Nitrogen removal by combined nitritation-anammox process in an upflow anaerobic sludge blanket (UASB) reactor
Anaerobic ammonium oxidation (anammox) process, where anammox bacteria convert ammonium to N2 with nitrite as the terminal electron acceptor in the absence of O2, has been extensively studied since its discovery in the late 1990s. The combined nitritation-anammox process represents a promising innovative biological nitrogen removal technology, especially for treating wastewater with low chemical oxygen demand (COD)/ammonium ratio. Due to the low growth rates of both aerobic and anaerobic ammonium-oxidizing bacteria (i.e. AOB and anammox bacteria), efficient biomass retention is critical for successful reactor start-up and stable operation.
In this thesis, two similar laboratory-scale upflow anaerobic sludge blanket (UASB) reactors have been developed for cultivating anammox bacteria under complete absence of oxygen (UASB #1), and nitritation-anammox biomass with low oxygen levels (UASB #2), respectively. The ratio of influent NO2--N to NH4+-N was optimized to evaluate the long-term performance of the reactor. The observed NO2--N to NH4+-N ratios under different influent NO2--N to NH4+-N ratios did not agree with the proposed ratio of 1.32 (Eq. (1.3)), but showed a positive correlation with influent ratios. Synthetic wastewater with a NO2--N/NH4+-N ratio of 1.2 achieved the highest total nitrogen removal efficiency of 96-97%. The average ratio of observed NO3--N/NH4+-N was much smaller than proposed ratio of 0.256.
The nitritation-anammox reactor operated under the low oxygen level (≤0.5 mg/L) for over 250 days was able to remove more than 85% of the supplied total nitrogen loads without nitrite accumulation. The nitritation-anammox granules were successfully enriched with typical colors of brown-yellow, reddish, light red, red, etc., depending on the microbial community compositions. Granules with porous structures had a mean diameter of 3 mm and featured good settling ability. The microbial community compositions in UASB #2 reactor, investigated by fluorescence in situ hybridization (FISH), showed the coexistence of AOB and anammox bacteria in the granules. In addition, the two groups of bacteria exhibited an overlapping growth style, which can improve the availability of ammonium for anammox bacteria and facilitate the immediate consumption of the nitrite produced by AOB by anammox bacteria. FISH results also proved that most nitrite oxidizing bacteria were eliminated under high temperature and oxygen-limiting conditions.