CSO Resources

So far this June has been pretty rainy, so it seems timely to provide some resources on a problem that New York City and many other municipalities are struggling to deal with: Combined Sewer Overflows or CSOs.

Per Capita Water Consumption in the US 1950-2005, from USGS, 2005.

Per Capita Water Consumption in the US 1950-2005, from USGS, 2005.

Why should this be of interest to you? Well, you don’t swim in your toilet, but do you poop in your waterway? It might be nice to know before going for a swim at the beach or eating some locally sourced invisible shrimp.

Let’s start with a few key terms…

Greywater: Used water from sinks, washers, etc… (but not from toilets). Since this water is not potable but still not extremely dirty, many people believe that using it (and/or rainwater) to water plants and flush toilets, for instance, would help conserve our clean/potable water supply. There are regulatory limitations to doing this (less so with rainwater), so please check to see what your local municipality’s building code will permit.

Blackwater: Used water containing human waste (dirty toilet water and everything in it). Note that one option for reducing the number/severity of CSO events is to reduce the amount of waste going into the system. Aside from removing greywater, conceivably one could also implement composting toilets which would take human waste out of the water cycle. Regulations tend to be much more strict on composting toilets (they are not allowed in NYC), but there are many commercial models of these types of systems available if the DIY version is not to your taste and you live where they are permitted.

Combined Sewer System: A system where rain water and grey/blackwater both utilize the same pipes to go to a treatment facility. This type of system was developed in flat areas with good rainfall so that the rain might wash the more solid waste to the treatment facility. Note that this type of system was built before the denser population levels and the increase of water use per capita during the mid twentieth century which has resulted in higher levels of waste water output from cities. When there are severe weather events the system may not be able to handle the rainwater + wastewater and the excess water may flow out of the system into a waterway without being treated. When this happens it is called a Combined Sewer Overflow or CSO event. The New York Times did an amazing article on this in 2009.

Separate Sewer System: Essentially the rainwater and street runoff is collected in a separate pipe system from grey/blackwater. This runoff can be more quickly filtered and released back into the water cycle and does not have the dangerous bacteria that is present in blackwater. For more information on SSS or CSS this EPA publication (also the source for the above image) goes into more detail.

BMP: Best Management Practices. A bland term for technology-based controls to reduce CSOs and storm water runoff. BMPs are small interventions of a variety of types that when added together create a system that effectively addresses runoff. The EPA has a menu of BMPs.  These are often rainwater retention and cleaning systems that are implemented at a small scale (an individual home owner might have one to reduce the runoff from her/his property into a sewer or waterway). They usually don’t require major permitting processes. Many landowners in the Chesapeake Bay watershed are required to have them. Rainwater retention ponds, rain gardens, constructed wetlands, etc… are all considered forms of BMPs. What most places commonly call BMPs, are usually referred to as green infrastructure in New York City.

Some resources:

History on the development of sewer systems can be found at sewerhistory.org. And specific history on New York City’s Sewer System can be found at NYC Department of Environmental Protection. For more background on water issues try these NYT articles.

How much water does the average person in the US use per day? and other info can be found at the United States Geological Survey (USGS) website.

Where are the CSO’s? Good question! And here is a good map to answer that: OASIS! Here is a screen shot of some of the CSO locations (pipe outfalls) at Newtown Creek.


Who regulates Sewers and CSOs? Regulations are based on the Clean Water Act (CWA). Nationally, the EPA regulates. Here is their 2011 sewer report on Region 2. In New York State the Department of Environmental Conservation (DEC) does. And in New York City the DEP does. There are also local groups like Riverkeeper or dontflush.me that are advocating for cleaner waterways, and communities near the Gowanus Canal and Newtown Creek have been very vocal about CSOs due to flood issues and due to those industrial waterways being designated as Superfund sites.

There is a lot of legalese involved in CSOs. Consent order? Long term control plan (LTCP)? SPDES? Yeah, it is not always fun reading, but delve into those links if you are into poop!

Keep in mind while going through this information that major infrastructure like NYC’s sewer system is not only incredibly large, but also very old. Upgrading a system like this takes a heck of a lot of money, engineering and time. Not to mention the disruption to streets and other infrastructure that occurs when new sewer lines or storm water systems are needed. Ultimately tax dollars go to fund these improvements and political will directs that money. Another thing to consider is that we are in an era of unprecedented technological innovation, and if engineering and research is focused on these types of issues, we could potentially see some innovative new ways to treat waste water. Somethings to think about come election time in November.

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