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III.E Wastewater Collection and Treatment
Local governments are responsible for designing, planning, constructing, financing, operating and maintaining wastewater treatment facilities, and the conveyance systems that direct water to the treatment facility or directly to bodies of water. These systems manage point source wastewater including storm water runoff from local government facilities; and domestic, commercial, and industrial sites. The publicly owned treatment works is comprised of the collection systems and the wastewater treatment plant (WWTP). Water passes through the sanitary sewer systems, combined sewer systems, and storm sewer systems on its way to local WWTP, or directly to bodies of water.
Overall, POTWs are responsible for the collection, treatment, analysis, and discharge of wastewater received from separate sanitary or combined sewer systems, and the disposal of sludge generated from the treatment process. Wastewater collection and treatment activities may include NPDES permit compliance (including NPDES monitoring program), laboratory operations, biosolids management and disposal, chemical storage/hazardous materials management, and vehicle/equipment management. Exhibit 51 presents common operations for wastewater collection and treatment.
The system through which water is conveyed can be one or more of three types, any or all of which a local government may be responsible. The three types are separate sanitary sewer systems, municipal separate storm water systems, and combined sewer systems. These systems may be regulated under the NPDES, pretreatment, or storm water provisions of the CWA.
Defining "Municipal" Sewer Systems
EPA uses a broad definition of "municipal" in defining municipal sewer systems. Municipal systems are defined as conveyances that are owned or operated by a state, city, town, borough, county, parish, district, association or other public body having jurisdiction of disposal of sewage, industrial wastes, storm water, or other wastes, including special districts under state law such as a sewer district, flood control district or drainage district, or other similar entity, or an Indian tribe or an authorized Indian tribal organization, or a designated and approved management agency under Section 208 of CWA.
Sanitary Sewer Systems
Local governments design, construct, operate and maintain sanitary sewer systems to convey wastewater from homes and businesses to a wastewater treatment plant. Activities of local governments include installing new sewer lines, cleaning blocked lines, repairing lines that are subject to leaks and infiltration, maintaining root control, repairing manholes, operating and maintaining pump stations, and conducting all maintenance activities necessary to prevent sanitary overflows and assure that wastewater is conveyed to the treatment plant.
Maintaining sanitary sewer systems is a significant responsibility for local governments. Leaks, or the infiltration of water into the sewer system, can occur through cracks and improperly sealed joints of piping. Local governments are responsible for repairing or upgrading these systems as needed. As wastewater collection systems age, cracks develop in pipes through which groundwater can seep. Leaks can also occur in new systems that are improperly installed thus having open joints.
What part of the sanitary sewer system is most likely to leak?
Sanitary sewer capacity is reduced by groundwater seepage through leaky pipes, and storm water flow through leaky and missing manhole covers and domestic and industrial roof drains. While much of the leakage occurs in main trunk sewers, as much as fifty percent of groundwater seepage in certain areas may come from holes in pipes on private property.
Overall, seeping groundwater raises the volume of wastewater in sewers. The more leaks in the system, the less efficient the operation of the system becomes due to decreased capacity. During excess rainfall events, the sewer system is unable to carry the excess water, and flooding can occur. Diluted and untreated sewage can back up through private sewers into basements, spill into storm drains and creeks, and wash up onto public beaches. This results in the movement of raw sewage, bacteria, and chemicals into homes, into lakes and rivers, and onto beaches. To assure maximum system capacity and to prevent sanitary sewer overflows, local governments must undertake an active monitoring and preventive maintenance programs to identify and repair leaky sewer lines, in addition to conducting any major upgrades or restorations.
Local governments that operate the treatment plants to which the sanitary sewers flow are required to report all overflows and flooding, whether from sanitary or combined sewage systems, so that repairs and preventive action can be taken to minimize the extent of environmental and human health impacts.
Combined Sewer Systems
Municipal combined sewer systems convey both storm water and sanitary sewage through the same pipe.
Local governments may be responsible for operating and maintaining combined sewer systems (CSS). CSS collect and convey municipal sanitary wastewater, industrial wastewater, and storm water to a treatment plant, or directly to bodies of water depending on the size of the CSS and the amount of rainfall. Similar to sanitary sewers, activities of local governments include installing new sewer lines, cleaning blocked lines, repairing lines that are subject to leaks and infiltration, and conducting all maintenance activities. Additional activities of local governments to reduce the volume of silt and solids being transported to the sewer systems and reduce water contamination include street cleaning, yard waste removal, and screen cleaning.
When the size of a rainfall event exceeds the capacity of the CSS, the system overflows, directing combined sanitary sewage, industrial wastewater and storm water directly to bodies of water. Although some systems include primary wastewater treatment for these overflows prior to the discharge, many systems discharge untreated sewage directly to the receiving waterbody. These events are known as CSOs. (When moderate rainfall events occur and the system does not overflow, combined systems convey the wastewater to the POTW where storm water that would not otherwise receive treatment is treated.)
EPA's CSO control program requires municipalities to take steps to reduce CSO events, and to reduce the impact of these events on water quality. Recognizing that rerouting or separating sewer lines could potentially be prohibitively expensive to some municipalities, the CSO Control Policy promotes the use of nine minimum controls (NMC), and the development of long term control plans to reduce contaminated discharges from CSS. The NMC, which includes activities such as (1) maximizing the use of the collection system for storage, (2) preventive maintenance, and (3) pollution prevention, were developed to reduce contaminated discharges while avoiding major capital expenditures. In the long term control plan, municipalities determine which controls, including capital projects, will be developed to help meet water quality standards. EPA encourages both regional and state permit writers to include these CSO provisions as part of the NPDES permit for municipal wastewater treatment plants.
Municipal Separate Storm Sewer Systems (MS4)
Operating and maintaining separate storm sewers is another of the activities for which local governments are responsible. Municipal separate storm sewer systems (MS4s) are designed to convey storm water from impermeable areas to bodies of water. In conveying storm water directly to streams, rivers, and lakes, MS4s also transport oil, grease, pesticides, herbicides, dirt and grit, all which have the potential to reduce water quality. Local government operations related to operating and maintaining storm sewer systems include clearing blocked sewer lines, preventing contaminants from entering the storm sewer system, constructing storm water controls, and sampling and analyzing storm water discharges. Additional activities of local governments to reduce the volume of silt and solids being transported to the sewer systems and reduce water contamination include street cleaning, yard waste removal, and screen cleaning.
EPA's NPDES storm water regulations require municipalities to apply for an NPDES storm water permit, characterize storm water discharges, implement management procedures to prevent contaminated storm water from discharging to waterways, and monitor storm water discharges.
Water Line Repair/Replacement
Separate, combined, and storm sewer systems require repair to eliminate conditions that interfere with their ability to convey sewage and storm water flows. Sewers, and other collection system components such as manholes, pump stations, and syphons must be repaired or replaced to address structural failure, infiltration (leakage of groundwater into pipes), exfiltration (leakage of sewage out of pipes), and blockages. In combined sewers, regulators must be repaired when they fail to divert combined wastewater flows at the intende flow rates. Replacement of portions of a sewer system may also be required to address inadequate capacity, which can result in separate sewer overflows during periods of high flow in separate systems, or dry weather overflows in combined systems.
Repairs may involve replacement of individual pipe sections, replacement of entire sewer segments, or repair of existing sewers. Repairs carried out may include grouting of leaking joints, lining of existing sewers, and rebuilding or lining of manholes and other structures.
SSOs, whether caused by sewer system component failure, or inadequate capacity, impact the enviroment through the discharge of raw sanitary sewage. These discharges often result in direct human exposure to raw sewage, as well as discharge of sewage to surface and ground waters. SSOs are unpermitted, illegal discharges under the CWA, and may subject the local government to enforcement action by the regulatory authority.
Combined sewer systems impact the environment during weather by discharging raw, or minimally treated, combined sewage to surface waters. Combined sewer systems that experience dry weather overflows have an even greater impact on the environment, in that relatively undiluted sewage is discharged untreated to surface waters. Combined sewer wet weather overflows are regulated by the local government's NPDES permit.
Separate and combined sewer system repairs can impact the environment through the discharge of raw sewage that may occur as a result of the need to bypass sewage around the line or system component being repaired. Repairs of separate, combined and storm sewers also have the potential to impact the environment through erosion and sedimentation, which takes place as a result of excavation, stockpiling, and backfilling; or through discharge of sediment-laden water from the repair excavation.
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