| Activated sludge process is the most commonly used technology in treatingmunicipal and organic industrial wastewater nowadays. The structure and characteristicsof floc, which dramatically affected by the extracellular polymeric substances (EPS),have significant implications in determining the efficiency of contaminants degradationand solid liquid separation in activated sludge systems. Nevertheless, there exist somedebates on the mechanism of floc formation and the role of particulate organics due tothe complicated aspects of bacteria aggregation. In addition, the precise relationshipsbetween the floc structure characteristics and EPS are still not very clear, and somecontradictory results were reported in the early studies. Therefore, the continuousin-depth investigation of EPS is very useful for us to understand the structurecharacteristics and aggregation mechanism of activated sludge flocs. Following themain line of floc structure and EPS, four research contents were performed in this study.Those were the implications of particulate substrate in determining the physicochemicalcharacteristics of EPS in activated sludge, the effects of particulate substrate on thesludge settleability and growth of extended filaments, separation and comparison ofEPS between the floc level and microcolony level, the cultivation and structure analysisof anoxic granular sludge. The main results and conclusions are made as follows:1. In sequencing batch reactor (SBR) system, the particulate substrate in theinfluent could significantly change the properties of activated sludge and thecharacteristics of EPS. More open and fluffy flocs with poorer settleability and dewaterability can be promoted by particulate substrate. The particulate substrate alsoleads to the activated sludge with higher nitrification activity. Hydrolysis process ofparticulate substrate introduced more protein and humic compounds into the EPS matrixand increased the contents of protein and humic compounds in EPS. The increments ofprotein and humic compounds in EPS then led to slight higher MW, higherhydrophobicity and lower zeta potential of EPS in particulate substrate system.2. In completely stirred tank reactor (CSTR) system, extended filaments werequickly induced with soluble substrate. Total extended filamentous length (TEFL) ofactivated sludge decreased and sludge settleability was significantly improved after thesoluble substrate was switched to particulate substrate. Due to the limitation ofhydrolysis products in the bulk liquid, the extended filaments were forced to growtowards the inside of the flocs and finally sink in the floc layer. A physical model wasproposed to explain the evolution of extended filaments caused by soluble substrate andcured by particulate substrate. The results provide new ways of thinking and newmethod for filamentous bulking in activated sludge system.3. On the basis of floc structure and EPS fiction, the EPS in activated sludge isclassified into extra-microcolony polymers (EMPS) and extra-cellular polymers (ECPS).Cation exchange resin treatment is efficient to disintegrate the flocs for EMPSextraction, however, inefficient to disaggregate the microcolonies for ECPS harvesting.The content of ECPS in activated sludge was1.5times of that of EMPS and theprotein/polysaccharide ratio of ECPS was1.3times of that of EMPS. The proteomes ofthe EMPS and the ECPS were much different. The ECPS contained higher molecularweight proteins and more hydrophobic substances than the EMPS contained. All resultsreported herein demonstrated that two different types of exopolymers exist in theactivated sludge flocs and the inter-particle forces for aggregation of activated sludgeare not identical between the floc level and the microcolony level. The cation bridginginteractions are more crucial in floc level flocculation, while the entanglement andhydrophobic interactions are more important in microcolony level cohesion.4. After approximately90days cultivation, the sludge in an anoxic SBR fed withsodium acetate was almost granulated. The granules in the reactor were almostyellowish-brown and had smooth surface with clear outline. The content of EPS and extracellular polysaccharides in the sludge were increased by75%and120%with thesludge granulation, respectively, while the content of extracellular protein was notchanged significantly. Structure observation showed that inorganic crystal like materialwas exist in the center of the granules, while coccus layer with compact bacterialaggregate was present outside the nuclear zone. In addition, large quantities offilamentous bacteria proliferated at the surface of the granules. A string-bag hypothesiswas proposed, which believed that the surface filamentous structure can enhance thehydraulic shear resistance of granules, increase the internal density of granules andreduce the risk of granules disintegration. |
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