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The influence of physicochemical properties of microbial floc, namely extracellular polymeric substances (EPS) and hydrophobicity, on ultraviolet (UV) disinfection of sequencing batch reactor effluent was studied. Samples were generated from bench scale sequencing batch reactors (SBRs) fed a synthetic media. The SBRs were fed a media composed of either glucose or a 50% glucose / 50% skim milk mixture. Escherichia coli was added to the reactors daily to serve as an indicator organism for UV disinfection assays of SBR effluent. Sludge retention time (SRT) was adjusted between 4 – 20 days to generate microbial floc with varying physicochemical properties. EPS was extracted from mixed liquor suspended solids using a cationic exchange resin. EPS composition and concentration did not change with SRT for reactors fed with a similar carbon source. However, EPS samples obtained from reactors fed with a glucose-skim milk mixture contained more protein, less carbohydrate and more total EPS than EPS from reactors fed primarily glucose. EPS showed absorbance at 253.7nm, indicating that EPS may serve to absorb UV and prevent penetration of UV light through microbial floc; however, there was no direct relationship drawn between SRT and UV absorbance. Hydrophobicity increased and surface charge decreased with increasing SRT, and hydrophobicity increased with the protein:carbohydrate ratio of EPS (rp = 0.89, p<0.05). The total number of enumerable E. coli in SBR effluent showed a negative correlation with SRT and hydrophobicity of microbial floc. As hydrophobicity has been related to bioflocculation, this phenomenon may indicate increased bioflocculation of E. coli at higher SRT, resulting in a reduction in the number of free living E. coli. UV disinfectability of SBR effluent, as measured by the dose required to achieve E. coli disinfection limits of 100 and 10 CFU/100mL, did not significantly differ for the range of SRTs tested.
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Document Type: Research Article

Publication date: 2002-01-01

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