Study of startup of aerobic granular system and characteristics of granules

The investigation was engaged in the performances of startup of aerobic granular system and characteristics of the granules under hydraulic selection pressure and microbial selection pressure by combining action of stepwise shortening settling time and increasing nitrogen loading rate. The results show that the mean size of granules during the system startup reaches 0.5 mm in 36 days, and then increases to 0.9 mm in 90 days. The average granular size growth rate is about 9.3 μm/d. The apparent sludge yield increases from 0.40 to 0.52 g · g-1 when the sludge loading rate is between 0.4 and 0.6 g · g-1 · d-1 in 53 days operation. Thereafter, due to great increase of biomass concentration, the apparent sludge yield decreases to the range of 0.17 to 0.25 g · g-1 as the sludge loading rate is about from 0.2 to 0.4 g · g-1 · d-1. After 40 days of culture, the sludge volume index maintains between 10 and 20 mL/g with excellent granular settleability. Good nitrification metabolic ability is achieved with NH4+-N removal efficiency above 98% after 10 days. The removal efficiency of COD maintains around 80%. Meanwhile, NH4+-N metabolic rate of the granules increases with nitrogen loading rate. The activities of heterotrophs, AOB and NOB also increase with nitrogen loading rate.