Continuous monitoring of the sludge/water interface or sludge blanket is central to the efficient operation of clarifiers, thickeners and settlement tanks. Reliable measurement enables a control system to maintain the optimum depth of sludge in the tank. A sludge interface that is too high can result in pollution incidents caused by carry-over from a final settlement tank into the final effluent channel. The sludge can also become too thick to extract easily or can turn septic if left too long. If there is insufficient sludge in the tank the settlement process cannot operate correctly and the solids will tend to remain in suspension.
Historically, settlement tanks have been monitored during routine site visits using the operators judgment or a portable blanket method such as the 715 Portable Sludge Blanket Detector. This spot method of control leaves the site exposed to changes in the process that occur when the site is unattended or while the operator is busy on other duties. This means that if a change happens to the site loading, the settlement tank can quickly become overloaded with sludge, or be drained of sludge entirely.
Use of Partech’s Sludge Blanket monitors will enable the site operators to set maximum and minimum interface levels. The control system can then combine this with other control parameters to improve the quality of both the final effluent and the sludge that is produced.
To ensure reliable operation of a Sludge Blanket Monitor, care must be taken to ensure that the correct sensing technique and the correct sensor is selected. Individual product data sheets and your local Partech Sales Engineer will help with this selection. The principle areas to consider are; type of solids or process, thickness of the sludge, clarity of the supernatant and the definition of the interface.
Another process parameter to consider is the settling characteristics of the sludge. If the interface that is produced is very well defined with thick sludge and clear supernatant the measurement is relatively straight forward, however when the interface becomes diffused then much greater care is required with the selection of the measuring device. In addition to process considerations, the physical features of the tank and site must be be taken into account. The size and shape of the tank, the presence of rotating bridges, scum boards and scrapers, will all affect the product selection.
In general on a circular tank the Sludge Blanket Monitor should be located half to two-thirds of the way from the centre to the outside of the tank. On half bridge scrapers where the bridge and scraper rotate together round the tank the sensor should be on the leading edge, ahead of disturbance caused by the scraper. Once the instrument has been installed, the signal must be transmitted to the control system. A rotating bridge can cause problems if there are insufficient slip rings to allow hard wiring. If this is the case, then Partech can offer low power radio telemetry options.
Once the system is up and running it is important that the sensor is kept clean. The infrared sensors will need routine cleaning, typically every fortnight. This is a very quick and simple process and will easily be accommodated in any good housekeeping procedure. The ultrasonic sensor supplied by Partech includes an integral wiper to prevent the build up of air bubbles on the transducer face.
Partech have been designing, promoting and maintaining Sludge Blanket Monitors for over 40 years. This experience combined with a product range that includes portable, fixed point and continuous monitors using infrared and utrasonic technologies makes us the leading supplier in this field.