| Biological nitrification performance and Biological characteristics of biological sand filter used for treating the effluent of WWTP were studied in this paper. Research results were as follows.In the initial stage of mature of biofilm the average removal rate of CODCr, NH4+-N, TN were26.4%, 35.91%, 25.72%, respectively. The time, when the removal rate of CODCr and NH4+-N was basically stable, was 25 days and 27 days, respectively. The stable removal rate of NH4+-N and TN was the symbol of mature of biofilm for nitrifying and denitrifying filter, respectively.When the hydraulic loading was 3.00m/h, gas water was 3:1 and the system operated steadily, the removal effect of pollutants by biological sand filter were as follows: the average removal rate of CODCr was 32.1% by nitrifying filter, and the removal rates of NH4+-N, TN and turbidity were 61.66%, 38.54% and 78.45% by the system, respectively.Influence of water temperature, hydraulic loading, gas water ratio, NH4+-N volume loading and organic volumatric loading on nitrification performance were analyzed. When hydraulic loading was 3.00m/h, gas water was 3:1, organic volumatric loading was 2.52kgCODCr/ (m3 filter medium·d) and NH4+-N volume loading was 0.84kgNH4+-N/ (m3 filter medium·d), the biological sand filter could have a good treatment effect. AHM was applied for analyzing influencing factors. The results showed that the weights of water temperature, hydraulic loading, gas water ratio, NH4+-N volume loading and Organic volumatric loading were 0.355, 0.289, 0.283, 0.263 and 0.118, respectively.When the water temperature was 26℃, gas water was 3:1, hydraulic loading was 3.00m/h and the system operated steadily, the biofilm, attached on quartz sand was super thin film, the thickness of biofilm in nitrifying and denitrifying filters were 84.02μm and 59.08μm , respectively. The biomass firstly increased then decreased along with flow direction in nitrifying filter, and the biomass gradually decreased along with flow direction in denitrifying filter. With the increasing depth of filter, biological activity firstly increased then decreased.Nitrification kinetic model of biological sand filter was KN1n(N/NO)+NO-N=qNh(in which the relation equation about degradation rate of ammonian"qN"and hydraulic loading"h′"was qN = -1 .6263h′+10.295 ). The verification revealed that the maximum absolute error and the maximum relative error were 0.32mg/L and 7.21% between the theoretical calculating value and experimental value. The results from calculation by nitrification kinetic model were similar to the measured values. Analysis of experimental result indicated that O/A two-stage biological sand filter was advanced in technology and reasonable in economy. It would have a broad application prospect in the field of actual engineering in the future.
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