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Flow Measurement of Activated Sludge

Flow Measurement of Activated Sludge

A wastewater treatment plant that serves 1,800 households includes several mechanical and biological wastewater treatment facilities.
The mechanical treatment consists of a screening system and an aerated circular grit trap. The biological treatment is carried out in a combi-tank (biological treatment stage outside, secondary clarifier-settler basin inside).

The plant also includes facilities for the removal of nitrogen which is carried out by intermittent nitrification/denitrification. In addition, the plant also has a static sludge thickener. The activated sludge, which has been formed in the biological treatment stage through the growth of microorganisms, slowly separates via gravity in the secondary clarifier basin.
A portion of the activated sludge is fed back into the biological treatment stage as return sludge and mixed with the newly added nutrient-rich wastewater. The amount of return sludge should always be in a defined ratio to the current feed quantity. For this reason, the flow rate of the recirculation pipeline has to be measured.

The installation of a wetted magnetic-inductive flowmeter, which is very common in the water and wastewater industry, would have required modifications to the piping and subsequent expensive civil engineering work. The only available manhole location on the buried recirculation pipeline has not been suitable for retrofitting a magnetic-inductive flowmeter. Moreover, the pipeline in the manhole is always submerged in water.

Retrofitting the measurement point with a FLEXIM non-invasive ultrasonic flow meter, including IP68 fully submersible transducers, proved to be a convincingly simple, accurate, reliable and cost effective solution.
There is no need to open the pipeline when mounting the transducers onto the outside of the pipe and therefore no interruption to operation. The cramped installation point in the flooded shaft does not pose a particular challenge to FLEXIM’s measuring system: the IP68 transducers can be operated while permanently submerged and, since they were installed, they have been providing highly reliable measurements for automatic control of the pumps that convey the return flow quantity of the activated sludge.


  • Reliable and accurate flow measurement of activated sludge for automatic pump control
  • Secure automatic control of the recirculation proportional to the demand
  • Easy to retrofit measuring point during ongoing operation, without any pipe work and without the need to modify the existing pipeline system
  • No expense for civil engineering and excavation work
  • Submersible IP68 transducers guarantee long-term stable measurements


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