The present invention relates to a composition for treatment of wastewaters in a membrane bioreactor comprising a mixture of inorganic coagulants and organic water soluble polymer(s), wherein the inorganic coagulants are selected from iron and/or aluminium containing compounds and the organic water soluble polymers are selected from the group consisting of anionic, cationic, nonionic polymers, polysaccharides and polyphenolic compounds and any combination thereof. The present invention further relates to a method for treating of wastewaters in a membrane bioreactor, and uses thereof.