Disclosed is a method of conditioning mixed liquor in a membrane bioreactor (MBR) system comprising adding an effective amount of a tannin containing polymer to the mixed liquor. Also disclosed is a method of improving flux in an MBR system comprising adding an effective amount of a tannin containing polymer to mixed liquor of the MBR. An effective amount of the tannin containing polymer may be added separately or in combination with other water soluble sludge filterability improvement polymers or sludge filterability improvement inorganic coagulants to the mixed liquor of the MBR. In one embodiment, the tannin containing polymer is a water soluble or dispersible copolymer of a tannin and a cationic monomer. In another embodiment, the tannin containing polymer is a water soluble or dispersible polymer of a tannin, a cationic monomer, and at least one monomer.