A new “third generation” sonar-based monitoring system uses a number of sonar transducer frequencies to monitor various sludge and flock layers in both drinking water and wastewater treatment applications. By choosing the correct transducer frequency, the instrument can measure both light and heavy density interfaces.
Most existing sonar transmitter technologies use only one sonar transducer frequency and are limited in their applications for sludge-level monitoring. The new ORCA Sonar Series of interface level transmitters from Hawk Measurement use seven different sonar transducer frequencies, so the appropriate frequency can be selected according to the density of the interface that is being controlled and monitored.
This flexibility allows for use of a different sonar transducer frequency for each of the main applications at a wastewater treatment plant: Primary Sedimentation Tanks, Secondary/Final Clarifiers, Thickeners, and DAF Tanks.
Typically, one of the more difficult tasks in monitoring an interface level lies in the correct selection of the transducer frequency. The higher the transducer frequency, the easier it becomes to see smaller particle sizes. The disadvantage is the lack of penetration and a higher likelihood of air bubbles forming on the sensor face, creating impedance to the transmit and received signals. Lower transducer frequencies will not see such small particles, but will see a more clearly defined interface.
There are many other variables to consider. They include particle size, flow, velocity, material, and air bubble retention. The frequency selection should be based upon the percent solids, plus a logical placement of the sensor taking into consideration the other process variables.
