Combined sewer systems (CSS) are sewers that collect storm water runoff, domestic sewage, and industrial waste water in the same pipe. During rain events, the storm water enters the sewers rapidly, and the capacity of the sewer system may be exceeded. The excess effluent is thus discharged directly to the receiving water. A combined sewer overflow (CSO) is this excess discharge. Here, we examine the history of New York City’s sewer system. We also analyze our Combined Sewer System and examine how we can start Eliminating CSO and finding more efficient ways to treat and deal with waste water in New York City.

NYC’s Sewer System

All across New York City, combined-sewer systems (CSS) are used to collect sewage, storm water, and industrial waste. All of this effluent flows through one pipe — hence the name combined sewer system. Approximately 70% of city sewers are combined. On a typical day, the CSS can hold all the matter that flows through its pipes. However, when rain is heavy or when snow begins to melt, the CSS begins to fill with the storm water. The mix of the storm water and waste begins to rise until the CSS reaches capacity. When the CSS exceeds capacity, all the waste — untreated — is offloaded directly into the water. This is combined sewage overflow (CSO).

Combined Sewer System

A closed sewer system (CSS) leads to combined sewage overflow (CSO).

But if CSSs are so unsafe for our water, and can possbily contaminate the single most important source of life, with raw sewage, why were they originally built? To understand why CSS are common throughout the United States, we must first examine the history of sewer systems in the Western world.

A History of Combined Sewer Systems (CSS)

Sewers in Western Europe and the US were originally built to hold surface water runoff only, though throughout time they accommodated other waste waters as well. However, with the combined stressors of a burgeoning population and the advent of the first flush toilets in the 19th century, existing separate receptacles for human waster were quickly overwhelmed; the refuse quickly found its way into the storm water pipes. This was not legal, though, until years after people began sending their waste down with the rain water — 1847 in London, 1880 in Paris.

In the United States, as in Europe, flush toilets rapidly overwhelmed private cesspools for waste. According to Steven J. Burian et. al, engineers elected to use a combined sewer system (CSS) instead of separate systems for three main reasons.

  • There was no European precedent for successful separate systems
  • Many engineers believed that combined systems were cheaper to build than separate systems
  • Engineers were unsure of what to do with sewage waste, if it was separated from storm water

There were dissenters to the use of closed-sewer systems, including Colonel George E. Waring, Jr., who designed and assembled possibly the first separate-sewer system in the US, for Lenox, Massachusetts. Waring also created a separate-sewer system for Memphis, Tennessee; the reduction of the frequency of yellow fever, which had ravaged the city for years, was accredited to his system. Unfortunately, the separate-sewer system in Memphis, and in many other cities, eventually failed, mainly because of blockages, backups, and flooding.

New York City currently uses a CSS to carry its sewage and storm water. The sewage in the pipes has been subject to treatment since the 1890s, when outbreaks of cholera and yellow fever and the contamination of public beaches generated concern about the health of city residents. The first three treatment plants were located near beaches, and were intended to keep beaches and visitors healthy, rather than preserve the safety of the water.

When building sewer systems and treatment plants, little attention was paid to population and projected population growth. This, however, became the single most important question of eliminating waste — how would the sewer system keep up with the growing population and its growing need for sewage receptacles and treatment?


All links last accessed April 10, 2010

Image 1: “Report to Congress on the Impacts and Control of CSOs and SSOs,” figure 2.1, U.S. Environmental Protection Agency. Web.

“New York City’s Wastewater Treatment System,” New York City Department of Environmental Protection, 2007. Web-PDF.

“Combined Sewage Overflow,” New York State Department of Environmental Conservation. Web.

Steven J. Burian, Stephan J. Nix, S. Rocky Durrans, Robert E. Pitt, Chi-Yuan Fan, and Richard Field. “The Historical Development of Wet-Weather Flow Management,” EPA Risk Management Research, 1999. Web-PDF.


Table of Contents:


o CSO in NYC
NYC Sewer System
Combined Sewer System
Eliminating CSO

o Drinking Water in NYC
Buried Streams

o Holland

o Water Around the World

o PLANYC Initiatives