Strategies for achieving partial nitrification by immobilized cells

Abstract

Shortcut biological nitrogen removal (SBNR) is a cost effective innovative process to treat low carbon or/and high nitrogen wastewater. Partial nitrification (PN) is believed to be the rate-limiting step of the overall SBNR and can be achieved by the oxidation of ammonia (NH₃) to nitrite (NO₂⁻) (or nitritation) without further oxidation of NO₂- to nitrate (or nitratation). The most two common strategies to promote activity of ammonia oxidizing bacteria (AOB) over nitrite oxidizing bacteria (NOB) under normal temperature condition are to maintain oxygen (O₂)-limiting or/and free ammonia (FA)-accumulating conditions in the systems. The study was divided into three main tasks. Task 1 is to examine whether and how the two most common strategies can be applied for entrapped cell system. Results from batch nitritation and nitratation kinetic study implied that FA inhibition or O₂ limitation can be used to maintain PN in entrapped cell but might not be effective strategy. Task 2 is to find out the strategies to achieve PN in continuous-flow entrapped cell nitritation reactors. This part of experiment was sectioned into two subtasks, the first subtask is to study the effect of different entrapped inoculums on accelerating PN during start-up periods. The results showed that high NOB entrapped cells inoculums, which has different ability to nitrifying and partial nitrifying, can achieve the stable PN at comparable level and timeframe (65 – 66% nitritation after 30 - 42 days of the start-up). This indicated that a step for preparing sludge which is readily for nitrifying or partial nitrifying, was not needed for entrapped cells. The control factor is expected to be the levels of O₂ in the gel beads under the presence of high NH₃ concentration. Therefore, cell entrapment can be an effective way to accelerate partial nitrification. The second subtask is to study effect of bulk dissolved oxygen (DO) or/and FA concentrations on PN during the long term operation period. Higher NO₂⁻ accumulation was found at the lower concentration of bulk DO and the higher concentration of FA. Because the accumulation of NO₂⁻ depended on both concentrations of bulk DO and FA, a relative ratio of both parameters (ratio of DO/effluent FA) rather than either one is recommended to use as a control parameter for PN. Task 3 is to study effect of heterotrophs on the activity of AOB in entrapped cell under condition simulated the shock load of a model toxic chemical, p-nitrophenol (PNP). Two sequentially tests, 1st and 2nd batch test, were used to investigate the PNP degradation and NH₃ oxidation under condition simulated the 1st and 2nd time of PNP shock load in batch reactor with nitritation entrapped cells which have a different amounts of heterotrophs in the gel beads. Results from task 3 (and a part of the results for suspended cells from task 1) implied that an inability to recover the AOB activity after experience with the toxic PNP shock can be partly prevented in the entrapped cell based-rather than the suspended cell based-reactor as a results from reducing the penetration of PNP by the outer layer of heterotrophs and subsequently reduce toxic sensitivity of AOB in the deeper part of the biofilm-like layer structure in the peripheral of the gel bead.