BACKWASHING FIRST-STAGE SAND - BAC FILTERS

The effect of filter backwash on the efficiency of first-stage sand-bi ological activated carbon (BAG) filters was evaluated by comparing, in first-stage sand-BAG filters, concentrations of ammonia, biodegradabl e dissolved organic carbon, aldehydes, and oxalate. These concentratio ns were measured before and after filter backwash. Filter position (fi rst-stage sand-BAG filters versus second-stage BAC filters) was also s tudied to understand how flocs and particle accumulation influenced th e efficiency of the first-stage sand-BAG filter. A first-stage sand-an thracite filter operated under similar hydraulic conditions and efflue nt turbidity acted as a reference. Filter backwash improved the effici ency of sand-BAG filters, especially in cold water sources. However, b oth filters produced similar water quality. First-stage sand-anthracit e filters did not adequately remove biodegradable organic matter and a mmonia.