Organics Removal and Nitrification

The feasibility of simultaneous removal of organics and nitrogen by microbial granules was investigated by Yang et al. (2003). Figure 8.8 shows the profiles of COD concentration and nitrification in the reactor operated at various substrate N/COD ratios. The salient points of the data are that (i) almost all influent COD is removed in the first hour; (ii) no nitrite and nitrate are produced in the reactor run at a substrate N/COD ratio of 5/100, while typical nitrification profiles were observed in the reactors operated at a respective substrate N/COD ratio of 10/100, 20/100, and 30/100; (iii) a complete nitrification occurred after the COD removal; (iv) ammonium-nitrogen removal in the first hour of the cycle was the result of microbial growth requirement for nitrogen source instead of nitrification because neither nitrite or nitrate was produced in this period; (v) no lag nitrate production with respect to nitrite formation was observed.

Basically, nitrification is completed by two kinds of bacteria, namely ammonia oxidizers responsible for nitrite formation, and nitrite oxidizers for converting nitrite to nitrate. The biological oxidation sequence can be simplified to two consecutive reactions:

It should be realized that nitrite is an intermediate of nitrification process. The complete nitrification observed in Fig. 8.8 indicates that both ammonia oxidizer and nitrite oxidizer present sufficiently in the aerobic granules. According to Liu and Tay (2001), at least three factors would influence the nitrification profiles: (i) the relative specific growth rate of ammonia oxidizer and nitrite oxidizer in the aerobic granules; (ii) the relative ratio

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Fig. 8.8. COD and nitrogen profiles in microbial granular sludge SBRs operated at the substrate N/COD ratios of 5/100-30/100 (Yang et al., 2003).

between ammonia oxidizers and nitrite oxidizers in the aerobic granules; and (iii) the level of free ammonia. It appears that the ammonia oxidation rate is proportionally related to the substrate N/COD ratios, i.e. the higher the substrate N/COD ratio, faster is the fall of ammonium-N concentration.

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