Study Of The Factors Affecting The Competition Between Floc-Formers And Filaments

The fact of filamentous bulking is the unbalance competition between floc-forming bacteria and filamentous bacteria in the activated sludge. The severe proliferation of filamentous would cause poor sludge settleability. However, if filamentous bacterias is defficient, the actived sludge will be lack of skeleton and become pin-point flocs. The turbidity of the supernatant is high. Recently, some research show that low DO could lead to limited filamentous bulking, which didn't affect the separation in secondary clarifiers. In addition, the capture mechanism of filaments during the settling period makes the supernatant more transparent. Meanwhile, low DO can save energy. Based on these phenomenons, Prof. Yongzhen Peng proposed"Energy saving achieved by limited filamentous bulking under low dissolved oxygen (DO)". Again, the key of this technology was keeping the balance of the competition between floc-formers and filaments. It is significant to further the study of the competition between floc-formers and filaments under different environment.The first part of this thesis studied the competition of floc-forming bacteria and filamentous bacteria. According to the pricinple factors of filamtous bulking, mechanisms of bulking in different environmental were investigated and the dominate filaments were indentified. Based on these studies, the second part of this thesis analyzed the feasibility of limited filamentous bulking under different operational remiges. The main research methods and results are as follows:1. Two groups of sequencing batch reactors (SBRs), adopting fully aerobic and anoxic-oxic operational regimes respectively, were employed to analyze the effects of operational remiges on the competition of floc-forming bacteria and filamentous bacteria. Fully aerobic system easily led to bulking. Under low substrate gradient, sludge was bulking rapidly however the DO concentration was, The bulking rates were more than 34.0 mL g-1d-1 in both SBRs. Sludge volume index (SVI) increased to 1000 mL/g within 2 sludge rentation time (SRT). Severe sludge bulking also occured in the environment of high substrate gradient with low DO, although the bulking rate was relatively slow. Sludge settleability was good under high gradient with normal DO, but was vulnerable to bulk in organic shocking. Anoxic-oxic operational mode, simulating anoxic selectors, had a significant effect to enhance sludge settleability. In the environment of high substrate gradient with low DO, sludge settleability was good, SVI was about 50 ~ 60 mL/g. Stable limited bulking could be achieved by appropriately decreasing substrate gradient under low DO. The SVI was between 120 ~ 220 mL/g. 2. The differences and similarities of bulking rate under different operational regimes were analyzed based on the level of substrate consumption and supply rate. A new hypothesis—the consumption-supply rate hypothesis was proposed. That is, for the bulking caused by improper DO or organic loading rate, two necessary conditions should be met: (1) microorganisms within the floc uptake organic substrate or DO, and the corresponding floc external supply are available; (2) the comsumption rate is less than the supply rate.3. In the environment of single-aerobic stage with low substrate gradient, two reactors using acetate and glucose as carbon source respectively were bulking severely both; the other reactor, using starch as carbon source, was not bulking severely (SVI remained at 300 mL/g or less) although a substantial number of filamentous bacteria proliferated. As to the bulking rate, Racetate > Rglucose > Rstarch. This result confirmed our consumption-supply rate hypothesis.4. In the environment of anoxic-oxic mode with high substrate gradient under low DO, separate N or P deficiency did not lead to bulking within 3 SRT. Non-filamentous bulking occurred in 2 SRT when the influent converts from P deficiency to P free; sludge settleability in N free system was also good. As to the substrate storage capacities, the N deficiency system was higher than the P deficiency system. And they decreased when N or P was empty.5. The floc of limited filamtous bulking sludge was relatively small. Filamentous bacteria linked flocs to form a network which cautured small particles during settling peroids. The effluent COD and turbidity were only 16.7 mg/L and 4.8 NTU, significantly lower than them of normal sludge. At least 10% aeration consumption could be saved under limited sludge bulking. Nitrification and phosphorus removal performances did not become poor under low DO. Fluorescence in situ hybridization (FISH) analysis showed that after a long term anaerobic/aerobic running, the number of nitrifying bacteria under low DO was even greater than that under high DO. P release was also more surfficient under low DO. The number of phosphate accumulating bacteria accounted to 20% of the total bacteria. Dominant filamentous bacteria in each bulking sludge were indentified through the conventional staining reactions and FISH. The dominant filamentous bacteria in the fully aerobic system were Thiothrix, Type 1851, Type 021N and S. natans, etc. The dominant filamentous bacteria in the anoxic-oxic were mainly Type 0092.