A pilot-scale Orbal oxidation ditch was operated for 17 months to optimize nitrogen removal from domestic wastewater of average COD to total nitrogen ratio of 2.7, with particular concern about the roles of dissolved oxygen (DO), mixed liquor suspended solids (MLSS) and return activated sludge (RAS) recycle ratio. Remarkable simultaneous nitrification and denitrification (SND) was observed and mean total nitrogen (TN) removal efficiency up to 72.1% was steadily achieved, at DO concentration in the out, middle and inner channel of 0.1, 0.4 and 0.7 mg/L, respectively, with an average MLSS of 5.5 g/L and RAS recycle ratio of 150%. Although the out channel took the major role in TN removal, the role of middle channel should never be ignored. The denitrification potential could be fully developed under low DO, high MLSS with adequate RAS ratio. The sludge settleability was amazingly improved under low DO operation mode, and some explanations were tried. In addition, a series of simplified batch tests were done to determine whether novel microorganisms could make substantial contribution to the performance of nitrogen removal. The results indicated that the SND observed in this Orbal oxidation ditch was more likely a physical phenomenon.
The study presented the method for isolating the heterotrophic nitrifiers and the characterization of heterotrophic nitrification. When influent Ammonia nitrogen concentration was 42.78-73.62 mg/L. The average ammonia nitrogen removal rate was 81.32% from the bio-ceramics reactor. Sodium acetate and ammonium chloride were used as carbon and nitrogen source. The COD removal rates by microorganisms of strain wgy21 and wgy36 were 56.1% and 45.45%, respectively. The TN removal rates by microorganisms of strain wgy21 and wgy36 were 65.85% and 67.98%, respectively. At the same time, the concentration of ammonium nitrogen was with the removal rates of 75.25% and 84.96%, and it also had the function of producing NO2-N. Sodium acetate and sodium nitrite were used as carbon and nitrogen source. Through the 12 days of the aerobic culture, the COD remoral rates by microorganisms of strain wgy21 and wgy36 were 29.25% and 22.08%, respectively. NO2-N concentration decreased slowly. Comparison, similarity of wgy21 and many Acinetobacter sp.≥99%, similarity of wgy36 and many Acinetobacter sp.≥99%. Refer to routine physiological-biochemical characteristic determination, further evidences showed that wgy21 and wgy36 belong to Acinetobacter sp.,respectively.
