A novel technology was developed to achieve partial nitrification at moderately low DO and short HRT, which would save the aeration cost and have the capacity to treat a wide range of low-strength real wastewater. The process enables a relatively stable whereas nitrite accumulation rate (NO2-AR) was stabilized over 94% in the last aerobic basin on average of each phase through a combination of short HRT and low DO level. Low DO did not produce sludge with poorer settleability. The morphology and internal structure of the granular sludge was observed by using a scanning electron microscope (SEM) analysis during a long-term operation. The images indicated that thick clusters of spherical cells and small rod-shaped cells (NOB and AOB are rod-shaped to spherical cells) were the dominant population structure, rather than filamentous and other bacteria under a combination of low DO and short HRT, which gives a good indication of nitrite accumulation achievement. MPN method was used to correlate AOB numbers with nutrient removal. It showed that an ammonia-oxidizing bacterium (AOB) was the dominant nitrifying bacteria, whereas high NO2-AR was achieved at AOB number of 5.33×108 cell/g MLSS. Higher pollutant removal efficiency of 86.2%, 98% and 96.1%, for TN, NH4+-N, and TP, respectively, was achieved by a novel six basin activated sludge process (SBASP) at low DO
level and low C/N ratio which were approximately equal to the complete nitrification-denitrification with the addition of sodium acetate (NaAc) at normal DO level of (1.5-2.5 mg/L).
Chinese SEPA, Water and wastewater monitoring methods, 4th Ed., Chinese Environmental Science Publishing House, Beijing, China, 2002
APHA, Standard methods for the examination of water and wastewater, 20ed, American Public Health Association, American Water Works Association and Water Environment Federation, Washington, DC, USA, 1998