A hybrid nanofiltration and reverse osmosis process for urine treatment: Effect on urea recovery and purity

Human urine can be treated and concentrated using reverse osmosis (RO), but this process also concentrates pharmaceuticals and undesirable salts along with valuable nutrients such as urea. The final fertilizer product, therefore, has limited use on salt-sensitive crops or edible crops as the effects of pharmaceuticals remain a concern. Tight and loose nanofiltration (NF) pre-treatment was investigated as a method to recover urea in the permeate (which could then be further concentrated using RO), whilst pharmaceuticals and undesirable salts would be removed in the brine. Both NF options removed pharmaceuticals (NF90 - 99%, NF270 - 70%). Using a loose NF membrane, 78% of the urea was recovered in the permeate (80% water removal), however, the salt removal was poor (44%), and the urea purity only increased from 37 to 56%. Tight NF membranes provided better rejection of salts and organics (96% and 90% removed respectively). The urea purity in the permeate (75% water removal) from the tight NF process was 89%, however, the urea recovered was lower (48%). The tight NF permeate was then further concentrated with RO. At an overall water removal of 80%, 32.7% of the urea could be recovered with a purity of 85%. A decision tree was also developed to determine the optimum treatment process based on the desired final product. This decision tree could be used to determine the economic feasibility of each treatment process based on the final product choice and product value.