Controlled sequential biocathodic denitrification for contaminated groundwater bioremediation.

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 gNO-N m d) at constant NO-N concentration (40 mgNO-N L), 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 NO loading rates between 66 and 200 gNO-N m d. The first stage reactor achieved lower values of effluent nitrate and nitrite than WHO guidelines for drinking water quality (<11.3 mg NO-N L, and 0.9 mgNO-N L, respectively) up to a nitrate loading rate of 167 gNO-N m d; in these conditions the second stage acted mainly as polishing step. From a loading rate of 200 gNO-N m d on, NO 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 gNO-N m 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) and volume treated (SEC). The described combination of two bioelectrochemical systems system hence proved to be effective for groundwater denitrification.