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7.0 (±0.4)

a Sieve size in microns.

b Mode of particle size distribution from Coulter particle size analyzer.

a Sieve size in microns.

b Mode of particle size distribution from Coulter particle size analyzer.

increases (i.e., the inactivation rate constant, k2, decreases), emphasizing that larger particles are harder to disinfect.

For any given size fraction, the ratio of the number of colony forming units obtained prior to the UV irradiation and the number concentration of particles obtained from the Coulter analyzer will provide an estimation of the percentage of viable flocs (Table 18.2). Based on this result, the percentage of colony forming flocs increased from 7% to 11%, when comparing 53/45 to 150/125 ¡xm sieve fraction. This observation emphasizes the importance of larger flocs in UV disinfection, that is although there is smaller number of large flocs in a typical wastewater compared to small flocs, a larger fraction of them are viable and they are harder to disinfect.

18.3.2 The Role of Floc Composition

Microbes that are embedded in flocs are shielded and receive reduced doses of UV light. The UV light intensity within a floc depends on the size and composition of floc. To understand better, the potential effect of floc composition and particularly the role of EPS on the light penetration into flocs, EPS was extracted from pure cultures of Klebsiella sp. and its UV absorbance was measured.18

Klebsiella cultures were grown to allow for the formation of flocs. Ethanolic extraction19 was used to extract EPS from the cultured samples. The broth samples were collected and the mixed liquor suspended solids (MLSS) was separated by centrifugation at 9000 rcf and 4°C for 15 min. The supernatant was decanted and the sludge pellet was dissolved in ethanol. These solutions were left in parafilm-sealed containers at ambient conditions for several days for extraction. The solution was then filtered using Whatman Microfibre GF/A filters and the filtrate was rotary evaporated under vacuum to remove ethanol. The remaining EPS was weighed and dissolved in a known amount of ethanol and the UV absorbance of EPS solution was measured at 253.7 nm using a UV-Vis spectrometer. This measurement was repeated for five concentrations of EPS.

To investigate the effect of carbon source on the UV absorbance of EPS, the above procedure was repeated for two different carbon sources, a lactose-fed culture and a

EPS concentration, wt%

EPS concentration, wt%

FIGURE 18.6 UV absorbance of EPS for (a) glucose-fed samples, (b) lactose-fed samples.

EPS concentration, wt%

FIGURE 18.6 UV absorbance of EPS for (a) glucose-fed samples, (b) lactose-fed samples.

glucose-fed culture. Each test was conducted in replicates. Figure 18.6(a) and 18.6(b) show the plot of absorbance versus EPS concentration for all runs. The slope of both curves is about 4 wt%, indicating a strong UV absorptivity for EPS. For comparison, the UV absorbance of protein (bovine serum albumin) and DNA (calf thymus) at 253 nm are 4.1 and 155 wt%, respectively (estimated based on data reported by Harm5). The results also indicate that the carbon source has a minimal impact on the absorbance of EPS produced by Klebsiella sp. as measured by this method.

The effect of EPS on the UV penetration into microbial flocs depends on its spatial distribution. To illustrate this point, we take the three idealized cases presented in Figure 18.7. We consider a 100 ^m spherical floc with a density of 1 g/cm3 and a porosity of 90%. Assuming an EPS concentration of 50 mg/g MLSS with an absorbance of 400 cm-1, and assuming that EPS accumulates around a single target organism within the floc (Figure 18.7a), 55% of the incident UV light would be absorbed by the EPS before reaching the shielded microbe. On the other hand, if EPS was assumed to be uniformly adsorbed on the surface of the floc while forming a thin film around it (Figure 18.7c), only 1% of the UV light will be attenuated in the EPS layer. Finally, if EPS was homogeneously distributed within the floc volume (Figure 18.7b), 3% oftheUV light will be absorbed by EPS before reaching the center of the floc. The above models are oversimplifications of the actual distribution of EPS

(a) Dense (b) Uniformly (c) Coating microsphere distributed layer layer

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