The 40,000 sewer overflows that occur across the United States each year are expensive. Plus, these overflows run the risk of daily U.S. Environmental Protection Agency fines of up to $27,500 and ill will from residents complaining about noxious fumes and exposure to potentially hazardous liquids.
Though it's never been fined, the Pima County Wastewater Management Department in Arizona has had its fair share of overflows. In 2001 alone, overflows cost the department more than $600,000 in response and cleanup expenses.
That same year, the department launched a multiyear program to install a supervisory control and data acquisition (SCADA) flow-monitoring system powered in part by the sun and linked by remote radio stations. The award-winning system, which couples flow-metering stations, telemetering equipment, and data analysis with enhanced preventive maintenance, was a big step up from the department's cell-phone-based alarm system that told operators only that there was a problem—not the nature of it.
Now employees monitor flows from virtually anywhere. They can detect developing blockages and get out in the field to fix them before the system overflows. The result: reduced spill volume.
In April 2005, for example, the system identified a 250,000-gallon spill in a remote section of the desert, which could have become catastrophic had the system not notified employees. Quick notification allowed workers to fix a blockage caused by construction debris that potentially could have spewed millions of gallons of sewage into the environment.
The Pima County Wastewater Management Department comprises two main treatment plants and several smaller outlying treatment facilities. The conveyance system collects about 65 mgd throughout its 370-square-mile sanitary service area, which includes the urbanized areas of Tucson, Marana, Oro Valley, and Sahuarita, and the unincorporated communities of Mt. Lemmon, Arivaca Junction, Avra Valley, Green Valley, Corona de Tucson, and Catalina (total population of about 1 million people).
With 3165 miles of public sanitary sewers—of which approximately 300 miles are considered trunks or interceptors (pipe diameter of 15 inches and larger)—the system incorporates 62,000 features in its urbanized area (reaches, manholes, cleanouts, etc.).
The system measures flow at solar-powered locations and transmits the data to a central server via 900 MHz spread spectrum radio links. Data are received at the central server from every site every 60 seconds.
By using archived data, expected values for the flow at a given time are established by averaging the observed values for flow from the same day of the week and same time of day for the previous four weeks. The system generates upper and lower control limits using statistical process control methods and then acquires real-time observed data from the field. It then compares the current real-time value with the control limits and transmits alarms if control limits are violated (see diagram). If a data point is determined to be outside of control limits, it is flagged in the database as invalid and isn't used for establishing future expected flows.
Data that do not fall within expected parameters require a response. The system waits to observe a minimum of five to seven successive control limit violations before sending out an alarm to field personnel, which minimizes errors of commission (false positives) from short-term flow variations or instrument noise. Upper control limit violations typically are encountered after rain events, while lower control limit violations are almost exclusively caused by blockages.