UV-based advanced oxidation process for nutrient stabilisation and organic micropollutant degradation in source-separated human urine

Abstract

Urine dehydration is one of the technological approach to recover nutrients in concentrated form from source separated urine. When drying fresh urine, nitrogen loss occurs due to hydrolysis of urea into ammonia unless methods to inactivate urease enzyme are employed. In addition, concerns arise when using urine-derived fertiliser due to the potential presence of organic micropollutants (pharmaceuticals). This thesis evaluated ultraviolet (UV) treatment as an alternative chemical-free nutrient stabilisation (urease inactivation) and organic micropollutant (OMP) degradation technology. Urease inactivation and OMP degradation in water and in urine (synthetic urine, real urine from human subjects) were studied in a photoreactor equipped with a low-pressure mercury UV lamp emitting light predominantly at 185 and 254 nm. Exposure of real urine to 80 min of UV irradiation resulted in more than 90% degradation of 18 out of 75 OMPs and 1-90% degradation of the remaining OMPs. Enzymatic activity fell below the detection limit for real urine exposed to 71 min of UV irradiation. However, electrical energy demand for reducing enzymatic activity below the detection limit in real fresh urine was 52-fold higher than for inactivation in synthetic fresh urine (without urea), while electrical energy demand was more than 10-fold higher for 90% OMP degradation in real fresh urine than in water. The inactivation and OMP degradation observed were probably due to direct photolysis and photo-oxidation. Presence of organic substances in real urine was the likely reason for less efficient inactivation of urease and OMP degradation, as such substances can competitively absorb incoming UV light and scavenge the free radicals formed during UV treatment. Although 20% urea was lost after UV treatment, there was no decrease in total nitrogen. In summary, UV treatment can stabilise urea-N and degrade OMPs in fresh urine and has potential for integration into urine diversion sanitation systems.