A horizontal pressure filter including a washing liquid recovery channel for receiving unused washing liquid from the washing liquid inlet channel and conducting said unused washing liquid to a washing liquid recovery outlet. A flow controller arranged to selectively open and close a flow connection between the washing liquid inlet channel and the washing liquid recovery channel, and respectively, between a compressed gas inlet and the washing liquid inlet channel, such that a gas flow from the compressed gas inlet to the washing liquid recovery channel via the washing liquid inlet channel may be induced, so as to flush unused washing liquid remaining in the washing liquid inlet channel to the washing liquid recovery outlet.