Nitrification of source-separated human urine to nitrate for discharging into sewers would provide a cost-effective option for inducing in-sewer denitrification as well as the control of sulfide production responsible for sewer odor and corrosion. Besides, human urine accounts for 75% of the nitrogen and 50% of the phosphorus in nutrient loadings for sewage treatment plant. Therefore, on-site nitrification of source-separated urine not only greatly reduces the nitrogen loadings on municipal sewage treatment works, but also saves substantial amount of nitrate dosing cost for controlling the hydrogen sulfide production in sewers....[ Read more ]

Nitrification of source-separated human urine to nitrate for discharging into sewers would provide a cost-effective option for inducing in-sewer denitrification as well as the control of sulfide production responsible for sewer odor and corrosion. Besides, human urine accounts for 75% of the nitrogen and 50% of the phosphorus in nutrient loadings for sewage treatment plant. Therefore, on-site nitrification of source-separated urine not only greatly reduces the nitrogen loadings on municipal sewage treatment works, but also saves substantial amount of nitrate dosing cost for controlling the hydrogen sulfide production in sewers.

To realize this option, full urine nitrification, namely nitrifying 100% of the urine nitrogen to nitrate, is essential. No attempt was made in the full nitrification of human urine. This study was therefore motivated to investigate the possibility and performance of the full nitrification of human urine in a sequencing batch reactor (SBR).

Human urine was collected from a specially designed non-flushing toilet serving students of the Hong Kong University of Science and Technology. Strategic cultivation of urine nitrifying sludge was adopted through synthetic urine-based inoculums, followed by progressive cultivation of sludge in a 3-L lab-scale SBR with human urine.

Full urine nitrification was achieved in the SBR after a transitional period of 24 days. At a steady state, 89.9% COD and 99.7% ammonium nitrogen in the urine were removed at the COD and TKN loading rates of 1.2 kg/m³/d and 1.1 kg N/m³/d, respectively. Benefiting from the multiple-feeding pattern of the reactor, inhibition of free ammonia (FA) or free nitrous ammonium (FNA) on the nitrification activity became insignificant with around 3 times diluted urine.

It was found that urine itself provided around 41% of the theoretical alkalinity required for the full urine nitrification process. The nitrogen content in the SBR sludge was found slightly higher than conventional activated sludge. Active nitrifying bacteria accounted for 9% of the total biomass in the SBR sludge, much higher than that in conventional activated sludge (only 3%). The SBR sludge was formed by dense and thick sludge flocs and around 10% of aerobic granules (around 1 mm in diameter), resulting in an excellent settleability (SVI₅=40).

Keywords: Human urine; full urine nitrification; nitrifying bacteria; aerobic granular sludge; sequencing batch reactor.