AN IMPROVED DYE-DILUTION FLOW MEASUREMENT PROCEDURE FOR SEWERS USING AN IN-SITU APPARATUS

Dye-dilution testing of sewage flow is being used in the Metropolitan Detroit area to obtain highly accurate flow rate estimates to verify meter accuracy. The types of flow meters being verifiedinclude: Parshall flumes; Palmer-Bowlus flumes; Leopold Lagco flumes; magnetic flow meters ("magmeter"); and multi-path area-velocity meters. Often, these sewage flow meters are used for billingpurposes, and meter accuracy is important for proper calculation of sewer bills. Dye-dilution testing also is useful for verification of pump performance curves.A dye-dilution testing procedure was established in 1996 by a Flow Metering Task Force that was organized by the Detroit Water & Sewerage Department (DWSD). The established dyedilution testing procedure involves injection of a known concentration of fluorescent dye into the sewage flow in an upstream manhole and withdrawal of a stream of sewage from a downstream manhole. This procedurerequires suitable upstream and downstream manholes in series with the flow meter and no active sewer connections at or between the manholes. The withdrawn sewage is filtered and pumped through tubing into a fluorometer and a spectrophotometer. The fluorescent dye is well mixed in the sewagein the sewer reach between injection and withdrawal manholes. The sewage flow rate is related tothe dilution of the dye and is compared to the flow rate estimated by the sewage flow meter.An improved, alternative dye-dilution testing procedure was developed that simplifies and significantly lowers the cost of performing dye-dilution testing by using an in-situ fluorometer/turbidity meter apparatus and provides highly accurate flow rate estimates similar to the established procedure. The in-situ apparatus is compact and may be inserted in the sewage flow in the downstream manhole/meter chamber and sewage withdrawal at the downstream manhole is not necessary. Consequently, the alternative procedure can be applied to sewage flow meter installations that otherwise do not have suitable downstream manholes.