Combined Sewer Systems (CSS) were designed to collect rainwater runoff, domestic sewage, and industrial wastewater in one pipe that, under normal or “dry” conditions, transports the water to a treatment facility. The volume of wastewater can sometimes exceed the system’s capacity, which most often occurs under “wet” conditions including heavy rainfall and snow melt. CSS were designed to deal with these large volumes through Combined Sewer Overflows, which discharge the excess wastewater directly into nearby streams, rivers, or lakes.
CSOs contain stormwater, untreated human and industrial waste, harmful materials such as oil and pesticides, and floating debris, making it a source of concern for many communities. In the Merrimack River, CSOs have been found to be a source of bacterial contamination, to contribute to the reduction of dissolved oxygen levels, and to increase nutrient levels to above normal ranges.
One of the main concerns associated with CSOs is the discharge of untreated human excrement. Although diluted, these wastes may contain pathogenic bacteria and viruses. These organisms may cause illnesses such as dysentery and hepatitis when people swim in CSO affected waters or eat contaminated shell fish.
When organisms break down biodegradable waste, they use the dissolved oxygen in water. An excess of biodegradable waste in a water body may therefore cause a reduction in the oxygen supply, resulting in fish kills, odor, and overall degradation of water quality.
Overloads of nitrogen and phosphorous, which stimulate plant growth, may lead to rapid growth of algae and other plants, which also contribute to the depletion of oxygen levels in the water.
In accordance with the District’s NPDES permit, CSOs within the District are regulated and monitored to limit their occurrences and impacts. To learn more about these regulations, see pages 11-13 of the District’s permit.
Additionally, GLSD is currently working to upgrade the River Side Pump Station to allow for greater pumping capacity of sewerage and reduce the amount of CSOs. For more information on this project, see Projects.