Woodchip-based Solid-phase Denitrification Bioreactor For Nitrate-contaminated Water Treatment

Nitrate?NO₃^-?is one of the intractable contaminants in water environment.Excessive NO₃^-released into the environment has potential hazard to human and has attracted the great attention.Furthermore,solid-phase denitrification?SPD?has widely used to remove NO₃^-in low organic carbon water as its promising,environmentally-friendly and efficient technology with low operation and maintenance costs.In this study,woodchips were used as the solid carbon source for biological denitrification of nitrate-contaminated low organic carbon water such as groundwater and sewage treatment plant secondary effluent.The performance of woodchips as a denitrifying carbon source was evaluated through batch and column experiments.The NO₃^-removal pathway and microorganisms change along with the height of an up-flow woodchip-based solid-phase denitrification?W-SPD?bioreactor was investigated to give insight into remediation process and mechanism inside the SPD bioreactor.Based on the studies of W-SPD bioreactor,the carbon gradient-filling SPD bioreactor has been developed to treat nitrate-contaminated low organic carbon water,which provided the theoretical reference for biological denitrification of the low organic carbon water and the new ways for the construction of SPD bioreactor.The results showed that:Woodchips as denitrification carbon sources could achieve complete denitrification.It is conducive to acclimate the simultaneous reduction of NO₃^-and nitrite?NO₂^-?environment,reducing the accumulation of NO₂^-in the system,thereby reducing the risk of the system.The release of total organic carbon?TOC?from woodchips denitrification process depends on microbial action,water-solubility occurs only in the presence of a TOC diffusion gradient.W-SPD bioreactor could realize efficient NO₃^-removal?92.5%-96.4%?without TOC and NH₄⁺-N accumulation as long as the reaction height was appropriate.In other words,no secondary pollution could be achieved as long as the influent flow rate matched the bioreactor height of the reactor.The NO₃^-removal rate matched for zero-order?R²>0.97?and first-order?R²>0.94?combination Michaelis-Menten kinetics,whereas microbial reaction rate suited for modified logistic model?R²>0.99?.The NO₃^-removal rate and the microbial reaction rate firstly increased and then decreased along the water flowing direction in the W-SPD bioreactor,and it was further confirmed by the microorganisms'quantity and their activity analysis.The aerobic bacteria,denitrifiers,carbonaceous compound degrading bacteria and fermentative bacteria co-existed in W-SPD system,which was vital for efficiently sustainable NO₃^-removal.Hence,aerobic degradation,heterotrophic denitrification and dissimilatory NO₃^-reduction to ammonium?DNRA?occurred successively along the water flowing direction in the bioreactor.Compared to uniform-filling method,the carbon source gradient-filling method enhances the NO₃^-removal rate at the inlet of the SPD bioreactor.The NO₃^-loading ranged from 7.78±0.10 to 47.82±0.53 g N·m^-3·d^-1,the NO₃^-removal rate of the gradient-filling bioreactor is approximately 1.40-1.56 times that of the uniform-filling bioreactor.The gradient-filling bioreactor can achieve low NO₂^-and TOC accumulation,thus reducing NO₂^-and TOC accumulated secondary pollution risks.