The invention relates to a device and a method for detecting particles, in particular parasites, in drinking water adapted to on-line application. In particular, the invention relates to a method for detecting parasites in water, said method comprising: Passing at least a part of the water through a filter; Applying indirect sonication with ultrasound to said filter to release parasites which have been collected in said filter without disrupting said parasites; Collecting parasites for detection; and Detecting the collected parasites. This serves to collect parasites in the filter and/or increase the concentration of parasites before the filter and/or disrupt aggregates without disrupting the parasites per se. The invention further relates to a concentration device for filter filtration concentration of particles from a volume of a fluid.; The concentration device comprises an ultrasonic transducer that is configured to clean the filter.