Influence Of Extracellular Polymers And Their Surface Characters On Flocculation And Settlement Of Activated Sludge

Solid-liuid separation is the key problem in running of activated sludge system, which influences the quality of effluent and the concentration of returned sludge. Extracellular Polymeric Substances (EPS) is an important part of activated sludge, which locates in interior and exterior of sludge flocs, and has important effect on flocculation and settlement of activated sludge. However, the exactly influence of EPS to flocculation and settlement is not clear. In the past research, the effect of EPS on flocculation and settlement of activated sludge was studied only from the total amount and components of EPS and the surface characters of the flocs, but not from the surface characters of EPS. Furthermore, most researchers took the EPS as a whole, ignoring the difference between interior EPS and exterior EPS. In the dissertation, the activated sludge in test scale system was taken as the subject investigated. At first, loosely bound EPS (LEPS) and tightly bound EPS (TEPS) were extracted by ultrasonic and cation exchange resin in turn. Subsequently, the components, amount and surface characters of LEPS and TEPS from different SRT activated sludge was tested, and the effect of them on flocculation and settlement of activated sludge flocs were discussed. Whereafter, LEPS and TEPS from low or high sludge retention time(SRT) activated sludge was used to reflocculate the suspended sludge liquid respectively to testify most part of the conclusions which includes the effects of amount and surface characters of LEPS and TEPS on flocculation and settlement of activated sludge. In the end, the structural model of activated sludge flocs was proposed from the viewpoint of macromolecular force, according to the discrepancy of the LEPS and TEPS surface characters. The main conclusions are as follows:①Adopting G6PD activity to value the degree of bacteria lyses in the course of extraction,the EPS extraction method by ultrasonic and cation exchange resin was systematically discussed. Borate terrariums were suggested to be used in the process of EPS extraction by ultrasonic, and the process had better be put in the ice bath without stirring. Moreover, the best ultrasonic frequency for extracting EPS was 21 kHz. Considering the extraction efficiency and the degree of bacteria lyses in the course of EPS extraction, the ultrasonic time should be lower than 3minutes, and the suitable scope of the power should be from 32W to 40W. 001×7 cation exchange resin as the extactant was suited to extract EPS. While value of pH was from 5.8 to 7.8, the extracted amount of the EPS was stable. The appropriate process of the extraction by cation exchange resin was 80g/g VSS of the addition amount of resin, 500rmp of the stir speed and 1h～3h of stirring time. By this means, the extraction amount of EPS could achieve 85.2～139.9mg/gVSS, and the bacteria lyses percentage could be controlled under 10%. Ultrasonic plus resin extraction method was proposed, which is more efficient, less chemical pollution and bacteria lyses and easier to process than heating method, EDTA method and formaldehyde-NaOH method. From different types of sludge, the amount of EPS extracted by ultrasonic followed by resin extraction method was about 8% to 13% of the volatile constituent of sludge.②LEPS and TEPS were extracted by ultrasonic and cation exchange resin in turn for the first time. The results showed that SRT had an important influence on the amount and surface characters of LEPS and TEPS. The amount of LEPS changed greatly with the change of SRT, which decreased from 62.7mg/g VSS of 3d SRT activated sludge to 25.6 mg/g VSS of 15d SRT activated sludge, while the amount of TEPS changed a little with the change of SRT.,which was about 80mg/g VSS. While the SRT increased from 3d to 20d, the value of surface negetive charge of LEPS decreased in the beginning, then increased a little latterly, which was from∣-1.59meq/g EPS∣ to∣-0.77meq/g EPS∣. When SRT was 15d, LEPS had lowest surface negative charge value and highest hydrophobicity. By contrast, the value of surface negative charge of TEPS changed a little with the increase of SRT from 3d to 15d, which was from∣-1.26 meq/g EPS∣ to∣-0.98meq/g EPS∣. Furthermore, TEPS was more hydrophobic than LEPS for different SRT activated sludge.③The influence of EPS and it's surface characters to the surface characters, flocculation ability and seattleability of activated sludge was discussed for the first time. Surface negative charge of activated sludge was mainly affected by the amount and surface negetive charge of LEPS, increasing with the increase of them. The hydrophobicity of sludge was more significantly related to the hydrophobicity of TEPS than that of LEPS, but less to the amount of TEPS. With the increase of the amount and surface negative charge of LEPS, ESS increased, but those of TEPS had less influence on flocculation ability of activated sludge. Furthermore, lower ESS and higher FA were significantly relateded to the increase of hydrophobicity of LEPS and TEPS. While the total amount and surface negative charge of LEPS increased or its hydrophobicity dropped, SVI increased and ZSV decreased. SVI and ZSV were not significantly related to the amount of TEPS, while hydrophobicity of TEPS had strong effect on SVI and ZSV. That was to say, higher hydrophobicity of TEPS was related to lower SVI and higher ZSV.④Suspended sludge liquid was reflocculated by LEPS and TEPS of low or high SRT activated sludge respectively, and the flocculation performance of LEPS and TEPS and settleability of the reflocculated flocs were disscused. The results showed that TEPS and LEPS had different floccultion performance, the former promote flocculation,and the latter was harmful to flocculation. With the incrcease of addition amount of four types of EPS, the upwards trend of reflocculated flocs SVI was 15dTEPS<3dTEPS<15dLEPS<3dLEPS. At the same time, the most conclusions of the influence of the amount and surface characters of LEPS and TEPS on the flocculation ability and settleability of activate sludge was verified.⑤According to the microstructure photos of activated sludge flocs and flocs reflocculated by LEPS and TEPS, and macromolecular force in interior and exterior of flocs, the structural model of activated sludge flocs was proposed. In the model, high valence cation, especially tri-valence cation, was the key component in the course of flocs forming, and surface characters of LEPS and TEPS were key factors affecting activated sludge flocs structure.