Ultraviolet disinfection of sequencing batch reactor effluent: A study of physicochemical properties of microbial floc and disinfection performance

The influence of physicochemical parameters of microbial floc on the ultraviolet disinfection of effluent from sequencing batch reactors (SBRs) was studied. Sludge retention time (SRT) and synthetic feed were adjusted in the SBRs to vary the physiochemical properties of microbial floc. Escherichia coli was added daily as an indicator organism for assaying UV disinfection. The composition and concentration of extracellular polymeric substances (EPS) did not change with SRT, but changed with the carbon source that the SBRs were fed. The estimated UV absorbance at a wavelength of 253.7 nm due to EPS was approximately 250 for a solid layer of floc compressed into 1 cm, which is insignificant compared to the total UV absorbance of microbial floc; in comparison to the cellular constituents of microbial floc, EPS does not appear to be a limiting factor in the UV disinfection performance of microbial floc. The total number of enumerable E. coli in the effluent from an SBR showed a negative correlation with SRT and hydrophobicity of microbial floc. Effluent with a high SRT may have increased the ability of E. coli to floc and resulted in less freely suspended E. coli. Key words: UV disinfection, microbial floc, sludge retention time, EPS, hydrophobicity, Escherichia coli.