Nitrification and microalgae cultivation for two-stage biological nutrient valorization from source separated urine

[Display omitted] •Full nitrification of undiluted urine at a conductivity of 75mScm−1 was obtained.•A halotolerant inoculum cut start-up time by 54% compared to sewage activated sludge.•Nitrite oxidation showed faster salt adaptation than ammonia oxidation.•Nitrobacter spp. became the dominant nitrite oxidizers in both nitrification systems.•Arthrospira platensis growth on nitrified urine led to a 62% biomass protein content. Urine contains the majority of nutrients in urban wastewaters and is an ideal nutrient recovery target. In this study, stabilization of real undiluted urine through nitrification and subsequent microalgae cultivation were explored as strategy for biological nutrient recovery. A nitrifying inoculum screening revealed a commercial aquaculture inoculum to have the highest halotolerance. This inoculum was compared with municipal activated sludge for the start-up of two nitrification membrane bioreactors. Complete nitrification of undiluted urine was achieved in both systems at a conductivity of 75mScm−1 and loading rate above 450mgNL−1d−1. The halotolerant inoculum shortened the start-up time with 54%. Nitrite oxidizers showed faster salt adaptation and Nitrobacter spp. became the dominant nitrite oxidizers. Nitrified urine as growth medium for Arthrospira platensis demonstrated superior growth compared to untreated urine and resulted in a high protein content of 62%. This two-stage strategy is therefore a promising approach for biological nutrient recovery.