Frequently Asked Questions


  • What is a CSO?
  • A combined sewer overflow (CSO) is a discharge of raw sewage mixed with stormwater into local waterways during a wet weather event, such as a rainstorm. Overflows occur when there is too much of this mixture for the sewer system or treatment plants to handle. To relieve pressure in the system and minimize backups into homes and businesses, excess sewage flows into local waterways.

  • What is a combined sewer?
  • A combined sewer is a one-pipe sewer that is designed to convey both stormwater and sanitary sewage. During dry weather, sanitary sewage alone is conveyed to a treatment facility. During wet weather, a mixture of stormwater and sanitary sewage is conveyed. Combined sewers can reach full capacity and begin to overflow to the Des Moines River.

  • Why are CSOs a concern?
  • CSOs contain raw sewage, which can be the source of disease-causing organisms. In addition, the pollutants in CSOs can adversely affect fish and other aquatic life and can create aesthetic problems, such as odors and sewage waste debris.

  • How does a CSO affect me and my water quality?
  • A CSO contains raw sewage and pollutants that include human bacteria and viruses, chemicals, oils, animal wastes and other contaminants that all have the potential to cause health concerns and illness. A CSO allows millions of gallons of untreated sewage and stormwater to enter the waterways in a typical year. Concerns associated with these discharges include:

    • The possibility of human contact with raw sewage that can carry disease-causing organisms.
    • Impacts to aquatic life, such as fish.
    • Impacts on stream and park recreation. Most area streams do not meet the state‚Äôs water quality standards for recreational activities, such as swimming or wading.
    • Offensive odors and unsanitary debris along banks of rivers and streams.

    The extent of the health concerns from a CSO discharge depend on the amount of water dilution from the size of the receiving stream, the amount of precipitation that causes the overflow, and if the overflow occurs during peak sewage periods such as the morning or evening.

  • What is a separate sanitary sewer system?
  • A separate sanitary system is a collection of pipes located under streets and easements that are designed solely to transport sewage away from the sanitary fixtures inside homes, businesses, and industry and convey it to the wastewater treatment plant. This system protects public health by treating human and industrial wastes to reduce pollutant concentrations so they can be safely discharged. Cities that have these systems must also have a separate sewer system to handle stormwater.

  • What is a separate storm sewer system?
  • A storm sewer system is a collection of inlets and pipes, located under streets and easements, designed to transport rainwater and snowmelt away from streets, homes and businesses and convey it to various receiving waters (such as streams and rivers). Storm sewers are usually much larger than sanitary sewer system pipes because peak stormwater flows from typical rain events greatly exceed sanitary flows. Water discharged through separate storm sewers generally receives no treatment.

  • What other communities are subject to CSO regulations?
  • There are currently about 770 U.S. communities subject to CSO regulations. Most of these communities are located in the Northwest, Great Lakes, and Northeast portions of the country.