Contact filtration of humic waters: performance of an expanded clay aggregate filter (Filtralite) compared to a dual anthracite/sand filter

Contact filtration performance of a coarse expanded clay aggregates filter (Filtralite) is compared to a conventional dual media anthracite/sand filter. In a contact filtration pilot plant, humic water was coagulated using aluminium sulphate (AS), iron chloride sulphate (ICS), poly-aluminium chloride (PAC) or the bio-polymer chitosan. Filtration experiments were performed at optimum pH and dose conditions for each coagulant found from previous optimisation experiments in the pilot filter plant. The results reveal two main problems in coagulation/filtration of humic waters: short filtration run times for the most commonly used metal salts, and metal residuals in treated water. The best ways to overcome these problems in raw waters high in organic matter, was found to be by the use of a coarse filter bed in combination with chitosan, or a metal salt with the addition of polymer as filter aid. An increased turbidity level in the raw water (as bentonite clay) was found to increase the removal of humics in most cases, as well as increase the filterability of the flocs formed.