Hydrogen producer microalgae in interaction with hydrogen consumer denitrifiers as a novel strategy for nitrate removal from groundwater and biomass production

Abstract

Low-soluble and hazardous hydrogen gas which is used in current water denitrification processes was substituted by microalgae as a novel approach for biological nitrate removal. Bioremediation of nitrate-contaminated groundwater was evaluated by three batch cultures of hydrogen consumer denitrifiers (HCD), microalgae, and HCD-microalgae consortium at a similar inoculated total biomass of about 0.025 g. Microalgae contained three species of Chlorella vulgaris, Ettlia sp., and Chlamydomonas reinhardtii with an initial cell number of 60 × 104 cell/mL and subsequently C. vulgaris as dominant strain. High nitrate concentration of about 221 mg/L was entirely removed in hydrogen-injected denitrification bioreactor containing HCD in <24 h with N removal rate of 86 mg/L/d. However, it released sCOD into the environment and did not remove phosphorus efficiently. The consortium of HCD and microalgae could offset the lack of hydrogen for dissimilation of nitrate and produced N removal rate of 78 mg/L/d at lowest HCD/microalgae inoculated mass ratio of 0.26. Microalgae played the main role for assimilation of phosphorus and mitigation of the produced sCOD in both photobioreactors containing microalgae alone or their consortium with HCD. The maximum biomass productivity of 0.32 g/L/d and settling efficiency of 0.6 were obtained at highest inoculated HCD/microalgae mass ratio of 2.33. However, the performance of the consortium in terms of nitrate removal was better in lower ratio of HCD/microalgae, which can be applied in large-scale application.