the mass transfer rate of the first chemical being removed from the unflushed membrane web 1 each of the lateral ends of the vacuum roller may be sealed with an end cap 104 which may act as a plug to seal the interior vacuum chamber 102 the suction source of a vacuum pump or other pump may be attached to a vacuum pressure inlet 106 so as to create a trans web pressure differential across the pores of the membrane web in one embodiment the one end of the vacuum pressure inlet 106 may connect to a vacuum pressure channel 110 that terminates at a channel opening 111 the vacuum pressure channel 110 may have one or more channel openings 111 to transmit vacuum pressure to the interior vacuum chamber 102 a inlet bearing 112 may separate the end cap 104 from the vacuum pressure inlet 106 alternatively a rotary coupling may be used the effective contact area for mass transfer between the unflushed membrane web 1 and the cylindrical element 103 of the vacuum roller 3 may be determined in part by the location of the first and second positioning rollers 2 a and 2 b the diameter of the vacuum roller 3 the size of the shield 107 or percentage of openings 101 transmitting vacuum pressure at any instant and the depth of submersion of the vacuum roller 3 and unflushed membrane web 1 in the flushing chemical 5 among other factors the mass transfer rate is affected by the effective contact area the vacuum pressure applied to the unflushed membrane web 1 through the openings 101 in the cylindrical element 103 of the vacuum roller 3 the rate of rotation of.