A laboratory filtration plant for drinking water treatment is constructed to study the conditions for purely mechanical in situ cleaning of fouled polymeric membranes by the application of ultrasound. The filtration is done by suction of water with defined constant contamination through a membrane module, a stack of five pairs of flat-sheet ultrafiltration membranes. The short cleaning cycle to remove the cake layer from the membranes includes backwashing, the application of ultrasound and air flushing. A special geometry for sound irradiation of the membranes parallel to their surfaces is chosen. Two frequencies, 35kHz and 130kHz, and different driving powers are tested for their cleaning effectiveness. No cleaning is found for 35kHz, whereas good cleaning results are obtained for 130kHz, with an optimum cleaning effectiveness at moderate driving powers. Acoustic and optic measurements in space and time as well as analytical considerations and numerical calculations reveal the reasons and confirm the experimental results. The sound field is measured in high resolution and bubble structures are high-speed imaged on their nucleation sites as well as during their cleaning work at the membrane surface. The microscopic inspection of the membrane surface after cleaning shows distinct cleaning types in the cake layer that are related to specific bubble behaviour on the membrane. The membrane integrity and permeate quality are checked on-line by particle counting and turbidity measurement of the permeate. No signs of membrane damage or irreversible membrane degradation in permeability are detected and an excellent water permeate quality is retained.
Ultrasonic cleaning of membranes used in water purification and waste water treatment is under investigation for avoiding fouling and scaling on the membranes. So far chemicals are used, but their use is under scrutinity for safety, waste removal and health issues. Chemicals often even do not solve the cleaning problem durably. For applications in the part of drinking water treatment it is necessary to ensure the filtered water is really clean. Therefore in the presented experiment the outflow is constantly controlled by turbidity measurements and by using a particle counter in an online system. A pilot plant for sonication of submerged membranes to produce drinking water from surface water was constructed and placed at the Rhine water works in Biebesheim. It could be shown that sonication with 130 kHz when backflushing the membranes only works with following air overflow. Then the permeability keeps high. No damage of the membranes occurs like often has been found in former studies. Only 30 seconds of sonication after 30 min of filtration are enough to keep high performance of the membranes. Thus energy demand is low, which is a pre-condition for an economical use in technical applications.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.