Eliminating CSO

The Problem:

As earth’s climate continues to change, the Combined Sewage Overflow problem in NYC grows as well, and we must either continue to deal with or adapt to the problem or we must face the problem head on by eliminating the CSO’s at their source. Programs have been established to help NYC deal with CSO’s after times of severe wet weather, however, as the CSO’s still encroach on our overall water quality, environmental protection agencies are trying to push for the elimination of the CSO’s altogether.  As the earth’s climate continues to change, the water cycle will continue to speed up and storms will become more severe. Obviously, we cannot control the weather, however, we can change the way the storm water is collected once it falls.  And so we must choose to either rebuild the entire NYC sewer system, which will definitely cost the city more money than it can afford, or we must enhance the current system through “greenstreet” type projects that will help collect more water and abate the storm water overflow.

The Combined Sewer System

The Options:

1. CSO Control – Technological Projects:

In 2004, the NYCDEC (the New York State Department of Environmental Conservation) negotiated a “CSO abatement program” with the NYCDEP (Department of Environmental Protection). The NYCDEP apparently discharges untreated sewage and storm water runoff from about 460 CSO outfalls during severe wet weather. These discharges are one of the major causes of serious water quality standard violations, and have a significant impact on the overall quality of the waters in and around New York City.  As a result of the NYCDEP’s numerous water quality violations, the abatement program consent order was made that would require the city to address the CSO concerns.  This new concern order required control of almost 76% of the wet weather flow – an improvement to the earlier 1992 order, which only covered about 70% of the wet weather flow.  The 2004 Order also required “the design, planning, and construction of over 30 city-wide projects, including: off-line retention tanks, sewer cleaning, in-stream aeration, floatables-containment booms, skimmer boats, sewer separation, flushing tunnels, vortex concentrators, throttling facilities, catch basin modifications, and numerous other projects designed to optimize the operation of the sewer collection system, pumping stations, and treatment plants during wet weather.”  In 2004, the estimated capital cost of these projects was over $ 2.2 billion.

Many NYC organizations have been proposing similar plans for years.  In the 1950’s, the City first recognized that CSO’s had an impact on water quality and began preliminary control plans. In 1972 the first CSO retention tank for excess wet weather flow was built at Spring Creek in Brooklyn and later that year, the Federal Clean Water Act, which required 85% removal of conventional pollutants for all plant discharges and provided more funding for pollution control plants.  In 1975 the City was divided into eight planning areas: the East River, Jamaica Bay, Inner and Outer Harbors, Flushing Bay, Paerdegat Basin, Newtown Creek, and the Jamaica tributaries. In 1980 the Regulator Improvement Project (RIP) began, followed in 1984 by the Flushing Bay CSO Facility Planning project. By 1988, the EPA had established its “nine minimum control program”, and since then many other projects – not all successful – have taken flight.

In 1988, the EPA composed the following list of CSO control steps:

Combined Sewer Overflows
The Nine Minimum Controls:

  1. Proper operation and regular maintenance programs for the sewer system and the CSO’s
  2. Maximum use of the collection system for storage
  3. Review and modification of pretreatment requirements to assure CSO impacts are minimized
  4. Maximization of flow to the publicly owned treatment works for treatment
  5. Prohibition of CSO’s during dry weather
  6. Control of solid and floatable materials in CSO’s
  7. Pollution prevention
  8. Public notification to ensure that the public receives adequate notification of CSO occurrences and CSO impacts
  9. Monitoring to effectively characterize CSO impacts and the efficacy of CSO controls [1]

Since 1995 and implemented until today, the New York/New Jersey Harbor Estuary Program (HEP) (authorized by the EPA) together with the Bight Restoration Plan named a step-wise approach to eliminating the CSO problem in the New York Harbor and in any connecting waters. The process includes implementation of “the nine minimum measures” (as set up by the EPA) to ensure a minimum acceptable level of CSO control, as well as additional CSO abatement measures – beyond the nine minimum measures – as necessary, to eliminate water quality standards violations and restore beneficial uses of the storm water impaired by CSO’s.   In 1999, United Marine International put out a “fact sheet” explaining the various technologies it was implementing to battle CSO’s. Most of these technologies were aimed at catching floatables during storms and stopping them from entering deep into the combined sewer system.  The list included: baffles in flow regulators, screen and trash racks for catching floatables within key points in the CSS, catch basin modifications at surface-level flow inlets (for example street grates), 2 different types of netting within the CSS, floating containment boom structures to capture buoyant materials, and skimmer vessel boas to collect debris – including materials captured by the booms. UMI also reported on the comparative applicability, cost effectiveness, and performance of the above-mentioned structures.  All structures helped to improve the cleanliness at its test site, but some, such as the netting, collected so much debris that they needed to be changed too often to be cost effective.  The EPA and other organizations continue to compare the costs and qualities of similar or better technologies and continue to upgrade the structures already in use.

The EPA’s long-term goals for CSO elimination:

Combined Sewer Overflows
Elements of a Long Term Control Plan

The CSO Control Policy identifies the following essential elements of a long-term control plan:

  1. Characterization, monitoring, and modeling of the combined sewer system
  2. Public participation
  3. Consideration of sensitive areas
  4. Evaluation of alternatives to meet CWA requirements using either the “presumption approach” or the “demonstration approach”
  5. Cost/performance considerations
  6. Operational plan
  7. Maximizing treatment at the existing POTW treatment plant
  8. Implementation schedule
  9. Post-construction compliance monitoring program [2]

2. CSO Control – Green Infrastructure:

Another important and effective way of dealing with the storm water overflow is by implementing “green infrastructure” which helps to collect excess storm water before it ever reaches the CSS.  One of its greatest benefits is that “Green infrastructure reduces storm water runoff volumes and reduces peak flows by utilizing the natural retention and absorption capabilities of vegetation and soils.”  In essence, by implementing increased amounts of foliage cover – like green roofs – we increase the amount of green surface area. Thus more rainwater is absorbed and the volume of runoff entering the CSS and other bodies of water is greatly reduced.  Additionally, plants have microbes that can naturally filter off any impurities or pollutants, and so green infrastructure would naturally reduce the amount of pollutants reaching the sewers.  Furthermore, greeneries and vegetation are generally helpful because they naturally reduce air pollutants, thereby increasing air quality as well, and plants remove carbon dioxide from the air through photosynthesis.

Green Infrastructure:

  1. Green Roofs – increases green surface area and may reduce annual storm water runoff by almost 50%
  2. Rain Harvesting – collecting the water, possibly to be reused
  3. Planter boxes – certain plants work better than others to increase storm water infiltration and, like roofs, reduce runoff
  4. Rain Gardens – areas of land covered in foliage to absorb rainwater
  5. Permeable Pavement – using permeable, interlocking pavement stones that allow water to seep into the ground rather than build upwards and cause flooding
  6. Vegetative Swales – filling street dividers with foliage to increase water infiltration
  7. Redeveloping brown-fields
  8. Green Parking Lots – adding plants to parking lot surfaces, especially at dividers
  9. Green Streets and Highways – planting trees along the roads, wherever possible


All references last accessed March 24-April 7

[1]: “Combined Sewer Overflows Nine Minimum Controls,” U.S. Environmental Protection Agency. Web. http://cfpub1.epa.gov/npdes/cso/ninecontrols.cfm?program_id=5

[2]: “Combined Sewer Overflows Elements of a Long Term Control Plan,” U.S. Environmental Protection Agency. Web. http://cfpub.epa.gov/npdes/cso/ltplan.cfm

“Sewage & Combined Sewage Overflows,” Riverkeeper.org. Web. http://www.riverkeeper.org/campaigns/stop-polluters/cso/

“State Enforcement Targets Health of Waters near New York City,” New York State Department of Environmental Conservation, 2004. Web. http://www.dec.ny.gov/press/12654.html

“Rainfall-Induced Discharges,” New York-New Jersey Harbor Estuary Program (HEP), 1995. Web. http://library.marist.edu/diglib/EnvSci/archives/hudsmgmt/ny-njharborestuaryprogram/rainfall.html

Stephane Gibbons and Cathy Yuhas. “Combined Sewer Overflows
What’s Happenning in New York City,” NY-NJ Harbor Estuary Program.. Web. http://www.harborestuary.org/TEautumn05.htm

“Combined Sewer Overflow (CSO) Technology Fact Sheet,” United Marine International. Web. http://www.trashskimmer.com/tp_cso.htm

“Managing Wet Weather with Green Infrastructure,” U.S. Environmental Protection Agency. Web. http://cfpub.epa.gov/npdes/home.cfm?program_id=298

“Types, Applications, and Design Approaches to Manage Wet Weather: Rain Harvesting,” U.S. Environmental Protection Agency. Web. http://cfpub.epa.gov/npdes/greeninfrastructure/technology.cfm#rainharvesting

Image 1: http://library.marist.edu/diglib/EnvSci/archives/hudsmgmt/ny-njharborestuaryprogram/figure9web.jpg

Image 2: http://ase.tufts.edu/uep/blogs/image.axd?picture=green-rooftop+chicago.jpg

Table of Contents:


o CSO in NYC
Combined Sewer System
Eliminating CSO

o Drinking Water in NYC
Buried Streams

o Holland

o Water Around the World

o PLANYC Initiatives

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