Optimised start-up strategy for bioelectrochemical systems operating on hydrolysed human urine

Key nutrients, such as nitrogen measured as total ammonium nitrogen (TAN), could be recycled from hydrolysed human urine back to fertiliser use. Bioelectrochemical systems (BESs) are an interesting, low-energy option for realising this. However, the high TAN concentration (> 5 g L^-1) and pH (> 9) of hydrolysed urine can inhibit microbial growth and hinder the enrichment of an electroactive biofilm at the anode. This study investigated a new strategy for bioanode inoculation by mixing real hydrolysed urine with thickened waste activated sludge (TWAS) from a municipal wastewater treatment plant at different volumetric ratios. The addition of TWAS diluted the high TAN concentration of hydrolysed urine (5.2 ± 0.3 g L^-1) to 2.6-5.1 g L^-1, while the pH of the inoculation mixtures remained > 9 and soluble chemical oxygen demand (sCOD) at 5.6-6.7 g L^-1. Despite the high pH, current generation started within 24 h for all reactors, and robust bioanodes tolerant of continuous feeding with undiluted hydrolysed urine were enriched within 11 days of start-up. Current output and Coulombic efficiency decreased with increasing initial hydrolysed urine fraction. The anodes inoculated with the highest sCOD-to-TAN ratio (2.1) performed the best, which suggests that high organics levels can protect microbes from inhibition even at elevated TAN concentrations.