Nutrient recovery from source-separated urine through ammonia stripping prior to membrane bioreactor

The high ammonia concentration in hydrolysed urine presents a major challenge for biological nitrification in membrane bioreactor (MBR) often leading to process instability due to nitrite accumulation and requires extended aeration and long hydraulic retention times (HRTs) compared to conventional wastewater treatment. This study evaluated the effectiveness of ammonia air stripping as a pretreatment strategy to reduce ammonia concentration in hydrolysed urine and thereby improve the subsequent performance of the MBR process. Air stripping was conducted at room temperature and pH 9.5 across a range of air flow rates from 0.5 to 3.0 L/min for this particular lab-scale setup. The optimal ammonia stripping efficiency of 51% was achieved at an air flow rate of 1.0 L/min after 48 hours of air stripping. Ammonia recovery using 0.5 M sulfuric acid was also evaluated, with a flow rate of 0.6 L/min·Lacid achieving up to 98% absorption efficiency, indicating high potential for recovery as ammonium sulphate fertiliser. The lab-scale MBR fed with stripped urine achieved stable nitrification without nitrite accumulation and demonstrated a shortened start-up phase of 44 days with a 40% reduction compared to previous studies using raw hydrolysed urine. During steady-state operation, the system achieved a nitrification rate of 482 ± 71 mgN/L·d, while reducing the HRT to 2.5 ± 0.3 days, representing a 75% reduction compared to MBR with untreated urine. This result significantly outperformed reported benchmarks for full-strength hydrolysed urine treatment. These findings demonstrate that ammonia stripping not only facilitates nutrient recovery but also significantly enhances the operational efficiency and stability of MBR systems treating source-separated urine. This hybrid approach shows strong potential for faster nutrient recovery, and the simultaneous production of two types of fertilisers, a urine-derived liquid fertiliser and ammonium sulphate.