Energetic valorization of ammonium resulting from nitrate electrochemical reduction—Feasibility of biohydrogen production

• The feasibility of coupling electrochemical and biological processes to treat concentrated nitrate solutions was examined.
• Selective and quantitative electroreduction of NO3− to NH4+ was shown.
• The produced NH4+ was then used as a nitrogen source in anaerobic batch culture of heat treated activated sludge.
• Complete assimilation of the obtained NH4+ with production of an acceptable H2 yield (0.35 mol H2 mol−1 glucose) was recorded.
• Environmentally friendly coupled process was developed.

The main objective of this work was to examine the feasibility of coupling electrochemical and biological processes to destroy nitrate ions (NO3−) while producing biohydrogen. In this integrated process NO3− was firstly converted to ammonium using an electrochemical flow cell. After only one pass of concentrated nitrate solutions (3 g NO3− L−1) through the flow cell, ammonium ions selectivity of 98.8%, corresponding to 0.86 g NH4+ L−1 was recorded. The obtained ammonium solution was then tested as a nitrogen source to produce H2 in a batch system involving heat-treated aerobic activated sludge.In the optimal conditions corresponding to pH 5.5 and initial glucose concentration of 15 g L−1, consumption yields were 97% and 82% for ammonium and glucose, leading to H2 yield of 0.35 mol H2 mol−1 glucose consumed. The H2 production was associated with acetic/butyric acids type fermentation. Obtained biogas contains only H2 and CO2 and was free of methane, hydrogen sulphide, and nitrous oxide. Therefore, the targeted objectives were achieved since on the one hand selective and quantitative conversion of NO3− to NH4+ was shown and on the other hand the obtained NH4+ was completely assimilated by activated sludge with the production of biohydrogen, a clean energy carrier.