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.
Storm Surge
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.
Unfunded Mandate
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.
South Bend Mayor Steve Luecke estimated in his 2008 State of the City address that the city would need to spend $200 million to $400 million over 20 years to meet EPA guidelines -- and that's in addition to the $120 million now being spent on South Bend's long-term sewer plan.
However, the city could save $110 million to $150 million of that mandate with the use of CSOnet, Luecke said.
Though many newer areas of South Bend have segregated sewer systems, the water and sewage is inevitably combined upon entering the city's older sewer network.
In normal circumstances, the mix is treated at a wastewater treatment plant before its release. But wet weather can put the plant at full capacity. Even dry weather can cause overflows when storm debris plugs sewer lines, thereby flooding other pipes, Gilot said. The resulting EPA fine is $27,500 for each incident. With the monitoring system in place, Gilot said the city has already detected and corrected many potential dry weather overflows.
He said public works estimates that 2 billion gallons of sewage per year empties into South Bend's waterways, which includes the St. Joseph and Wabash rivers. Cities like South Bend drain the sewage mix into natural waterways to prevent it from backing up into homes and businesses.
The EPA estimates that reducing CSO events by 85 percent nationwide would cost $50 billion using traditional technology, Montestruque said. Traditional fixes include building new separated sewer systems, expanding wastewater treatment plants and building large reservoirs, or holding tanks, to temporarily hold sewage, as was done in Chicago.
"These solutions are highly unpopular because this is taxpayer money," Montestruque said. Taxpayers can't use or see sewer improvements, so it's hard to justify spending money on them, he said.
A 2005 estimate for totally segregating South Bend's sewage and storm water was $650 million, Gilot said. Water rates would have to rise 80 percent to amass the necessary capital.
Research and Development
CSOnet originated when Lemmon and Jeffrey Talley, an associate professor of civil and environmental engineering at Notre Dame, began discussing South Bend's water management needs with city officials.
In 2004, Talley led a research and development team to create a sensor and control prototype for stopping the city's sewer overflow problem. After he landed a $1 million grant from the Indiana 21st Century Research and Technology Fund, Purdue University and environmental engineering firm Greeley and Hansen joined the project. Granger, Ind.-based EmNet was founded in 2004 to commercialize the research from Notre Dame and Purdue.
Successful test runs at St. Mary's Lake near Notre Dame paved the way for a pilot in November 2005. The pilot, a small retention basin deployment with six sensors and one controller, prevented an estimated 6 million gallons of sewage from entering the St. Joseph River that month and increased the basin's capacity by 110 percent for about 1 cent per gallon, Montestruque said.
A subsequent study conducted by environmental engineering firm Malcolm Pirnie determined that a citywide installation would reduce CSOs up to 30 percent, Montestruque said.
System Intricacies
Although there's been an occasional node malfunction, Gilot said, the system is robust because there's no single point of failure.
"We could see this had potential early," Gilot said. "[The pilot] showed that real-time control logic and communications worked and that the system was robust under tough, real-world conditions."
Battery-operated nodes and wireless communication make for fast implementation and less up-front cost. "It's very unique in that it doesn't require any structure to be in place beforehand," Montestruque said, "It's usable the moment it is installed."
Node-to-node communication that's linked to a hierarchical information structure lets the electronically simple system consume less power. The nodes also rely on highly efficient hardware and middleware to synchronize sleep and awake cycles, extending battery life to two
to three years. Purdue is working on the ability to reprogram the nodes wirelessly.
Additionally algorithms help optimize CSOnet's functions. "One allows communication using mesh network technology; another algorithm is responsible for utilizing sensor information to determine the optimal set points for the valves that control flows; yet another is responsible for energy management," Montestruque said.
The mesh network technology lets neighboring nodes communicate using many paths, thus bypassing obstacles.
The composite fiberglass manhole cover used to house the antenna and sensor adds to CSOnet's sturdiness by enabling radio signals to propagate easily from inside the sewer system. Another advantage of the retrofitted manhole cover is its easy installation.
There's already been interest in South Bend's CSOnet, including from places as far as India, Lemmon said. Gilot predicts interest will increase after the control phase ends and CSOnet can display its full value.
"There's a big potential here for this to have significant impact that goes beyond the horizon of academics," Lemmon said.
