There’s a good website I found called about Combined Sewage Overflows, I’m going to shamelessly pull some of the writing from it for this blog entry.

The definition of a combined sewer overflow (CSO), from Water Environment Federation Manual of Practice FD-17: Prevention and Control of Sewer System Overflows is: “A CSO is the intentional or unintentional discharge of untreated sanitary wastewater mixed with stormwater runoff or snow melt and occurs when the carrying capacity of a single conveyance system is exceeded by the instantaneous rate of flow within the single conveyance system. To control where overflows occur, engineers design diversion structures at strategic locations within the single conveyance system that ultimately discharge extraneous commingled flow to receiving waters.”

The US Environmental Protection Agency (EPA) has developed a national framework in the United States for control of CSOs called the ‘Combined Sewer Overflow Control Policy’. The Policy was developed through the National Pollutant Discharge Elimination System (NPDES) permitting program, which requires that all point sources discharging pollutants to surface waters must have a permit. The CSO Control Policy provides information on how to meet the pollution control goals of the Clean Water Act flexibly and cost-effectively. The Policy, published in 1994, contains four fundamental principles to ensure that CSO controls are cost-effective and meet local environmental objectives:

• Clear levels of control to meet health and environmental objectives
• Flexibility to consider the site-specific nature of CSOs and find the most cost-effective way to control them
• Phased implementation of CSO controls to accommodate a community’s financial capability
• Review and revision of water quality standards during the development of CSO control plans to reflect the site-specific wet weather impacts of CSOs3.

As part of the CSO Control Policy, all communities with combined sewer systems were required to implement ‘nine minimum controls’ by January 1, 1997. These controls are measures that can be implemented to reduce the effect of CSOs without large engineering studies. The ‘nine minimum controls’ are summarized below:

• Proper operation and regular maintenance programs for the sewer system and the CSOs
• Maximum use of the collection system for storage
• Review and modification of pre-treatment requirements to assure CSO impacts are minimized
• Maximization of flow to the publicly owned treatment works for treatment
• Prohibition of CSOs during dry weather
• Control of solid and floatable materials in CSOs
• Pollution prevention
• Public notification to ensure that the public receives adequate notification of CSO occurrences and CSO impacts.

So it’s not surprising that one of the biggest uses that we see for FlowWorks is clients who are looking for cost-effective monitoring and reporting of CSO’s. The equipment to do this is now WAY cheaper than it used to be, so much so that it’s now possible to instrument overflow manholes or chambers with battery powered, underground wireless level equipment for about the same price that just the basic programmable logic controller (PLC) would cost in a traditional SCADA system. As a result, putting monitors in every overflow location is now within striking distance of cities that have even modest funding available. We are seeing clients instrument their overflow points with ultrasonic level sensors, pressure transducers, and even good old-fashioned float switches. There are even some amazing load-rated all-in-one monitoring manhole lids available on the market!

Once you have those locations instrumented, watching near-realtime data coming into FlowWorks is where the real fun begins. When all is well, you see all of the locations as green symbols on the FlowWorks map. When levels are nearing critical values the symbols switch to orange, and finally to red as the event begins. At several stages before and during the event, FlowWorks will issue alarm emails and text messages to everyone that needs to know. At anytime during or after the event, the FlowWorks graphing and reporting functions can tell you how long the event was. Plus, if the station is setup correctly you can get an estimate of how much overflow volume has been spilled.

It’s even possible for us to customize the reporting functions to match the exact format and requirements of your regulatory agency. After the events are over, a few mouse clicks is all it takes to produce the CSO overflow duration and volume reports to meet your regulatory obligations.