[Study on rapid start-up of a nitrifying process using aerobic granular sludge as seed sludge]

Using synthetic ammonia-rich wastewater as influent, rapid start-up of the nitrification reactor was attained in a laboratory-scale column-type sequencing batch reactor (SBR) inoculated with aerobic granular sludge, by gradually increasing the influent NH4(+) -N concentration (100-1000 mg x L(-1)) and decreasing the hydraulic retention time (8-4 h) under mesophilic condition (28-30 degrees C). The influent loading rate of NH4(+) -N reached 3.9 kg x (m3 x d)(-1) and the average ammonia removal efficiency was above 95% within one month. Values of ammonia oxidizing rate (AOR) as high as 5.0 kg x (m3 x d)(-1) was obtained in the following operational stage with extremely high nitrogen loading rate. Nitrite accumulation obviously occurred during the start-up period. The nitrite accumulation rate reached 2-4.5 kg x (m3 x d)(-1) from day 25 to 70. In spite of the change in the feeding composition (COD/N ratio) and the frequent fluctuations of nitrogen loading rate, the granules maintained their structures, with the SVI of 30-40 mL x g(-1). The amount of granules with diameter larger than 0.21 mm was about 93% (mass fraction) of the total on day 36. The granular color changed from yellow to brownish-yellow, and some turned brown in this study. All these results suggested the critical role of aerobic granular sludge as seed sludge for the rapid start-up of nitrifying processes and the formation of nitrifying granules.