is treated by the POTW. Additionally, some facilities participate in source control that utilizes water reuse, waste minimization or pollution prevention to reduce or eliminate discharges of pollutants.

Industrial Wastewater Treatment Methods
There are three general treatment methods for industrial wastewaters: Physical, chemical and biological treatment. Physical treatment methods consist of processes such as membrane technologies, carbon adsorption, distillation, filtration, ion exchange, oil and grease skimming, oil/water separation, sedimentation, steam stripping, and solvent extraction. Membrane technologies, such as ultrafiltration and microfiltration are currently popular physical treatment technologies. Chemical treatment methods include: Chemical oxidation, chemical precipitation, chromium reduction, coagulation, cyanide destruction, dissolved air flotation electrochemical oxidation, flocculation, hydrolysis, and neutralization (pH control). Biological treatment methods include: biological nitrogen removal, bioaugmentation, activated sludge, extended aeration, anaerobic processes, rotating biological contactors, sequencing batch reactors and trickling filters.

Water Reuse, Pollution Prevention and Waste Minimization
For many industries water reuse, pollution prevention and waste minimization are being increasingly utilized. Pollution prevention, focuses on preventing the generation of wastes, while waste minimization refers to reducing the volume or toxicity of hazardous wastes. Water reuse involves reusing or treating and reusing wastewater in manufacturing or other processes at a facility. Many companies are also substituting materials used in production processes with more environmentally friendly materials. Industries are now realizing that a leaner, greener, and safer company can also contribute to cost savings, positive recognition and customer satisfaction. Regulatory agencies are finding ways to reduce regulatory burdens for "green industries".

POTW Pretreatment Program Requirements
POTWs that receive wastewater from industries may be required to develop a pretreatment program if they meet certain regulatory criteria. There are specific requirements for certain types of industries. Other industries or commercial businesses may not have specific requirements due to the volume of their discharge to municipal treatment plants, or their individual flow may not significantly impact the wastewater treatment plant. A pretreatment program has three primary objectives: 1) To prevent the introduction of pollutants into POTWs that will interfere with the operation of the POTW, including interference with its use or disposal of biosolids; 2) To prevent the introduction of pollutants into POTWs that will pass through or are incompatible with the treatment works; and 3) To improve opportunities to recycle and reclaim municipal and industrial wastewater and biosolids.

There are two primary regulations that govern development of pretreatment programs: 40 CFR Part 122 - National Pollutant Discharge Elimination System (NPDES) program; and 40 CFR Part 403 - General Pretreatment Regulations for Existing and New Sources of Pollution. The NPDES program requires that point source discharges from industries located in the United States obtain a National Pollutant Discharge Elimination System (NPDES) permit. Publicly Owned Treatment Works (POTWs) that receive: Flows greater than 5 million gallons per day (MGD), flow from categorical industrial users, or pollutants that pass through or interfere with treatment plants must develop a pretreatment program. Additionally POTWs that the Approval Authority requires to develop a program regardless of the above must develop a program.

Industry Permitting and Reporting Requirements
Two types of industries have specific permitting and reporting requirements:

  • Categorical Industrial Users (listed by 40 CFR, Part 403.6 and 405-471)
  • Significant Industrial Users (SIUs)

The following are SIUs:

  • Industries subject to Categorical Pretreatment Standards
  • Industries discharging 25,000 gallons per day (GPD) or more of process water (excluding sanitary, non-contact cooling and boiler blowdown wastewater)
  • Industries contributing 5% or more of the average dry weather hydraulic or organic capacity of the POTW
  • Industries having a reasonable potential for adversely affecting the POTW or for violating any standard or requirements

No industrial user shall introduce or cause to be introduced into a POTW any pollutant or wastewater which causes pass through or interference. These general prohibitions apply to all industrial users of a POTW whether or not they are subject to categorical pretreatment standards or any other National, State or local pretreatment standards or requirements. Furthermore, no industrial user may contribute the following substances to a POTW:

  • Pollutants which create a fire or explosive hazard to the municipal wastewater collection and POTW, including but not limited to, wastestreams with a closed- cup flashpoint of less than 140 degrees F (60 degrees C) using the test methods specified in 40 CFR 261.21.
  • Any wastewater having a pH less than 5.0 or more than 9.0, or otherwise causing corrosive structural damage to the POTW or equipment, or endangering District personnel.
  • Solids or viscous substances in amounts which will cause obstruction of the flow in the POTW resulting in interference, but in no case solids greater than one-half(1/2") inch.
  • Any wastewater containing pollutants, including oxygen-demanding pollutants (BOD, etc.), released in a discharge at a flow rate and/or pollutant concentration which, either singularly or by interaction with other pollutants, will cause interference with either the POTW or any wastewater treatment or sludge process, or which will constitute a hazard to humans or animals.
  • Any wastewater having a temperature greater than 110 degrees F, or which will inhibit biological activity in the treatment plant resulting in interference, but in no case wastewater which causes the temperature at the introduction into the treatment plant to exceed 104 degrees F (40 degrees C).
  • Petroleum oil, non biodegradable cutting oil, or products of mineral oil origin, in amounts that will cause interference or pass through.
  • Any pollutants which result in the presence of toxic gases, vapors or fumes within the POTW in a quantity that may cause acute worker health and safety problems.
  • Any trucked or hauled pollutants, except at discharge points designated by the local governing authority.
  • Any noxious or malodorous liquids, gases, solids, or other wastewater which, either singularly or by interaction with other wastes, are sufficient to create a public nuisance, a hazard to life or to prevent entry into the sewers for maintenance and repair.
  • Any wastewater which imparts color which cannot be removed by the treatment process, such as, but not limited to, dye wastes and vegetable tanning solutions, which consequently imparts color to the treatment plant's effluent thereby violating the governing NPDES permit. Color (in combination with turbidity) shall not cause the treatment plant effluent to reduce the depth of the compensation point for photosynthetic activity by more than 10 percent from the seasonably established norm for aquatic life.
  • Any wastewater containing any radioactive wastes or isotopes except as specifically approved by the POTW Manager in compliance with applicable State or Federal regulations.
  • Storm water, surface water, ground water, artesian well water, roof run off, subsurface drainage, swimming pool drainage, condensate, deionizes water, noncontact cooling water, and unpolluted industrial wastewater, unless specifically authorized by the POTW Manager.
  • Any sludges, screenings, or other residues from the pretreatment of industrial wastes
  • Any medical wastes, except as specifically authorized by the POTW Manager in a wastewater discharge permit.
  • Any wastewater causing the treatment plant's effluent to fail a toxicity test.
  • Any wastes containing detergents, surface active agents, or other substances which may cause excessive foaming in the POTW.
  • Any discharge of fats, oils, or greases of animal or vegetable origin is limited to 100 mg/l.

Prohibited wastes shall not be processed or stored in such a manner that they could be discharged to the POTW. All floor drains located in the process or materials storage areas must discharge to the industrial user's pretreatment facility before connecting with a POTW.

Table 1 - Pretreatment Standards for Existing Sources (PSES)

Pollutant or
Pollutant Property
Single Day - Maximum
Miligrams per liter (mg/l)
Monthly Average - Maximum
Milligrams per
liter (mg/l)
Total Cadmium 0.69 (0.11*) 0.26 (0.07*)
Total Chromium 2.77 1.71
Total Copper 3.38 2.07
Total Lead 0.69 0.43
Total Nickel 3.98 2.38
Total Silver 0.43 0.24
Total Zinc 2.61 1.48
Total Cyanide 1.20 0.65
Total Toxic
Organics (TTO)
2.13 ...........................

Additional restrictions on wastewater pH levels and the amount of oil/grease and total suspended solids allowed in the discharge will likely be established by the local POTW.
*PSNS for cadmium. PSNS for all other parameters are the same as those listed for PSES.

The EPA provides “A Guide to the Biosolids Risk Assessments for the EPA Part 503 Rule” on their site at:

National Center for Manufacturing Sciences. Environmental Roadmapping Initiative.

Pacific Northwest Pollution Prevention Resource Center.

Water Environment Federation, 601 Wythe St., Alexandria, VA 22314-1994.

West Knox Utility District.


The Topic Hub™ is a product of the Pollution Prevention Resource Exchange (P2Rx)

The Publicly Owned Treatment Works Topic Hub™ was developed by:

Southwest Network for Zero Waste
Southwest Network for Zero Waste
Contact email:

Hub Last Updated: 5/22/2007