Sludge Monitoring

July 15, 2002
Challenge Monitoring sludge levels in clarifiers is important for efficient operation of a wastewater treatment plant, but accurate measurement has traditionally been a challenge. In Brisbane, Australia, Brisbane Water sought a better solution for its Gibson Island wastewater treatment plant.

The plant used infrared optical sensors to detect sludge levels in each of its six final settling tanks and two thickening tanks. Three sensors were needed per tank for a range of measurement points. Sensors were expensive at AU$2500 each and needed replacing every 12 months. Plant management wanted a more reliable, cost-effective approach.

In 1998, the plant installed a Siemens-Milltronics InterRanger™ DPS 300 interface detection system on a trial basis on one of its final settling tanks. The system uses ultrasonic technology with Siemens-Milltronics Echomax® transducers mounted just below the liquid surface. The sensor does not contact the sludge, and can be submerged for extended periods of time without fouling or maintenance. It never interferes with the rakes in the bottom of the clarifier.

After a successful trial, Brisbane Water installed InterRangers on each of the five remaining settling tanks, feeding the signal to the SCADA system for easy monitoring. When sludge levels reach a certain height, InterRanger automatically increases pump speeds moving more sludge to the contact tank where it is mixed with influent. This keeps sludge levels at their optimum levels in the final settling tanks.

In November 2000, they installed InterRangers on the two thickened waste activated sludge tanks to monitor level and communicate the information to a Siemens PLC. The PLC then controls the sludge pumps based on sludge level, balance tank level and belt press operation. These are all used in concert to dry and treat the sludge before removal.

The plant’s rotating-bridge clarifier design made it difficult to connect the 4-20mA signal into the SCADA system. Brisbane Water used radio telemetry, a very cost-effective solution as all eight transmitters can communicate with a single receiver.

“Using InterRanger DPS 300 systems on all the sludge settling tanks made level monitoring easy and improved efficiency”, said Michael Benfer, Plant Electrical Coordinator. It also reduced maintenance costs by thousands of dollars in the first year. Continuous monitoring and automatic pumping of return activated sludge into the contact tank maintains optimum performance levels in the final settling tanks. Monitoring sludge levels in the thickened waste tanks provides the data needed by the Siemens PLC to efficiently manage the sludge treatment process, including the balance tank and belt press. It eliminated the need for manual checking of sludge levels, freeing operators’ time for other tasks. If a final settling tank must be taken offline for maintenance, operators can easily determine which tanks have capacity to handle the diverted flows. By checking sludge levels in the final settling tanks each morning, operators can inform the sludge-removal contractor how many trailers will be required for sludge removal that day.
(Photo caption) Brisbane Water in Brisbane, Australia, reduced maintenance costs and improved efficiency by using Milltronics InterRanger DPS 300 to monitor sludge levels.