A method of controlling a spinning membrane separator during filtering of a bulk fluid, in particular a suspension of biological cells, to limit fouling of the membrane by changing the rotation rate of the spinning membrane in response to the rate of change of transmembrane pressure (TMP) over time while maintaining a constant outlet cellular concentration, comprising the steps of: a) setting an initial spin rate for the spinning membrane; b) operating the spinning membrane at the initial spin rate; c) flowing bulk fluid through the spinning membrane; d) measuring a fouling rate (FR) corresponding to the change in transmembrane pressure over time ( ΔTMP / ΔT ); and either e 1 ) comparing the FR to a predetermined maximum fouling rate (FR MAX ); f 1 ) if FR > FR MAX , determining a cumulative time above limit during which the FR has been > FR MAX (RSTAL); g 1 ) if RSTAL is > a predetermined time limit, increasing the spin rate by a predetermined value; h 1 ) waiting a predetermined time to allow for the TMP to stabilize; and i 1 ) returning to step d); or e 2 ) comparing the FR to a predetermined maximum fouling rate (FR MAX ) and a predetermined minimum fouling rate (FR MIN ); f 2 ) if FR > FR MAX , determining a current time above limit during which the FR has been > FR MAX (TAL); g 2 ) if FR < FR MIN , determining a current time below limit during which the FR has been < FR MIN (TBL); h 2 ) determining a total cumulative time outside the limit (RSTOL) by subtracting TBL from TAL; i 2 ) comparing RSTOL to a negative and positive time limit (TL NEG and TL POS ); j 2 ) if RSTOL < TL NEG , decreasing the spin rate by first preset value; k 2 ) if RSTOL > TL POS , increasing the spin rate by a second present value; l 2 ) waiting a predetermined time to allow for the TMP to stabilize; and m 2 ) returning to step d). a) setting an initial spin rate for the spinning membrane; b) operating the spinning membrane at the initial spin rate; c) flowing bulk fluid through the spinning membrane; d) measuring a fouling rate (FR) corresponding to the change in transmembrane pressure over time ( ΔTMP / ΔT ); and either e 1 ) comparing the FR to a predetermined maximum fouling rate (FR MAX ); f 1 ) if FR > FR MAX , determining a cumulative time above limit during which the FR has been > FR MAX (RSTAL); g 1 ) if RSTAL is > a predetermined time limit, increasing the spin rate by a predetermined value; h 1 ) waiting a predetermined time to allow for the TMP to stabilize; and i 1 ) returning to step d); or e 2 ) comparing the FR to a predetermined maximum fouling rate (FR MAX ) and a predetermined minimum fouling rate (FR MIN ); f 2 ) if FR > FR MAX , determining a current time above limit during which the FR has been > FR MAX (TAL); g 2 ) if FR < FR MIN , determining a current time below limit during which the FR has been < FR MIN (TBL); h 2 ) determining a total cumulative time outside the limit (RSTOL) by subtracting TBL from TAL; i 2 ) comparing RSTOL to a negative and positive time limit (TL NEG and TL POS ); j 2 ) if RSTOL < TL NEG , decreasing the spin rate by first preset value; k 2 ) if RSTOL > TL POS , increasing the spin rate by a second present value; l 2 ) waiting a predetermined time to allow for the TMP to stabilize; and m 2 ) returning to step d). <EOS>