Low-energy ammonium recovery by a combined bio-electrochemical and electrochemical system

Bio-electrochemical systems (BESs) can recover ammonium at low specific energy inputs and can produce hydrogen gas. A reason hampering development of large-scale applications for ammonium recovery is the general instability of the bio-generated current and the thereby variable TAN (ammonia and ammonium) effluent concentrations. Electrochemical systems for ammonium recovery, such as the hydrogen recycling electrochemical system (HRES), can fine-tune the applied current, but require higher specific energy inputs than BES, and may require an additional external hydrogen supply in case of HRES. This research presents for the first time an integrated BES and HRES system for ammonium recovery to combine the advantages of both types of systems to achieve high TAN removal efficiency at low specific energy input. The HRES was able to partially compensate for BES instability, which resulted in an overall high TAN removal efficiency (89–95 %) at an increased energy demand of 5.2–10.2 MJ/kg_N. When the BES was performing stably and efficiently, the HRES removed almost all of remaining TAN at up to 99.8 % overall TAN removal efficiency, requiring 9.2 MJ/kg_N. The combined system can remove TAN down to much lower effluent concentrations at little to no additional energy input. These results indicate that combining BES and HRES in one system can result in TAN recovery that is more efficient than in each system separately, which could facilitate new application possibilities for (bio-)electrochemical ammonium recovery