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Katronic clamp-on flowmeters in sand and grit trap optimisation

Posted: Monday 28th June 2021

Ultrasonic flowmeter measures turbidity and flow for grit trap control at wastewater treatment plants, reducing pump and scraper operation by up to 85%.

In classic three-stage wastewater treatment plants, grit and grease traps are indispensable as a process step in the first mechanical pre-treatment stage, as all substances that can lead to operational disturbances (wear, clogging) within the plant are filtered here. Sand, stones, glass, mineral solids and other impurities with grain sizes of 0.2 mm to 1 mm settle to the bottom of the grit chamber, from where it is removed with, depending on type, various versions of pump and scraper. The grit removal process is usually initiated on a timed sequence, meaning that the process is often operating when it is not necessary, increasing wear and wasting energy.

Technisches Büro Holger Pinta uses Katronic’s fixed ultrasonic flow measurement systems to detect turbidity due to sand build-up in sand trap pressure lines, and uses the measurement to control pumping processes more efficiently.

The clamp-on transducers of the permanently installed flow measurement units - which are usually used for flow measurements of liquids with low solids content - detect turbidity due to sand as an expression of reduced signal strength as the sand in the line scatters the ultrasonic pulse. Models of the way that increased turbidity relates to grit levels in the pressure line have meant that the grit pump and scraper can be initiated at an optimum point, reducing operating times of the sand trap pump and scraper by up to 85%. Another advantage of this application is to be able to continuously monitor the pump delivery rate. Users who know their process peaks better can better control the energy input for their grit trap pumps and thus counteract early, high wear.

Two special ultrasonic sensors mounted on the sand trap pressure line detect the turbidity non-invasively after the floating sludge pump is switched on. They are hard-wired to the Katronic transmitter and a 4-20mA signal proportional to turbidity is supplied to the central control system, which also takes weather information into consideration. The standard programme can be modified for each installation according to the specific conditions of a grit chamber.

The Katronic flowmeters installed on the pressure line also fulfil a second purpose on wastewater treatment plants: monitoring the flow rate. As soon as the flow rate permanently falls below an adjustable limit value (wear on the impeller/clogging), a visual warning is given. With the use of this control optimisation developed by the Technisches Büro Holger Pinta, all processes around grit chambers in wastewater treatment plants become clear. Plant operators can not only control the pump and scraper more efficiently, but they can have a continuous view of the process at any time if there is a risk of wear of the pumps and/or the impeller or if too much energy is being used.

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July 2021

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