Ammonia volatilisation and solids formation during urine transport in pipelines

Processed urine is suitable for use as fertiliser, but vehicular transportation of urine from the source to the processing facility for further processing is costly. Pipeline transport offers a potential way to reduce these costs. Still, ammonia volatilisation and solids deposition during pipeline transport of urine present challenges, and the extent of these challenges during both short and long pipeline transport remains uncertain. This study investigated ammonia volatilisation and solids formation during pipeline transport of fresh urine (FU), partially hydrolysed urine (PHU), and hydrolysed urine (HU) over 5 to 291 km of time velocity equivalent conveyance in a lab-scale recirculated sewer-like system. Foam developed extensively on the surface of all types of urine within 4 h of the experiment. This foam layer appeared to act as a barrier that suppressed ammonia volatilisation. Compared with the pre-foam phase, ammonia volatilisation rate was reduced by 86% in PHU and 60% in HU during the first 5 km, and by up to 99% during the remaining 291 km. Ammonia volatilisation in FU was initially negligible, but increased as urea hydrolysis progressed, eventually reaching rates comparable to PHU and HU. Overall, with foam present, ammonia loss remained below 3% during 5 km and below 15% after 296 km of conveyance. FU and PHU produced large amounts of solids that deposited and adhered to pipe surfaces, whereas HU produced minimal solids. These findings suggest that allowing urine to fully hydrolyse before pipeline transport may be beneficial, provided that foam is effectively utilised and managed.