| Denitrifying poly-phosphate accumulating organisms?DPAOs?utilize an external carbon source to store poly-?-hydroxyalkanoates?PHA?under anaerobic condition.Then,DPAOs oxidize intracellular PHA and utilize nitrate/nitrite as electron acceptor for P uptake under anoxic condition.Compared with conventional biological nutrient removal,the process has tremendous advantages in saving carbon sources and aeration,and reducing the production of residual activated sludge,especially applicable for treating low C/N ratio wastewater.However,denitrifying phosphorus removal process will release large amounts of greenhouse gases?N2O?.Therefore,it is crucial to understand emission characteristics and influencing factors of N2O during denitrifying phosphorus removal process,which has a great significance for its practical application.Inoculated with conventional activated sludge,denitrifying phosphorus removal was successfully started up in a anaerobic/anoxic sequencing batch reactor?SBR?with decoupled acetate and nitrate additions.Sludge were sampled from the SBR to investigate the electron acceptor?NO3--N,NO2--N respectively as electron acceptor and NO2--N dosing method?and the carbon source?carbon source types,C/N and external carbon source dosing quantity under anoxic phase?on denitrifying phosphorus removal and nitrous oxide emission.The main research results are as follows:?1?During the start-up process,the influent COD in anaerobic phase kept at 250mg·L-1,and the NO3--N and PO43--P in anoxic phase were adjusted according to the reactor operation situation.Denitrifying phosphorus removal was successfully started up in around a month.During a single cycle,the removal efficiencies of NO3--N and PO43--P were 99.95 % and 72.07 %,respectively.The N2O conversion rate?ratio ofN2O-N to nitrogen removal?was 16.41 %.The SBR showed stable denitrification and phosphorus removal performance.?2?Sludge were sampled from the SBR at the end of anaerobic phase to investigate the nitrate concentration on denitrifying phosphorus removal and nitrous oxide emission.As the concentration of NO3--N increased from 50 to 65 and 90 mg·L-1,the removal efficiency of PO43--P was 71.43,67.90 and 68.53 %.The average denitrification rates were 0.0456,0.0452 and 0.0415 mg·min-1·g-1SS.The emission of N2O was 2.27,2.48 and 2.25 mg.And the N2O conversion rates were 5.72,6.28 and6.15 %,respectively.The results showed that if there were enough nitrate used as electron acceptor,the process of phosphorus uptake,nitrite reduction and the emission and conversion rate of N2O won't be effected by nitrate concentration.?3?Sludge were sampled from the SBR at the end of anaerobic phase to investigate the nitrite concentration on denitrifying phosphorus removal and nitrous oxide emission.As the concentration of NO2--N increased from 15 to 25 and 50 mg·L-1,the N2O emission increased from 3.08 to 5.62 and 6.67 mg.The average 60 min PO43--P removal rates were 0.0625,0.0489 and 0.0268 mg·min-1·g-1SS.And the average 60 min NO2--N removal rates were 0.0814,0.0763 and 0.0589 mg·min-1·g-1SS.The results revealed that high concentration of NO2--N had a strong inhibitory effect on phosphorus uptake and nitrite reduction,and it could induce a higher emission of N2O.?4?Sludge were sampled from the SBR at the end of anaerobic phase to investigate the nitrite dosing method on denitrifying phosphorus removal and nitrous oxide emission.When changed the pulse addition frequency to 2(influent concentration of NO2--N was 20 mg·L-1),the removal efficiency of PO43--P was only 56.56 %,and the nitrogen removal efficiency was 100 %.The N2O conversion rate was 27.25 %.When changed the pulse addition frequency to 4(influent concentration of NO2--N was 10mg·L-1)and 3(influent concentration of NO2--N was 15 mg·L-1),the P removal efficiencies reached 81.82 and 80.70 %,and the nitrogen removal efficiencies were98.97 and 96.66 %.The N2O conversion rates were 27.05 and 23.93 %,respectively.The findings indicated that more frequent pulses of nitrite addition at smaller concentrations may have led to higher nitrogen removal efficiency and P removal efficiency,and won't cause N2O conversion rate increase.?5?Sludge were sampled from the SBR at the end of anoxic phase to investigate the C/N on denitrifying phosphorus removal and nitrous oxide emission.When changed the C/N from 2 to 5 and 8,the COD removal rates were 82.86,93.60 and 75.71 %.The P removal efficiencies decreased from 50.82 to 17.14 and-60.29 %.The N2O conversion rates increased from 9.48 to 18.62 and 17.97 %.The results showed that high organic matter concentration would lead to low COD removal efficiency,and occurring “endogenous” P release in the anoxic phase.At the same time,the findings indicated that higher organic shock loads increased the maximum concentration of NO2--N in the anoxic phase,leading to a higher emission of N2O.?6?Sludge were sampled from the SBR at the end of anaerobic phase to investigate the external carbon source dosing quantity under anoxic phase on denitrifying phosphorus removal and nitrous oxide emission.Without adding external carbon source,it present a typical denitrifying phosphorus metabolism process,and the removal efficiencies of PO43--P was 33.54 %.The nitrogen removal efficiency was55.44 %.And the N2O conversion rate was 25.08 %.With the increase of COD concentrations?Cexternal/N from 0.75 to 2.5?,the external carbon source were preferred for denitrification and hypothesis of poly-?-hydroxybutyrate?PHB?when both internal and external carbon coexisted.The nitrogen removal efficiencies increased to 63.50 and96.44 %,while the P removal efficiencies decreased to 26.87 and-0.016 %,respectively.The N2O conversion rate decreased from 25.08 to 22.96 and 11.85 %,which revealed that PHB could induce a higher emission of N2O.?7?Sludge were sampled from the SBR at the end of anoxic phase to investigate the carbon source types on denitrifying phosphorus removal and nitrous oxide emission.A single cycle changed carbon source to propionate,the COD removal rate slightly increased.While changed carbon source to butyrate,the COD removal rate is greatly reduced,only 29.59 %.When used acetate,propionate and butyrate as carbon source,the N2O conversion rates was 16.76,9.65 and 15.87 %,respectively.The result showed that if used propionate as carbon source for denitrifying dephosphatation,the largest accumulation of NO2--N compared with acetate and butyrate cut in half,and the N2O emission was lowest?2.47 mg?. |
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