Combined sewer overflows: causes, controls, and reporting

Combined sewer overflows (CSOs) discharge raw sewage into rivers and coastal waters during heavy rain. Older sewer networks generate them by design. The question is how often, how much, and what the operator does about it, because the numbers are now public and increasingly headline material.

This guide explains what a CSO is, why it happens, how it is measured and reported, what interventions actually reduce the volume, and what the CMMS role is in the operational response. If you run a combined sewer network or you are trying to understand a regulator or campaign group report, everything you need to interpret the numbers is here.

What is a combined sewer overflow

Combined sewer systems carry both sanitary sewage and stormwater in a single pipe. In dry weather, the pipe delivers a low volume of concentrated sewage to the treatment plant. In heavy rain, the flow can rise 10 to 50 times above dry weather. If the pipe or the treatment plant cannot handle the surge, the excess volume overflows to a receiving watercourse via a designed overflow point. This is a CSO.

Combined systems were the standard urban sewer design from the mid 1800s through roughly 1950. Most large cities in Europe and older US cities (Boston, New York, Philadelphia, London, Paris) still operate significant combined sewer networks. Newer development areas typically use separated systems with distinct sanitary and storm pipes. The EPA combined sewer overflow programme covers the US regulatory approach.

Why CSOs happen

How CSOs are measured

Regulatory reporting on CSOs has moved from paper based estimates to continuous monitoring in most developed jurisdictions. In England and Wales, the Environment Agency requires event duration monitors on every CSO discharge point, reporting through the Environment Agency storm overflow performance datasets. In the US, states differ but continuous monitoring is now the default at metropolitan systems under consent decree.

What actually reduces CSO volume

Storm storage

Underground storage tanks or tunnels hold peak flow until the downstream system can process it. This is the largest single lever at most combined sewer utilities. London new Thames Tideway tunnel is the most publicised example; smaller versions exist at hundreds of cities. Storage typically reduces CSO volume 60 to 90 percent when sized to the design storm.

Real time control

Dynamically adjustable throttles, gates, and weirs at network junctions can shift flow from constrained pipes to pipes with capacity. This uses existing storage more efficiently and typically reduces CSO volume 15 to 30 percent for a fraction of the cost of new storage.

Green infrastructure

Bio swales, permeable paving, green roofs, and rain gardens reduce runoff at source. Effective per acre treated but slow to scale up. Typically reduces CSO volume 5 to 15 percent when deployed across a significant fraction of the catchment.

Sewer separation

Converting a combined system to separate sanitary and storm pipes eliminates CSOs from the separated area. Very expensive per mile but eliminates the problem structurally. Typically deployed on a partial basis where sewer replacement is happening anyway.

Illegal connection removal

Foundation drains, downspouts, and sump pumps illegally connected to the sanitary side raise dry weather flow and shrink wet weather headroom. Systematic surveys and removal can reduce CSO volume 5 to 20 percent depending on the extent of the problem.

Plant capacity uplift

Raising the peak wet weather flow the treatment plant can accept directly reduces the volume that needs to overflow. See our companion article on capacity utilization for the plant side.

What CSO reduction actually costs

The CMMS role in CSO management

A CMMS does not eliminate CSOs directly, but it plays a decisive role in three ways.

Preventing CSOs caused by asset failure

A meaningful fraction of CSOs happen not because of storm surge but because of pump station failure, blocked pipe, or malfunctioning gate. A mature CMMS reliability programme catches these before they happen. See our article on pumping station downtime reduction.

Rapid response to CSO events

When a CSO does happen, the response workflow matters. The CMMS holds the incident template, dispatches the technician, tracks the notification clock (regulator notification typically within 24 hours), and generates the required report.

Evidence for the regulator

Under an enforcement action or consent decree, the regulator wants to see the maintenance programme, the response, and the corrective action. A CMMS backed record is far stronger than a spreadsheet based one.

Reporting cadence and transparency

Public reporting on CSO performance is now the norm in most developed jurisdictions. UK water companies publish annual event and duration data by CSO. US metropolitan systems under consent decree publish quarterly or annual reports. The EPA CSO Report to Congress and subsequent updates provide the regulatory context in the US.

Climate change and CSOs

Rainfall patterns are shifting toward higher intensity, shorter duration storms in most temperate regions. The design storm that a combined sewer network was built for in 1950 is not the storm the network faces in 2025 or 2050. CSO reduction programmes should now design to future climate projected storms, not historical ones. The IPCC assessment reports provide the projected precipitation extremes for major climate regions.

The community side

CSOs are increasingly a community and campaign issue. Swimmers, anglers, riverkeepers, and downstream residents want to know when and where discharges are happening. Real time public reporting (event notification via app or website) is now a common utility offering. It builds trust when done well and destroys it when done selectively.

Water quality impact of CSOs

The public health impact of a CSO discharge depends on the receiving water use, the discharge volume, the dilution ratio, and the persistence of the pollutants. Indicator bacteria (E coli, enterococci) spike sharply in the hours after a discharge, with typical return to baseline within 24 to 72 hours in flowing rivers, longer in enclosed waters. Nutrient loading from CSOs contributes to eutrophication in receiving lakes and estuaries. Priority pollutants including pathogens, oils, and heavy metals arrive in the first flush and can cause acute ecological damage. Long term water quality monitoring at CSO influenced water bodies typically shows the cumulative pattern rather than individual event impacts.

Real time public notification

Real time public notification of CSO discharges is now standard practice in the UK, mandated in England for consumer water companies. Notifications reach subscribers via email, SMS, and mobile app within minutes of a discharge starting. Similar programmes are emerging in the US, driven by public interest litigation and voluntary utility action. Effective notification requires reliable event detection (event duration monitors on every discharge point), a robust notification pipeline, and clear message content that lets recipients make informed decisions about downstream water use. Community groups increasingly track and publish CSO data in near real time, creating an independent accountability layer utilities cannot control.

Metrics worth tracking

• Number of CSO events per year, by discharge point.
• Duration hours per year, by discharge point.
• Volume discharged per year, by discharge point.
• CSO events attributable to asset failure vs storm surge.
• Regulator notification compliance rate (within 24 hours).
• Corrective action closure rate (post event).
• Public complaint volume per event.
• Downstream water quality trend at receiving points.

A typical CSO reduction programme timeline

Frequently asked questions

Summary

Combined sewer overflows are a legacy of 19th century urban infrastructure that continues to shape modern water quality. Reducing them requires a multi decade combination of storage, real time control, green infrastructure, sewer separation, and treatment plant uplift, sized and phased to the specific catchment. The CMMS role sits alongside these capital programmes: preventing asset failure driven CSOs, managing the operational response when events happen, and holding the evidence trail regulators and communities increasingly demand.

Next reading

• Capacity utilization: why 80 percent is the danger zone
• How a CMMS reduces unplanned downtime in pumping stations
• Treatment plant climate resilience
• Reading a discharge permit
• Browse the wastewater plants directory