An experimental study of the interrelationships between filter operation, floc properties, and filter performance for water clarification was conducted. Two different clarification systems were investigated. One system had flocculation and sedimentation ahead of the filters. In the other, the pretreatment consisted of chemical addition and a brief period of rapid mixing. When operated in this latter manner, the filters were actually serving as contact flocculators. The technical and economic feasibility of water or waste water clarification by contact flocculation are demonstrated. It was concluded that a cationic polyelectrolyte, cat-floc (dimethylpentamethylene ammonium chloride), is more effective than alum. A dual media filter, consisting of a very coarse, uniform coal layer on top of a shallow sand layer, performed better than others. The dependence of filter performance on such factors as chemical dose, filtration rate, and raw water suspended solids was also examined.