Abstract
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 (DNBFS) and the other of 4–6 mm (DNBFL), 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 DNBFL, 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 DNBFS and 1.08 kg/(m3·d) in DNBFL. 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 DNBFS but with a slightly high decreased flow rate during the operation.
Highlights
Eutrophication is a serious environmental issue nowadays and nitrogen is one of the limiting factors inducing the occurrence of eutrophication
The two tertiary denitrifying biofilters (DNBF) were operated under empty bed retention times (EBRTs) of 30 min, 15 min and 7.5 min, respectively
Long-Term Performance under Different EBRTs. Both biofilters were started with the EBRT of 30 min and the dosed carbon to nitrogen ratio of 3.42, and after a period of stable operation, the EBRT was decreased to 15 min and 7.5 min
Summary
Eutrophication is a serious environmental issue nowadays and nitrogen is one of the limiting factors inducing the occurrence of eutrophication. Due to the shortage of organic carbon in influent wastewater and limits of the secondary biological treatment process, nitrogen cannot be removed to achieve a very high standard. Nitrate is the main nitrogen component in the WWTP’s secondary effluent, and post-denitrification or tertiary denitrification may be required to further remove the oxidized nitrogen so as to achieve a high discharging standard. The low biomass yield in denitrification filters will reduce the effluent turbidity and the energy consumption for backwashing, and alleviate the clogging problem of biofilters. The filter with media sizes above 6 mm is commonly used in pretreatment, 3–6 mm used in the secondary treatment process, and around 3 mm used in tertiary biofilters [9,10,11,12,13,14]. Long-term performance of the two DNBFs, nutrient removal along the biofilter depth and performance of biofilters within a backwashing cycle were investigated so as to clarify the denitrification performance for tertiary nitrogen removal
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