Rejection of nitrogen species in real fresh and hydrolyzed human urine by reverse osmosis and nanofiltration

Urea and unionized ammonia are valuable nitrogen compounds with numerous industrial uses such as fertilizer and diesel exhaust fluid. Human urine is a local source of both urea and ammonia depending on the age of the urine. Nitrogen recovery from urine is possible through membrane separation when the nitrogen rejection is low (i.e., permeation is high) and the rejection of salts and other contaminates is high. Yet, there is limited research on the rejection of urea and unionized ammonia in real human urine by reverse osmosis (RO) and nanofiltration (NF). Therefore, this research investigated the rejection of urea in fresh urine at varying pH and the rejection of unionized ammonia and the ammonium ion in hydrolyzed urine by RO, NF, and microfiltration (MF). The average urea rejection for RO was 57% and pH did not significantly affect rejection. The average urea rejection for NF ranged from 42 to 54% with a significant decrease in rejection at pH 12.5 compared to pH 5. The average ammonia rejection for both RO and NF at pH 6.5 and 9 were 94 and 80%, respectively. At pH 11.5 when all the nitrogen was unionized ammonia, RO rejected 36% of the ammonia and NF rejected 10% of the ammonia without a statistical difference in permeate conductivity. Therefore, when these different membrane separation processes are compared, NF is found to be a promising technology to recover up to 90% of ammonia from hydrolyzed urine with a high rejection of salts and organics.