Isolation is the name of the game in Naperville, Ill. The city—a rapidly growing community of 140,000—is in the process of isolating problem sections of its wastewater collection system to make it more efficient and prioritize maintenance.
The wastewater collection system is a separate sanitary sewer system—most of it gravity-fed—with 469 miles of sewer lines and 19 pump stations. The treatment plant also receives flows from nearby Warrenville, Ill.; average daily flow entering the plant is 22.5 mgd.
In recent years, the system has experienced tremendous expansion and hydraulic loadings. Clay lines installed as early as the 1920s are deteriorating. Reducing inflow and infiltration (I/I) to avoid overload—thus avoiding higher treatment costs, excessive pumping costs, and increased pump wear—is critical.
The Water Distribution and Collection Division of the city's Department of Public Utilities has developed a flow monitoring program that provides “snapshots” of current conditions, letting them identify, isolate, and rehabilitate sections that aggravate the I/I problem. The agency monitors flows at 24 permanent and 10 temporary sites. Two of the permanent sites measure flows from Warrenville for billing; the rest are used primarily for I/I monitoring and determining I/I versus rehab costs.
Field supervisor Tony Conn says permanent meters help determine what lines have the most serious I/I problems. “We break these areas down into smaller sections using our temporary flow meters, and we keep these in place during the entire rehab process,” he says. “We compare the flows in these sections before and after rehab and can also easily determine the amount of I/I we knocked off at what cost.”
Naperville is replacing the older sensors with Sigma AV flow and level sensors from Loveland, Colo.-based Hach Co. The sensors—developed to withstand harsh environments—avoid problems that the older sensors displayed. About every two weeks, technicians had to clean fouled sensors or level-adjust ones that were drifting.
In six permanent monitoring sites, the city has installed new sensors with oil-filled cover plates, designed for sites susceptible to extreme fouling. The high-viscosity silicone oil prevents fouling for up to a year and can be easily replenished onsite. Scattergraph plots of depth and velocity readings from the sensors reveal hydraulic conditions and can expose bottlenecks and obstructions. Based on its success, the city plans to replace all its permanently mounted flow and level sensors with the AV units over the next several years, funding the effort through user fees. Most importantly, the new measurement devices are improving upon the I/I mitigation program's positive results.
“In the areas that we have been working, the payback in rehabilitation costs is about eight years,” says Conn. “Also, a $6 million treatment plant expansion will not be necessary because flows have been reduced.”
— Jim Caruso is product application specialist of flow and sampling for Hach Co., Loveland, Colo.