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By Jennifer Muller and Wayne Lem, Trojan Technologies Inc.

Ultraviolet (UV) light is used for disinfecting many types of waters including low quality waters like primary treated wastewater and combined sewer overflows (CSO). Combined sewer systems are sewers that are designed to collect rainwater runoff and domestic wastewater in the same pipe. Most of the time, combined sewer systems transport all of their wastewater to a sewage treatment plant, where it is treated and then discharged to a water body. During periods of heavy rainfall or snowmelt, however, the wastewater volume in a combined sewer system can exceed the capacity of the municipal wastewater treatment plant. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers, or other water bodies. These overflows, called combined sewer overflows (CSOs), contain not only stormwater but also raw sanitary wastewater. CSOs may be thought of as a type of urban wet weather discharge. As untreated (or partially treated) effluent flows directly into the river or water bodies, the receiving body can be microbiologically disturbed for months before the river and marine ecosystem recovers. For this reason, treatment of CSOs is becoming a regulatory requirement in many regions in order to protect receiving water bodies and the ecology within those areas.