Combined Sewer Overflows

The Everett wastewater collection system is divided into two areas: the north end combined sewer system and the south end separated sewer system.

What are combined sewer systems?

Combined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe. Most of the time, the combined sewer system conveys all of the wastewater to Everett's wastewater treatment plant, called the Everett Water Pollution Control Facility (EWPCF), where it is treated and then discharged to either the Snohomish River or Port Gardner Bay. However, during periods of heavy rainfall or snowmelt, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system. This is why the combined sewer system is designed to occasionally overflow and discharge directly to the Snohomish River or Port Gardner Bay. These overflows are called combined sewer overflows (CSOs).

  1. Causes of overflows
  2. Reduction program


The combined sewer system in the north end of the city was largely constructed between 1890 and 1963. The system was designed to convey sewage, horse manure, street and rooftop runoff, and garbage from city streets to the nearest receiving body of water.

Old system

Prior to 1960, the combined sewer system discharged directly to Port Gardner Bay and the Snohomish River through numerous outfalls without treatment. A system of gravity sewers, lift stations, force mains, and regulators was constructed in the early 1960s to intercept these outfalls and convey the sewage to treatment lagoons. The interceptor sewers and lift stations were sized to accommodate all of the dryweather flows and part of the stormwater runoff. Excess combined sewage resulting from stormwater, overflows either to Port Gardner Bay or the Snohomish River. 

Older Combined Sewer System Graphic

Advantages of new system

The advantage of the combined sewer system is that, most of the time when rainfall is low to moderate; both the storm water and waste water go to the treatment plant before being discharged to Puget Sound. The disadvantage is that during heavy rains, untreated stormwater and waste water may be discharged at combined sewer outfall locations. 

Typical modern combined sewer system graphic