CSO vs SSO: Understanding Sewer Overflows

Two acronyms dominate the smart sewer conversation: CSO and SSO. Both refer to sewer overflows, but they have fundamentally different causes, regulatory treatment, and solutions. Understanding the distinction is essential for anyone involved in wastewater infrastructure.

Combined Sewer Overflows (CSOs)

A combined sewer overflow occurs in cities with combined sewer systems — systems where stormwater runoff and sanitary sewage flow through the same pipes. About 860 communities in the US have combined sewer systems, mostly older cities in the Northeast and Midwest built before the 1940s.

During dry weather, combined systems work fine — they carry sewage to the treatment plant. But during heavy rain, the combined flow of stormwater plus sewage exceeds pipe and treatment plant capacity. The excess — a mix of diluted sewage and stormwater — is discharged through overflow points directly into waterways.

Key CSO Facts

• Occur at permitted outfall points (most are authorized under NPDES permits)
• Triggered by wet weather events
• The overflow is a mix of stormwater and diluted sewage (typically 90%+ stormwater)
• An estimated 850 billion gallons of CSOs occur annually in the US
• EPA's 1994 CSO Policy establishes the regulatory framework

Sanitary Sewer Overflows (SSOs)

A sanitary sewer overflow occurs in separate sanitary sewer systems — systems designed to carry only domestic and commercial wastewater (no stormwater). SSOs are never expected or permitted and are always a violation.

SSOs are typically caused by system failures:

• Blockages — Grease, debris, root intrusion, or collapsed pipes
• Capacity exceedance — Inflow and infiltration (I&I) overwhelming the system during rain
• Equipment failure — Pump station breakdowns, power outages
• Pipe deterioration — Structural failures in aging pipes

Key SSO Facts

• Occur in separate sanitary sewer systems
• Never permitted — always a violation of the Clean Water Act
• The overflow is undiluted sewage
• An estimated 75,000+ SSOs occur annually in the US
• Can result in significant fines, enforcement actions, and consent decrees

Environmental Impact

Both types of overflow pollute waterways, but their impacts differ:

• CSOs release large volumes of diluted sewage. The environmental harm comes from the sheer volume — pathogens, nutrients, and pollutants entering rivers and lakes during every significant rain event.
• SSOs release smaller volumes but of concentrated raw sewage. They're often more immediately dangerous, especially when overflows reach areas where people have direct contact (parks, beaches, neighborhoods).

How Smart Sewers Address Both

For CSOs: Real-Time Control

Smart sewer technology addresses CSOs primarily through real-time control (RTC) — redistributing flow across the network to maximize use of available capacity during storms. By storing water in underutilized pipes and timing flows to match treatment plant capacity, RTC systems can reduce CSO volume by 50-80% without building new infrastructure.

For SSOs: Monitoring and Early Detection

Smart technology prevents SSOs through continuous monitoring that detects problems before they cause overflows:

• Level monitoring — Sensors detect rising water levels that indicate developing blockages
• Flow anomaly detection — AI identifies unusual patterns suggesting I&I, root intrusion, or grease buildup
• Predictive maintenance — ML models predict which pipes and pump stations are most likely to fail
• Real-time alerts — Crews are dispatched to clear blockages before overflows occur

Learn more about the terminology or see how cities are solving both problems.