Abstract: An apparatus for filtering machining liquid of an electrical discharge machine. The apparatus includes a plurality of cross flow filters placed in series with one another and with a pump drawing dirty liquid from a capacity tank. The permeate from the cross flow filters is provided to the electrical discharge machine. The permeate outlets of the cross flow filters are provided with valves that are selectively opened only upon demand for machining liquid by the electrical discharge machine. The permeate valve of the cross flow filter having the smallest cross member pressure is opened prior to opening other permeate valves. A capacity tank includes a baffled area and sludge accumulating within the capacity tank is drawn from the baffled area to paper filters. The cross flow filters are back flushed on a timed interval and the particulates dislodged during back flushing are dumped into the baffled area of the capacity tank.