Combined sewer overflows discharge raw sewage during storms. Here is how older networks generate them, what control measures actually work, and what regulators want to see reported.
Combined sewer overflows discharge raw sewage into rivers and seas during heavy rain. Older networks generate them by design — the question is how often, how much, and what the operator does about it.
A combined sewer carries domestic wastewater and stormwater runoff in the same pipe. That made sense in the 19th and early 20th centuries: one trench, one pipe, half the cost. The problem is hydraulic. Domestic flow on a normal day in a typical urban catchment runs at maybe 200 litres per person per day. A heavy rainfall event can push the same network to 10 to 20 times that flow within an hour.
No treatment plant can absorb that surge, and no pipe can be sized for it without exploding the capital budget. The Victorian engineers who built the networks knew this. They designed in weir overflows — points where, once the pipe filled past a set level, the excess water (a mixture of dilute sewage and stormwater) spilled into the nearest river or harbour. The plant downstream stayed protected. The receiving water took the hit.
Separation — building a parallel storm sewer so the foul system carries only sewage — was the policy in most new development from the 1960s onward. Retrofitting separation in already-built cities turned out to be ruinously expensive: dig up every street, find every connection, double the asset base. Most older European and North American cities concluded it was not feasible at scale and looked for cheaper interventions instead.
UK Environment Agency event duration monitoring data, published since 2020, shows a typical CSO point spilling 30 to 60 times per year, with total spill duration in the low hundreds of hours. The worst-performing points spill over 200 times per year. The best-performing networks — those with significant storage built in over the last two decades — sit in the single digits.
US data is less consistent. EPA Long-Term Control Plans require permitted CSO communities to track and reduce events, but reporting cadence and units vary. A typical mid-Atlantic CSO community runs 40 to 80 events per year across the network as a whole, with individual outfalls firing 5 to 30 times.
European reporting under the Urban Waste Water Treatment Directive is currently being tightened. The 2024 UWWTD revision introduces stricter spill-reporting and reduction targets that take effect through the 2030s.
The reporting framework in the UK is Event Duration Monitoring (EDM): a sensor at every storm overflow records start and stop time of every spill, with annual totals published by the Environment Agency and the equivalents in Wales, Scotland, and Northern Ireland. Coverage was around 5% of overflows in 2015 and is now over 95%, which makes inter-year comparisons unreliable but recent absolute numbers more credible.
The Water Industry National Environment Programme (WINEP) obliges English water companies to address every flagged overflow on a regulator-defined timetable. Investment plans run in five-year cycles aligned with Ofwat price reviews.
In the US, EPA Long-Term Control Plans are negotiated under the Clean Water Act and typically embedded in consent decrees with the Department of Justice. They are court-enforceable, multi-decade, and expensive — a single major-city consent decree commonly runs $2 billion to $5 billion over 20 years.
Four families of intervention dominate every CSO programme:
For deeper context on the plant-side capacity dynamics that connect to CSO behaviour, see the capacity utilization danger zone guide.
If you order the four families by typical pound-per-cubic-metre of overflow eliminated, the ranking is consistent across the dozens of programme reviews published in the last decade:
Most programmes deploy three of the four in combination. Separation is reserved for areas where digging is happening anyway — major redevelopment, road reconstruction, deep utility renewal.
The regulatory environment is tightening rapidly. The UK's Storm Overflows Discharge Reduction Plan sets a 2050 target of substantial elimination at high-priority sites and significant reduction everywhere else, with interim 2035 milestones. The cost is north of £60 billion over the period.
The EU's revised Urban Waste Water Treatment Directive, agreed in 2024, requires Member States to publish detailed integrated urban water management plans for agglomerations above 100,000 PE and tightens overflow monitoring obligations. Implementation rolls through national law over 2025–2027.
In the US, the EPA continues to negotiate consent decrees with the remaining major non-compliant CSO communities. A handful of high-profile public-health crises — surfer illness clusters, beach closures, swimmer infections — have shifted public opinion enough that local elected officials now actively push utilities to over-deliver against permit minimums.