| Because of its advantages activated sludge process becomes the most widely used biological treatment process. Solid-liquid separation by gravity is very important in the process of activated sludge processing units, and the activated sludge flocculation settleability directly affects the effect of solid-liquid separation. Now the control of the sludge flocculation settleability is adjusted from the process conditions directly to the sludge sedimentation effect mostly based on the practical engineering experience. Actually process conditions are influenced by the surface physical and chemical properties of microbial flocs thus indirectly influence the sludge flocculation settleability. Now, the research on surface physical and chemical properties of sludge and its mechanism is very few, that greatly limits the sludge flocculation settleability. In order to solve this problem, this research adopts the main idea of "autotrophic and heterotrophic and mixotrophic reactors up and running, influence analysis of the surface physical and chemical properties of sludge flocculation settleability, key factors affecting sludge flocculation settleability, key factors affecting mechanism based on the XDLVO theory". Through the analysis of flocculation settleability, surface physical and chemical properties and microscopic material composition of three different sludge through laboratory culture, it’s easy to find out the key factors affecting sludge flocculation settleability. With the XDLVO theory, this research aims at interaction mechanism of three kinds of sludges, with further discussion on the mechanism of action of EPS. The main research contents and research results are as follows.(1) Cultivate three kinds of activated sludge:autotrophic sludge R1, heterotrophic sludge R2 and mixotrophic sludge R3, and monitor its concentration, sedimentation and the surface physical and chemical properties, then find out the key control factors in sludge settling. The results showed that sludge R1, with the lowest value of SVI and the best sludge settling; sludge R2, with the highest value of SVI and the worst sludge settling; sludge R3, with the value of SVI and sludge settling lies between R1, R2; Three are obvious differences among the Zeta potential and relatively hydrophobic of the three, with the least surface negative charge and the best relatively hydrophobic of sludge R1, the most surface negative charge and the lowest relatively hydrophobic of sludge R2, and sludge R3 lies between. Therefore, this work concludes that the sludge flocs surface Zeta potential and relatively hydrophobic are the most important physical and chemical properties resulting in the differences of settling performance. To analyze the sludge flocs content and composition of EPS of the three, we found that the total content of EPS, polysaccharide and humic acid have nothing significant to do with the Zeta potential on the surface of the sludge and the relative hydrophobicity, and the micro reasons of the differences on the surface of the sludge and relatively hydrophobic are different protein components content in EPS. the combination of proteins and cationic reduce negative charge on the surface of the sludge, enhance the relative hydrophobicity, therefore reduce the repulsion between the sludge and promote the flocs that gathered themselves together, and then affect the sludge settling performance.(2) Based on the observation of hree kinds of sludge flocculation sedimentation in different concentration of Na+, Mg2+, we discover that flocculating settling of auto trophic sludge R1, due to its structure is compact, is very good, resulting that the influence of ion concentration was not significant; and flocculation sedimentation performance of heterotrophic sludge R2 and mixed nutrition sludge R3 did not increase with the increase of ionic strength, but it will deteriorate sharply when it comes over one streme due to the "salting out" effect and the function of ion exchange. At the same time, we also found that the addition of divalent metal ions of Mg2+ can effectively improve the settling performance of sludge flocs. Combined surface thermodynamics analysis with XDLVO theory, we found the main reason for the differences is:the same sludge under different ionic strength has different Zeta potential value and diffusion of double shell thickness which can lead to differences of WR. Throught the analysis of XDLVO theory of three types of sludge flocculation sedimentation differences, it shows that sludge R1 flocculation sedimentation performances the best due to the larger effective Hamaker constant ABLB value, negative lewis acid-base hydration and smaller Zeta potential values; sludge R2 flocculation sedimentation performances the worst due to the smaller effective ABLB, positive lewis acid-base value of hydration and larger Zeta potential values; sludge R3 nutrition sludge flocculation sedimentation performances in between due to the ABLB, lewis acid-base hydration and Zeta potential values lie between Rl and R2.(3) Through the extraction experiments of LB-EPS and TB-EPS on the surfaces of the three, we can observe the sludge turbidity change, thus judge the changes of sludge flocculation sedimentation performance. The results show that the LB-EPS content of three sludgeare relatively less than the TB-EPS content; it is conducive to the sludge flocculation sedimentation, no matter if it is LB-EPS or TB-EPS. By conducting further survey into the sludge flocculation settlement mechanism with LB-EPS, TB-EPS and EPS based on the theory of XDLVO, we found that the LB-EPS and TB-EPS of sludge Rl and sludge R3 have strong adsorption, within the range of 0-20 nm between microbial; That of sludge R2 has rejection, within the range of 0.5-3.5 nm; and performances adsorption within the range of 3.5-20 nm between microbial. and found that LB-EPS contributes more to the sludge R1 (within the range of 2.5-20 nm between microbial) and sludge R3 (within the range of 0-20 nm between microbial) than that of TB-EPS; LB-EPS and TB-EPS contributes the same to the heterotrophic sludge R2. |
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