Locating the interface between the high solids region of a settlement tank or clarifier and the relatively lower solids region above it is a constant requirement in the operation of sewage, wastewater and water treatment works, as detecting and monitoring the sludge blanket is essential in both the primary and final stages of water treatment.

Partech has supplied sludge blanket monitoring systems for all the clarifier tanks at a drinking water treatment plant in the West. The system comprises two Soli-Tech 20 sensors and an 8200 fixed point detector for each clarifier, one suspended in the cone and the other above the cone but below the surface of the water. This arrangement with the second sensor was implemented as a back up in the unlikely event of the lower sensor failing.

Because of the water flow at the plant, the sludge blanket tends to rise and fall in the tank, resulting in a certain amount of fine sludge rising up the clarifier. Partech decided to set the upper sensor to a more sensitive level (0-200mg/l) so that plant managers would detect the presence of rising sludge and activate an alarm condition, automatically opening the de-sludging valve.

Suspended at a fixed point below the surface of the water, Partech’s Soli-Tech 20 sensor is attached by cable to the 8200 fixed point detector, which provides a reliable method of detecting the sludge blanket. Suitable for mounting outdoors on the bridge of settlement tanks, the enclosure is IP65 rated, so an outer enclosure is not required.

The two sensors linked to the 8200 each produce a high alarm signal to indicate the presence of a sludge blanket at their location points. An additional system fault alarm is also provided, and the alarm signals can be used to provide either a two-stage alarm or to start and stop a pump.

The correct selection of infrared sensor enables the detection of the sludge/water interface on final settlement tanks or primary tanks even when the settlement process is “upset” and the interface has become disturbed.

At this plant, the double sensor arrangement linked to the 8200 monitors positioned at intervals above the clarifiers allows the plant operating team to prevent the finer sludge particles entering the outlet channel. It also provides a real-time view on the SCADA system of what is actually happening in the clarification tanks.

Remedial action

With the plant being a fully automated site, the software supplied to work with the Partech detectors has been configured to provide a visual record of which clarifiers are de-sludging at any one time. Should there be a situation where a clarifier is constantly or too frequently de-sludging, this is indicated in the main control room. This enables remedial action to be taken, including visual inspection of the appropriate Soli-Tech 20 sensor by site engineers.

“Using a reliable sludge blanket detector monitor can give the process control system the information that it needs to take automatic corrective action or give an operator an indication of a problem and the data to justify emergency process adjustments,” says Partech’s Angus Fosten.

“The detection of the sludge blanket in a settlement tank provides vital information about the performance of the site and can help prevent pollution problems. Blankets that are too high can cause problems in the next treatment stage, while blankets that are too low will not settle properly.”

Sludge blanket detection can be undertaken using fixed or portable infrared instruments. Partech’s infrared sensors measure infrared absorption across a fixed path length, with the path length varying depending on the application. This set-up provides an economic solution, with periodic cleaning of the optical surfaces being the only maintenance required. Installation of a fixed system like this is simple, with the sensor being suspended from a cable using a standard mounting bracket for sludge blanket detection.

All the sensors work with a monitor that communicates with the outside world, and the output can be used to control pumps or valves and to interface directly with telemetry, PLC and SCADA systems. Provision must be made for transmission of the output signal to the control system, and where spare slip rings cannot accommodate this, the radio telemetry option allows direct transmission to the control system without the need for any cable installation.

The presence of a detector in the primary clarifier ensures that the once the blanket has formed it does not get to a level where it becomes too heavy to be removed, being discharged for further treatment once it has reached a predetermined level. The objective of monitoring should be to ensure that the sludge blanket in the final clarifier is in the region of 4% prior to it being removed, either to a digester or sludge thickener.

Where sludge blanket detection is most critical is in the final clarifier. The final effluent that reaches this point does not go into the works for further treatment but is discharged into a river, so it is essential that the blanket formed in the clarifier does not rise and spill over the top.

The purpose of installing detectors at this stage of the process is to monitor the position of the sludge blanket and to instigate an alarm should it reach a critical level. Alternatively, the detector could activate a pump for removing the sludge blanket.


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