05Where it goes after you flush (or it rains)

The short version: In many older cities, toilets and storm drains empty into the same pipe, a combined sewer, and in heavy rain that pipe is designed to overflow untreated into the nearest river or harbor.⁸¹ EPA counts about 700 US communities with combined systems, and London, Chicago, Washington and Milwaukee have built deep storage tunnels to catch the overflow.^131132133 Boston went from a harbor a presidential candidate called a "harbor of shame" in 1988 to a sewage plant on Deer Island that is now a case study in recovery.¹³⁴¹²⁴ Cities increasingly pair the big pipes with green infrastructure that soaks up rain where it falls.¹³⁵

Where does sewage go after you flush?

The route is a branching tree run in reverse. A building's drain joins a lateral pipe under the sidewalk, the lateral joins a street sewer, street sewers join larger trunk sewers and interceptors, and the interceptors carry everything to a treatment plant. The treated water leaves through an outfall, the pipe or tunnel end where it is released into a river, lake or sea. Most of the trip runs downhill by gravity, with pumping stations where the ground does not cooperate.

There are two basic designs:

• Separate systems carry sewage in one set of pipes and street runoff in another. Sewage goes to the treatment plant; stormwater usually goes straight to a stream or harbor.
• Combined systems carry sewage and stormwater in the same pipes.⁸¹

In the US, EPA lists combined sewers as clustered in the Northeast and Great Lakes, including Illinois, Indiana, Maine, Michigan, New York, Ohio, Pennsylvania and West Virginia.¹³⁶ London's Victorian network, built by Joseph Bazalgette between 1859 and 1875, is a combined system too.¹³³

What is a combined sewer overflow?

A combined sewer works fine in dry weather. In wet weather, the mix of sewage and rain can exceed what the pipes or the treatment plant can handle, and the excess discharges untreated through relief outfalls into rivers, lakes and harbors.⁸¹ That discharge is a combined sewer overflow, or CSO.

The overflow is a designed relief valve. Without it, the same water would back up into basements and streets. The problem is what it carries.

How many US communities have combined sewers depends on which EPA page you read:

• EPA's current figure is about 700 communities.⁸¹ It goes with EPA's National CSO Inventory, first published in May 2023; about 90% of outfalls in it had been verified by state permitting authorities as of September 2023.¹³¹
• EPA's older Frequent Questions page, still online and updated in April 2026, gives about 860 communities serving about 40 million people.¹³⁶

The safest summary is about 700 US communities as of the 2023 inventory, down from the roughly 860 EPA long cited. The 40 million figure belongs to the older count; no EPA page pairs it with the 700.

The problem is not only American.

England's storm overflows, which are monitored under the Environment Agency's Event Duration Monitoring program, recorded 450,398 spills in 2024, for a record total of 3,614,427 hours, according to the Rivers Trust's analysis of the 2024 data.¹³⁷ That was 8,558 more hours than in 2023, though the average number of spills per overflow fell from 33 to 31.8.¹³⁷ The Environment Agency reported on March 26, 2026 that every storm overflow in England now has an event duration monitor, and that 2025 brought 291,492 spill events, 35% fewer than in 2024, or an average of 20.5 spills per overflow.¹³⁸

How is sewage treated?

Treatment happens in stages:

• Primary treatment holds sewage in tanks so solids settle and grease floats off.
• Secondary treatment uses bacteria to consume the dissolved organic pollution left behind.
• Tertiary or advanced treatment removes nutrients such as nitrogen and phosphorus, or polishes the water further for reuse.

An EPA-hosted training deck, "Wastewater Basics 101" by John R. Buchanan, defines the stages the same way: primary treatment is the separation of liquids from solids, secondary treatment supplies oxygen to aerobic microorganisms to "finish the job," and tertiary treatment is mainly nutrient removal.¹³⁹

The solids that settle out become sludge, which also needs treatment. At Boston's Deer Island plant, sludge goes into 12 egg-shaped anaerobic digesters, each about 130 feet (40 m) tall and 90 feet (27 m) across, sealed tanks where bacteria break it down without oxygen.¹⁴⁰ Since 1991, MWRA has shipped the digested sludge to a plant on the Fore River that turns it into fertilizer pellets.¹²⁴

At the far end of advanced treatment, some cities clean wastewater all the way back to drinking quality (for where drinking water usually starts, see Where your tap water comes from).

Singapore is the best-known example. Its national water agency, PUB, introduced NEWater to the public on August 9, 2002; it is made from treated used water by microfiltration, reverse osmosis and ultraviolet disinfection.¹⁴¹ PUB says NEWater and desalination together will meet up to 85% of Singapore's water demand by 2060.¹⁴² When its fifth NEWater plant opened in January 2017, NEWater was reported to meet 40% of Singapore's daily water demand.¹⁴³ The used water reaches the plants through the Deep Tunnel Sewerage System, a planned 206 km network that runs by gravity; its first phase, including a 48 km tunnel from Kranji to Changi, was finished in 2008, and the second phase is due to be commissioned in stages from 2027.¹⁴⁴

How do cities stop sewer overflows?

There are three broad options, and most large cities use all of them: separate the pipes, store the overflow until the plant can treat it, or keep rain out of the sewer in the first place. EPA's case studies show the range:¹³²

Indianapolis is building DigIndy, a 28-mile (45 km) tunnel system with a planned cost of $1.9 billion.¹⁴⁵

Deep tunnels also handle floodwater that has nothing to do with sewage.

North of Tokyo, Japan's Metropolitan Area Outer Underground Discharge Channel, often called G-Cans, diverts floodwater from smaller rivers to the Edogawa River.¹⁴⁶ Its tunnels total 6.3 km at a depth of about 50 m, and its surge tank is 177 m long, 78 m wide and 18 m high, held up by 59 pillars that each weigh 500 tons.¹⁴⁶ Construction began in March 1993, and from June 2006 water could flow all the way from the Otoshifurutone River to the Edogawa; four pumps can discharge up to 200 cubic meters a second.¹⁴⁶

What is green infrastructure?

EPA uses the Clean Water Act definition: green infrastructure means "plant or soil systems, permeable pavement or other permeable surfaces" that manage stormwater.¹³⁵ The everyday versions are rain gardens, planter boxes, green roofs and permeable pavements, which, in EPA's words, "help reduce and treat stormwater where it falls."¹⁴⁷

EPA's case for it is cost and scale. Green infrastructure "can be a cost-effective approach that reduces and treats stormwater at its source," it "can complement gray infrastructure to control CSOs," and it can shrink the size of the pipes, tanks and tunnels needed downstream, along with their operating and energy costs.¹³⁵ The word EPA uses is complement, not replace: every city on EPA's green-and-gray case study page combines the two.¹³²

Atlanta's version includes parks that double as storage. Historic Fourth Ward Park is built to hold stormwater, and Cook Park can hold up to 10 million gallons (about 38,000 cubic meters).¹³²

Each rain garden is small. The idea is that many of them together take the early part of a storm off the sewer, which lowers the odds that a combined pipe tips into overflow. (Rain is also central to How cities are adapting to a changing climate.)

London's super sewer: the Thames Tideway

London faces the same problem as Boston at a larger scale. Bazalgette's sewers, built from 1859 to 1875, were designed to overflow into the Thames, and by a 2011-based estimate the tidal river was receiving about 39 million cubic meters (about 10 billion gallons) of storm sewage a year.¹³³

The fix is the Thames Tideway Tunnel:

• 25 km (15.5 miles) long, from Acton in the west to Abbey Mills in the east, with a 7.2 m (24 ft) internal diameter.¹³³
• 30 to 70 m below ground, per Wikipedia; Thames Water gives a maximum depth of 67 m.¹³³¹⁴⁸
• It intercepts 34 of the most polluting CSOs on the Victorian network.¹³³
• It cost about £5 billion.¹³³

Construction began in 2016.¹³³ Thames Water marks the tunnel's connection to the Lee Tunnel in May 2024 as the key milestone; the last of 21 connections was finished in February 2025, and King Charles III officially opened it on May 7, 2025.¹⁴⁸¹³³ The dates describe different stages and are consistent with each other.

Tideway feeds the 7 km Lee Tunnel, completed in 2016, which runs from Abbey Mills to the Beckton treatment works and captures over 5 million tonnes of sewage a year.¹⁴⁸ Together, the two tunnels can hold 1.6 million cubic meters.¹⁴⁸ Thames Water expects spills at the intercepted outfalls to fall from over 50 a year to about five.¹⁴⁸

Our reading: Tideway is the closest parallel to Boston's approach. Both cities kept their old combined pipes and added deep storage and interception rather than digging up every street to separate them.

In Boston: from "harbor of shame" to Deer Island

Until 1991, Greater Boston's sewage sludge was discharged into its harbor.¹²⁴ The legal pressure began in 1982, when the city of Quincy sued; the Conservation Law Foundation and the US government followed with suits of their own.¹⁴⁹ Governor Michael Dukakis then proposed moving water and sewer functions out of the state's Metropolitan District Commission into a new agency.¹⁴⁹

The legislature passed MWRA's enabling act in 1984, and MWRA took over the water and sewer systems in 1985.¹²⁴ In 1986, federal Judge A. David Mazzone ordered a 13-year schedule to build a new treatment plant on Deer Island, and he oversaw the project for the court until 2004.¹²⁴

The harbor became national politics in the 1988 presidential race.¹⁴⁹ George H.W. Bush toured it and called it a "harbor of shame" in his campaign against Dukakis.¹³⁴

The rebuild came in stages:¹²⁴

• 1991: Sludge discharges to the harbor end; sludge goes to the Fore River pelletizing plant instead.
• 1995: New primary treatment begins at Deer Island.
• 1997 and 1998: The first two secondary treatment batteries start up; all three are running in the 2001-2005 period.
• 1998: The inter-island tunnel ends discharges from the Nut Island plant.
• 2000: The 9.5-mile Massachusetts Bay outfall opens.

Today Deer Island serves 43 communities, with an average flow of about 380 mgd (1.4 million cubic meters a day) and a peak capacity of 1.2 billion gallons a day.¹⁴⁰ Its outfall tunnel is 9.5 miles (15 km) long and 24 feet (7.3 m) across, and it releases treated water through 52 risers along its last mile, far out in Massachusetts Bay.¹⁴⁰

The ecological results are measurable. In studies from around 1984, liver tumors turned up in 8% to 15% of sampled winter flounder; after 2004 they were no longer detected.¹³⁴

What it cost depends on the source: Wikipedia's Deer Island article gives $3.8 billion, its Boston Harbor article gives $4.5 billion, and an article in The Conversation gives $4.7 billion, with different scopes and cost bases.^140149134 Call it roughly $4 billion. The same Conversation article values the resulting ecosystem services at $30 billion to $100 billion.¹³⁴

Boston's combined sewers have not gone away. MWRA's CSO program cut annual overflow volume by more than 2.8 billion gallons between 1987 and 2015, from about 3.3 billion gallons a year in the 1980s.¹²⁴ By subtraction, that leaves on the order of half a billion gallons a year still overflowing as of 2015.

How it changed

• 1859-1875: Bazalgette builds London's intercepting sewers, a combined system with overflows to the Thames.¹³³
• 1982: Quincy sues over Boston Harbor pollution.¹⁴⁹
• 1985: MWRA takes over Greater Boston's water and sewer systems.¹²⁴
• 1986: Judge Mazzone orders the Deer Island construction schedule.¹²⁴
• 1988: Boston Harbor becomes a presidential campaign issue.¹³⁴
• 1994: Milwaukee's deep tunnels begin keeping overflow out of Lake Michigan.¹³²
• 2000: Deer Island's Massachusetts Bay outfall opens.¹²⁴
• 2016: London's Lee Tunnel is completed and Tideway construction begins.¹⁴⁸¹³³
• 2023: EPA publishes its National CSO Inventory.¹³¹
• 2025: The Thames Tideway Tunnel becomes fully operational and is officially opened.¹³³

Taken together, the 19th-century goal was to get sewage away from houses as fast as possible, and rivers took the result. The late 20th century added treatment and legal deadlines. The current phase is about rain: storing it, slowing it, and keeping it out of the sewer.

What's changing now (2026)

• Tideway's first full years. London's super sewer has been fully operating since February 2025, and its performance against the target of about five spills a year at intercepted outfalls is the number to watch.¹³³¹⁴⁸
• US tunnel deadlines. Chicago's final reservoir is due in 2029, and DC Water's system-wide CSO reduction target of 96% is set for March 2030.¹³²
• The count keeps shrinking. EPA's current pages put the number of US combined sewer communities at about 700, down from the 860 it cited for years.¹³¹¹³⁶
• Green plus gray as the default. EPA's 2026 guidance pages present green infrastructure as a standard part of CSO control plans, alongside tunnels and tanks.¹³⁵¹⁴⁷
• Sewers as health sensors. Testing sewage to track disease in the population that flushed it has become a newer job for a very old system.

MWRA approved a pilot with Biobot in 2020 to test Deer Island wastewater for the virus that causes COVID-19.¹⁵⁰ Biobot had run an initial study at the plant in early March 2020, and MWRA's board then approved a $200,000 contract for a six-month pilot.¹⁵¹ It reports results separately for its North and South systems; as of mid-2026 sampling had dropped to once a week, Biobot testing was set to end in late July 2026, and the Massachusetts Department of Public Health planned its own twice-weekly program.¹⁵²

Nationally, CDC has run the National Wastewater Surveillance System since 2020, largely on one-time COVID-19 funds.¹⁵³ Water Online reported in February 2026 that the FY2026 spending law signed February 3, 2026 appeared to secure its funding, while the American Society for Microbiology wrote in April 2026 that CDC "will have no funding to sustain the national program beyond Sept. 30, 2026" and asked Congress for at least $120 million for FY2027.¹⁵³¹⁵⁴

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