Chicago’s sewage overflows are a warning for London’s combined sewers

Chicago’s Racine Avenue pumping station has discharged more than 10 billion gallons of combined sewage and stormwater into the Chicago River this year, according to reporting by the Chicago Sun-Times summarised by Planetizen. The North Branch pumping station, also operated within the city’s system, has released a further 2 billion gallons into the river’s north branch over the same period. Both stations are designed to do this: when heavy rain overwhelms the pipes, the overflow goes to the river rather than into basements.

The story matters in London because the underlying design is the same. Bazalgette’s sewer network, built between 1859 and 1875, is a combined system: foul water from homes and surface water from streets share the same pipes. When rainfall exceeds what the network can carry, storm overflows discharge diluted sewage into the tidal Thames and its tributaries. London’s answer to that problem is the Thames Tideway Tunnel, and Chicago’s numbers are a useful stress test of the claims made for it.

What Chicago’s figures actually show

The Chicago material comes from Chicago Sun-Times reporting by Brett Chase, relayed through Planetizen’s summary. The figures are specific: over 10 billion gallons at Racine Avenue, about 2 billion at North Branch, this year. Residents near Bubbly Creek, an arm of the river’s South Branch, report smells after heavy rain and debris left on the banks.

The context is that Chicago has spent decades and billions of dollars on the Tunnel and Reservoir Plan, known as the Deep Tunnel: more than 100 miles of underground tunnels and large reservoirs intended to capture floodwater before it reaches the river and Lake Michigan. The criticism in the Sun-Times reporting is not that the tunnels do nothing, but that they have not stopped overflows at these stations. That distinction matters for any city justifying a large storage tunnel as the fix for river pollution.

Why London’s network has the same design flaw

London’s combined sewers were a public health intervention against cholera and the “Great Stink” of 1858, not a pollution-control system. They were sized for a smaller city and a different rainfall pattern. The overflows that relieve them — combined sewer overflows, or CSOs — were built as safety valves to protect properties from flooding. Every discharge is a trade: sewage in the river instead of sewage in a hallway.

Two things have changed since Bazalgette. London’s population and impermeable surface area have grown, which increases the volume and speed of runoff reaching the pipes. Rainfall is also becoming more intense in shorter bursts, which is exactly the pattern a combined system handles worst. Neither trend is unique to London, and neither is reversed by building more storage alone.

What the Tideway Tunnel does — and does not — change

The Thames Tideway Tunnel is a 25-kilometre storage and transfer tunnel running under the tidal Thames from Acton Storm Tanks to Abbey Mills Pumping Station in east London, where it connects to the Lee Tunnel, opened in 2016. Its purpose is to capture overflow that would otherwise discharge to the river and hold it until Beckton sewage treatment works can process it. It is the largest single intervention in London’s sewerage since the Victorian system and has been moving through commissioning.

The tunnel intercepts the largest and most polluted CSOs on the tidal Thames. It does not separate London’s sewers, and it does not remove the overflows on tributaries such as the Lee, Wandle, Brent and Beverley Brook. It also does not add treatment capacity to a network that new development keeps loading. Readers should treat “Tideway is finished” and “the overflows have stopped” as two different claims, and check which one a press release is making.

The Chicago comparison is a caution against assuming a tunnel ends the argument. Chicago’s Deep Tunnel is a storage scheme of comparable ambition, and the overflows have continued at the stations named in the Sun-Times reporting. Storage buys time and cuts the volume and frequency of spills. It does not fix a network that receives more water than it was designed for.

What Londoners can check

Storm overflow discharges are monitored. Water companies publish event duration monitoring data annually, and Thames Water operates a near-real-time map showing discharges from its overflows. The Environment Agency publishes company-level spill data that can be compared year on year. The government’s storm overflows reduction plan, published in 2022, sets targets including restricting discharges to unusually heavy rainfall by 2035 and near elimination at bathing waters and sensitive sites by 2050; progress against those targets is reported rather than assumed.

For readers who want to test local claims, the useful questions are narrow. Which specific overflow discharges to this stretch of river? How many hours did it spill in the last reporting year, and the year before? Is the outfall on the tidal Thames, where Tideway intercepts, or on a tributary, where it does not? Is the discharge monitoring certified, or estimated?

What it means for planning and new housing

London’s sewer capacity is a development constraint as well as an environmental one. New homes connecting to an already overloaded combined network add foul flows and, unless surface water is managed separately, add storm flows too. The London Plan’s sustainable drainage policy, Policy SI 13, requires major developments to manage rainfall close to where it falls and to limit runoff to greenfield rates. That policy is the main lever against compounding the overflow problem through incremental development.

The practical consequence for readers following planning applications is that drainage strategy and sewer capacity correspondence are material, not technical appendages. Where a borough’s planning committee papers include a drainage statement, surface water calculations or a response from the sewerage undertaker, those documents indicate whether a scheme reduces or adds to the load on an overflow. Schemes that rely on connection to a combined sewer without attenuation are the ones to question.

Key facts

What remains unresolved

The Chicago figures are attributed to one newspaper’s reporting and a Planetizen summary; no separate official discharge dataset was reviewed for this draft, so the exact accounting period and the method behind the gallon totals should be confirmed against the Metropolitan Water Reclamation District of Greater Chicago’s own reporting. The London position is better documented but moves quickly: Tideway commissioning milestones, Thames Water’s spill figures and Environment Agency enforcement all change, and any figure quoted here should be checked against the current published data before being used in a planning representation or a public meeting.

Source: Planetizen, “City officials are dumping billions of gallons of sewage into the Chicago River”, https://www.planetizen.com/news/2026/11/138772-city-officials-are-dumping-billions-gallons-sewage-chicago-river