| The partial denitrification coupled with ANAMMOX(PD/A)process has become the mainstream nitrogen removal process nowadays.This study aims to enhance the feasibility as well as the stability of the PD/A process,and a stable partial denitrification(PD)process was initiated under laboratory conditions,where the effects of the influences of C/N ratio and carbon source type in combination with nitrate concentration,substrate adding methods and pH on PD process were investigated.Furthermore,the microscopic mechanism of the stable accumulation of nitrite(NO2--N)was clarified from the microbial perspective.Finally,the performance of advanced nitrogen removal by PD/A process was investigated,and the feasibility of the coupled process was evaluated based on Nitrogen loading rate(NLR)and microbial community changes.The main studied contents and related conclusions are as follows.(1)Combined effects of influent C/N ratio and NO3--N concentration on partial denitrificationBy comparing the PD performance under different C/N ratios(0.8,1.5,2.5,3.5,4.5),it was found that the NTR and NO2--N production reached up to 76.79%and 43.0 mg/L with the C/N ratio of 2.5 in the R80(initial NO3--N:80 mg/L)system.The insufficient carbon source supply at lower C/N ratios(0.8,1.5)resulted in incomplete denitrification with effluent NO2-N concentration and NTR values of 3.59,16.20 mg/L and 34.23%,63.51%,respectively.However,the higher C/N ratio(3.5,4.5)conditions inhibited the accumulation of NO2--N(37.0,29.0 mg/L)and the increase of NTR(76.57%,52.73%).In the R40(NO3--N:40 mg/L)system,the best PD performance was achieved at a C/N ratio of 3.5,with NO2--N accumulation and NTR values reaching to 20.0 mg/L,66.89%.Compared with seed sludge(SS),the functional bacteria with different C/N ratios in the two systems were well enriched,among which R403.5 and R80-2.5 shwed the highest enrichment and the lowest species diversity in the corresponding systems.Thauera and Flavobacterium were the main functional bacteria promoting NO2--N accumulation in the PD process,and the total proportions of the two dominant bacteria in R40-3.5 and R80-2.5 reached 42.79%and 52.13%,respectively,which was much higher than other sludge samples.(2)Combined effects of carbon source type and NO3--N concentration on partial denitrificationThe preferred order of carbon sources in PD process was HAc,HPr,Glu+HAc,and Glu,and the best NO2--N accumulation(67.03 mg/L)and NTR(99.14%)were achieved in the R80(NO3--N:80 mg/L)system with HAc as the carbon source compared to the R40(NO3--N:40 mg/L)system.HAc-R80 also corresponded to higher NO3--N reduction rate(SN3RR:29.94 mgN/(gVSS·h)),NO2--N accumulation rate(SN2AR:29.68 mgN/(gVSS·h))and COD depletion rate(SCCR:58.46 mgCOD/(gVSS·h)).In contrast,Glu-R40 corresponded to the lowest NO2--N accumulation(1.76 mg/L)and NTR(38.98%).For the same type of carbon source,the effluent NO2--N contents of the R80 system was found to be significantly higher than those of the R40 system,indicating that higher initial NO3--N concentrations were more favorable for the NO2--N accumulation.The genera playing important roles in the HAc system were Terrimonas(4.24-14.54%),Bacillus(7.42-21.63%)and norank_f_Saprospiraceae(4.95-15.01%),while norank_f_norank_o_Saccharimonadales was the dominant genus in Glu samples with to the proportion of 56.96-67.77%.Compared to the single Glu system,the addition of HAc promoted floc aggregation and sludge retention with a corresponding increase in NO2--N content and NTR values,and the Glu+HAc system was enriched with a dominant bacterium named OLB13,which presented a positive effect of HAc on PD formation with the content increasing to 4.76-9.36%indicating.(3)Effect of substrate dosing methods and pH on partial denitrificationThe effect of NO3--N concentration(40,80,120 mg/L)and the adding method(1,2,3,4 times)on the PD was investigated,where the accumulation of NO2--N was highest at 1 time adding,reaching to 16.51,40.06,50.49 mg/L under the same NO3--N concentration,respectively.The NO2--N content in the effluent gradually increased with the increase of NO3-N concentration,but the differences between three types of adding methods were not significant.It was worth noting that the peak time for stable NO2--N accumulation was gradually shortened when NO3--N was added in batches,which was 90,60,60 and 45 minutes,respectively.When examining the effect of carbon source adding methods(1,2 and 4 times)on PD process,it was found that NO2--N accumulation and NTR gradually decreased with the increase of carbon source adding times,which were 43.99,40.98,38.65 mg/L and 60.45%,59.20,57.94%,respectively.The time for the maximum NO2--N accumulation was 60,90 and 120 min when carbon source was added by 1,2 and 4 times,indicating that multiple adding could prolong the peak time of NO2--N accumulation and reduce the consumption of carbon sources.The effect of pH value(6.0,7.5,9.0,10.0)on PD process showed that the best NO2--N accumulation(40.30 mg/L),NTR(82.70%)and NRE(53.58%)were obtained at pH 9.0,followed by pH 7.5,while lower and higher pH conditions(pH=6.0,10.0)inhibited the increase of NO2--N,NTR and NRE.(4)Nitrogen removal performance of PD/A coupling processDuring the stable stage of nitrogen removal performance of PD/A process,the average influent COD/NH4+-N was within the range of 1.96-2.13,and the highest removal rate of NO2--N,NH4+-N were 87.87%,90.77%,where the average TN removal rate and contribution rate of ANAMMOX nitrogen removal reached to 72.25%and 53.85%,respectively.However,the influent COD/NH4+-N was higher than 2.5,which was not conducive to deep nitrogen removal.The denitrification performance deteriorated in the later operation of the PD/A coupled process with NO2～-N,NH4+-N and TN removal rates decreasing to 38.35%,29.15%and 25.91%,resulting in the dominated nitrogen removal by denitrification.When exploring the effect of NLR(0.22,0.46,0.70 kgN/(m3·d))on ANAMMOX,it was found that lower NLR(0.22 kgN/(m3·d))was effective for nitrogen removal and the TN removal rate was up to 83.94%,where ANAMMOX played the main role in nitrogen removal with an average contribution of 70.67%.With the increase of NLR and the disintegration of ANAMMOX granular sludge,the contribution rate of ANAMMOX nitrogen removal decreased from 70.67%to 33.31%and 5.23%,and the balance of denitrification by denitrifying bacteria and AnAOB was destroyed. |
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