Mechanism And Optimization Of Nitrogen Removal Treating Landfill Leachate Based On Denitrification Functional Gene

Landfill leachates with cocktail of components are difficult to treat. Their high organic ammonia contents usually frustrate conventional biological denitrification process. This research mainly studied the mechanisms and optimization strategies regarding nitrogen removal of landfill leachate via landfill bioreactor and two-stage systems of A/O. The distribution characteristics of functional genes including ammonia oxidizing archaea and ammonia oxidizing bacteria (amoA), anaerobic ammonium oxidation bacteria (hzsA) and denitrifying bacteria (nirS, nirK and nosZ,) in different operational conditions. During the periods of whole experiments, study different organic loading rates, BOD/TN ratio and NO2--N/NH4+-N ratio, normal operation and two-stage system of A/O suffered the influent load. The discussion of denitrification mechanism of landfill bioreactor can provide optimization strategies for nitrogen removal process. The main conclusions are:(1) Landfill bioreactor denitrification system(1) Landfill bioreactor had higher level of AOA than AOB. Their highest ratio (AOA/AOB) of 125.82 indicated that the AOA was more advantageous over AOB in functioning nitrification in landfill bioreactor. Likewise, higher level of nirK, compared to nirS in most reactors could imply that nirK is predominant in denitrifying bacteria. The highest abundance of nosZ and hzsA gene was detected as 1.63%.and 27.7% respectively in our bioreactors where ANAMMOX belonged to Candidatus Kuenenia stuttgartiensis.(2) When the organic load was 0.04 kg/(m3, d), BOD/TN was 0.2 and NO2--N/NH4+-N was 1.5, the the sum of hzsA, nirS and nirK was the highest, up to 30.2%, and total nitrogen removal efficiency reached to 92.66%. This indicates the existence of synergic effects of denitrification and ANAMMOX. The correlation analysis shows that BOD/TN significantly affected the ANAMMOX (hzsA), while the conventional denitrification (levels of nirK and nosZ) were correlated with the ratio of NO2--N/NH4+-N. and AOA were negatively correlated with both BOD/TN and NO2--N/NH4+-N. Among these genes, hzsA was analyzed as the only determinant of nitrate and total nitrogen removal rates (p< 0.05). This imply that ANAMMOX was an important nitrogen pathway in the landfill bioreactor.(3) The landfill bioreactor inoculated sludge shows the average removal rates of ammonia nitrogen and total nitrogen are 89.58% and 86.29%, higher than that of uninoculated reactor by 8.38%and 8.19%. The molecular tests shows that average level of AOB-amoA and nirS was 1.61 x 103 copies/ngDNA and 2.36 x 103 copies/ngDNA respectively, higher than uninoculated reactor by 150% and 66%. The highest abundance of nirS, nirK, nosZ were 4.88%,7.46% and 0.96% respectively. Average of nirS+nirK in inoculated reactor was 6.1 x 103 copies/ngDNA, higher than that of uninoculated reactor by 3%. And meanwhile the inoculated bioreactor shows increased nirS, which could provide oxidant (NO) for Candidatus Kuenenia stuttgartiensis during the ANAMMOX process.(II)The two-stage of A/O denitrification system(1) AOB-amoA gene ranges from 1.06 x 103-2.17 x103 copies/ngDNA. AOA-amoA genes and hzsA was three magnitude orders lower than AOB-amoA. The abundance of denitrification functional genes-nirS was highest, up to 19.62%,the maximum ratio to nirK was 26.25. The highest abundance of nosZ gene is 1.76%.(2) The abundance of AOB-amoA gene (2.17 x 103-1.04 x 104 copies/ngDNA) increased to 9.19% after the influent loading shocks. During this period, the ammonia content in effluent was kept stable and lower than discharge standard. However, the level of nirS,nirK,nosZ ranged from 6.97 x 103-9.61 x 103 copies/ngDNA, and their relative abundance reduced by 10.89%,10.89% and 1.35%, respectively. This probably led to the high content of nitrate nitrogen and total nitrogen in effluent.To sum up, by controlling the water quality, coexistence of denitrification and anaerobic ammonia oxidation (mainly by Candidatus Kuenenia stuttgartiensis)can be achieved in landfill bioreactors. Sludge inoculation can speed up the startup of bioreactors, and improve the nitrogen bioremoval efficiency. When the A/O system treating leachate suffered the influent shocks, the nitrification functional genes increased, but denitrification functional genes decreased significantly, which made the denitrification system underperformance.