Sanitary Sewer Overflows

What are Sanitary Sewer Overflows?

Sanitary Sewer Overflows (SSOs) are discharges of raw sewage from municipal sanitary sewer systems. SSOs can release untreated sewage into basements or out of manholes and onto city streets, playgrounds, and into streams before it can reach a treatment facility. SSOs are often caused by blockages in sewer lines and breaks in the sewer lines.

Why do sewers overflow?

SSOs occasionally occur in almost every sewer system, even though systems are intended to collect and contain all the sewage that flows into them. When SSOs happen frequently, it means something is wrong with the system.

Problems that can cause SSOs include:

  • Infiltration and Inflow (I&I): Too much rainfall or snowmelt infiltrating through the ground into leaky sanitary sewers that are not designed to hold rainfall or drain properly and excess water inflowing through roof drains connected to sewers, broken pipes, or badly connected sewer service lines.
  • Undersized Systems: Sewers and equipment that are too small to carry sewage from subdivisions or commercial areas.
  • Pipe Failures: Can be caused by blocked, broken, or cracked pipes; tree roots growing into the sewer; sections of pipe settling or shifting so that pipe joints no longer match; and sediment and other material building up causing pipes to break or collapse.
  • Equipment Failures: Include pump failures and power failures.
  • Sewer Service Connections: Discharges occur at sewer service connections to houses and other buildings; some cities estimate that as much as 60% of overflows comes from the service lines
  • Deteriorating Sewer System: Can be a result of improper installation or improper maintenance. Widespread problems that can be expensive to fix develop over time, some municipalities have found severe problems necessitating billion-dollar correction programs; often communities have to curtail new development until problems are corrected or system capacity is increased.

How can SSOs be reduced or eliminated?

A few SSOs may be unavoidable. Unavoidable SSOs include those occurring from unpreventable vandalism, some types of blockages, extreme rainstorms, and acts of nature such as earthquakes or floods.

Many avoidable SSOs are caused by blockages such as grease and other debris, inadequate system capacity, and improper system design and construction. These SSOs can be reduced or eliminated by:

  • Sewer system cleaning and maintenance.
  • Reducing infiltration and inflow through system rehabilitation and repairing broken or leaking service lines.
  • Enlarging or upgrading sewer, pump station, or sewage treatment plant capacity and/or reliability.
  • Eliminating or reducing the amount of Fats, Oils and Grease (FOG) or other non-biodegradable debris that is disposed of via the sewer system.