Optimising nitrogen recovery from reject water in a 3-chamber bioelectroconcentration cell

With the growing demand for macronutrients, such as nitrogen, and environmental issues related to their production, there is increasing need for efficient nutrient recycling. Reject waters from the dewatering of anaerobically digested sewage sludge are potential sources for nutrient recovery due to their high ammonium nitrogen (NH4-N) concentration (ca. 1 gNH4-N L−1) and low volume (ca. 1% of incoming sewage). In this study, a 3-chamber bioelectroconcentration cell was used for NH4-N recovery into a liquid concentrate from both synthetic and real reject water. NH4-N recovery efficiency and rate were optimised based on NH4-N loading rate, varying from 1.4 to 9.4 gNH4-N L−1 d−1 with synthetic reject water. The obtained NH4-N recovery efficiencies are the highest reported to date for bioelectroconcentration, peaking at 75.5 ± 4.6% (recovery rate of 728 ± 117 gN m−3 d−1) at loading rate 1.9 gNH4-N L−1 d−1. A loading rate of 2.9 gNH4-N L−1 d−1 led to the most optimal ratio between NH4-N recovery efficiency (68.2 ± 11.6%) and recovery rate (965 ± 66 gN m−3 d−1), with NH4-N up-concentrated 7.4 ± 0.9 times to 7483 ± 625 mg L−1 in the concentrate. With real reject water, NH4-N recovery efficiency of 53.2 ± 4.0% and recovery rate of 556 ± 37 gN m−3 d−1 were obtained at loading rate 2.5 gNH4-N L−1 d−1, with a specific energy consumption of 6.1 ± 1.1 kWh kgN−1. 16S rRNA amplicon analysis showed the dominance of phyla Bacteroidetes and Firmicutes in the anodic biofilms, with a significant change in the enriched microbial communities after transitioning from synthetic to real reject water. This study indicates the potential of bioelectroconcentration for nitrogen recovery from reject water without the need for an external organic carbon source or other chemical additions.