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The effects of aerobic pH on nitrification in a single-sludge pre-anoxic system treating high ammonia leachate Charlette, Gerard Ian Juan

Abstract

The Environmental Engineering group in the Civil Engineering Department has, for the past two decades, been studying high ammonia leachate that is usually produced during both the early and late stages of sanitary landfills. This research makes use of these past achievements to study the effects of pH on the nitrification process in the aerobic tank at 20°C. The main objective was to understand how nitrification reacted to two series of pH changes, each involving a step up from 7.5 to 8.0, and back to 7.5 and then finally to 7.0. Each of the steps lasted around 10 to 15 days. The aerobic production of nitrous oxide (N₂O) was also monitored. N₂O is a greenhouse gas and is more potent and persistent than carbon dioxide. The pre-anoxic set-up (Modified Ludzack Ettinger process), that was used was made up of a 5L anoxic tank, 10L aerobic and a 4L clarifier. The hydraulic retention time in the anoxic, aerobic and clarifier was 1.9 hours, 3.7 hours and 1.5 hours, respectively. Leachate from the Burns Bog Landfill in the Delta Municipality was fed at 9L/day and supplemented by ammonium chloride to reach a target of 1,200 mg N/L or a load of 10kg N/day. The clarifier recycle ratio was adjusted to 6:1. The aerobic pH was maintained at 7.5 during the acclimatization period by the addition of 80g/L sodium bicarbonate. Upon reaching steady state, the system had an MLSS of around 6,000mg/L, a VSS of 5,500mg/L, and an airflow of 2L/min. The COD loading of 40 to 44g COD/day into the anoxic tank reduced the anoxic NOx to less than 5mg N/L. The ratio of nitrite to nitrates in the aerobic tank was less than 1. As the pH was lowered, it consistently led to lower nitrate concentration in the effluent, but a slightly higher nitrite concentration after 10 days. The nitrite accumulation is consistent with the suggestion that nitrite is taken to the cellular exterior, after it is formed from the oxidation of hydroxylamine. The decrease in nitrate concentration is believed to be due to the lower amount of nitrite being oxidized and a loss of nitrogen in gaseous forms, and not necessarily a decrease in nitrate production. At the three different pH values, there was no significant change in the levels of nitrous oxide that was produced after 10 days, after the pH change was made. When the pH was raised (from 7.5 to 8.0), the nitrous oxide decreased for the first two to three days and when the pH was decreased, the emission of the gas was increased. After steady state had been reached at the three different pH values, the amount of nitrogen entering the system that is converted to nitrous oxide was estimated to be around 20%; this is considerable when compared to reported values of less than 1% during municipal wastewater treatment. Under non-steady state, as would be the case in a full-scale system, gas production reached 40%. Other observations made were an increase in bicarbonate consumption, but a decrease in specific anoxic denitrification rate and nitrification rates. The results indicate that, for a continuous flow treatment process, pH most probably exerts its influence on the dissociation of free nitrous acid (FNA) that can compete with nitrite for the enzyme binding sites. However, during this project, the calculated concentration of FNA, as well as free ammonia, was lower than values at which these two inhibitors have been reported to affect nitrification. It was evident that the system used in this project can withstand pH changes while maintaining almost 100% nitrification and almost 100% anoxic denitrification. However, the influence that the pH change has on nitrite and nitrous oxide release in the environment was not fully addressed in this undertaking and requires further study.

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