Effect Of Floc Characteristics On Disinfection Kinetics

18.3.1 The Role of Floc Size

To systematically investigate the effect of floc size on disinfection, UV disinfection of model samples with narrow floc size distributions was studied.17 Wastewater samples were collected from the main treatment plant of the city of Toronto located at Ashbridges Bay and fractionated using 150, 125, 90, 75, 53, and 45 ^m sieves. Three size fractions were chosen for further study with nominal ranges of 150/125, 90/75, and 53/45. Each size fraction was prepared by continuous washing of sieved particles with distilled water for at least 15 min or until a narrow size distribution is achieved. A Coulter particle size analyzer, Multisizer 3 (Beckman Coulter, Miami, FL), was used to count the number concentration of particles and to ensure the effectiveness of the fractionation process. Figure 18.4 shows the floc size distribution of the three fractions obtained using this technique. Each fraction was diluted with distilled water and 20 ml of diluted sample was transferred into a petri dish for exposure to UV light. For accurate estimation of UV dose, an IL 1700 radiometer (International Lights Co., Newburyport, MA) was used to measure the intensity at 33 points within the region irradiated by the lamp. To correct for the UV absorption of sample, the absorbance of each sample was determined using Lambda 35 UV/Vis spectrometer (Perkin Elmer, Boston, MA) at 253.7 nm. Based on these measurements, the exposure times were determined using the Beer-Lambert law. The sample was irradiated using a low-pressure collimated beam system (Trojan Technologies Inc., London, Ontario). The irradiated sample was filtered using a 0.45 ^m filter paper and was cultured for a day in the dark. The number of colony forming units was then counted for each sample. In addition, a blank sample (nonirradiated) from each fraction was cultured to determine the concentration of viable microorganisms in the original sample. All experiments were conducted in replicates.

Figure 18.5 shows the dose-response curves for the three floc size fractions. Although there is a considerable variability in the results, a distinct increase in the average UV dose demand with increased floc size is observed. For comparison, the

Size (microns)

FIGURE 18.4 Size distribution of various sieve fractions used for disinfection studies.

Size (microns)

FIGURE 18.4 Size distribution of various sieve fractions used for disinfection studies.

FIGURE 18.5 Dose-response curve for various sieve fractions.

FIGURE 18.5 Dose-response curve for various sieve fractions.

dose-response curve for free fecal coliforms is also shown in this figure. The initial slope for flocs is significantly smaller than that of free coliforms. This indicates that there are very few, if any, free microbes in the sieved samples. At higher UV doses, the slope of dose-response curve decreases as the floc size increases, indicating an increase in the UV resistance of the larger flocs in the sample. Using nonlinear regression analysis (Mathematica, v5.1), the double-exponential model parameters were estimated for the three sieve fractions (see Table 18.2). By increasing the particle size, both the fraction of resistant flocs (P) and their resilience to the ultraviolet light

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