A Primer on Combined Sewer Overflows

 

By Cathy Yuhas, NJ Sea Grant Extension Program/NY-NJ Harbor Estuary Program

 

What are Combined Sewer Overflows?

Combined sewers are systems that use a single pipe to transport both stormwater runoff from streets and sewage from households, businesses, and industries to sewage treatment plants. When the treatment plants and collection systems become overloaded with water during a heavy rainstorm, raw sewage and stormwater are released through an outfall into local waterways. This overflow is known as a combined sewer overflow (CSO). During a CSO event, untreated human waste, industrial waste, toxic materials, stormwater and street litter are discharged into the waterways. There are 212 in New Jersey, and 29 of them are on the Hackensack River.

 

Why were combined sewers built?

Although combined sewers are now known to be a problem, they were originally built to alleviate public health concerns. Combined sewers are found throughout older cities. New York City built more than 6,000 miles of sewers starting in the late 1600s and continuing through the early 20th century. In Newark, NJ, underground sewer pipes were installed in the late 1880s.

Combined sewers were designed to carry both stormwater from streets and human wastes from homes and industries. These sewers not only ended problems of flooded basements and gutters, they also protected people from exposure to disease-causing organisms by carrying away stormwater, garbage, human waste, animal waste, and other refuse that collected on city streets. Combined sewers made sense at the time - there was no treatment of sewage and therefore no reason to build two separate sewer systems.

By the turn of the century, combined sewers comprised 84% of existing sewers in the New York and northern New Jersey region. Water quality conditions were declining because of the high concentration of untreated sewage. To address this problem, municipalities began constructing facilities to treat sewage in the 1920s, connecting the existing sewer systems to new sewage treatment plants (STPs).

To account for high volumes of rainfall, the sewage treatment plants were usually designed to handle twice the average flow of wastewater. As both population and water consumption have grown over time, this extra capacity has proven to be inadequate. As a result, the combined flow from a storm has often been more than the treatment plants could handle; so despite improved sewage treatment, CSOs continue to be a major source of pollution.

 

What are the affects of CSOs?

CSOs can affect the quality of local waters because they introduce pollutants such as pathogens, floatable materials, organic matter, nutrients, and toxic substances.

Untreated sewage often contains pathogens, a group of bacteria, viruses and protozoa that are responsible for many human diseases. When pathogens are present in the water they can be transferred to people through shellfish consumption or accidental ingestion of water while swimming. Elevated levels of pathogens therefore lead to bathing beach closures and restrictions on the harvesting of shellfish. Pathogens can also limit other recreational activities in waterways, including swimming, boating, kayaking, fishing and crabbing. Total and fecal coliforms and enterococcus, all indicators human waste, are used to measure attainment of water quality standards in waterways.

Floatable materials from CSOs can wash up on beaches making them unattractive and sometimes unsafe. Floating plastic can be harmful to birds and marine animals, while larger floatable debris can pose navigational hazards.

Excess organic matter from domestic, commercial and industrial sewage can lead to reduced levels of dissolved oxygen (DO) in the water. Since most marine organisms require oxygen for respiration, adequate DO levels are essential for sustaining aquatic life.

CSOs can contribute excess nutrients to the Harbor in the form of human sewage, fertilizers, and animal wastes. These excess nutrients can increase algae growth, which in turn may increase organic matter and decrease DO levels.

Toxic substances can enter the Harbor through CSOs from runoff, industrial effluents discharged into the sewer system, or household products poured down the drain. Toxic contamination can be found in Harbor sediments and the water column and can accumulate in the tissue of fish and shellfish.

 

Reprinted from The Tidal Exchange: Newsletter of the New York-New Jersey Harbor Estuary Program, Spring 2005, with permission of the New York-New Jersey Harbor Estuary Program (www.harborestuary.org).