Municipal wastewater

Scope

It is the responsibility of the operator to obtain an NPDES permit. Effluent limitations are the main mechanism in NPDES permits for controlling discharges of pollutants to receiving waters. Municipalities use two major kinds of sewer systems:

1. Combined sewers, and
2. Separate sanitary sewers.

Regulatory citations

• 40 CFR Part 122 — EPA administered permit programs: the National Pollutant Discharge Elimination System
• 40 CFR Part 401 — General provisions

Key definitions

• Combined sewers: Systems designed to collect both sanitary sewage and stormwater runoff in a single-pipe system. These systems are designed to convey sewage and wastewater to a treatment plant during dry weather.
• Effluent limitation: Any restriction imposed by the Director on quantities, discharge rates, and concentrations of “pollutants” which are “discharged” from “point sources” into “waters of the United States,” the waters of the “contiguous zone,” or the ocean.
• Publicly owned treatment works (POTW): A treatment works as defined by section 212 of the Act, which is owned by a 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. The term also means the municipality as defined in section 502(4) of the act, which has jurisdiction over the indirect discharges to and the discharges from such a treatment works.
• Sanitary sewers: Systems installed to collect wastewater only and do not provide widespread drainage for the large amounts of runoff from precipitation events. They are typically built with some allowance for higher flows that occur when excess water enters the collection system during storm events.

Summary of requirements

EPA establishes secondary treatment standards for POTWs, which are minimum, technology-based requirements for municipal wastewater treatment plants. These standards are reflected in terms of five-day biochemical oxygen demand, total suspended solids removal, and pH.

Secondary treatment standards also provide for special considerations regarding:

• Treatment plants that receive flows from combined sewers,
• Treatment plants that received industrial wastes,
• Waste stabilization ponds, and
• Treatment plants that receive less concentrated influent wastewater for combined and separate sewers.

Municipal wastewater treatment plant NPDES permit application. Each NPDES permit applicant with combined sewer overflow (CSO) systems must supply the following information:

• A system map that includes CSO discharge points, sensitive use areas potentially affected by CSOs, and waters supporting threatened and endangered species potentially affected by CSOs;
• A system diagram with the location of major sewer trunk lines (both combined and separate sanitary), locations of points where separate sanitary sewers feed into the combined sewer system, in-line and off-line storage structures, locations of flow-regulating devices, and locations of pump stations;
• A description of outfall with information such as location (state, county, and city or town in which the outfall is located and latitude and longitude), distance from the shore (if applicable), and depth below the surface (if applicable);
• CSO events which include information such as the number of events in the past year and the average duration per event, if available;
• A description of receiving waters with information such as the name of receiving water and name of the watershed/river/stream system and the United States Soil Conservation Service watershed (14-digit) code (if known); and
• CSO operations which include information such as the description of any known water quality impacts on the receiving water caused by the CSO.