Physicochemical studies of microbial flocs

grantor: University of TorontoThis thesis investigates the influence of sludge retention time (SRT) on surface properties and interparticle interactions of sludge flocs, and the role of surface properties and interparticle interactions in bioflocculation, compaction and stability. Four laboratory-scale sequencing batch reactors (SBRs) were operated in parallel at different SRTs (4 to 20 days) for two years. Surface properties of sludge flocs were examined by a number of state-of-the-art techniques, including physicochemical extraction, chemical analyses and ultrafiltration of extracellular polymeric substances (EPS), contact angle measurement (hydrophobicity) and colloidal titration (surface charge). Interparticle interactions (electrostatic, ionic interactions and hydrogen bonds) and the stability of sludge flocs at different SRTs were evaluated in batch experiments by manipulating the water chemistry of the surrounding solutions. At all SRTs, proteins and carbohydrates were the dominant components of EPS, followed by a small amount of DNA. Acidic polysaccharides were not detected in the EPS. Molecular weights of proteins, carbohydrates and DNA in the EPS covered a broad range, from less than 1,000 daltons to more than 100,000 daltons. The majority (>85%) of EPS components had molecular weights larger than 10,000 daltons. The total amount of EPS was independent of the SRT. Sludge surfaces were more hydrophobic (a higher value of water contact angle) and less negatively charged at higher SRTs (16 and 20 days) than those at lower SRTs (4 and 9 days). The contact angle values were strongly correlated to the surface charge density. Changes in dissociation constants of sludge flocs in batch experiments indicated that electrostatic interactions were involved in disrupting the stability of sludge flocs, and ionic interactions and hydrogen bonds were present to compensate for the negative influence of electrostatic interactions on the stability of sludge flocs and to keep flocs together. Ionic interactions and hydrogen bonds were two dominant forces that maintained the stability of sludge flocs at lower SRTs; while other mechanisms, such as physical enmeshment and hydrophobic interactions, were likely more important than ionic interactions and hydrogen bonds in controlling the stability of sludge flocs at higher SRTs. In summary, the results from this study demonstrate that it is possible to operationally and chemically manipulate surface properties and interparticle interactions of sludge flocs for effective floc separation. A transition in floc surface properties was found to occur in the SRT range of 9 to 12 days. This transition in floc surface properties was linked to improved bioflocculation and a more stable floc structure. (Abstract shortened by UMI.)Ph.D