When sewers were first being built in the United States, they were designed to carry both storm water runoff and wastewater in one pipe rather than two separate pipes.  During dry weather, only municipal wastewater flowed through the sewer, and all of this water was diverted to the wastewater treatment plant (WWTP), treated, and released with no problem. 

However, during wet weather, all of the storm water runoff also entered the sewer.  This additional flow exceeded the capacity of the WWTP and could not be treated.  Rather than flooding roads and basements with this combined wastewater, it was instead released directly into a nearby river or lake at set overflow points called outfalls.  Currently, most Combined Sewer Systems (CSSs) are passive systems, meaning that each of these outfalls has a static set point.  Whenever flow exceeds this outfall set point, an overflow occurs, even if there is still capacity in the rest of the CSS.  With each overflow, more untreated wastewater enters the environment.  In fact, each year in the United States, Combined Sewer Overflow (CSO) events discharge 850 billion gallons of untreated wastewater into America’s waterways – enough to cover downtown Chicago in 17 feet of sewage. 

There are several factors that make solving the CSO issue difficult.  Most CSSs were built over 100 years ago, were designed using now outdated methods and loads, and have been piecemealed together over the last century.  This means that most municipalities don’t understand how their sewers operate, where bottlenecks occur, and where all of the problem areas are.  Many communities have done some temporary flow monitoring in order to create a computer model of the system, but even the best models have error ratings of up to 20% and do not possess the detail needed to focus in on certain problematic areas.  Hence, many municipalities develop CSO abatement plans that do not effectively and efficiently address the problems in their systems.  The difficulties continue after these municipalities develop plans because most traditional abatement technologies, such as sewer separation and WWTP expansion, are very expensive and disruptive to the public.  CSOnet, however, can greatly reduce these difficulties and expenses through real time monitoring and control (RTMC).  Using remote sensing and coordinated, automated valves CSOnet can maximize in-line and off-line storage and flows to the WWTP.  The goal of CSOnet is to maximize the potential of the existing infrastructure and minimize the need for new construction, thereby cutting the cost of any CSO abatement plan.