Research On The Performance And Mechanism Of Efficient NO₂^--N Accumulation By Sludge Fermentation And Partial Denitrification Process

With the gradual development of economic society,domestic wastewater treatment processes have become more complete,but urban domestic wastewater still often arises from insufficient carbon sources during the treatment process and the remaining sludge after biological treatment is difficult to be resourced and harmless.And the addition of an external carbon source would additionally increase the sludge yield,and it is still customary to separate the biological denitrification from the treatment of sewage and the treatment of the remaining sludge,which not only neglect the potential“energy reservoir”of the remaining sludge but also aggravate the hazard of secondary pollution caused by the remaining sludge in the disposal for subsequent treatment.Therefore,a sludge fermentation coupled short-range denitrification process（sludge fermentation+partial denitrification,SF+PD）was proposed to examine the effect of pre-treatment on the rapid start-up of PD performance of residual sludge in relation to its microbial changes;Changes of long-term operation characteristics and acid producing microorganisms of medium temperature alkaline fermentation system in surplus sludge;And the effect of sludge fermentation broth stepwise replacing traditional carbon source（sodium acetate）on the performance of short-range denitrification system and the dynamic changes of flora.The main research content is as follows:（1）The effect of pre-treatment on the performance and microbial change profile of fast start up of denitrification sludge was studied with the residual sludge as the test sludge and the wastewater was artificially synthesized to simulate municipal wastewater.Control variables were pretreatment hypoxia duration of 0,3,6 and 9 h;Hypoxic NO₂^--N dosing was 5,10,20 and 40 mg/L;p H and dosing frequency difference test:without adjusting p H,dosing buffer solution,after normal operation for 1 h,substrate was re-dosed and p H was adjusted to 9.The results indicated that a certain pre-treatment could activate the short-range denitrification performance of the remaining sludge quickly,and when the anoxic duration of the pre-treatment was increased from 0 h to 9 h,the maximum accumulation of NO₂^--N could be increased from 0.60 mg/L to 4.20 mg/L,and the NTR_max could reach 24.90%after 9 h of anoxic pre-treatment.Low concentration hypoxic NO₂^--N dosing（≤20 mg/L）showed positive effects on PD（maximum NO₂^--N accumulation was 4.92 mg/L）,while whether higher NO₂^--N（>20 mg/L）could be effective still needs further study.Increased p H can also facilitate the initiation process of PD;Hypoxia lasted for 9 h compared with 3 h.at phylum level,Proteobacteria abundance decreased by 1.23%（23.9%）,Bacteroidota abundance increased by 1.52%（19.37%）,Actinobacteriota abundance increased by 2.07%（13.8%）,Acidobacterta abundance decreased by 1.93%（5.77）,and Chloroflexi changed less（18.77%）;Some denitrifier function bacteria genera were also changed by hypoxia lasting 9 h compared to3 h for norank＿f＿＿Saprospiraceae（5.49%）,ottowia（1.19%）and norank＿f＿＿PHOS-HE36（2.53%）Were elevated 0.62%,0.59%,and 0.55%,andidatus＿Microthrix（4.3%）and Thauera（1.12%）were slightly decreased.（2）In order to study the formation characteristics of COD and volatile fatty acids（VFAs）,sludge reduction,and changes in various acid-producing microorganisms in the medium temperature alkaline fermentation system of excess sludge.a long-term experiment of medium temperature alkaline fermentation of excess sludge was conducted using an 8 L SBR fermentation tank.During the experiment,the temperature was controlled at 35℃,the p H was 10,and the daily sludge discharge was 660 m L.The daily sludge inflow was adjusted to maintain the sludge concentration in the fermentation tank at 10±1 g/L.The results showed that during the long-term stable operation of the medium temperature alkaline fermentation system（21-344 days）,the amount of COD generated was between 1500-2600mg/L.Sludge reduction was proportional to the amount of COD generated,and the sludge reduction per cycle was approximately 5.24%.The content of VFAs was approximately 77%of total COD,with acetic acid accounting for 71.54%to 79.57%of VFAs,occupying an absolute advantage,followed by n-valeric acid（11.94%to 15.51%）,indicating that efficient acid production and sludge in situ reduction can be achieved simultaneously during the fermentation process.The enrichment of hydrolytic acidification bacteria such as Corynebacterium（0.01%to6.07%）,Guggenheimella（0 to 4.17%）,and Candidatus＿Microthrix（2.62%to 4.01%）was the reason for the better hydrolytic acid production and sludge reduction performance of the sludge fermentation system.（3）The influence of the sludge fermentation supernatant as carbon source to stepwise replace the traditional carbon source（sodium acetate）process on the performance of partial denitrification（SF+PD）NO₂^--N accumulation and microbial flora was investigated.The C/N was fixed at 5（the influent mixed COD was 150,NO₃^--N was30）,and the supernatant of sludge fermentation was used as carbon source stepwise（the COD of fermentation broth as a proportion of total COD was 0,20,40,60,80 and 100%）to replace CH₃COONa.Experiments showed that the PD system could reduce NO₃^--N with high efficiency in the carbon source replacement process（31-261d）.average r（NO₂^-）was 7.22 mg N/（g SS h）;r（NO₃^-）was 12.61 mg N/（g SS h）;The NTR of the full process was56.38%.High throughput analysis shows that Proteobacteria and Bacteroidota have poor adaptability in the process of carbon source replacement at the microbial phylum level,with little variation in the abundance of Chloroflexi.Acidobacterota and Firmicutes have strong adaptability to sludge fermentation broth.The abundance of denitrification functional bacteria Thauera（3.28%）increased by 2.79%,Hyphomicrobium（0.15%）decreased by 0.93%,Phaeodactylibacter（1.70%）increased by 1.70%,and Deniratisoma（0.09%）decreased by 0.62%.Thauera is the main functional bacterium in the PD process.To explore the effect of sodium acetate directly replaced by fermentation solution on the nitrate reduction performance and microbial changes in PD system.The results are as follows:the average r（NO₂^-）is 9.24 mg N/（g SS h）;r（NO₃^-）was 13.13 mg N/（g SS h）;ΔCOD was between 46.3-65.3 mg/L and NTR was 72.61%.At the level of microorganism,the abundance of Proteobacteria and Bacteroidota decreased,while Chloroflexi（11.04%）had little change.Acidobacteriota（13.21%）increased by 6.67%and Firmicutes（28.10%）increased by 27.55%.Acidobacteriota and Firmicutes showed strong adaptability to sludge fermentation solution.The abundance of dominant denitrification bacteria Thauera（7.81%）increased by 6.89%,Exiguobacterium（1.29%）increased by 1.29%,Phaeodactylibacter（1.13%）increased by 1.13%.The Denitratisoma（0.11%）was reduced by 0.62%.The results showed that Thauera was the main functional bacteria in the nitrate reduction process in the SF+PD system,and the high efficiency accumulation of sodium nitrite could be achieved by the SF+PD process.