Study On Conditionality Of Denitrification In AO Process For Municipal Wastewater Treatment

At present, about70%domestic sewage treatment plants use A/O process forbiological denitrification, but in practice, total nitrogen in the effluent of the A/Oprocess contains a large number of nitrate nitrogen. The denitrification efficiency isonly50%to70%that is difficult to improve. In order to improve the denitrification rateand reduce NO3–N concentration in effluent, many researches have been done all overthe world, but they are mainly focused on the affection of dissolved oxygenconcentration on the denitrification rate, types of carbon source and C/N. A lab-scaleA/O reactor was used in this thesis, changing the dissolved oxygen concentration in theaerobic zone and COD-fractions in the influence to study the affection of the mixturewith different dissolved oxygen concentration on denitrification. The denitrificationeffiency with different soluble readily biodegaradable COD (ie. Ss) fractions wereanalyzed.The results drawn from the study are as follows:(1) In the A/O reactor, the DO concentration in the aerobic zone was maintained at3mg/L,2mg/L,1mg/L respectively by controlling aeration. It is showed that theaverage TN concentration of effluent was6.92mg/L,5.63mg/L,4.530mg/L, and theaverage TN removal rate was82%,85%and88%with the DO concentration of3mg/L,2mg/L and1mg/L respectively. The result shows that the denitification rate wasimproved gradually as the DO concentration of aerobic zone decreased. It is due toincrease of DO concentration in anoxic zone as after DO concentration in aerobic zonewas increased that destroyed the anoxic condition and affected denitrificationefficiency.DO in the anoxic zone will enforce competion in carbon source betweenheterotrophic bacteria and denitrifying bacteria, that leds to the decrease of the denitrification efficiency. At the same time, as DO concentration in aerobic zonedecreased, the SND rate was increased, of which is20%,38%and20%respectively,and this is another important reason on the increase of denitrification effiency.(2)Denitrification rate and denitrification potential were investigated by changingthe fractions of soluble readily biodegaradable CODand slowly biodegradable COD (ie.Xs) in the influents in bach tests. It shows that the total nitrogen removals of A/Oprocess were increased obviously when the fraction Ss in the influent was incresed.After Ss fraction of influent was increased from15%to50%, the total nitrogenremoval was improved from82%to89%. The denitrification kinetic information wasobtained with anoxic batch tests. When the Ss fraction of influent is50%,30%,15%respectively, the denitrification rate is0.027gNO3--N/(g VSS·min),0.022gNO3--N/(gVSS·min),0.020gNO3--N/(g VSS·min), and the denitrification potential is16.49mg/L,13.99mg/L,13.74mg/L respectively. The result shows that thedenitification rate and denitrification potential were improved as the Ss fraction ofinfluent increased. As a consequence, the variation of COD-fractions in influent,especially the ratio of Ss, is a key factor to influence the denitirfication.