Analysis of microbial flocs collected from a municipal wastewater treatment plant shows that by increasing the floc size fraction from 53/45 ¡xm to 150/125 ¡xm, the percentage of viable flocs increases from 7% to 11%. At the same time, the dose demand of samples to achieve one log inactivation more than doubled, increasing from ~25 to ~60 mJ/cm2 with increased floc size. Analysis of EPS extracted from pure cultures of a Klebsiella sp. shows that EPS is a strong absorber of ultraviolet light with absorbance of about 400 cm-1; however, the reduction in the UV light intensity within the floc due to the presence of EPS could vary from less than 1% up to ~55%, depending on whether the EPS was all surface associated (an extreme) or forming a dense microsphere within the floc (another extreme).

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