Introduction

The presence of pathogenic bacteria, viruses, and parasites in recreation waters is a potential source for the spread of diseases. To protect the public health and the quality of water resources, wastewater is often disinfected by chemical or physical means prior to discharge to the receiving water.

Waterborne pathogens might exist as dispersed (or free) organisms or could be embedded within microbial flocs. In a typical wastewater, microbial flocs vary in size from several microns up to hundreds of microns. The floc structure acts as a barrier to the penetration of chemical and physical disinfectants and therefore reduces the disinfection efficiency. Flocs also provide a vehicle for the transport and spreading of pathogens in the environment. In this chapter we focus our attention on the ultraviolet (UV) disinfection and the effect of flocs on this process.

The antimicrobial effects of ultraviolet light were discovered in early 1900s.1 Ultraviolet light is part of the electromagnetic spectrum and is often divided into four regions, UVA (315 to 400 nm), UVB (280 to 315 nm), UVC (200 to 280 nm), and vacuum UV (<200 nm).2 It is the high energy UVC photons that are responsible for the germicidal action of light, for example the photon energy at 253.7 nm is 7.8 x 10-19 J or 4.9 eV with a high germicidal efficiency.

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Disinfection of water with UV light is considered to be a photochemical process that results in the alteration of DNA and RNA and therefore prevents microorganisms from reproduction.3 In this process, the main mechanism for the microbial inactivation is believed to be the formation of pyrimidine dimers (thymine dimers in the case of DNA). Insufficient irradiation results in partial damage to the nucleic acid that may be either repaired by cellular repair mechanisms or cause mutant progeny.4

The germicidal effectiveness of inactivation of pathogens exhibits a peak at around 264 nm (Figure 18.1).5 Protein and DNA also absorb strongly in the UVC region.6'7 Therefore, the disinfection of floc-associated pathogens can be adversely affected by the shielding effect of adjacent microbes and by the UV absorption of extracellular polymeric substances (EPS) present within the floc matrix. Additionally, flocs can alter the light intensity field by absorption and scattering of UV light. Thus, the presence of flocs not only reduces the average ultraviolet dose in the sample but also modifies the apparent kinetics of disinfection. Figure 18.2 shows the schematic diagram of such interactions.8

Wavelength (nm)

FIGURE 18.1 Action spectrum of E. coli and DNA absorbance. (From Harm, W., Biological Effects of Ultraviolet Radiation. Cambridge University Press, New York, 1980.)

Wavelength (nm)

FIGURE 18.1 Action spectrum of E. coli and DNA absorbance. (From Harm, W., Biological Effects of Ultraviolet Radiation. Cambridge University Press, New York, 1980.)

UV light

UV light

Complete penetration

Incomplete penetration

Region of limited cellular damage

FIGURE 18.2 Interaction of suspended particles with light. (From Snider, K., Tchobano-glous, G.G., and Darby, J., Evaluation of Ultraviolet Disinfection for Wastewater Reuse Applications in California. University of California, Davis, 1991.)

Complete penetration

Incomplete penetration

Region of limited cellular damage

FIGURE 18.2 Interaction of suspended particles with light. (From Snider, K., Tchobano-glous, G.G., and Darby, J., Evaluation of Ultraviolet Disinfection for Wastewater Reuse Applications in California. University of California, Davis, 1991.)

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