Denitrification Performance By Thermophilic Chelatococcus Daeguensis TAD1and Its Preliminary Study On The Molecular Mechanism Of Aerobic Denitrification

Recently, the pollution of reactive nitrogen is growing in intensify, which includes notonly NO-3-N, NO-2-N and NH+4-N in the water but also NO_x in the gas. Accordingly, how toremove reactive nitrogen has became a research focus. As a green technology with low costand no pollution, denitrificaion by microbial methods has attracted more and more attention.However, the oxygen inhibits denitrification and decreases removal efficiency according tothe traditionary denitrifying theory. As a result, the initial bioreactor all performed under theanaerobic condition, which obviously increased the operation cost and thus limited itsindustrial application on a large scale. In addition, the temperature of some industrial wastewater containing nitrogen and flue gas after scrub achieves to about50℃, but the presentrelative biotechnology functions well at common temperature. Therefore, it has the value ofengineering application to explore the biological nitrogen removal technology at hightemperature. In this study, thermophilic Chelatococcus daeguensis TAD1was used andinoculated to the different bioreactors. Then, the denitrification performances under differentconditions were deeply analyzed and the molecular mechanism that TAD1denitrified underaerobic condition was preliminarily discussed with the aim to provide practice and theoryguidance for the real industrial application.First of all, Plackett-Burman design and response surface method based on centralcomposite design were utilized to optimize the medium for recycling liquid of a bio-tricklingfilter. Results showed that the most important factors affecting denitrification rate wereammonium citrate and ferrous sulfate. With the data subject to regression fitting, thefollowing second-order polynomial equation was obtained, depicting relationship between thedenitrification rate （Y） and variables （concentrations of ammonium citrate （X1） and ferroussulfate （X2））: Y=-2.15+3.13X₁²+204X₂-0.657X₁-1844X₂²+5.68X₁X₂. On the basis of thisequation, extreme points were: ammonium citrate2.62g/L and ferrous sulfate0.059g/L, andthe corresponding predicted maximum denitrification rate by the model was8.0318mg/（L·h）.Then, a lab-scale bio-trickling filter was set up to treat simulated flule gas after theformation of biofilm with TAD1under the open condition using the optimized recyclingliquid. The filter could remove NO_x efficiently under conditions of different loadings and oxygen concentrations. When the inlet NO concentration was600ppm and EBRT was112.5s,the removal efficiency （RE） of NO_x achieved to80.2–92.3%and oxygen had no negativeeffect on RE. Meanwhile, TAD1predominated in the biofilm all the time during the wholeoperation, indicating that it is feasible to treat the real flue gas by TAD1.Thirdly, the denitrification performance of simultaneous nitrification and denitrificationby TAD1at high temperature was analyzed, and then it was applied to an aerobic biofilter.Results showed that TAD1exhibited a high performance of heterotrophic nitrification-aerobicdenitrification in both pure culture and aerobic biofilter. In the pure culture, the eliminatecapacity of NH₄⁺-N got up to6.97mg/（L·h）, and about32.3%of total nitrogen was convertedto nitrogen gas. In the aerobic biofilter, all the nitrogen removal efficiencies of threeconditions after12h reached100%, and the eliminate capacity （average denitrification rate） ofnitrogen was12.67mg/（L·h）、3.62mg/（L·h） and16.53mg/（L·h）, respectively, suggesting thatTAD1has a potential for nitrogen removal from wastewater at high temperature.Fourthly, the best process parameters and conditions affecting denitrification efficiency ina lab-scale bio-trickling filter were further investigated. Thereafter, TAD1was inoculated to apilot-scale bio-trickling filter in a power plant （Guangzhou） using the flue gas afterdesulfuration and dedusting as an object of study. Results revealed that the initial NO₃^-concentration and air flow had no influence on the denitrification, and taken together, sodiumacetate was the optimum carbon source in a lab-scale bio-trickling filter. In a pilot-scalebio-trickling filter, the removal efficiency of NO_x attained to84.3-86.7%. The maximumpollutional loading and processing capacity were159.4g/（m³·h） and137.3g/（m³·h）,respectively, and the corresponding inlet concentration of NO_x was558mg/m³. AlthoughTAD1did not predominate in the biofilm, it coexisted with other microorganisms to removeNO_x together.Finally, expression of four denitrificaiotn genes in TAD1under different levels ofdissolved oxygen （DO） was systematically analyzed by means of real-time fluorescentquantitative PCR, aiming to preliminarily discuss molecular mechanism of aerobicdenitrification by TAD1from gene level at high temperature. Results manifested that genetype of nitrate reductase, nitrite reductase and nitric oxide reductase was napA, nirK andcnorB, respectively, and there was nosZ in TAD1. With DO increasing, expressions of napA, nirK and cnorB all decreased, suggesting that high level of DO exerted an inhibitory effect onexpression of four denitrification genes, whereas oxygen-limited or microaerobic environmentprobably more favored gene expressions.