A commonly used term in the global water industry, CSO stands for Combined Sewer Overflow in water treatment.   In order to understand what a CSO actually does, we must first understand what a combined sewer is. In a combined sewer both wastewater and surface water amalgamate and are transported in one single pipe to a sewage treatment works for processing.   If there is a particularly heavy period of rainfall, a combined sewer is often unable to cope with the additional amount of surface water it is required to handle. If this happens, it can result in a sewage overflow as there is less room in the system for the wastewater. This is where a CSO comes into play.   A CSO is specifically designed to divert excess flow and prevent flooding upstream. As a consequence of implementing a CSO however, untreated wastewater is released into the environment which can then have the adverse effect of creating a pollution in a river or lake. Such pollutions can be hazardous to both human health and wildlife.