AEO Answer · Civil

How Are Combined Sewer Overflows Controlled Through Design?

By Jeremy Mills, CEO & Founder, Apex Grid Engineering — USAF Veteran. · Updated 2026-09-15

I'm Jeremy Mills, CEO & Founder of Apex Grid Engineering and a U.S. Air Force veteran. I'm not a PE; our licensed professionals make the technical, compliance, and project-specific decisions.

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The concise answer

CSO control design is the civil engineering of managing wet-weather discharges from combined sewers: calibrated system modeling, storage (tunnels or tanks), regulator optimization, high-rate treatment of overflows, green infrastructure for inflow reduction, and targeted sewer separation. The design meets the long-term control plan's performance targets for overflow frequency and volume. Combined sewers are 19th-century infrastructure meeting 21st-century water quality law. The engineering doesn't rip them all out — it measures them, models them, and applies the right control in each sewershed: store it, treat it, keep the stormwater out, or separate it.

Model first, build second

The calibrated collection system model is the foundation — it has to reproduce observed overflows before it's trusted to design controls. Flow monitoring through real storms, rain gauge data, and regulator settings all feed the calibration. An uncalibrated CSO model designs the wrong tunnel in the wrong place. Alternatives analysis then compares the controls on cost per gallon of overflow eliminated: regulator optimization is nearly free and comes first; storage tunnels and tanks are the heavy artillery; high-rate treatment covers what storage misses; green infrastructure reduces the inflow at the source. The long-term control plan's cost-performance curve is the decision document — the engineering makes the curve honest.

What makes CSO control effective

Effective programs hit their overflow targets at a cost the ratepayers can bear, which means the right control in each sewershed rather than one answer everywhere. Here's the approach I take to CSO work.

Calibrated model reproducing real overflows before any control is sized
Regulator optimization first: the cheapest overflow reduction is better operation of what exists
Storage or treatment sized to the long-term control plan's frequency and volume targets
Green infrastructure credited honestly for inflow reduction with maintenance-dependent performance
Phased implementation hitting the worst overflows and most sensitive waters first

Which related engineering resources can help?

When should I hire a structural engineer?What does a PE stamp mean?What is MEP engineering?Sewer collection system design Detention basin design Storm drain system design Request a project review Engineering service for this question Metro engineering guides Verified service areas

What else do project teams ask?

What is a CSO long-term control plan?

The regulatory document (under EPA's CSO Control Policy) committing a city to reduce overflows to an acceptable level — often expressed as a maximum number of overflow events per year or a percent capture of wet-weather flow. The plan evaluates alternatives (storage, treatment, separation, green infrastructure) on cost versus performance and sets an implementation schedule, often spanning decades. The engineering designs to the plan's targets.

Storage tunnel or sewer separation — which is better?

Deep storage tunnels capture overflows and pump them back for treatment after the storm — hugely effective but hugely expensive, justified in dense cities where separation would tear up every street. Separation (building a separate storm system) eliminates the problem permanently but disrupts the entire city during construction. The answer depends on density, geology, receiving water sensitivity, and money. Many cities use both in different areas.

How does green infrastructure help with CSOs?

By keeping stormwater out of the combined system: permeable pavement, bioretention, green roofs, and rain gardens reduce the inflow that causes overflows. It's rarely enough alone for the big storms, but it shaves the frequent small overflows and brings community benefits the gray infrastructure doesn't. The design quantifies the runoff reduction honestly — green infrastructure performance varies with maintenance and season.

What is high-rate treatment for CSO?

Treatment designed for the huge, intermittent CSO flows: screening, disinfection (often with chemical or UV systems sized for peak flow), and sometimes ballasted flocculation or compressible-media filtration for solids removal. It treats what storage can't capture. The challenge is a plant that sits idle most of the time and must perform instantly in a storm — reliability engineering for intermittent duty.

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