Competition of electrogens with methanogens for hydrogen in bioanodes

Bioelectrochemical systems (BES) can provide an energy efficient way to recover nutrients from wastewaters. However, the electron donors available in wastewater are often not sufficient to recover the total amount of nutrients. This work investigates hydrogen (H₂) as an additional substrate for bioanodes. This hydrogen can be produced in the fermentation of complex organic waste or could be recycled from the cathode. Understanding how to influence the competition of electroactive microorganisms (EAM) with methanogens for H₂ gas from different sources is key to successful application of H₂ as additional electron donor in bioelectrochemical nutrient recovery. Ethanol (EtOH) was used as model compound for complex wastewaters since it is fermented into both acetate and H₂. EtOH was efficiently converted into electricity (e^-) by a syntrophic biofilm. Total recovered charge from 1 mM EtOH was 20% higher than for the same amount of acetate. This means that H₂ from EtOH fermentation was converted by EAM into electricity. Low EtOH concentrations (1  mM) led to higher conversion efficiencies into electricity than higher concentrations (5 and 10  mM). Thermodynamic calculations show this correlates with a higher energy gain for electrogens compared to methanogens at low H₂ concentrations. Cumulatively adding 1 mM EtOH without medium exchange (14 times in 14 days) resulted in stable conversion of H₂ to e^- (67%-77% e^-) rather than methane. With H₂ gas as electron donor, 68 ± 2% H₂ was converted into e^- with no carbon source added, and still 53 ± 5% to e^- when 50  mM bicarbonate was provided. These results show that under the provided conditions, electrogens can outcompete methanogens for H₂ as additional electron donor in MECs for nutrient recovery.