Comammox bacteria are capable of catalysing the full nitrification pathway – oxidation of ammonium to nitrate – and have been encountered in many ecosystems recently (Lawson & Lücker, 2018). What the ecological role of comammox bacteria is in hypoxic enrichment cultures remains unclear. Based on the thermodynamics and biochemistry of known nitrogen cycle conversion, we propose that comammox is oxidizing ammonium to nitrite with both oxygen and nitrate as electron acceptor in hypoxic condition. Our hypothesis suggests that when comammox cooperates with anammox, they can harvest most energy per unit of oxygen supplied. We tried to cultivate bacteria toward a community of anammox and comammox consortium by limiting the oxygen and supplying ammonium and nitrate. Although the predicted optimal state has not been achieved during this work, we did observe that the community indeed developed towards higher consumption of ammonium under limited oxygen supply.

Extracellular polymeric substances (EPS) extracted from aerobic granular sludge (AGS) have recently gained commercial interest as a valuable resource. The alkaline EPS extraction yields roughly 25% of soluble EPS, and about 75% of solid alkaline residual sludge. This study aims to give the first outlook to implement treatment of this alkaline residual sludge with anaerobic digestion (AD) at haloalkaline conditions (pH 9.5, Na+ [0.6 M]). Batch AD enrichment experiments were performed with an inoculum of haloalkaline anaerobic microorganisms, originating from soda lake sediments. The residual sludge was successfully digested and 40-50% of the theoretical production of biogas was obtained. A higher biodegradability was obtained compared to the digestion of AGS at haloalkaline conditions. Due to the alkalinity of the medium, the majority of the CO2 remained in solution, which yielded a high methane content in the gas phase, on average 98%. Environmental factors such as the cation composition of the medium, the salt concentration and substrate preparation showed a negative influence on the biogas yield. In addition, the first implications for continuous AD operation are discussed.