Urine dilution with a synthetic wastewater (Syntho) boosts the electricity production in a bio-electrochemical system powered by un-pretreated human urine

[Display omitted] •Anodic current densities of 0.1 A.m-2 were obtained from pure un-pretreated urine.•Precipitation of salts on the anode and high FAN levels inhibited electro-active response.•Acclimation with un-pretreated urine was not useful to increase bacterial electro-activity.•Dilution of un-pretreated urine allowed avoiding microbial inhibition events.•Anodic current densities of 3.6 A.m-2 were obtained by diluting un-pretreated urine. Human urine can be turned into electricity in bio-electrochemical systems. The acclimation of electro-active bacteria to culture media with increasing urine concentrations has led to raising the obtained current densities, which typically followed a Monod-like evolution profile as a function of urine concentration. However, the acclimation protocol has been so far evaluated using pretreated urine samples (fermented or precipitated), not raw (un-pretreated) urine. We demonstrate that, when un-pretreated urine is used, the microbial adaptation to increasingly concentrated urine leads to a current density profile that does not reach a saturation-like phase, but follows a Han/Levenspiel-type trend (bell-shaped). By diluting un-pretreated urine with a synthetic domestic wastewater (Syntho) up to concentrations matching those of the maximum in the Han/Levenspiel-like current profile (15–20% v/v) it is possible to avoid the drop in the electro-active response, generating anodic current densities as high as 3.6 ± 0.2 A.m−2 (per actual surface area), 35-fold higher than those reached in pure un-pretreated urine.