This work involves a laboratory‐scale investigation of the effects of suspended particle size and coagulant type on the performance of contact or inline direct filtration (no flocculation). Dilute monodisperse and polydisperse suspensions of polystyrene particles (0.27‐, 1.24‐, 1.32‐, and 10‐μm diameters) were applied to shallow beds of 0.4‐mm glass‐bead filter media after destabilization with either cationic polymer or calcium chloride. The particle removal and head‐loss results show dramatic effects of particle size on filtration performance. Submicron particles significantly improve the removal of larger particles in mixed size suspensions and also dominate head‐loss development. Head‐loss development is typically linear with time and for mixed suspensions is the same as, or somewhat lower than, head loss for monodisperse suspensions of the smaller‐sized particle. Polymer destabilization generally causes more head loss than calcium chloride destabilization for a similar extent of particle deposition. - See more at: http://ascelibrary.org/doi/abs/10.1061/(ASCE)0733-9372(1993)119%3A3(520)?src=recsys#sthash.11H22xts.dpuf