Kinetics Of Ultraviolet Disinfection Of Microbial Flocs

The kinetics of ultraviolet disinfection governs the scale and the operation of UV reactors. Therefore, an understanding of disinfection kinetics will help to improve the design and performance of disinfection processes.

18.2.1 Dose-Response Curves

The kinetics of ultraviolet disinfection is quantified by exposing the sample to various doses (= UV intensity x time) of UV light and enumerating the survived colonies. The sample is a stirred liquid suspension and the irradiation is carried out using a collimated beam apparatus. The purpose of collimating the UV beam is to provide a parallel beam of light perpendicular to the surface of the sample.

In the case of wastewater disinfection, a common technique for the enumeration of survived organisms is the membrane filtration method.9 In this method, the irradiated sample is filtered, cultured in an appropriate medium, and the number of colonies is counted after an incubation period. A plot of the log of number of colony forming units (CFU) per 100 ml of the sample versus the applied dose of UV light is called the dose-response curve. This plot represents the kinetics of inactivation and quantifies the UV demand of wastewater to achieve a certain level of disinfection.

The shapes of dose-response curves that typically occur are given in Figure 18.3. The inactivation of dispersed or free organisms usually follows first order kinetics (curve 1). However, in some cases, the inactivation of free microbes results in an apparent lag or a shoulder at low doses (curve 2). This phenomenon may be explained by the clumping of microbes to form flocs10 or by the action of cellular repair mechanisms.3 The most common kinetics for municipal wastewaters is shown schematically by curve 3. At low doses, the shape of the curve is governed by the UV response of free microbes. However, at higher doses, the curve exhibits a plateau or a tailing effect. There is strong evidence that the tailing phenomenon is primarily due to the presence of microbial flocs.11 Curve 4 illustrates a case for which the disinfection kinetics exhibits both an initial shoulder and a subsequent tailing phenomenon.12

Response of wastewater to UV radiation depends on the type of target organism. The most common indicator organisms used for wastewater disinfection are total and fecal coliform, E. coli, and enterococci.13 In the present study, all dose-response data are based on the enumeration of the surviving fecal coliforms unless it is stated otherwise.

18.2.2 Mathematical Models for UV Disinfection

Kinetic models are often used for estimating the impact of wastewater quality on the reactor performance and for effective reactor design. A summary of kinetic models that are published in the literature is given in Table 18.1.

The one-hit model assumes that a single harmful event (hit) is sufficient to inac tivate a biological unit.14 This model represents a Poisson process where the mean

FIGURE 18.3 Schematic survival curves showing the kinetics of UV disinfection with and without the presence of microbial flocs.

FIGURE 18.3 Schematic survival curves showing the kinetics of UV disinfection with and without the presence of microbial flocs.

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