The role of backwash in start-up of full-scale drinking water biofilters

Abstract

The purpose of backwashing during the start-up period of rapid biofilters at drinking water treatment plants differs significantly from the purpose of backwashing during normal operation. During startup, finished water quality is secondary, detachment of micro-organisms must be minimized, dust present in the original filter media must be removed and inherent inoculation of the biofilter with water used for backwashing must be considered. Investigations over a 9-week start-up period at a full-scale waterworks showed a strong correlation between suspended solids and turbidity, and that fine particles present in the original filter media were removed over a period of several weeks. In addition, after an initial period in which iron accumulated on the filter was not removed by backwash, iron was removed with 80% efficiency. Backwashing had 0% efficiency in removing manganese from the filter media. Practical conclusions include that shortened backwash procedures during start-up may be preferable, turbidity may be used as a surrogate for suspended solids in backwash effluent, filter media may be pre-treated by exaggerated backwashing to remove fine particles before start-up, and that a reasonable backwash procedure for future operation of the filter may be defined early in the start-up period.