| The activated sludge process becomes the most widely used technology for biological wastewater treatment in the world due to its efficiency and low investment. Sludge bulking as a serious problem frequently occurred in the system since this system was created.It will influence the operation and effluent of activated sludge process directly. Internal factors of microorganism may affect the settleability of the sludge, such as the evolution of filamentous bacteria group, extracellular polymeric sub stances (EPS) in the sludge. The detailed relationship between these factors and the sludge settleability is still unclear. The influence of the sludge density on the sludge settleability, the relationship between the sludge density and the efficiency of phosphorus removal also need to be studied.In this study, we established a series of continuous flow anaerobic/aerobic (A/O) activated sludge systems and changed the influent substrate (proportion of particulate matters and nitrogen loading), temperature and operating conditions. EPS content, sludge density, and the succession of filamentous bacteria group were analyzed in different conditions to reveal the effect of operation parameters on the sludge settleability. We will put forward the strategies to resolve sludge bulking in anaerobic-aerobic systems of phosphorus removal then provide theoretical basis of scientific design and stable operation for BNRAS systems.The results obtained in this study are as follows:(1) Three systems of phosphorus removal were operated to investigate the effect nitrogen loading on the sludge settleability at the same time. When the concentration of TN18mg/L, the settleability of the sludge was good, the type of filamentous are Type0092, Type0675and H.hydrossis. But it deteriorated rapidly when the concentration decreased to8mg/L. When the concentration of the TN was lacking, the EPS content of the sludge increased remarkably and became a major factor for sludge bulking. As the increase of EPS, Thiothrix Ⅱ and Type021N grew and proliferated in the systems. When the TN concentration was raised to18mg/L, the growth of filamentous was restrained and sludge settleability was improved significantly. It seemed that lack of nitrogen will lead to the proliferation of Thiothrix Ⅱ and Type021N, resulting in serious sludge bulking. We can restrain the growth of filamentous by increasing the concentration of TN and then inhibit the sludge bulking.(2) We setted up three anaerobic-aerobic systems of phosphorus removal at the same time, with constant concentration of COD, the proportion of parti culate matters of inflow matrix was raised from40%to70%to investigated the effect of slowly biodegradable substrate on sludge settleability. When the proportion rose to70%, the SVI value increased remarkably. At this time, both the EPS content and the filaments increased significantly and sludge density decreased gradully. Furthermore,Type0041, Type0092, Type0961filamentous bacteria proliferated in the three systems by the end of the experiment, resulting in serious sludge bulking. The result shows that the type of substrate has a strong impact on Type0041, Type0092, Type0961filamentous and then induced the sludge bulking.(3) We setted up two anaerobic-aerobic systems of phosphorus removal at the same time and changed temperature to study the impact of temperatur on the sludge settleability when reactors were stable at20℃. Operation temoerature of one reactor was25℃and the other one was15℃, At25℃, SVI value decreased from240mL/g to110mL/g. At15℃, SVI value rapidly increased to480mL/g, and then decreased to250mL/g. At20℃, the filamentous were Type0961and type0041. At25℃, Type0041was washed out gradully. There was only Type0961in the system and sludge settleability was improved. At15℃, the filaments of Type0961and Type0041were washed out of the systems slowly, while Type0914grew and proliferated. As the temperature increases, the EPS content of zoogloea decreased slightly. On the contrary, The content of EPS increased rapidly at15℃and then decreased to the content of20℃. To sum up, the change of the temperature will lead to the conversion of EPS, sludge density, filamentous bacteria group and so on, and thus affected the sludge settleability.(4) We setted up two anaerobic-aerobic systems of phosphorus removal at the same time and changed operation process to study the impact of anaerobic zone on the sludge settleability when reactors were stable. No.1reactor operated with alternating anaerobic/aerobic condition and No.2reactor changed to CAS systems. The SVI of sludge in No.1system was relatively good, always maintaining around100mL/g. The SVI value of No.2increases remarkably and sludge settleability deteriorated dramatically. The different operation process will result in different sludge settleability. There are a few filaments in the No.1reactor, including Type0961, Type0041and Type0914. Filaments of Type0961proliferated quickly in No.2reactor. Additionally, there are a few Type1851filamentous bacteria. The sludge density in No.1reactor was much higher than that in No.2reacto. It seemed that the incorporation of anaerobic zone acted like the selector can inhibit the growth of filamentous bacteria remarkably and the performance of the sludge settling was much better than the traditional aerobic sludge process. |
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