There are three types of sewer systems: wastewater, stormwater and combined. Combined sewer systems collect domestic and industrial wastewater, as well as stormwater together in one pipe. The purpose of combined sewer systems is to transport this combination of sewage and rainwater to a wastewater treatment plant for processing.
Combined sewers are less of a concern in dry weather and light rain because the volume of water is small enough to be contained within the system and travel to the treatment plant. However, during heavy storm events, or following snow melts, the volume of water in the combined sewer system rises.
When the volume of wastewater in the system increases, it can deviate to holding tanks if they exist. Unfortunately, the more common outcome is a Combined Sewer Overflow (CSO). A CSO occurs when the combination of sewage and stormwater exceeds the capacity of the sewer system or the wastewater treatment plant and is released directly into nearby lakes, streams, rivers, or other natural bodies of water. This water has either not yet received treatment from a wastewater treatment facility or has only received primary treatment.
Combined sewer overflows are harmful to human health as well as the natural environment. The wastewater discharged can contain raw sewage, industrial waste, toxins, and debris.
Sewage-polluted waters contain pathogens, heavy metals, pharmaceuticals, and chemicals that can lead to illness and infection. Exposure to contaminated water from a CSO poses health risks to humans. After a CSO, bacteria such as E.coli, viruses, and parasites can be found in the water, leading to diseases and infections. Gastroenteritis, rashes, and ear, nose, and throat infections are the most common illnesses associated with contaminated water. More severe health outcomes include cholera, dysentery, infectious hepatitis, and severe gastroenteritis.
Sewage and stormwater pollution are also harmful to aquatic life. Stormwater contains sediments, which after a CSO turn the water murky, affecting the growth of fish and aquatic plants. CSO’s also create an increase in nutrients in the water, resulting in algae blooms and low oxygen levels in the water. This is detrimental to fish and other aquatic life, as they do not survive when the oxygen levels in the water are too low.
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