A new classification paradigm of extracellular polymeric substances (EPS) in activated sludge: separation and characterization of exopolymers between floc level and microcolony level.

Extracellular polymeric substances (EPS) play a crucial role in the formation of activated sludge flocs. However, until now, the EPS are rather classified by the method used for extraction than by a theoretical consideration of their function and composition. In this paper, a new classification paradigm of EPS was proposed, which offered a novel approach to identify the role of EPS in the formation of activated sludge flocs. The current study gave an exploration to distinguish the EPS in the floc level (extra-microcolony polymers, EMPS) and in the microcolony level (extra-cellular polymers, ECPS). It was found that cation exchange resin treatment is efficient to disintegrate the flocs for EMPS extraction, however, inefficient to disaggregate the microcolonies for ECPS harvesting. A two-steps extraction strategy (cation exchange resin treatment followed by ultrasonication-high speed centrifugation treatment) was suggested to separate these two types of EPS in activated sludge flocs and the physicochemical characteristics of EMPS and ECPS were compared. The protein/polysaccharide ratio of ECPS was higher than that of EMPS and the molecular weight of proteins in EMPS and ECPS were found to be different. The ECPS contained higher molecular weight proteins and more hydrophobic substances than the EMPS contained. The result of excitation-emission matrix fluorescence spectroscopy analysis also showed that the EMPS and the ECPS have different fluorescent expressions and the components of EMPS were more diverse than that of ECPS. All results reported herein demonstrated that two different types of exopolymers exist in the activated sludge flocs and the inter-particle forces for aggregation of activated sludge flocs are not identical between the floc level and the microcolony level. It suggested that cation bridging interactions are more crucial in floc level flocculation, while the entanglement and hydrophobic interactions are more important in microcolony level cohesion.

The important implications of particulate substrate in determining the physicochemical characteristics of extracellular polymeric substances (EPS) in activated sludge.

Since the notable amount of particulate substrate in wastewater, the implications of particulate substrate on treatment efficiency have been a topic of major interest in the field of biological wastewater treatment. The particulate substrate has to be hydrolyzed by the extracellular enzymes, which are mainly embedded in extracellular polymeric substances (EPS) matrix of microbial aggregates, prior to consumption. Therefore, the important relevance between the particulate substrate and the characteristics of EPS can be expected. In this study, two lab-scale sequencing batch reactors were performed in parallel to investigate the effects of particulate and soluble substrate on the physicochemical characteristics of EPS in activated sludge. The results showed that the particulate substrate in the influent could significantly change the properties of activated sludge and the characteristics of EPS. More open and fluffy flocs with poorer settleability and dewaterability were formed with particulate substrate. More protein and humic compounds were introduced into the EPS matrix due to the deep involvement of protein and humic compounds in hydrolysis process of particulate substrate. The increments of protein and humic compounds then caused the slight higher molecular weight, higher hydrophobicity and lower zeta potential of EPS in particulate substrate system. The results in this study permitted for obtaining answers to understand the significant implications of particulate substrate in determining the physicochemical characteristics of EPS in biological wastewater treatment systems.

Hydrogen utilization rate: A crucial indicator for anaerobic digestion process evaluation and monitoring.

Hydrogenotrophic methanogens had been considered as key species for the anaerobic digestion (AD) of industrial wastewater and municipal sludge. However, how to evaluate the activity of the hydrogenotrophic methanogens was less studied. In this study, a volumetric device and a test procedure were developed for measuring the specific hydrogen utilization rate (HUR) of anaerobic sludge. Results showed that HUR values were highly influenced by sludge concentrations because of limitation on H2 mass transfer. The critical value of sludge concentration in the test bottle should not be higher than 1 gVSS/L. Under such condition, the kinetics of HUR would not be limited by H2 mass transfer and the maximal value of HUR could be obtained. Field survey confirmed that HUR exhibits a good relationship with specific methanogenic activity (SMA) and reactor performance. An anaerobic system with a relatively high HUR was found to be beneficial for maintaining H2 partial pressure in an appropriately low level. Moreover, such system was thermodynamically favourable for the syntrophic degradation of volatile fatty acids. As a crucial parameter of the anaerobic process, HUR could be used as a key indicator for evaluating and monitoring AD processes.