An anaerobic two-layer permeable reactive biobarrier for the remediation of nitrate-contaminated groundwater

Abstract

In this paper, an anaerobic two-layer permeable reactive biobarrier system consisting of an oxygen-capturing layer followed by a biodegradation layer was designed firstly for evaluating the remediation effectiveness of nitrate-contaminated groundwater. The first layer filling with granular oxygen-capturing materials is used to capture dissolved oxygen (DO) in groundwater in order to create an anaerobic condition for the microbial denitrification. Furthermore, it can also provide nutrition, such as carbon and phosphorus, for the normal metabolism of immobilized denitrifying bacteria filled in the second layer. The second layer using granular activated carbon as microbial carrier is able to biodegrade nitrate entering the barrier system. Batch experiments were conducted to identify the effect of DO on microbial denitrification, oxygen-capturing performance of zero valent iron (ZVI) powder and the characteristics of the prepared oxygen-capturing materials used to stimulate growth of denitrifying bacteria. A laboratory-scale experiment using two continuous upflow stainless-steel columns was then performed to evaluate the feasibility of this designed system. The first column was filled with granular oxygen-capturing materials prepared by ZVI powder, sodium citrate as well as other inorganic salts, etc. The second column was filled with activated carbon immobilizing denitrifying microbial consortium. Simulated nitrate-contaminated groundwater (40 mg NO3–N/L, pH 7.0) with 6 mg/L of DO content was pumped into this system at a flow rate of 235 mL/d. Samples from the second column were analyzed for nitrate and its major degradation byproduct. Results showed that nitrate could be removed more than 94%, and its metabolic intermediate, nitrite, could also be biodegraded further in this passive system. Further study is necessary in order to evaluate performance of its field application.