The paper presents evidence to demonstrate that filter backwash sequences should include a combined air and water phase to maintain the long-term performance of rapid gravity filters when treating lowland river water without the use of coagulants or pre-chlorination. Techniques for assessing filter backwash effectiveness in maintaining clean media included laboratory analysis for retained organic and inorganic material, scanning electron microscopy and examination of trends in clean bed head-loss data. Backwashing with air scour followed by a water wash, without a combined air and water stage, failed to keep the filter media clean. It led to increased starting head-losses, fluctuating filtrate quality and ultimately the appearance of cracks in the bed. Backwashing with combined air and water at a sub-fluidising rate ensured consistent filtrate quality and maintained low clean bed head-losses. Data were obtained from pilot plants and full-scale works (200-790 Ml/day range) treating London＇s reservoir stored water prior to further biological filtration by slow sand filters. Air scour rates investigated were from 20 to 50 m/hr tree air and were combined with water at 7-12 m/hr. These rates were successful for several different media types, including standard (0.7 mm) and coarse (1.2 mm) fitter sand and dual-media (anthracite/sand) filters. Backwash launder position and configuration were evaluated at large scale and found to be critical to successful retention of media during combined air and water washes. The results confirm, for water containing living organisms, theoretical and laboratory findings of other workers using predominantly inert suspensions in clean water. The degree of filter maturation attributed to biofilm and inorganic coating of media was affected by backwashing conditions. The impact of filter maturation on filtrate quality is discussed.