CSO = Combined Sewage Overflow

Yesterday Riverkeeper released a 5 year study on water quality in the Hudson River and it’s tributaries. I applaud all the efforts of this organization and am glad they are on the scene, in the waters and putting pressure on our legislature bodies and officials. The report defines CSOs (Combined Sewage Overflows) on page 20 and includes lots of specific information on the exact amounts of sewage released along the estuary study area (the Hudson is not actually a river). I’m not going to rehash all this, you can read the report for that. But I would like to go a little more in depth about what exactly a CSO is on a broader sense because this is not a phnomeneon that only occurs in NYC or even New York State. So I’ll be going a bit in depth on why overflows happen and some potential ways of reducing this occurrence. After all, a major take way from this report and many others is that CSOs are one of the most noxious, persistent and intractable environmental problems. So lets get into it shall we…

A CSO is a Combined Sewage Overflow but sometimes it is (incorrectly) used to refer to a specific location where the overflow occurs these are called outfall locations  (AKA) Combined Sewage Outfalls. A Combined Sewer is a waste water collection system that does not separate sanitary waste water produced in buildings from the urban runoff that enters storm drains from streets and open areas. This sort of combined sewage system (CSS) is typical in many older cities. Most younger cities have a sanitary sewer system which separates the two types of water (but at times can also suffer from overcapacity resulting in Sanitary Sewer Overflow, SSOs). The sanitary waste is piped to a sewage (or domestic wastewater) treatment plant, the system employs another set of pipes to collect and send stormwater runoff to adjacent retention ponds, lakes, streams or other open bodies of water (which could cause a separate set of issues, see the non-point pollution discussion below). This image (image link) really does a lot to explain the phenomenon visually.

Combined Sewage Overflows (CSOs) occur when the capacity of the sewer system is exceeded. This usually happens during wet weather storm events. Since sewage pipes can only handle a certain volume of effluent and the sewage treatment plant also has limited capacity to treat incoming wastewater. Any amount of water above this threshold will eventually exit the system. This escape is typically made at outfall points, if not, it would rise up and flood city streets via storm drains. The outfall points drain excess stormwater combined with municipal effluent directly into adjacent lakes, streams, marshes or other open bodies of water. For information on what exactly is flowing into our natural waters during storm events check out the”What’s in it? section on Riverkeeper web page about CSOs. In short, there is raw sewage, industrial byproducts, pathogenic bacteria and such all mised up with debris and non-point source pollutants from the streets to create a toxic cocktail.

You can see how this could be an issue worth addressing. In fact the Environmental Protection Agency (EPA) release a CSO Control Policy in 1994 in order to help cities comply with the Clean Water Act which regulated the discharge of pollutants into US waters. Interestingly the first legislation only concerned itself with point source pollution, that is pollution can be traced back to a specific source, like a toilet or a factory, but the legislation didn’t include the regulation of non-point source pollution (which the EPA claims is the #1 source of water pollution nationally), which would include nebulous toxins found on a street such as automotive fluids coming from many sources, that are washed into drains during a storm event, but a lawsuit brought non-point source pollution under the Clean Water Acts purview. Except, remarkably, for the exemption the EPA granted to agricultural uses. These encompass the application of herbicide, pesticide and fertilizers on agricultural lands which are all to often washed into adjacent waterways and cause ecological havoc downstream and have created a dead zone at the mouth of the Mississippi River (but that is another blog post all together ).

There are several ways of dealing with CSOs. Many of these include infrastructure improvements like tunnels and catchments basins to store excess stormwater, increasing capacity or number of wastewater treatment plants or retooling the entire system to move to a sanitary sewer system that separates the two effluent flows. The problem with all these proposals are primarily cost related. Not only the capitol improvement cost of upgrading the systems at the outset but the continuing maintenance costs associated with their upkeep. Another issue with this is it promotes a system that tries to tame nature by corralling it and whipping it into shape. This false sense of dominance over the naturally occurring systems leads to continued unsustainable growth. OK so I don’t like to toss the “S” word around without some context, so let me qualify my use of unsustainable by saying that it refers to the rampant expansion of impermeable surfaces such as rooftops and pavement. You see once these grey infrastructure improvements are made (increased sewage capacity) we think we solved the problem and continue to grow in the same way that cause the problem thus increasing the amount of water rolling off the built environment putting us back at square one with more rounds of improvements needing to be made. It is a vicious cycle that could be avoided with some forethought.

You may have picked up on my use of the term “grey infrastructure”, by this I am referring to the drains, pipes, streets, treatment plants and any number of other infrastructure elements commonly deployed by engineers and architects to deal with nature and provide the creature comforts we are accustom to in our modern cities like puddle free streets and sidewalks, cool building interiors during the summer and warm ones in the winter. So why the “grey” designation? Because there is an entire class of other infrastructures available to us, these are the green infrastructures (AKA Low Impact Development or LID). You see a forest or open meadow does not have to deal with stormwater runoff because the rainwater is used on site. This is how nature deals with stormwater and we would be wise to take note and mimic this behavior. This is biomimicry in it’s most basic format.

Despite the many wonderful and exciting uses and types of green infrastructure, we will stay on topic and discuss those useful in alleviating the CSO issue at hand. These include: permeable paving, porous concrete and resin bonded gravel,  bioretention via vegetated or bioswales, rain gardens, tree box filters, constructed wetlands, pocket wetlands, soil amendments and aeration, eco-roofs such as, greenroofs (both intensive and extensive), blueroofs, brownroofs (and hybrids thereof), green-cloaking, living walls, green screens and other green veneers, rainwater harvesting, downspout disconnection, grey water recycling, blackwater sewage reduction, on-site sewage biodigestors, Living Machines, rock-reed systems and more. So there are lots of options and various ways to deal with water on different scales and intensities across the transect of human development typologies. I have provided links to these so I won’t go into a detailed explanation of each in this post, but I do want to outline some ways that these techniques work and the benefits of employing these as opposed the the grey infrastructures already outlined above.

Green infrastructure for rainwater mitigation primarily seeks to absorb, retain or at the very least delay the release of water during a storm event. If you have clicked through any of the associated links you can see how this is done in straightforward ways (i.e. rain harvesting) and very nuanced ways (i.e. green-cloaking)

Why is delaying the water important? Because Urban Stormwater Runoff is classified by the State of New York as the #1 water quality issue in New York and is the reason CSOs occur. If you can give the sewage treatment plant time to treat the water, there is less chance of overflow occuring. So the primary goal it extends the time between the beginning of a precipitation event and the peak flow, which, along with treatment plant capacity and system design capacities, determines when and how CSOs happen as the sewage pipes and plant are overwhelmed. Thus, the longer you can delay peak flow, the longer and/or more intense a storm can be without overwhelming the system. So here is a super simplified explanation of peak flow. When it rains water lands on different surfaces, permeable surfaces absorb the water at different rates until saturation is reached, impermeable surfaces don’t absorb water but they do have different textures that can speed or slow the flow of water. These rates are calculated using runoff coefficients and all this is take into consideration along with areas of different porosity when determining the peak flow of a storm (here is a real life example). Peak flow is the time during a storm event when a drop of water from the furthest point from the drain or inlet catches up to the water at the closest point. When those two drops combine with all the water in between peak flow is reached. Other important factors are the length and intensity of the storm, but I digress.

CSOs occur for at different overflow rates at different places. For example, in New York City there are 5 tiers of CSOs outfalls…

DEP groups all outfalls into “Tiers” according to their ranked CSO volumes among all CSOs in the City. The
“Tier 1” outfalls have the largest volumes and together comprise 50% of the annual Citywide CSO volume; the
“Tier 2” outfalls are the next largest and together comprise the next 25% of the annual Citywide CSO volume; the
“Tier 3” outfalls are the next largest and together comprise the next 15% of the annual Citywide CSO volume; the
“Tier 4” outfalls are the next largest and together comprise the next 10% of the annual Citywide CSO volume; and
“Tier 5” outfalls are smallest as they do not overflow in response to the JFK 1988 rainfall record.

This outfall information was pulled from this report which outlines the intricate detail a large development must go into in order to account for the added CSO events and outfall occurrences during a single event due to increased impermeable surfaces.

In addition to helping solve the problem of CSOs during and after precipitation events, the green infrastructure typologies mentioned are living systems which means that they will grow and become more productive, bio-diverse and stable over time, unlike grey infrastructure that performs at its optimum level the day after installation and slowly deteriorates over time. If this wasn’t enough, green infrastructure also brings with it a myriad of other benefits including increased biodiversity and wildlife habitat, decrease ambient air temperatures, resulting in decreased urban heat island, less sedimentation and turbidity in lakes and streams as well as reduced thermal impact on open waters, denser urban forest canopy cover, improved air quality, stronger sense of place and fostered feeling of community, physical, visual and mental connection to nature and the systems that sustain our activities.

Of course there are costs associated with green infrastructure. Just like any other thing, it requires maintenance but you are not only simply maintaining the stormwater infrastructure, you are maintaining infrastructure that perform multiple functions while providing habitat and green space within the urban landscape. It is hard to compare costs between green and grey infrastructures due to the vast array of green infrastructure techniques. For instance a greenroof will likely cost two to three times as much as a standard shingle and tar roof but the greenroof will extend the life of the roof by two to three time and it will reduce the amount of energy needed to cool and heat the interior of the home as well as reduce the sewage cost if this is a utility the building owner must foot. Not to mention the comfortable living space that is created on the roof deck and the various environmental benefits that cannot be monetarily quantified but are nevertheless of great importance. On the other hand a swale is infinitesimally less expensive to install than a storm drain and pipe. The maintenance as well is much less of a burden to bear.

In wrapping this up I would like to say that, although my professional life has me concerned with this issue and I would like to believe I have some understanding of the it, I do not fully understand all the intricacies. For example, try as I did, I couldn’t dig up any really good explanation as to why there are 5 outfall tiers. I could postulate that is has to do with what pipes go where, their capacity, how much area they service and the clunky equation I outlined earlier when discussing peak flow, etcetera but the fact is, I don’t know so I cannot pretend that I do. That said, if any of you have any knowledge or resources you would lie to share I would be appreciative.

I hope you have learned a bit in reading this. I know I’ve learned a lot in researching it. I’d like to end by admitting that this is actually a well worn issue and I’m by far not the only one talking and thinking about it. So here are a few links that can further inform us on the topic. Plus a video that really drives it all home…

From my friend Mitch Waxman at the Newtown Pentacle: http://newtownpentacle.com/2011/01/10/gentle-manner/

HuffPo: http://www.huffingtonpost.com/2011/08/08/rain-sewer-oveflows-dirty-water_n_92…


From RiverKeepers http://www.riverkeeper.org/campaigns/stop-polluters/sewage-contamination/cso/





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