An Anaerobic Biological Permeable Reactive Barrier For The Remediation Of Nitrate-contaminated Groundwater

In the last decade, there has been an explosion of activities directed at thedevelopment and implementation of a most promising remediation technology–permeable reactive barriers (PRBs). In this study, an in situ anaerobic two-layerpermeable reactive biobarrier system consisting of an oxygen-capturing layerfollowed by a biodegradation layer was designed to evaluate the remediationeffectiveness of nitrate-contaminated groundwater. The first layer containingoxygen-capturing reagent (zero valent iron, ZVI), sodium citrate as well as otherinorganic salts, is used to consume dissolved oxygen (DO) in groundwater in order tocreate an anaerobic condition for the denitrification. Furthermore, it can also providenutrition, such as carbon and phosphorus for the normal metabolism of immobilizeddenitrifying bacteria filled in the second layer. The second layer using granularactivated carbon as microbial carrier is able to biodegrade nitrate entering the barriersystem. Higher removal efficiency may be obtained with immobilized bacteria as aresult of their higher local bacteria concentration.Batch experiments were conducted to identify the effects of DO on thebiodegradation, effectiveness of ZVI capturing oxygen and the characteristics of theprepared oxygen-capturing materials used to stimulate growth of denitrifying bacteria.At DO=0.02mg/L, the biodegradation efficiency of nitrate was the maximum andapproximately85.31%in the batch experiment. When500mg/L ZVI powder wasadded, the DO value was decreased significantly (from6mg/L down to1.61mg/L).Denitrifying bacteria in the reactor which was added the prepared materials grewbetter than the control group which added the mineral medium. The results suggestthat the prepared oxygen-capturing materials can provide an anaerobic condition fordenitrifying bacteria and nutrient for microbial metabolism. A laboratory-scaleexperiment using two continuous upflow stainless-steel columns was then performedto evaluate the feasibility of this designed system. The first column was filled withgranular oxygen-capturing materials prepared by ZVI, sodium citrate as well as otherinorganic salts, etc. The second column was filled with activated carbon immobilizingdenitrifying microbial consortium. Simulated nitrate-contaminated groundwater, inwhich dissolved oxygen content was6mg/L, was pumped into this system at a flow rate of236.75ml/d. Samples from the second column were analyzed for nitrate and itsmajor degradation byproduct–nitrite.Results showed that nitrate could be removed more than94%, and its metabolicintermediate, nitrite, could also be further degraded in this passive system. Furtherstudy is necessary in order to evaluate performance of its field applications.