Improvement Of Nitrogen And Phosphorus Removal For Oxidation Ditch Process Of Sewage Treatment Plant

Considering the present situation of water environmental quality in our country,requirements of effluent indexes of municipal wastewater treatment plants are further enhanced.Currently,the technical upgrading is necessary for municipal wastewater treatment plants which TN and TP concentrations in effluent can not be up to the the first level-A of the Pollutant Discharge Standard of Municipal Wastewater Treatment Plant（GB18918-2002）.With oxidation ditch of the wastewater treatment plant of Southwest University of Science and Technology as the subject,this study investigated the operating effect and existing problems through analysis of water indexes data on influent and effluent from 2012 to 2014.On this basis,the study explored feasible process parameters and optimization measures.The effects of increased aeration time,reduced settling time and adjusted hydraulic retention time（HRT）were investigated by the static study.On this basis,two improved operation modes for the oxidation ditch were put forward.By using a self-made test device for oxidation ditch,the effects of the two improved operation modes on domestic sewage treatment were compared,and HRT,sludge discharge method and other operating parameters were optimized.In addition,nitrogen and phosphorus in the supernatant of sludge thickening tank were recovered by struvite precipitation,and the effect of reflux of the supernatant on nitrogen and phosphorus loading was preliminarily studied.The results obtained are as follows:The analysis of water indexes data on influent and effluent showed that BOD5,TN,NH₃^-N and TP concentrations in influent could not meanwhile meet the design requirements of the influent indexes.The TN,NH₃^-N and TP concentrations in effluent could not stably meet the requirements of the first level-A of the Pollutant Discharge Standard of Municipal Wastewater Treatment Plant（GB18918-2002）.Therefore,oxidation ditch process upgrading was necessary.The static experiment demonstrated that lengthening aeration time from 90 min to 110 min in the pool A（side ditch）could reduce TN,NH₃^-N and TP concentrations in the sewage.Properly shortening settling time from 5h to 3h in A/C（side ditch）and HRT time from 18.2h to 16 h showed a good effect for the removal of pollutants in the sewage.Consequently,two improved operation modes for the oxidation ditch were put forward.The dynamic experiment indicated that after operational stabilization of the test device,with HRT set at 18.2h,improved operation mode（2）presented a superior effect in the removal of CODCr,TN,NH₃^-N and TP to improved operation mode（1）.When HRT was set at 16 h,improved operation mode（1）and（2）showed an optimal effect for the removal of the CODCr,TN,NH₃^-N and TP.With the same discharge amount of sludge,pool A/C（the side ditch）and pool B（the median ditch）discharging sludge simultaneously presented a better effect for the removal of TP as compared with pool B（the median ditch）discharging sludge alone.The operation results of the test device under the optimized parameters showed that the concentrations range of the CODCr,TN and NH₃^-N in effluent was 26 mg/L-40mg/L,6.2mg/L-12.7mg/L and 2.5mg/L-4.2mg/L,respectively,namely that all of the CODCr,TN and NH₃^-N concentrations in effluent could meet the standards of the first level-A of the Pollutant Discharge Standard of Municipal Wastewater Treatment Plant（GB18918-2002）.However,the TP concentration in effluent ranged from 0.33 mg/L to 0.80 mg/L,which indicated that the TP concentration in effluent could not be stably reach the standards of the first level-A of the Pollutant Discharge Standard of Municipal Wastewater Treatment Plant（GB18918-2002）.Struvite precipitation demonstrated that the suitable reaction conditions of struvite formation were as follows: initial pH at 9.5;reaction time on 10min;temperature from 10℃ to 30℃.Under these reaction conditions,PO₄^3--P and NH₄⁺-N contents（mean）in the supernatant of sludge thickening tank reduced from 0.46 mmol/L and 2.0 mmol/L to 0.15 mmol/L and 1.74 mmol/L.respectively.The average removal of PO₄^3--P and NH₄⁺-N in the supernatant of sludge thickening tank were 0.31 mmol/L（9.61 mg/L）and 0.26 mmol/L（3.64 mg/L）,respectively.The recovery rates of PO₄^3--P and NH₄⁺-N in the supernatant of sludge thickening tank were 68% and 13%,respectively.The purity of struvite in this precipitate was 44%.The supernatant yield of the sludge thickening tank in this wastewater treatment plant was 100m³/d.The removal amouts of PO₄^3--P and NH₄⁺-N in the supernatant of sludge thickening were 961g/d and 364g/d by struvite precipitation reaction,respectively.According to mean concentration of PO₄^3--P and NH₄⁺-N in influent of this wastewater treatment plant in 2014,the struvite precipitation reaction could reduce TP concentration by about 0.2 mg/L and NH₄⁺-N concentration by about 0.08 mg/L in influent of this wastewater treatment plant.The recovery of nitrogen and phosphorus by the struvite precipitation reaction could reduce the nitrogen and phosphorus loading of the oxidation ditch units.