Formation and Stability of Nitrifying Granules under High Loading Rates

Abstract

Abstract. Nitrifying granules are generally believed to have high nitrification ability due to the immobilization of large quantities of nitrifying bacteria with low growth rate. However, high loading rate is normally not recommended for nitrifying granules due to the high inhibition statue. In this study, nitrifying granules, able to treat ammonia nitrogen as high as 1000 mg N/L (2 kg N/m 3 ·d), were cultivated in the laboratory scale sequencing batch reactor. During around 200-day operation, the granules exhibited good performance with 99% ammonia removal efficiency. In the meantime, MLSS increased from initial 6 g/L to the final 10 g/L, and the value of SVI 30 decreased from around 100 to 15 ml/g. However, it was observed that the nitrifying granules gradually disintegrated into small aggregates with mean size decreased from 286 to 138 μm from the operation day 50 to 120, after which they gradually recovered by themselves with increased mean size of 235 μm at the end of the operation. Notably, this disintegration did not compromise performance and characteristics of the nitrifying granules. These results demonstrated that nitrifying granules may have the capability to undertake ammonia loading rates as high as 1000 mg N/L with good self-healing ability, which make the process more stable and promising in practice.