Skip to main content
Most King County offices will be closed on Monday, May 29, 2017, for Memorial Day.  
King County logo
In Seattle, like many older cities, sewer pipes carry both wastewater (used water and sewage that goes down the drain in homes and businesses) and stormwater (rain or snow that washes off streets and parking lots) to a sewage treatment plant. In many parts of Seattle, the mixed wastewater and stormwater flow together in a single pipe. This is called a Combined Sewer System.

During a heavy rain, the pipes may get too full and start to overflow into Lake Union, Lake Washington, the Duwamish River, or Puget Sound. When this happens, it's called a Combined Sewer Overflow (CSO). This provides a "safety valve" that prevents back-ups of untreated wastewater into homes and businesses, flooding in city streets, or bursting underground pipes.

The Combined Sewer System was built as Seattle grew during the early 1900s, as an economical way to handle wastewater and stormwater. One advantage of this system is that, most of the time when rainfall is low to moderate, both the stormwater and wastewater go to the treatment plant before being discharged to Puget Sound.

When the combined sewer system was designed over 100 years ago, it was less expensive than building two entirely separate systems of pipes and tunnels to carry stormwater and wastewater. Before the sewage treatment plants were built, all sewage and stormwater was discharged into the nearest body of water.

The CSO locations were left in place when the present sewer system was updated to act as safety valves when the pipes get too full to handle the high volume of water during heavy rains.

  • The advantage of a combined sewer system is that both stormwater and wastewater are treated most of the time.
  • The disadvantage is that during heavy rains, untreated stormwater and wastewater may be discharged at CSO locations. In fact, there are far fewer overflows now than in the past due to construction projects to control the overflows.

When the combined sewer system was designed over 100 years ago, it was less expensive than building two entirely separate systems of pipes and tunnels to carry stormwater and wastewater. Before the sewage treatment plants were built, all sewage and stormwater was discharged into the nearest body of water.

The CSO locations were left in place when the present sewer system was updated to act as safety valves when the pipes get too full to handle the high volume of water during heavy rains.

  • The advantage of a combined sewer system is that both stormwater and wastewater are treated most of the time.

  • The disadvantage is that during heavy rains, untreated stormwater and wastewater may be discharged at CSO locations. In fact, there are far fewer overflows now than in the past due to construction projects to control the overflows.

Swimming near a CSO sign

Most people will not get sick from just getting contaminated water on their skin. The biggest risk is from getting tainted water from an overflow in your mouth. The bacteria in untreated sewage might make you sick, especially if you are already sick or have low immunity. In general, young children and elderly people may have a higher risk of getting sick.

Public Health recommends that people not go in the water near these signs for 48 hours after a heavy rain.

Wetsuits are an added protection for your skin. If you do go in the water near an overflow location during or after a heavy rain, don't get water in your mouth and wash yourself and your wetsuit with hot water and soap as soon as possible.

Animals are usually not affected, but if your pet does go in the water during an overflow, be sure to give it a good bath as soon as possible. If your pet is very young or old, it could be at higher risk. If your animal develops diarrhea, you should withhold food, and consult your veterinarian about what to do next.

Combined Sewer Overflow sign posted near an outlet

The Combined Sewer Overflow sign warns people of the dangers of swimming or fishing in water that might be polluted because of a sewer pipe overflowing in the area during and after heavy rain.

Bacteria and chemicals from CSOs can increase the risk of getting sick from swallowing the water or eating the fish.

Public Health recommends that people not go in the water near these signs for 48 hours after a heavy rain.

Public Health advises that you not to eat bottomfish caught near a CSO sign, because the sediments are contaminated with chemicals from many years of pollution. Bottomfish (such as flounder, sole, rockfish and cod) may carry varying levels of contamination from long exposure to contaminated sediments. Eating these fish frequently may increase your risk of cancer and other chronic diseases.

If you decide to eat bottomfish anyway, you can lower your risk by eating smaller, younger fish that may have fewer pollutants.

When you prepare the fish:

  • remove and throw away the head, skin, fat, dark meat, and guts
  • be sure to wash your hands with hot water and soap after cleaning the fish
  • as with any fish, keep it in the refrigerator until you cook it
  • always cook fish to 140 degrees to kill bacteria
  • broil, grill, or bake the fish, and pour off any oil left in the pan.

King County and the City of Seattle operate the combined system, and have substantially controlled more than 100 of the 151 overflow locations in Seattle. Building bigger pipes and underground tanks to store the water will control the remaining CSOs. After a storm is over, the combined wastewater and stormwater will go to the sewage treatment plant for treatment before being discharged into Puget Sound.

At Alki and Carkeek Park, treatment plants have been converted to CSO treatment sites. During storms, these plants provide primary treatment before discharging to Puget Sound, significantly reducing the amount of untreated wastewater and stormwater going into the Sound.

Both the County and the City are planning major projects to control more CSOs in the next few years. The biggest construction project will begin in year 2000 between South Lake Union and Myrtle Edwards Park. This project will control the largest overflows and allow for treatment of CSOs from the South Lake Union area.

Related sites:

Remember that what goes down drains may go into lakes, streams, or Puget Sound.

We can all help keep the water clean by:

  • keeping paints, oils, and pesticides out of storm drains
  • fixing leaks from vehicles
  • taking cars to a commercial car wash, where the water is often recycled before going to the treatment plant.

If you wash your own car:

  • wash it with biodegradable soap over grass or gravel
  • use a bucket and a hose nozzle to limit the amount of soap and water used
  • wash only the outside of the car, not the engine.

The amount of rain varies between CSO locations, depending on how the system has been designed and built. Some locations will have CSO discharges during a moderate summer storm, while other locations will only overflow during the worst winter storms.

After the rain stops, it takes up to 48 hours (2 days) for the water near the CSO to return to "normal". During this time, the bacteria die off and the currents dilute the germs and chemicals in the CSO.

Since there are over 100 CSO locations, it would be very difficult to get samples during every CSO event; it also takes about a day to get the test results. Based on a major sampling study, we can estimate that you should stay out of the water near the CSO sign for about 48 hours (2 days) after a storm.

One way to eliminate CSOs would be to add a separate system of pipes to drain stormwater directly to lakes, rivers, or Puget Sound. This would be very expensive and would also release untreated stormwater in all storms, which would add chemicals from surface runoff to the water bodies.

The preferred way to reduce CSOs is to store the stormwater during heavy rainstorms and slowly release it to the treatment plant. This provides treatment to most of the wastewater and stormwater before it goes into lakes, rivers or the Sound. The State of Washington standard requires controls to reduce CSOs to an average of no more than one per year per location. Both the County and the City are working to reduce CSOs to that standard.

Related: Protecting Our Waters program, King County Wastewater Division