Plant Automates Sludge Measurement in Clarifier

The wastewater industry is no place for people who are afraid to get their hands dirty, but taking hand measurements of the sludge blanket in primary clarifiers is distasteful even to industry veterans.

Hand measurements have long been a necessary evil at wastewater treatment plants, a dirty job that someone had to do several times each day. New technology is changing that, providing alternative ways to monitor clarifier operations.

One such system is the Milltronics InterRanger DPS 300, a liquid interface detection unit that uses ultrasonic technology in much the same way that submarines use sonar. The devices ultrasonic transducer continuously emits sonic pulses that are reflected in the form of echoes from suspended solids, revealing the interface between light liquids and semi-solid sludge. A built-in microprocessor digitally converts the round-trip time of each pulse and uses it to calculate distance. The dual-point unit can track levels and control pump functions in two clarifiers.

Level is displayed numerically and can be sent directly to the plant?s control system. The unit also creates a composite graphic of the tank, displaying interfaces between density layers as peaks.

At Lumberton, Texas, about 90 miles east of Houston, the InterRanger is installed on the primary clarifier at the No. 2 Wastewater Treatment Facility. The clarifier is a standard circular design with a capacity of 295,000 gallons. The InterRanger?s transducer is suspended on a rod so that it sits just below water level, and the control unit is located on the clarifiers catwalk. The rod is hinged so that it does not interfere with the rotation of the clarifier?s rake/skimmer.

?Before the InterRanger was installed, my workers used a Sludge Judge® (a lightweight pipe measurement tool) several times each day to take hand measurements,? said Robb Starr, Wastewater and Lift Station Superintendent for the Lumberton Municipal Utility District. ?In order to avoid clogs and inaccurate readings, the workers had to clean the device carefully after each reading.?

To take readings now, workers read the numbers at the controller?s display. Starr estimates the unit saves about 1.5 man-hours per day. Within the year, Starr plans to automate all clarifier pump functions and route the InterRanger?s output into the plant control system.

Lumberton currently is consolidating treatment operations and Starr expects effluent throughput to nearly double, approaching 2 million gallons per day. A new equalization basin is expected to solve tubidity problems from high flows that introduce measurement errors up to four inches, no greater than hand measurement errors.

Starr will begin using the unit to control sludge pump operations on both clarifiers to improve sludge and wastewater consistency. A PLC will bring the InterRanger output directly into the plant, and workers will read the sludge blanket level on a computer screen.

About the Author: Doug Duncan, Milltronics? ultrasonics product manager, holds a bachelors degree in electrical engineering from Oklahoma State University and has more than 10 years experience in process instrumentation.