and state agencies to enforce cso-mitigating initiatives and the efficiency of their schedules. municipalities' sewage departments, engineering and design firms, and environmental organizations offer different approaches to potential solutions. sewer separation some us cities have undertaken sewer separation projects — building a second piping system for all or part of the community. in many of these projects, cities have been able to separate only portions of their combined systems. high costs or physical limitations may preclude building a completely separate system. in 2011, washington, d.c., separated its sewers in four small neighborhoods at a cost of $11 million. (the project cost also included improvements to the drinking water piping system.) cso storage another solution is to build a cso storage facility, such as a tunnel that can store flow from many sewer connections. because a tunnel can share capacity among several outfalls, it can reduce the total volume of storage that must be provided for a specific number of outfalls. storage tunnels store combined sewage but do not treat it. when the storm is over, the flows are pumped out of the tunnel and sent to a wastewater treatment plant. one of the main concerns with cso storage is the length of time it is stored before it is released. without careful management of this storage period, the water in the cso storage facility runs the risk of going septic. washington, d.c., is building underground storage capacity as its primary strategy to address csos. in 2011, the city began construction on a system of four deep storage tunnels, adjacent to the anacostia river, that will reduce overflows to the river by 98 percent, and 96 percent system-wide. the system will comprise over 18 miles (29 km) of tunnels with a storage capacity of 157 million us gallons (590,000 m3). the first segment of the tunnel system, 7 miles (11 km) in length, went online in 2018. the remaining segments of the storage system are scheduled for completion in 2023. (the city's overall "clean rivers" project, projected to cost $2.6 billion, includes other components, such as reducing stormwater flows.) the south boston cso storage tunnel is a similar project, completed in 2011. indianapolis, indiana, is building underground storage capacity in the form of a 28-mile (45 km) 18-foot (5.5 m) diameter deep rock tunnel system which will connect the two existing wastewater treatment plants, and provide collection of discharge water from the various cso sites located along the white river, eagle creek, fall creek, pogue's run, and pleasant run. citizens energy group is managing the efforts to construct the first phases of the work, which includes a 250-foot (76 m) deep deep rock tunnel connector between the belmont wastewater treatment plant and the southport wastewater treatment plant. additional tunnels will branch under the existing watercourses located in indianapolis. the planned cost for the project will total $1.9 billion dollars. fort wayne, indiana, is constructing a 4.5-mile (7.2 km), 14-foot (4.3 m) diameter, $180m tunnel under the 3rport (three rivers protection and overflow reduction tunnel) to address the myriad csos which outfall into the st. mary's, st. joseph, and maumee rivers. the 3rport is approximately 160 feet (49 m) below grade, and is anticipated to enter service in 2023. expanding sewage treatment capacity some cities have expanded their basic sewage treatment capacity to handle some or all of the cso volume. in 2002 litigation forced the city of toledo, ohio, to double its treatment capacity and build a storage basin in order to eliminate most overflows. the city also agreed to study ways to reduce stormwater flows into the sewer system. (see reducing stormwater flows.) retention basins main article: retention basin retention treatment basins or large concrete tanks that store and treat combined sewage are another solution. these underground structures can range in storage and treatment capacity from 2 million us gallons (7,600 m3) to 120 million us gallons (450,000 m3) of combined sewage. while each facility is unique, a typical facility operation is as follows. flows from the overloaded sewers are pumped into a basin that is divided into compartments. the first flush compartment captures and stores flows with the highest level of pollutants from the first part of a storm. these pollutants include motor oil, sediment, road salt, and lawn chemicals (pesticides and fertilizers) that are picked up by the stormwater as it runs off roads and lawns. the flows from this compartment are stored and sent to the wastewater treatment plant when there is capacity in the interceptor sewer after the storm. the second compartment is a treatment or flow-through compartment. the flows are disinfected by injecting sodium hypochlorite, or bleach, as they enter this compartment. it then takes about 20‑30 minutes for the flows to move to the end of the compartment. during this time, bacteria are killed and large solid materials settle out. at the end of the compartment, any remaining sanitary trash is skimmed off the top and the treated flows are discharged into the river or lake. the city of detroit, michigan, utilizes a system of nine cso retention basins and screening/disinfection facilities that are owned and operated by the great lakes water authority. these basins are located at original combined sewer outfalls located along the detroit river and rouge river within metropolitan detroit. these facilities are generally designed to contain two inches of stormwater runoff, with the ability to disinfect overflows during extreme wet-weather rainfall events. screening and disinfection facilities screening and disinfection facilities treat cso without ever storing it. called "flow-through" facilities, they use fine screens to remove solids and sanitary trash from the combined sewage. flows are injected with sodium hypochlorite for disinfection and mixed as they travel through a series of fine screens to remove debris. the fine screens have openings that range in size from 4 to 6 mm, or a little less than a quarter inch. the flow is sent through the facility at a rate that provides enough time for the sodium hypochlorite to kill bacteria. all of the materials removed by the screens are then sent to the sewage treatment plant through the interceptor sewer. reducing stormwater flows communities may implement low impact development techniques to reduce flows of stormwater into the collection system. this includes: constructing new and renovated streets, parking lots and sidewalks with interlocking stones, permeable paving and pervious concrete installing green roofs on buildings installing bioretention systems, also called rain gardens, in landscaped areas installing rainwater harvesting equipment to collect runoff from building roofs during wet weather for irrigating landscapes and gardens during dry weather implementing graywater collection and use on site to reduce sewage discharges at all times green infrastructure cso mitigating initiatives that are solely composed of sewer system reconstruction are referred to as gray infrastructure, while techniques like permeable pavement and rainwater harvesting are referred to as green infrastructure. conflict often occurs between a municipality's sewage authority and its environmentally active organizations between gray and green infrastructural plans. the 2004 epa report to congress on cso's provides a review of available technologies to mitigate cso impacts.: ch. 8 real-time decision support systems recent technological advances in sensing and control have enabled the implementation of real-time decision support systems (rt-dss) for cso mitigation. through the use of internet of things technology and cloud computing, cso events can now be mitigated by dynamically adjusting setpoints for movable gates, pump stations, and other actuated assets in sewers and storm water management systems. similar technology, called adaptive traffic control is used to control the flow of vehicles through traffic lights. rt-dss systems take advantage of storm temporal and spatial variability as well as varying concentration times due to diverse land uses across the sewershed to coordinate and optimize control assets. by maximizing storage and conveyance rt-dss are able to minimize overflows using existing infrastructure. successful implementations of rt-dss have been carried out throughout the united states and europe. real-time control (rtc) can be either heuristic or model based. model-based control is theoretically more optimal, but due to the ease of implementation, heuristic control is more commonly applied. generating sufficient evidence that rtc is a suitable option for cso mitigation remains problematic, although new performance methods might make this possible. regulations united kingdom there is in the uk a legal difference between a storm sewer and a surface water sewer. there is no right of connection to a storm-water overflow sewer under section 106 of the water industry act. these are normally the pipe line that discharges to a watercourse, downstream of a combined sewer overflow. it takes the excess flow from a combined sewer. a surface water sewer conveys rainwater; legally there is a right of connection for rainwater to this public sewer. a public storm water sewer can discharge to a public surface water, but not the other way around, without a legal change in sewer status by the water company. history main article: history of water supply and sanitation § sewer systems a medieval waste pipe in stockholm old town formerly deposited sewage on the street to be flushed away by rain. sewage canal of a medieval house as depicted in 1447 st. barbara altarpiece in the national museum in warsaw. combined sewer systems were common when urban sewerage systems were first developed, in the late 19th and early 20th centuries. this section is an excerpt from history of water supply and sanitation § sewer systems. archaeological discoveries have shown that some of the earliest sewer systems were developed in the third millennium bce in the ancient cities of harappa and mohenjo-daro in present-day pakistan. the primitive sewers were carved in the ground alongside buildings. this discovery reveals the conceptual understanding of waste disposal by early civilizations. society and culture a combined sewer-pipe being laid by the city's sewerage company in ghent, belgium. the image of the sewer recurs in european culture as they were often used as hiding places or routes of escape by the scorned or the hunted, including partisans and resistance fighters in world war ii. fighting erupted in the sewers during the battle of stalingrad. the only survivors from the warsaw uprising and warsaw ghetto made their final escape through city sewers. some have commented that the engravings of imaginary prisons by piranesi were inspired by the cloaca maxima, one of the world's earliest sewers. in fiction the theme of traveling through, hiding, or even residing in combined sewers is a common plot device in media. famous examples of sewer dwelling are the teenage mutant ninja turtles, stephen king's it, les misérables, the third man, ladyhawke, mimic, the phantom of the opera, beauty and the beast, and jet set radio future. the todd strasser novel y2k-9: the dog who saved the world is centered on a dog thwarting terroristic threats to electronically sabotage american sewage treatment plants. sewer alligators a well-known urban legend, the sewer alligator, is that of giant alligators or crocodiles residing in combined sewers, especially of major metropolitan areas. two public sculptures in new york depict an alligator dragging a hapless victim into a manhole. alligators have been known to get into combined storm sewers in the southeastern united states. closed-circuit television by a sewer repair company captured an alligator in a combined storm sewer on tape.