Bio-electrochemical degradability of prospective wastewaters to determine their ammonium recovery potential

Bio-electrochemical ammonium recovery (BEAR) can close the cycle between anthropogenic emission of reactive nitrogen and energy intensive nitrogen fixation in the Haber-Bosch process. BEAR is currently limited by the bio-electrogenic degradability of the treated wastewater. Here, we investigated the degradability of blackwater, hydrolyzed human urine, cow manure and pig manure as prospective wastewaters for BEAR in a standardized experimental design. We found that bio-electrochemical conversion efficiencies ranged from 63% (blackwater) to 42% (cow manure) and 41% (urine) to 26% (pig manure) after 5 days. These values correspond well with the relative VFA content of soluble COD for blackwater and cow manure, while additional compounds must have been converted for urine and pig manure. The degradability of blackwater and cow manure was sufficiently high to theoretically be able to remove all TAN already after < 0.5 d. The actual recovery potential (consisting of conversion efficiency and COD/TAN ratio) of pig manure was just high enough to remove all TAN. Human urine would require additional electron donor to remove all TAN in BEAR. Therefore, combining the maximum recovery potential with the relative VFA content of soluble COD can give a good estimate of the actual recovery potential of a wastewater.