The need for proper stormwater management should now be obvious in light of the environmental and public health concerns around urban stream syndrome discussed in the previous blog post. The problem is hammered home whenever our sewer systems fail us, flooding our streets and homes. A quick search in the news for “flooded streets” invariable returns a myriad of examples across the country and the world where our systems failed to protect the city from being inundated with water. This can represent not only significant property damage but can also pose a huge health risk to the public. However, not all sewer systems are created equal, and we will start our discussion of stormwater management by highlighting the history of sewer systems and the two main types of sewer systems in America today.
American Sewers Today: CSS v SSS/MS4
Today there are two main types of systems in play in America based on how their pipes are connected underground; both are regulated by the US Environmental Protection Agency (EPA) through a permitting process called the National Pollutant Discharge Elimination System (NPDES). Combined Sewer System (CSS), as the name suggests, are a system where stormwater and raw sewage are combined via share pipes and are all water is sent to a treatment plant before discharge. The trouble with CSS is that rain events inundate the treatment plant and can easily lead to Combined Sewer Overflow (CSO), detailed in the next section, that contains raw, untreated sewage, obviously posing a risk to human and environmental health. In fact, these systems are designed to overflow in such conditions to avoid overrunning the treatment plant (EPA, 2018).
These systems are old and outdated and are legacies from a time when officials believed dumping our waste into big water bodies was the best thing to do as it would be diluted and rendered harmless. Although we now know this thinking to be false and that dumping our waste directly into the environment has severe environmental and public health impacts, their legacy lives on as it would require major, system-scale reconstruction to switch major cities like New York, Atlanta, Philadelphia, and Boston away from CSSs. It is not just major cities in the Eastern US; in fact, of the approximately 740 CSS active in America, a vast majority serve fewer than 10,000 people (EPA, 2004).
Today under the EPS’s 1994 CSO Control Policy, CSS cities are encouraged to phase out their CSS pipes in stages, and municipalities applying for NPDES permits to open new sewer facilities would not be allowed to use CSS techniques (EPA, 1994). Today the standard is Sanitary Sewer Systems, or SSSs. SSSs, unlike CSSs, are not designed to convey large volumes of stormwater (Nakagawa, 2015). Typically, cities served by SSS also have a Municipal Separate Storm Sewer System (MS4) that conveys stormwater (Nakagawa, 2015), in this way, sewage and stormwater are both controlled in entirely separate systems. When rain events inundate MS4s beyond their capacity, the resultant flooding is just rainwater runoff and not raw sewage. However, even though stormwater and sewage are separated in the system, they are still subject to occasional overflow events that release raw sewage, called Sanitary Sewage Overflows (SSO). Although these types of overflow events are largely due to infrastructure malfunction, it is estimated that 3-10 billion gallons of SSO discharge occur in America every year, mostly associated with high precipitation events (EPA, 2004).
Health Risks Associated with Sewer Overflows
Both stormwater and raw sewage carry a number of harmful substances that can impact public health when released into the environment. The major classes of pollutants present in overflows are toxins (ex. heavy metals), microbial pathogens (ex. E. coli), nutrients (fertilizers), organic matter, and floatables (debris) (EPA, 2004). While the types of toxins present will vary with such factors as industry and agriculture, common toxins include copper, cadmium, zinc, lead, and pesticides which cause a number of neurological, muscular, and reproductive disorders /deficiencies in both people and wildlife (EPA, 2004). Raw sewage and runoff can contain untreated fecal matter from humans or other warm-blooded mammals which have the potential to carry a number of infectious diseases from bacteria, viruses, or parasites such as Salmonella, Hepatitis virus, and Giardia (EPA, 2004). Paths of exposure to these substances following overflow events include swimming in or accidentally injecting contaminated water, inadequate treatment of drinking water, consuming fish from contaminated waters, and sewer backups into streets, homes and businesses.
Nutrients and organic matter also cause major issues with water quality and have the potential to impact public health by inciting blue-green algae growth. The input of potassium and nitrogen to water systems from such things as fertilizer or raw sewage help favor aquatic plant growth, especially algae (Calderon et al., 2017). Meanwhile, the addition of organic matter promotes decomposition, a process that removes oxygen from the water (Miskewitz & Uchrin, 2013). Zooplankton feed on algae and keep algal growth under control; however, when oxygen is reduced zooplankton begin to die out. The result is an algal bloom, where water may turn green and thick with the amount of algae present. Not only is this ugly to look at and potentially smelly, but certain kinds of algae (the blue-green kind) produce cyanotoxins, a neurological toxin that can cause permanent damage or even death in humans (Scholz et al., 2016).
Gray Infrastructure and Sewer Overflows
Sewer overflows are an inevitable part of our modern systems, and our goals are to minimize the number and volume of occurrences, as well as the risk to public and environmental health. Gray infrastructure, be it roads, sidewalks, parking lots, or buildings, increase the total amount of surface runoff in our cities and therefore increase runoff. Since stormwater loading to our sewer systems is the primary cause of CSOs and is highly associated with SSOs, it seems that gray infrastructure, in general, is counter to this goal of minimizing overflow occurrences. While gray infrastructure solutions specific to stormwater management, such as expanding channels to increase flow capacity, may temporarily reduce overflows, since they do not address the main problem of excessive runoff, they cannot accommodate city expansion and do not perform well with high rainfall events for reasons explained above.
Moving Forward
Here we discussed the difference between Combined Sewer Systems and Sanitary Sewer Systems in America today and some of the policies and drawbacks these systems have. We know that high amounts of impervious surface lead to rapid, high volume inundation of rainwater to our sewer systems, and that this can cause sewer overflow. We also now know the environmental and health risks associated with these overflows, and that gray infrastructure is not well suited to address these problems long term. In the next post, we will take a harder look at green infrastructure, what it is, and what it looks like in practice.
References:
Calderon, O., Porter-Morgan, H., Jacobs, J., & Elkins, W. 2017. Bacterial diversity impacts as a result of combines sewer overflow in a polluted waterway. Global Journal of Environmental Science and Management, 3(4), 437-446.
Environmental Protection Agency. 1994. Combine Sewer Overflow Control Policy. Retrieved from: https://www.epa.gov/sites/production/files/2015-10/documents/owm0111.pdf on 04/10/2019.
Environmental Protection Agency. 2004. Report to Congress: Impacts and Control of CSOs and SSOs. Retrieved from: https://www.epa.gov/sites/production/files/2015- 10/documents/csossortc2004_full.pdf on 04/10/2019
Environmental Protection Agency. 2018. Retrieved from: https://www.epa.gov/npdes/combined- sewer-overflows-csos on 04/10/2019.
Miskewitz, R. & Uchrin, C. 2013. In-stream dissolved oxygen impacts and sediment oxygen demand resuling from combined sewer overflow discharges. Journal of Environmental Engineering, 139(10), 113-121.
Nakagawa, T. 2015. Urban Sewer System History in North America. Castagra. Retrieved from: http://www.castagra.com/2013/10/urban-sewer-system-history-in-north-america/ on 04/10/2019.
Scholz, S., Esterhuizen-Londt, M., & Pflugmacher, S. 2016. Rise of toxic cyanobacterial blooms in temperate freshwater lakes: causes, correlations and possible countermeasures. Journal of Toxicological & Environmental Chemistry, 99(4), 543-577.