Abstract
HypothesisMicrobial extracellular polymeric substances (EPS) produced from wastewater are generally heterodispersed, which is expected to influence their flocculation performances and mechanism, particularly in mixed particle systems. The different molecular weight (MW) fractions should contribute to the overall adsorption affinity and flocculation mechanism of EPS in single and dual clay systems. ExperimentsEPS harvested from bioreactors were size-fractionated into high, medium and low MW fractions (HMW, MMW, LMW, respectively). The harvested mixed EPS and its fractions were characterised by diverse analytical techniques coupled with optical reflectometry to investigate the role of each EPS fraction in the overall flocculation mechanism of EPS in kaolinite and montmorillonite clay systems. FindingsIn single clay systems, both the harvested mixed EPS and the HMW-EPS fraction showed comparable flocculation performances. However, mixed EPS proved to be more efficient than the HMW-EPS fraction for dual clay flocculation. Site blocking effects were observed in mixed EPS: the LMW and MMW EPS first adsorbed to the surface due to higher diffusivities and faster mass transfer to the interface, while the HMW-EPS were slowly transported but were attached to the surface irreversibly and stronger than the LMW/MMW-EPS. We propose from this, a mixed EPS adsorption mechanism: extended anionic polymer tails in solution, thereby enhancing particle flocculation.
Highlights
The coagulation/flocculation process has been employed for centuries as a simple and effective way to destabilise, agglomerate and remove particles from water and wastewater
S-extracellular polymeric substances (EPS) were harvested via centrifugation: 1000 mL sludge was centrifuged at 17,000 g and 4 °C for 1 h, after which the supernatant containing the Soluble extracellular polymeric substance (S-EPS) was dialysed against demineralised water to remove salts and ultra-low molecular weight (MW) compounds
The mixed EPS, HMW EPS and MMW EPS showed similar absorption bands, revealing the presence of polysaccharides and proteins as the main components. This Fourier transform infrared spectroscopy (FTIR) analysis mainly gives qualitative information, a clear difference is observed between the low MW (LMW) EPS spectrum and the other spectra
Summary
The coagulation/flocculation process has been employed for centuries as a simple and effective way to destabilise, agglomerate and remove particles from water and wastewater. The usual approach is the enrichment of pure cultures with single organic substrates (such as glucose) to obtain a single type of EPS, usually polysaccharides [8,10,11,12] This strategy produces biodegradable EPS, the disadvantage is that it necessitates sterile conditions and expensive carbon sources. One outcome of the mixed-culture approach is the production of a heterogeneous and heterodispersed EPS matrix, comprising different compounds with different molecular weights (MWs) and charge densities (CDs) [7,13]. These two characteristics (MW and CD), especially the former, govern the flocculation process and performance of anionic flocculants such as EPS [14]. Previous studies reported wastewater-produced EPS to comprise a mixture of varying MWs [7,13,15,16], which according to Bolto and Gregory [17], can be generally classified as high (˃1000 kDa), medium (1000– 100 kDa) and low (
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