Membrane Cleaning with Cavitation by Ultrasound –Application in Drinking Water Treatment

Abstract

The cleaning of membranes used for drinking water production or for separation processes in industrial applications is an essential task, because membrane fouling can generate enormous costs. These costs may result in a shutdown of the membrane filtration plant and large amounts of cleaning chemicals leading to chemical waste. Cleaning of fouled membranes with ultrasound induced cavitation has been investigated in former studies. The main problem encountered was that membrane damage may occur. The integrity of the membranes could not be secured. Researchers saw themselves far away from the construction of a pilot plant for industrial use that could work reliably and economically (low energy consumption). In a new approach, whose results are documented in this paper, a pilot plant has been constructed and operated under various loads for an extended time. The pilot plant consists of a submerged membrane stack in an adequate plant configuration operated under real conditions filtering water of different quality including surface water directly from the river Rhine. In the altogether three years of operation the reliable ultrasonic cleaning of the membranes could be demonstrated. The membrane integrity was documented by turbidity measurements and particle counting of the permeate. For optimal membrane performance, a specific plant configuration, adjustment of the parameters frequency, intensity, sonication duration and of process sequences are necessary and will be reported. With this pilot plant developed, membranes can be cleaned by ultrasonic cavitation without damage and with low energy consumption leading to even better permeate quality as compared to a parallel running plant without ultrasonic cleaning connected to the same water feed. The cleaning mechanism is a mechanism of soft cavitation by bubble oscillations. This may be one reason that no membrane damages could be detected. For regions without infrastructure a plant as described above in container size driven by solar and wind energy could bring clean water to the people.