be used solely for cleaning semipermeable membrane 32 or may be used for pressing coating and or impregnating paper web 34 in addition to cleaning of semipermeable membrane 32 and permeable layer 36 semipermeable membrane 32 and permeable membrane 36 are provided for carrying paper web 34 semipermeable membrane 32 has a low air permeability specially designed for displacement dewatering permeable membrane 36 has a high air permeability and may be a felt a wire a press belt drying fabric or an anti rewet layer air press 40 includes a first main roll 44 a second main roll 46 a first cap roll 48 and a second cap roll 50 which conjunctively define a pressurized air chamber 52 the pressure of the air in pressurized air chamber 52 serves to flush the cleaning fluid through semipermeable membrane 32 the pressure of the air therein is greater than atmospheric pressure about 1 bar advantageously more than about 2 bar and preferably greater than approximately 5 bar first main roll 44 is vented e g blind drilled grooved etc so as to promote removal of water used cleaning fluid and debris from semipermeable membrane 32 at least one of cleaning fluid sprayers 42 a 42 d is provided as part of papermaking machine 30 cleaning fluid sprayer 42 a is.