Urine Separation, Nitrification and Sewer-Discharge for Sewer Gas Control – Performance and Adaptation of a Urine Nitrifying Batch Reactor
Authors: Mackey Hamish R. , Chen Guang-Hao 
Published: July 2019

Lack of sufficient oxygen within sewer networks leads to anaerobic bioprocesses occurring, including hydrolysed organic degradation by methanogensis and sulfate reduction. These anaerobic processes produce methane, a greenhouse gas twenty-one times more potent than carbon dioxide; and hydrogen sulfide, a toxic and corrosive gas responsible for severe sewer corrosion and public odour nuisance. These gases are typically controlled by chemically dosing oxygen or nitrate into the sewers. Urine contains roughly 80% of the nitrogen in wastewater and can be easily separated at the household using specialized toilets. If nitrified decentrally it could be discharged to sewer as nitrate to control sewer gas while achieving simultaneous nitrogen removal in the underground sewer network.

In this study a 13.8 L lab-scale urine nitrification sequencing batch reactor was operated for 335 days to assess its performance treating real urine diluted to 30%, a concentration that could be expected from urine-source separating toilets. The reactor had a daily 1 hr anoxic fill and was aerated at 5 L/min for the remainder of the day by coarse bubble diffuser. The influent volume was increased from 1 to 2.65 L/d over the first 170 days of operation. The reactor was settled for 10 mins and decanted once the exchange volume capacity was exceeded, which occurred every 2–5 days depending on influent volume. Alkalinity as NaHCO3 was added stoichiometrically in the influent at a ratio of 1.05 mol:mol-N to allow complete nitrification. Seeding sludge was taken from a municipal wastewater treatment plant.

Between days 227–335 twelve batch tests were done to understand the activity of the various microbial groups including ammonia oxidizing bacteria (AOB), nitrite oxidizing bacteria (NOB) and heterotrophic bacteria (HB) under varying pH (7–9), free ammonia (42–194 mg-N/L) and free nitrous acid concentration (0–0.205 mg-N/L). In these tests a 5 L reactor was operated for 6 hours with approximately 700 mg-VSS/L of biomass measuring changes in ammonia, nitrite, nitrate and chemical oxygen demand (COD) which were used to calculate biomass specific removal/production rates.

The diluted urine in this study had an average total nitrogen concentration of 1790 mg-N/L, a COD of 1460 mg/L and a pH of 9.3. Nitrification was stable throughout the study with a maximum volumetric rate of 450 mg-N/L.d achieved and nitrate as the final nitrification product.

In the batch inhibition testing it was found the optimal pH of the system for ammonia oxidation was at 8.5 with only an 11% reduction in oxidation rate at the highest pH tested of 9. This reflected operational conditions of the reactor where the pH exceeds 9 at the start of a reactor cycle when nitrification begins. Conversely, nitrite oxidation rates were greatest at the lowest tested pH of 7. In a normal reactor cycle ammonia oxidation reduces the pH by consuming alkalinity. Due to the faster growth of AOB and high demand for oxygen the NOB were generally suppressed in a typical cycle until ammonia oxidation was complete, at which time the pH was near 7. This indicated a niche group of organisms adapted to the operational conditions and surrounding microbial community within the reactor.

Under varying free ammonia concentrations it was shown that inhibition for ammonia oxidation and organic oxidation began somewhere near 100 mg-N/L which corresponded roughly with the maximum levels encountered in the reactor. On the other hand free nitrous acid caused a 24% reduction in nitrite oxidation at a concentration of 0.1 mg-N/L.

This was not significant as free nitrous acid in a typical reactor cycle did not exceed 0.04 mg-N/L but could prevent NOB growing if nitrite build-up occurred, in which case concentrations could exceed 0.4 mg-N/L. Organic oxidation was inhibited by free nitrous acid somewhere above 0.1 mg-N/L, but during the initial two hours of a cycle when organics are predominantly degraded free ammonia concentrations are typically less than 0.001 mg-N/L. This study demonstrates the feasibility of decentralized urine nitrification to produce nitrate under stable conditions. The features and ability of the microbial community in such a reactor is strongly associated with the operational conditions imposed.