combined sewer combined sewer a combined sewer is a type of gravity sewer with a system of pipes, tunnels, pump stations etc. to transport sewage and urban runoff together to a sewage treatment plant or disposal site. this means that during rain events, the sewage gets diluted, resulting in higher flowrates at the treatment site. uncontaminated stormwater simply dilutes sewage, but runoff may dissolve or suspend virtually anything it contacts on roofs, streets, and storage yards.: 296 as rainfall travels over roofs and the ground, it may pick up various contaminants including soil particles and other sediment, heavy metals, organic compounds, animal waste, and oil and grease. combined sewers may also receive dry weather drainage from landscape irrigation, construction dewatering, and washing buildings and sidewalks. a combined sewer system. during dry weather (and small storms), all flows are handled by the publicly owned treatment works (potw). during large storms, the relief structure allows some of the combined stormwater and sewage to be discharged untreated to an adjacent water body. combined sewers can cause serious water pollution problems during combined sewer overflow (cso) events when combined sewage and surface runoff flows exceed the capacity of the sewage treatment plant, or of the maximum flow rate of the system which transmits the combined sources. in instances where exceptionally high surface runoff occurs (such as large rainstorms), the load on individual tributary branches of the sewer system may cause a back-up to a point where raw sewage flows out of input sources such as toilets, causing inhabited buildings to be flooded with a toxic sewage-runoff mixture, incurring massive financial burdens for cleanup and repair. when combined sewer systems experience these higher than normal throughputs, relief systems cause discharges containing human and industrial waste to flow into rivers, streams, or other bodies of water. such events frequently cause both negative environmental and lifestyle consequences, including beach closures, contaminated shellfish unsafe for consumption, and contamination of drinking water sources, rendering them temporarily unsafe for drinking and requiring boiling before uses such as bathing or washing dishes. mitigation of combined sewer overflows include sewer separation, cso storage, expanding sewage treatment capacity, retention basins, screening and disinfection facilities, reducing stormwater flows, green infrastructure and real-time decision support systems. this type of gravity sewer design is less often used nowadays when constructing new sewer systems. modern-day sewer designs exclude surface runoff by building sanitary sewers instead, but many older cities and towns continue to operate previously constructed combined sewer systems. development the earliest sewers were designed to carry street runoff away from inhabited areas and into surface waterways without treatment. before the 19th century it was commonplace to empty human waste receptacles, e.g., chamber pots, into town and city streets and slaughter animals in open street "shambles". the use of draft animals such as horses and herding of livestock through city streets meant that most contained large amounts of excrement. before the development of macadam as a paving material in the 19th century, paving systems were mostly porous, so that precipitation could soak away and not run off, and urban rooftop rainwater was often saved in rainwater tanks. open sewers, consisting of gutters and urban streambeds, were common worldwide before the 20th century. in the majority of developed countries, large efforts were made during the late 19th and early 20th centuries to cover the formerly open sewers, converting them to closed systems with cast iron, steel, or concrete pipes, masonry, and concrete arches, while streets and footpaths were increasingly covered with impermeable paving systems. most sewage collection systems of the 19th and early to mid-20th century used single-pipe systems that collect both sewage and urban runoff from streets and roofs (to the extent that relatively clean rooftop rainwater was not saved in butts and cisterns for drinking and washing.) this type of collection system is referred to as a "combined sewer system". the rationale for combining the two was that it would be cheaper to build just a single system.: 8 most cities at that time did not have sewage treatment plants, so there was no perceived public health advantage in constructing a separate "surface water sewerage" (uk terminology) or "storm sewer" (us terminology) system.: pp. 2–3 moreover, before the automobile era, runoff was likely to be typically highly contaminated with animal waste. further, until the mid-late 19th century the frequent use of shambles contributed more waste. the widespread replacement of horses with automotive propulsion, paving of city streets and surfaces, construction of municipal slaughterhouses, and provision of mains water in the 20th century changed the nature and volume of urban runoff to be initially cleaner, include water that formerly soaked away and previously saved rooftop rainwater after combined sewers were already widely adopted. when constructed, combined sewer systems were typically sized to carry three: pp. 2–4 to 160 times the average dry weather sewage flows.: 136 it is generally infeasible to treat the volume of mixed sewage and surface runoff flowing in a combined sewer during peak runoff events caused by snowmelt or convective precipitation. as cities built sewage treatment plants, those plants were typically built to treat only the volume of sewage flowing during dry weather. relief structures were installed in the collection system to bypass untreated sewage mixed with surface runoff during wet weather, protecting sewage treatment plants from damage caused if peak flows reached the headworks. combined sewer overflows (csos) combined sewer outflow into the anacostia river in washington, d.c. photo of the interior of a combined sewer in brighton, england. these relief structures, called "storm-water regulators" (in american english - or "combined sewer overflows" in british english) are constructed in combined sewer systems to divert flows in excess of the peak design flow of the sewage treatment plant. combined sewers are built with control sections establishing stage-discharge or pressure differential-discharge relationships which may be either predicted or calibrated to divert flows in excess of sewage treatment plant capacity. a leaping weir may be used as a regulating device allowing typical dry-weather sewage flow rates to fall into an interceptor sewer to the sewage treatment plant, but causing a major portion of higher flow rates to leap over the interceptor into the diversion outfall. alternatively, an orifice may be sized to accept the sewage treatment plant design capacity and cause excess flow to accumulate above the orifice until it overtops a side-overflow weir to the diversion outfall.: 112–114 cso statistics may be confusing because the term may describe either the number of events or the number of relief structure locations at which such events may occur. a cso event, as the term is used in american english, occurs when mixed sewage and stormwater are bypassed from a combined sewer system control section into a river, stream, lake, or ocean through a designed diversion outfall, but without treatment. overflow frequency and duration varies both from system to system, and from outfall to outfall, within a single combined sewer system. some cso outfalls discharge infrequently, while others activate every time it rains.: pp. 2–3, 2–4 the storm water component contributes pollutants to cso; but a major faction of pollution is the first foul flush of accumulated biofilm and sanitary solids scoured from the dry weather wetted perimeter of combined sewers during peak flow turbulence. each storm is different in the quantity and type of pollutants it contributes. for example, storms that occur in late summer, when it has not rained for a while, have the most pollutants. pollutants like oil, grease, fecal coliform from pet and wildlife waste, and pesticides get flushed into the sewer system. in cold weather areas, pollutants from cars, people and animals also accumulate on hard surfaces and grass during the winter and then are flushed into the sewer systems during heavy spring rains. health impacts cso discharges during heavy storms can cause serious water pollution problems. the discharges contain human and industrial waste, and can cause beach closings, restrictions on shellfish consumption and contamination of drinking water sources. comparison to sanitary sewer overflows csos differ from sanitary sewer overflows in that the latter are caused by sewer system obstructions, damage, or flows in excess of sewer capacity (rather than treatment plant capacity.): ch.4 sanitary sewer overflows may occur at any low spot in the sewer system rather than at the cso relief structures. absence of a diversion outfall often causes sanitary sewer overflows to flood residential structures and/or flow over traveled road surfaces before reaching natural drainage channels. sanitary sewer overflows may cause greater health risks and environmental damage than csos if they occur during dry weather when there is no precipitation runoff to dilute and flush away sewage pollutants. csos in the united states most of the us combined sewer systems are in the northeast and great lakes regions, and the pacific northwest. about 860 communities in the us have combined sewer systems, serving about 40 million people. pollutants from cso discharges can include bacteria and other pathogens, toxic chemicals, and debris. these pollutants have also been linked with antimicrobial resistance, posing serious public health concerns. the u.s. environmental protection agency (epa) issued a policy in 1994 requiring municipalities to make improvements to reduce or eliminate cso-related pollution problems. the policy is implemented through the national pollutant discharge elimination system (npdes) permit program. the policy defined water quality parameters for the safety of an ecosystem; it allowed for action that are site specific to control csos in most practical way for community; it made sure the cso control is not beyond a community’s budget; and allowed water quality parameters to be flexible, based upon the site specific conditions. the cso control policy required all publicly owned treatment works to have "nine minimum controls" in place by january 1, 1997, in order to decrease the effects of sewage overflow by making small improvements in existing processes. in 2000 congress amended the clean water act to require the municipalities to comply with the epa policy. mitigation of csos mitigation of combined sewer overflows include sewer separation, cso storage, expanding sewage treatment capacity, retention basins, screening and disinfection facilities, reducing stormwater flows, green infrastructure and real-time decision support systems. for example, cities with combined sewer overflows employ one or more engineering approaches to reduce discharges of untreated sewage, including: utilizing a green infrastructure approach to improve storm water management capacity throughout the system, and reduce the hydraulic overloading of the treatment plant repair and replacement of leaking and malfunctioning equipment increasing overall hydraulic capacity of the sewage collection system (often a very expensive option). the united kingdom environment agency identified unsatisfactory intermittent discharges and issued an urban wastewater treatment directive requiring action to limit pollution from combined sewer overflows. in 2009, the canadian council of ministers of the environment adopted a canada-wide strategy for the management of municipal wastewater effluent including national standards to (1) remove floating material from combined sewer overflows, (2) prevent combined sewer overflows during dry weather, and (3) prevent development or redevelopment from increasing the frequency of combined sewer overflows. rehabilitation of combined sewer systems to mitigate csos require extensive monitoring networks which are becoming more prevalent with decreasing sensor and communication costs. these monitoring networks can identify bottlenecks causing the main cso problem, or aid in the calibration of hydrodynamic or hydrological models to enable cost effective cso mitigation. municipalities in the us have been undertaking projects to mitigate cso since the 1990s. for example, prior to 1990, the quantity of untreated combined sewage discharged annually to lakes, rivers, and streams in southeast michigan was estimated at more than 30 billion us gallons (110,000,000 m3) per year. in 2005, with nearly $1 billion of a planned $2.4 billion cso investment put into operation, untreated discharges have been reduced by more than 20 billion us gallons (76,000,000 m3) per year. this investment that has yielded an 85 percent reduction in cso has included numerous sewer separation, cso storage and treatment facilities, and wastewater treatment plant improvements constructed by local and regional governments. many other areas in the us are undertaking similar projects (see, for example, in the puget sound of washington). cities like pittsburgh, seattle, philadelphia, and new york are focusing on these projects partly because they are under federal consent decrees to solve their cso issues. both up-front penalties and stipulated penalties are utilized by epa