Abstract: A fluid treatment device is disclosed, including a sludge separator. The separator includes an inner funnel and an outer funnel, arranged with an intermediate, at least partial, gap between the funnels. According to another aspect, a fluid filtering device includes a filter portion containing a filtering material, a fluid inlet and a fluid outlet. The inlet is in the upper part of the device and the outlet is in the lower part of the device. Aeration nozzles are arranged in the device between the inlet and the outlet at a distance from the inlet and the outlet, respectively, enabling the creation of a vertical division of the filter portion into an aerobe section and an anaerobe section. A method is disclosed for filtering a fluid is also disclosed, as well as a method for regenerating a filtering device.