Controlled sequential biocathodic denitrification for contaminated groundwater bioremediation

Abstract

Nitrate groundwater contamination is a worldwide concern. In this study, a novel 2-stage, sequential biocathodic denitrification system was tested to perform autotrophic denitrification of synthetic groundwater. The system was operated at different nitrate loading rates (66-301 gNO₃^--N m^-3_NCC d^-1) at constant NO₃^--N concentration (40 mgNO₃^--N L^-1), by varying hydraulic retention time (HRT) during different trials from about 14 to 3 h. The system was able to achieve almost complete removal of nitrate (>95%) and Total Nitrogen (TN) (>92%) at NO₃^- loading rates between 66 and 200 gNO₃^--N m^-3_NCC d^-1. The first stage reactor achieved lower values of effluent nitrate and nitrite than WHO guidelines for drinking water quality (<11.3 mg NO₃^--N L^-1, and 0.9 mgNO₂^--N L^-1, respectively) up to a nitrate loading rate of 167 gNO₃^--N m^-3_NCC d^-1; in these conditions the second stage acted mainly as polishing step. From a loading rate of 200 gNO₃^--N m^-3_NCC d^-1 on, N₂O accumulation was observed in the first stage reactor, afterwards successfully removed in the second stage. Maximum nitrate removal rate of the 2-step process was 259.83 gNO₃^--N m^-3_NCC at HRT of 3.19 h. The specific energy consumption of the system (SEC) decreased with decreasing HRT, both in terms of mass of nitrate removed (SEC_N) and volume treated (SEC_V). The described combination of two bioelectrochemical systems system hence proved to be effective for groundwater denitrification.

Keywords: Autotrophic denitrification; Biocathode; Bioelectrochemical system; Controlled biocathodic denitrification; Groundwater remediation; Resource recovery.