Flocculants are used in combination with coagulants to agglomerate suspended particles for removal by filtration. This technique is used extensively in all types of surface waters to reduce the silt density index (SDI) and minimize membrane fouling. Although organic cationic flocculants are particularly effective, their widespread use in membrane applications is limited because of perceived problems with flocculent fouling at the membrane surface causing irreparable damage. Reference material from some of the major membrane manufacturers on the use of flocculants is given. Autopsies show that more than 50% of membrane fouling is caused by inadequate, deficient or poorly operated pre-treatment systems.