Understanding the properties of aerobic sludge granules as hydrogels

Abstract

Aerobic sludge granules are larger, denser microbial aggregates than activated sludge flocs with a smoother and more regular surface, which facilitates greater wastewater treatment intensity. Factors important in their growth are still poorly understood, which is an impediment to the construction and operation of full-scale aerobic sludge granule processes. Data in this article obtained with granules treating an abattoir wastewater provide evidence that aerobic sludge granules are hydrogels. The results also demonstrate a method for characterizing macromolecular associations. The rheological profile of these granules was found to be analogous with that of typical polymer gels. Water uptake or swelling reflects an equilibrium between granule elastic modulus and osmotic pressure, whereby uptake is increased by reducing solute concentration or the elastic modulus. A weakening of the extracellular polymeric substance (EPS) matrix as demonstrated with mechanical spectroscopy was induced by several environmental factors including temperature, pH and ionic strength. Uniform and elastic deformation was observed at low strain. Enzymatic degradation studies indicate that proteins and alpha-polysaccharides were the major granule structural materials. The aerobic sludge granules in the current study were therefore protein-polysaccharide composite physical hydrogels. While aerobic sludge granules treating an abattoir wastewater are used as a case study, many of the fundamental principles detailed here are relevant to other granulation processes. The paradigm established in this study can potentially be applied to better understand the formation of aerobic sludge granules and thus overcome a hurdle in the acceptance of aerobic sludge granulation as an alternative to more traditional wastewater treatment processes.

Rheological properties of sewage sludge during enhanced anaerobic digestion with microwave-H2O2 pretreatment

“…The characteristics of the adsorption ability, hydrophilicity/hydrophobicity and content of the main components, such as proteins and polysaccharides, of EPS critically affect the properties of microbial aggregates, e.g., mass transfer, surface characteristics and stability (Sheng et al, 2010). Moreover, EPS also has a strong relationship with sludge rheology according to previous studies Ma et al, 2014;Seviour et al, 2009).…”

Extraction of Structural Extracellular Polymeric Substances from Aerobic Granular Sludge

“…The granulation process has been regarded as a gel-forming phenomenon with glycosides as the gelling agent 4,9,25,28 . Normally, Ca 2+ is one of the most common cations in wastewater.…”

“…Unlike biofilms, AGS is formed spontaneously without involvement of any carrier material 4 . In AGS, like in biofilms, microorganisms produce a significant amount of highly hydrated extracellular polymeric substances (EPS) 5 to form a hydrogel matrix in which they are self-immobilized [4][5][6] .…”

“…In AGS, like in biofilms, microorganisms produce a significant amount of highly hydrated extracellular polymeric substances (EPS) 5 to form a hydrogel matrix in which they are self-immobilized [4][5][6] . EPS are a complex mixture, consisting of polysaccharides, proteins, nucleic acids, (phospho)lipids, humic substances and some intercellular polymers 5,7,8 .…”

“…5 , forming a dense and compact tertiary network structure. The polymers in EPS which are able to form hydrogels 4,9 and contribute to the formation of the tertiary network structure are in this respect considered as structural EPS, a subset of the total EPS. EPS are responsible for the chemical structure and physical properties of granules 5 .…”