Each year, 850 billion gallons of raw sewage overflow into U.S. streams and rivers. If that amount was poured over New York City, it would create a pool 9 inches deep, said Luis Montestruque, CEO of EmNet, a startup designer of wastewater control systems.
Much of the overflow is caused by combined sanitary and stormwater sewer systems, which are prone to floods during storms; the channels for sewage and storm water runoff are only partially separated. South Bend, Ind., one of more than 700 U.S. cities grappling with the issue, is implementing a unique solution to direct sewage and rainwater to unused parts of its sewer system, preventing unnecessary spills.
The system, called CSOnet, is a "cyber-physical system" because it integrates computation with control, said Michael Lemmon, University of Notre Dame professor of electrical engineering. It watches and alters its own world, similar to how a traffic controller monitors traffic congestion and orchestrates light timing. "Probably what's unique about this is it includes actuation, so that we're actually controlling something," Lemmon said.
Engineers from Notre Dame, Purdue University, EmNet and the city of South Bend began work in 2004 to create a wireless sensor actuator network (WSAN) for the city's wastewater system.
"The advantage of CSOnet is that the intelligence is distributed throughout the system," Montestruque said. "This allows CSOnet to use local data more efficiently and robustly than conventional centralized systems."
A computer network communicates over wireless radio and is integrated into system components called "nodes" -- flow sensors, pressure sensors and smart valves that act in a feedback loop to efficiently store sewage and rainwater.
The system incorporates engineering innovations and is garnering interest for its cost-effective control of wastewater overflows. "It is arguably the largest permanently installed urban-scale wireless sensor network and one of the first cyber-physical systems in the world," Montestruque said.
During a storm, sensors in manhole covers detect high water levels and calculate amount of available sewer storage space, Montestruque said. The system sends a command signal to valves, pumps and gates to prevent overflows and maximize conveyance capacity. When the storm passes, sewage is slowly released into a wastewater treatment facility.
CSOnet's citywide installation was completed in February 2008 and has 110 wireless sensors installed throughout almost 40 square miles of South Bend. "This will allow the city to understand the details of the inner works of the sewer in preparation for the control phase," Montestruque said.
The real-time sensor information is collected by an EmNet server, for monitoring and archiving purposes, and is regularly accessed on the Internet by work crews that inspect for sewer changes or node malfunctions, said Gary Gilot, South Bend's director of public works.
EmNet will install 10 smart valve controllers to reduce dry weather overflows and flooding, and maximize storage in basins. That project is slated for completion in summer 2009. Beyond that, control may be extended to 30 other sites, he said.
The projected cost for CSOnet is $4 million, Gilot said.
Sewage overflows into nearby streams and rivers often occur during heavy rainfall, when excess water floods pipes in combined sewer systems. The resultant discharge, imbued with biological and chemical contaminants, is called a combined sewer overflow (CSO) event.
The overflows are toxic and can result in hefty fines for cities. Under the Clean Water Act of 1972, the U.S. Environmental Protection Agency (EPA) requires cities to monitor and reduce sewer overflows, and prepare long-term control plans. Fines are levied for not implementing and following a plan, or because of overflow events.
"It's a huge problem; it's basically like a federally unfunded mandate that all these cities are trying to address and are not sure how to," Lemmon said.