Impact of backwashing on nitrification in the biological activated carbon filters used in drinking water treatment

Abstract

Nitrification during biological filtration is currently used in drinking water production to remove ammonia, which is the source of several water quality problems during treatment and distribution. We evaluated here the impact of backwashing on nitrification efficiency in filters used for drinking water treatment. Two different granular activated carbon (one open and one closed carbon superstructure) were tested. Ammonia removal and fixed nitrifying bacterial biomass before and after backwashing were compared in first‐stage pilot filters and full‐scale second‐stage filters. Backwashing has a greater impact on nitrification on first‐stage than on second‐stage filters. Backwashing improved the ammonia removal in warm (≥ 18°C) water in a first‐stage filter containing an open‐superstructure granular activated carbon, whereas a closed‐superstructure support showed a removal capacity that is less after a regular backwashing than before, or similar to it. In cold water (≤ 4°C), backwashing had a negative impact on nitrification capacity in an open‐superstructure medium first‐stage filter. In full‐scale second‐stage filters, backwashing had a slight negative impact on filter performances for both open‐ and closed‐superstructure media at temperatures between 8°C and 12°C. In colder waters (≤ 3°C), nitrification was very poor both before and after backwashing. Sampling of fixed, nitrifying biomass in the pilot filter columns showed that in all cases (low and high expansion backwash; both media tested) and at 20±3°C, the overall biomass levels remained unchanged before and after backwash. In the full‐scale second‐stage filters, nitrifying biomass was similar before and after backwashing for both types of media tested.