Tertiary denitrification of the secondary effluent by denitrifying biofilters packed with different sizes of quartz sand

Tertiary denitrification of the secondary effluent in wastewater treatment plants is necessary to control the eutrophication of receiving water bodies. Two denitrifying biofilters (DNBF), one packed with quart sand with sizes of 2-4 mm (DNBF_S) and the other of 4-6 mm (DNBF_L), were operated for tertiary denitrification under empty bed retention times (EBRTs) of 30 min, 15 min and 7.5 min, respectively. Under EBRTs of 30 min, 15 min and 7.5 min, the NO₃-_-N removal percentages were 93%, 82% and 83% in DNBFS, and were 92%, 68% and 36% in DNBF_L, respectively. The nitrogen removal loading rates increased with decreasing EBRTs, and at the EBRT of 7.5 min, the rate was 2.15 kg/(m³·d) in DNBF_S and 1.08 kg/(m³·d) in DNBF_L. The half-order denitrification coefficient of DNBFS increased from 0.42 (mg/L)^1/2/min at the EBRT of 30 min to 0.70 (mg/L)^1/2/min at the EBRT of 7.5 min, while did not vary much in DNBFL with values from 0.22 to 0.25 (mg/L)^1/2/min. The performance of both DNBFs was stable within each backwashing cycle, with the NO₃ -_-N removal percentage variation within 5%. Better denitrification was achieved in DNBF_S but with a slightly high decreased flow rate during the operation.