| The biological nitrogen removal and phosphorus removal systems always suffer from the sludge bulking. Many studies demonstrated that the incorporation of biological nitrogen removal or phosphorus removal favor the sludge bulking. Nowadays, the cause of sludge bulking in the biological nitrogen removal and phosphorus removal system is still unclear. This study investigated the pollutant removal and sludge properties(especially settleability) in the biological nitrogen removal and phosphorus removal systems under different operating conditions, through operating multiple continuous flow reactors. Using polymerase chain reaction-denaturing gradient gel electrophoresis(PCR-DGGE) and ordination analysis(Correspondence Analysis and Canonical Correspondence Analysis), the bacterial community structure and its influence on sludge settleability in the activated sludge systems were analyzed. The main results and conclusions are as follows:(1) When in the influent contrained large quantity of slowly biodegradable substrate, two convential activated sludge systems were operated to investigate the influence of seed slude on sludge properties. The height to diameter ratio of both reactors was 1.0, and the settling time was approximately 2 h. Mixed liquor suspended solid(MLSS) was kept within a range of 2800–3500 mg·L, and the organic loading was maintained between 0.38 and 0.48 kg COD/(kg MLSS·d). In the No. 1 Reactor, the seed sludge contained some filamentous bacteria, and aerobic granular sludge was successfully cultivated in the system. The granular sludge had good settleability, and the SVI value was between 50 and 90 m L/g. The diameter of the granules was 0.18–1.25 mm. The SEM view showed sphere- like and rod-like bacteria were predominant. The dominant species in the granules were Proteobacteria, Bacteroidetes, and Firmicutes. In the No. 2 Reactor, the seed sludge contained no filaments, and no granular sludge was formed during the entire experiment although the sludge settleability was relatively good(SVI<100 m L/g).(2) When the influent contained large quantity of slowly biodegradable substrate, the sludge settleability in the CAS system was relatively good(SVI was approximately 122 m L/g). However, in the Modified Ludzack-Ettinger(MLE) system fed with the same influent substrates, sludge settleability was relatively poor, with SVI value between 175 m L/g and 260 m L/g. MLSS and the organic loading in both systems were approximately 3500 mg/L and 0.40 kg COD/(kg MLSS·d). In the MLE system, when the anoxic to aerobic volume ratio was 1:3, the nitrogen removal efficiency in the MLE system was relatively low, serious sludge bulking happened, and SVI was approximately 258 m L/g. As the volume ratio increased to 1:2 or 2:1, the mitrogen removal efficiency increased significantly, and sludge bulking was mitigated, with SVI value decreasing to 175 m L/g. In the MLE system, when the anoxic zone increased, the diversity and richness of bacterial species decreased notably. Correspondence Analysis indicated that the bacterial community structure in both CAS and MLE systems was closely related to sludge settlieability. Sludge with different settleability clustered into distinct groups on the graphical map of CA.(3) In the enhanced biological phosphorus removal(EBPR) system, when COD:N :P was 100:1.90:2.86, the nitrogen was the growth-limit factor, and the filament Type 021 N was stimulated greatly, with SVI value of sludge increasing slowly to approximately 240 m L/g. When the COD:N :P was 100:4.3:2.86, the filament Type 021 N was slowly washed out, and the sludge bulking was controlled, with SVI value decreasing to 100 m L/g. During the entire experiment, MLSS and the organic loading in both systems were approximately 3000 mg/L and 0.63 kg COD/(kg MLSS·d),. Correlation analysis indicated that filament content was closely correlated with sludge settleability(r=0.858,p<0.001). PCR-DGGE analysis showed that when sludge bulking began to break out, the evenness, diversity and abundance of bacterial species increased remarkably. CA indicated that the nitrogen deficiency greatly influenced the bacterial community structure of the sludge, and suppressed the growth of Firmicutes. The sludge settleability had a certain relation with the bacterial community structure, the sludge samples with poor and good settleability clustered into the upper half and lower half of the graphical map of CA, respectively.(4) In the EBPR systems, sludge settleability depended both on the filament content and on Pns/VSS and NVSS/VSS contents at varying temperature. During the entire experiment, the MLSS ranged from 1992 mg·L to 3000 mg·L, and the organic loading was maintained between 0.42 and 0.63 kg COD/(kg MLSS·d). When the temperature increased from 20 °C and 25°C, the phosphorus removal of the system decreased significantly(p<0.01), the filaments decreased remarkably, and sludge setteability improved substantially(p < 0.01). The influence of Pns/VSS and NVSS/VSS was obscured by the decrease in filament content. When the temperature decreased from 20 °C to 15 °C, the the phosphorus removal increased significantly(p<0.01). The sludge settleability deteriorated instantly in the short term, but slowly recovered in the long term(p<0.01). Because the temperature rarely affected the filament content of sludge, sludge settleabiliy was mainly determined by the Pns/VSS(r=-0.647,p<0.01) and NVSS/VSS(r=-0.463,p<0.05). A common core of bacterial phyla was observed in the EBPR systems with different temperatures via PCR–DGGE analysis. The CCA results indicated that bacterial community structure was closely correlated with the temperature and sludge properties(F=1.738,p<0.01), and the bacterial community structure shifted notably along the arrow of SVI. |
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