Pilot-scale membrane bioreactor for source-separated urine: Impact of hydraulic retention time on fertiliser production

Prolonged hydraulic retention time (HRT) in urine-treating membrane bioreactors (MBR) remains a challenge as it increases system footprint and costs. This study investigated the effects of HRT conditions in a pilot-scale compact MBR system on urine nitrification performance, aiming to determine the optimal HRT threshold ensuring the effectiveness of the produced liquid fertiliser on hydroponic plant growth. The start-up phase of the MBR successfully achieved stable nitrification at a 7-day HRT under pH-controlled feeding, with a high enrichment of Nitrospira as the predominant nitrite-oxidising bacteria (NOB) and Nitrosococcus as the dominant ammonia-oxidising bacteria (AOB). However, the transition to continuous urine feeding at systematically reducing HRTs of 5 days, 3 days, and 1 day resulted in a decreasing ammonia-to-nitrate conversion rate, dropping from 40 % to 10 % along with a significant nitrite accumulation caused by the high enrichment of AOB over NOB. The urine fertiliser produced under each HRT condition presented distinctive formulations, with a fixed total nitrogen concentration and varying nitrogen species proportions. The fertilisers were applied to hydroponic growth of basil and orchard grass. Both basil and orchard grass showed optimal growth, in terms of roots-to-shoots ratio, at HRTs of up to 5 days. However, orchard grass showed more resilience to the variations in HRT, displaying similar fresh biomass yields across the different conditions. This study offers valuable insights into optimising HRT in urine MBR systems to enhance nutrient recovery as a liquid fertiliser, paving the way for more compact and cost-efficient on-site nutrient recovery and fertiliser application at scale. • Pilot-scale compact MBR system was developed for urine to fertiliser conversion. • pH-based urine feeding achieved stable nitrification with 7-day of HRT. • Systematic HRT reduction led to gradual nitrite accumulation and AOB enrichment. • Hydroponic basil and orchard grass growth was optimal at HRTs of up to 5 days. • Orchard grass tolerated high-nitrite solution, showing potential at suboptimal HRT.