| Because the typical characteristics of granular sludge are fast settling ability, fine reaction activity, stronger resistant impact load capability and keeping high concentration of biomass in reactor, the advantageous capacity of treatment and better removal efficiency could be achieved, and which make the granular sludge became research hot point. In this study, the granulation, physical and chemical features, denitrification characteristic, distance between bacteria, substrate storage and usage and influence of extracellular polymers （EPS） on granular sludge formation were deeply discussed under anoxic conditions, when SBR was used and fed with synthetic wastewater. The main results and conclusions are as follows.（1） Stirring intensity and precipitation time have effect on granulation process and characteristics of granular sludge. The floc anaerobic sludge was used as seed sludge in this experiment. The granular sludge was observed after 100 days and formed was characteristic with 1.53.0mm in diameter, 54.6% MLVSS in MLSS, 25.9mL/g in SVI, 38.3 m/h in settling velocity, 1.0456×103 kg/m3 in wet density and 93.7% in concentration of water when the speed of agitator was 80 r/min. The granular sludge was observed at about 50th day and was characteristic with 1.52.0mm in diameter, 50.4% MLVSS in MLSS, 18.4mL/g in SVI, 44.9m/h in setting velocity, 1.0509×103 kg/m3 in wet density and 93.7% in concentration of water when the speed of agitator was 150 r/min.（2） Anoxic granular sludge has a better denitrifying activity. The max. specific denitrifying rate （SDNR） was 0.0325 gNOx-N/（g·h） （g,caculated as MLVSS, hereinafter the same） with floc active sludge and the max. SDNR was 0.2230 gNO3--N/（g·h）, 0.1290 gNOx-N/（g·h） and average SDNR was 0.0936NOx-Nmg/L after granular sludge was developed. The anoxic granular sludge also has a strong adaptability to high pH value, and the max. SDNR was 0.1723gNO3--N/（g·h）, 0.0718 gNOx-N/（g·h） when pH value in the effluent was 8.3, and the max. SDNR was 0.0824gNO3--N/（g·h）, 0.0430gNOx-N/（g·h） even though pH value in the effluent was 9.2.（3）The short distance between bacteria of anoxic granular sludge has positive effect on removal of nitrite, denitrification intermediate product, and then higher denitrifying activity and lower concentration of nitrite can be achieved. In tests, the ideal horizontal distance between bacteria of dispersed sludge was 58μm and which is 0.20.5μm for the granular sludge presented on SEM pictures with the same sludge concentration. The accumulating concentration of nitrite by dispersed sludge and granular sludge were 31.36mg/L and 21.98mg/L, and average SDNR of them were 0.0574gNOx-N/（g·h） and 0.0754gNOx-N/（g·h） in contrasted experiment with 80 mgN/L of primary nitrate concentration and 4.5 for C/N. Compare with dispersed sludge, the granular sludge has a relatively high capacity of nitrite reducing. it value was the ratio of nitrite reduced rate to nitrate reduced rate, an efficiency of nitrite reduction. The it value during denitrification shows that average it values of granular sludge were 0.69 and 0.75 and that of dispersed sludge were 0.62 and 0.68 when C/N ratios were 4.5 and 6 respectively. The granular sludge has a high level of nitrite reduction and the higher C/N ratio, the higher nitrite reducing efficiency.（4）There is rapid substrate storage during denitrification by organisms of granular sludge which inhibited the growth of filaments. While the higher C/N ratio and primary concentration of substrate were provided, the more storage would be produced with stronger competitive ability for substrate. When C/N ratio was 5 and primary COD concentration were 200mg/L, 300mg/L and 400mg/L, the max. concentration of poly-β-hydroxybutyric acid （PHB）in sludge were 55.4 mg/L, 66.2 mg/L and 79.6mg/L （PHB, caculated as COD, hereinafter the same）, and average synthesis rate of PHB were 0.251 g/（g·h）, 0.283 g/（g·h） and 0.297 g/（g·h）, the max. ratio of PHB synthesis rate to COD consumption rate （YSTOD） was 0.35. When C/N ratio was 7.5 and primary COD concentration were 300mg/L, 450mg/L and 600mg/L, the max. concentration of PHB in sludge were 125.3 mg/L, 166.8 mg/L and 232.5mg/L, and average synthesis rate of PHB were 0.228 g/（g·h）, 0.245 g/（g·h） and 0.281 g/（g·h） , and the max. YSTOD was 0.45. By comparison with the anaerobic- aerobic granular sludge, the synthesis rate of PHB was higher and the max. YSTOD was lower. When internal PHB was use as sole carbon source for energy supplement, the max. SDNR were 0.0232gNO3--N/（g·h） and 0.0128gNO2--N/（g·h）, which were smaller than that observed with acetate as substrate.（5）In granular sludge, polysaccharide is main component in EPS, accounting for 70.6%79.0% of EPS. The content of protein was lower with accounting for 10%18% of EPS and PS/PN was 4. Compare the content of polysaccharide with that of protein in municipal active sludge, there in a little difference between them and PS/PN was 0.831.14. Polysaccharide in EPS significantly rose during granulation, and protein elevated by 2.2mg/gMLVSS with polysaccharide increasing from 10.36mg/gMLVSS to 23.19mg/gMLVSS. Polysaccharide blocked in the space between the bacteria and stuck them together with loosely bound, which promoted the granulation of active sludge under anoxic condition. |
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