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CSOnet: An Innovative Solution for the Real Time Monitoring and Control of Combined Sewer Systems

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Each year in the United States, combined sewer overflows (CSO) events result in the release of 850 billion gallons of untreated wastewater into our rivers, streams, and lakes. These releases cause extensive human illness, fish and animal kills, and the eutrophication of the receiving waters. In 1994, the United States Environmental Protection Agency (USEPA) mandated that the approximately 772 American municipalities with combined sewer systems (CSSs) drastically reduce the frequency and severity of their CSO events. Current solutions to the CSO problem mainly involve large and expensive infrastructure improvement and expansion projects. Typically, these projects attempt to reduce CSO events by expanding the WWTP capacity or by separating the storm and wastewater sewers. These solutions are highly unpopular because they are expensive and disruptive to the public. CSOnet provides municipalities with the data they need to determine how to fix their CSO problems and then with the tools to implement those control solutions - all at a fraction of the cost of traditional CSO abatement programs.

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 Most CSSs were built over 100 years ago, were designed using now outdated methods and loads, and have been piecemealed together over the last century.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 have enough information on 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 due to the lack of useful information. However, CSOnet can greatly reduce these difficulties and expenses through real time monitoring and control (RTMC), which uses remote sensing and coordinated, automated valves to 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.

CSOnet is a modular, decentralized approach to RTMC, which sets it apart from many existing systems. CSOnet uses real-time flow information from the critical points in the sewer system to actively control the passage of water through the CSS with the use of valves and gates. The process is similar to that of traffic control using traffic lights. Modern traffic control systems are able to actively control traffic light green/red light times to avoid congestion during rush hours. In a similar way, CSOnet monitors the CSS and actuates valves and gates distributed throughout the CSS to avoid surcharged conditions that cause overflows. Most existing RTMC systems utilize programmable logic controllers (PLCs) to gather data from sensors mounted in the sewer, which is then sent to a centralized computing center via a supervisory, control, and data acquisition (SCADA) system. While this system has been shown to be effective, it is also limited and cumbersome. For example, each monitoring site requires an external power source, an electrical cabinet, a PLC, high power radios, and an open area of land for the equipment. This setup is expensive, which limits the number of monitoring sites a municipality can have, and the power and space requirements limit where the sites can be.

With its decentralized approach to RTMC, CSOnet avoids all of the limitations of the centralized system. In its most basic form, CSOnet is a wireless network of monitoring points (Logicovers) and data acquisition points (also known as Gateways). A Logicover collects data from a sensor mounted in the sewer and transmits it to a nearby Gateway. The Gateway then uploads this data to a secure website via cellular or Wi-Fi connection, where it can be accessed anywhere at any time by the municipality. This wireless network approach enables CSOnet to be quickly and inexpensively deployed and to be incrementally implemented, meaning the system can easily grow from one monitoring point to 10 points to 100 points.

Rather than needing a PLC, a power source, and a SCADA system, the Logicover is all selfcontained in a manhole cover assembly. The Logicover's antenna is encased inside of the highway-rated manhole cover, which is made of a composite fiberglass material. The necessary electronics and batteries are contained in an explosion-proof enclosure mounted to the underside of the manhole cover. A Logicover can be installed in any manhole in a matter of minutes and can start sending data instantly.

The Gateway effectively replaces the centralized computing center. The coffee-can sized node is mounted aboveground, typically on a utility or traffic signal pole, so that it can collect data from surrounding Logicovers, which it then uploads to the Internet. In addition to collecting data, the Gateway can be connected to and control an actuated valve. The Gateway contains an embedded PC, which analyzes the data collected from the surrounding Logicovers, and then adjusts the valve accordingly. If more than one control point exists in a CSS, the Gateway communicates with the other Gateways via the Internet to ensure that the Gateway's actions will not cause flooding or CSO events downstream.

After a successful pilot demonstration in 2005, the City of South Bend, IN decided to implement the CSOnet system across the entire city. South Bend is a medium sized city of 107,000 people located in north-central Indiana. Its CSS covers 13,100 acres and contains 36 CSO outfalls. The City decided to use CSOnet to fulfill the following objectives, many of which will fulfill the Nine Minimum Controls required by the USEPA:

Monitor every CSO outfall to determine when and how much overflow occurs.

Provide an early warning and prevention system for dry weather CSO events.

Determine the potential areas for inline storage in larger pipes.

Collect data to further calibrate South Bend's SWMM model.

Determine the locations of possible bottlenecks in the interceptor and trunklines.

Determine the effective storage capacity of retention ponds.

Prevent basement back-ups.

Maximize flows to the WWTP.

Maximize storage in in-line and off-line storage areas.

The CSOnet system is implemented in two primary phases. Phase 1, which was deployed in Spring of 2008, is a real time monitoring system that monitors 110 locations throughout the CSS. This system is able to accomplish the first six objectives through the monitoring of all 36 outfalls, 42 locations throughout the major trunklines, 27 locations along the interceptor, and 5 retention basins.

Phase 2 of the South Bend CSOnet project will implement the control of several critical points throughout the CSS. The data gathered by CSOnet after the Phase 1 implementation will serve to further refine the control strategy. Not only will CSOnet be used to maximize the storage potential of in-line and off-line storage areas, but it will also be used to increase flows to the WWTP during storm events. Preliminary studies have determined that by actively controlling the amount of water diverted to the interceptor line at each CSO outfall could reduce CSO discharge volumes by up to 25%. Larger savings could be accomplished through the integration of existing retention ponds and in-line storage to the CSOnet system.

Preliminary studies conducted by independent environmental consulting firms indicated that CSOnet system has the potential to reduce South Bend's $400 million CSO Long Term Control Plan budget by 110 to 150 million dollars in new construction by maximizing the use of the existing infrastructure and making the system adaptable to changing environmental and sewer conditions.
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Keywords: CSO; embedded sensor network; monitoring; real-time control

Document Type: Research Article

Publication date: 01 January 2009

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