Particle Association Effects on Microbial Indicator Concentrations for CSO Disinfection

EPA investigated the effects of blending disinfected combined sewer overflow samples to expose particle-associated microorganisms to the water column before measuring microbial indicator concentrations by membrane filtration. Before blending, samples were diluted in the mixture of chemicals identified by Camper et al. to enhance bacterial dissociation from solids. The resulting solution contained 10^–6 M Zwittergent 3-12, 10^–3 M EGTA, 0.01 M Tris buffer, and 0.01% peptone by weight, buffered to a pH of about 7. The results show that measured fecal coliform (FC) and Enterococcus (EC) concentrations in blended samples were up to 10 times greater than FC and EC concentrations measured in the untreated samples, i.e., unblended samples with no chemicals added. Both blending speed and time affected measured indicator concentrations, with increased concentrations observed in samples blended at 22,000 rpm from 0.5 to 3.5 min. Blending decreased mean particle size, although no correlations between increased indicator microorganism concentration and decreased particle size were observed.