Pilot-scale demonstration and feasibility study of nutrient recovery from human urine in a membrane bioreactor under acidic pH and elevated MLSS conditions

Membrane bioreactors (MBRs) that combine the stabilisation of human urine with nutrient recovery offer a promising strategy for producing renewable fertilisers towards sustainable agriculture. Nonetheless, MBRs employed in the biological nitrification of human urine can be disrupted by adverse environmental conditions that affect sensitive autotrophic nitrifiers, such as acidic pH and oxygen competition mechanisms with heterotrophic bacteria. To overcome these challenges, this study investigated the performance of a pilot-scale MBR for nutrient recovery from human urine via biological nitrification, operated at controlled acidic pH (5.75-5.8) and high mixed liquor suspended solids (15.3 g⋅L -1 ) and mixed liquor volatile suspended solids (13.8 g⋅L -1 ) under fixed dissolved oxygen concentration (4.5 mg⋅L -1 ). The membrane bioreactor exhibited good stability within this operating envelope, achieving a maximum nitrification rate of 26.8 mgN⋅gMLVSS -1 ⋅d -1 while removing up to 89% of total organic carbon (TOC). Taxonomic and predicted functional profiling based on 16S rRNA gene data revealed that ammonia oxidising bacteria (AOB) Candidatus Nitrosoglobus outcompeted Nitrosomonas during operation while the Nitrospira genus dominated the nitrite oxidising bacteria (NOB) community. Biological nitrification conducted under challenging conditions correlated with a high NOB:AOB ratio of 9.9 at steady state, which was reflected by a predicted prevalence of nitrite oxidoreductase genes and the plausible onset of a nitrite loop. The nutrient-rich solution obtained from the MBR permeate, containing essential macronutrients and protein-rich organic fractions, was investigated on the hydroponic growth of Dactylis glomerata. The urine-derived nutrient solutions presenting the highest nitrogen content across the series of treatments enhanced the wet shoot yield by 18% compared to a commercial hydroponic fertiliser, demonstrating that MBRs can efficiently transform human waste into a value-added product under a broad range of conditions. This study served as an essential demonstration step for the recovery of nutrients from human urine in a pilot-scale MBR operation, supporting the future deployment of such technology in applications that can contribute towards a circular economy of nutrients.