Abstract: The invention relates to a method for the purification of a polluted liquid, especially a washing liquid used at a washing installation, by sucking the polluted liquid and leading it through at least one filter and next leading back the liquid purified thus to a take-off point, whereby the liquid flow carried through the filter is maintained at a substantially constant level. The invention further relates to an apparatus for carrying out the method comprising a suction line connected to a source of polluted liquid, a discharge line for the purified liquid debouching at a take-off point, at least one filter unit positioned between the suction line and the discharge line, a pump connected therewith in series and a device for limiting the amount of liquid flowing across the filter unit positioned in series with the filter unit.