Urea recovery from fresh human urine by forward osmosis and membrane distillation (FO–MD)†
Urea is widely used as fertilizer and has other valuable uses such as diesel exhaust fluid and for resin fabrication. Human urine is a readily available and local source of urea that is overlooked due to the rapid hydrolysis of urea in fresh urine and wastewater, which makes its recovery challenging. Moreover, urea is a compound without an established method for recovery from urine or other waste streams. In this research, a novel two-step process of forward osmosis (FO) and membrane distillation (MD) was developed to recover the urea in fresh human urine. Specifically, FO was used to selectively separate urea from the other components in urine and MD was used to concentrate the separated urea. Five pre-treatment conditions were investigated for urea stabilization. For samples of fresh urine, fresh urine with acetic acid, fresh urine with calcium hydroxide, fresh urine with sodium hydroxide, and synthetic fresh urine with sodium hydroxide, FO recovered 20%, 15%, 12%, 11%, and 21% of the urea in urine, respectively. MD was able to concentrate the product draw solutions from FO containing urea by a factor of 1.9 to 3.3. The combined process was able to produce a product solution that had an average urea concentration that is 45–68% of the urea concentration found in the fresh urine with greater than 90% rejection of TOC. The proof-of-concept study illustrated that FO–MD provides a technology platform for urea recovery from fresh human urine, which currently does not have an established method for recovery.