Abstract: This invention relates to a method of batch filtration using a filter aid such as diatomaceous earth for example. The invention is characterized in that the filtration cycle of the filter includes a pulsed reversed flow stage in which fluid is pumped into the filter outlet with all the filter inlets closed. The filter drain is opened for a short time during the pulse. Loosening, breaking up and mixing of the filter cake and layer of filter aid is thus effected thereby extending time between cleaning or backwash stages when the filter aid is replaced. The life of each batch of costly filter aid is thus extended.