AWWA JAW46259

Since the 1960s, many rapid sand filters have been modified to become high-rate filters, and dual media have replaced sand. A common problem caused by this change is excessive loss of anthracite coal medium (5-10 percent annually) under regular filter washing practices. This happens because the space between the top of the filter bed and the top of the wash troughs is insufficient for the lightweight coal grains, even though it was adequate for the rapid sand filter beds. Also, rather tight spacing of wash troughs for old filter designs aggravates the loss of coal grains. The authors evaluate the loss of granular activated carbon (GAC) after existing rapid sand beds are converted to GAC beds without raising the concrete wash troughs. Results of the pilot study indicate that a simple baffle configuration can reduce media loss by as much as 70 percent. Includes tables, figures.