Portable Monitoring Instruments Solve Flow Rate Mystery

Concerned about excess flow in the Glen Walter Sewage Collection System, staff in the Township of South Glengarry, Ontario, conducted a flow monitoring survey to determine the cause.

The Glen Walter system was built to serve 1080 customers in 1989. At the time the population was 850 but by 1995 the sewage treatment plant had already reached capacity. Shawn Killoran took over the Operations Manger position in 1999 and had immediate concerns about excess flow in the system.

To investigate the problem, the Township videotaped all of the lines in the collection system. The pipe was found to be in good condition so significant infiltration was unlikely. As a next step, Killoran decided to monitor flow in high volume areas within the system. The Township selected Stingray Level-Velocity Loggers from Greyline Instruments because they are portable, work in partially filled pipes, and allow technicians to retrieve wastewater flow data without entering manholes.

The study of the Glen Walter collection system was conducted from May 16th to June 9th 2006. The ultrasonic sensors were installed in seven different manholes for periods of 3 to 5 days. To capture detailed flow information, the Stingray loggers were set to take readings at 10 second intervals.

To deploy each unit, a township technician selected a manhole location, attached a stainless steel bracket in the influent pipe, then mounted the sealed, ultrasonic sensor. The sensor cable was then attached to the watertight electronic logging unit and hung it inside the manhole. The loggers recorded the date and time, water level, velocity and temperature.

During the study, Township staff recorded weather conditions on a daily basis in the Glen Walter Collection log book. The number of homes upstream of the flow monitors was also noted. When a Stingray was removed from a specific manhole, the information was transferred to Greyline Logger software and a graph was created which displayed the total flow for that specific area.

At the end of the survey Killoran prepared a flow monitoring report for the Mayor and council on high volume areas within the collection system.

The loggers captured spikes in flow that matched typical run times from domestic sump pumps.

An average of one cubic meter a day per household was used as a guide for the amount of water used in a 24 hour time interval. However, the loggers recorded flow rates 15-70% higher than statistical flows. The loggers captured spikes in flow that matched typical run times from domestic sump pumps. Because the Stingray is able to monitor water temperature, Killoran also observed corresponding temperature drops matching the pump cycles. The evidence was clear: residents were connecting sump pumps to the municipal collection system.

Municipal agents went door-to-door informing residents that improper sump pump connections had been identified. They requested that any sump or eve trough discharges be disconnected from the municipal system.

“After the flow survey we observed a reduction in influent at the sewage treatment plant,” Killoran said.

Since the Glen Walter study, the Greyline Stingrays continue to be used to measure flow in other applications in the Township of South Glengarry. WW

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