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CE 521 Environmental Biotechnology

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the mechanism of inactivation of microorganisms by Ultraviolet irradiation

 

 Matt Kasper

 Abstract

 Ultraviolet (UV) disinfection has been in water treatment practice for hundreds of years.  This paper summarizes the biological mechanism of UV disinfection, the pros and cons when it is compared to chlorinated disinfection., and inactivation doses of typical waterborne pathogens.  The inactivation doses of Giardia lamblia, Giardia muris, Cryptosporidium parvum, Bacillus subtilus, MS2 phage, and Escherichia coli are compared in detail.  Reactor and lamp characteristics that influence the dose necessary for inactivation were also discussed.  These characteristics demonstrated that baffled and wavy reactor sidewalls increased the inactivation by a minimum of 0.12-log for the same UV radiation dose.  It was also found that low-pressure lamps are more germicidal per unit energy expended than medium-pressure lamps. 

 KEYWORDS

 Ultraviolet, disinfection, inactivation, Giardia lamblia, Giardia muris, Cryptosporidium parvum, chlorination

 Introduction

 The idea of using ultraviolet (UV) radiation as a disinfectant is surely not a cutting-edge or futuristic notion.  In fact, an ancient Hindu source written at least 4000 years ago suggested that raw water be boiled, exposed to sunlight, filtered, and then cooled in an earthen vessel.  However, the implementation of UV disinfection came in 1906 in Nice, France.  At this same time, the less expensive and less complicated method of chlorination was becoming popular in the United States, but the extensive use of chlorine gas in World War II warfare caused Europeans to shy away from it (Hall, 2000).  UV technology was first introduced in the USA in Henderson, KY in 1916 with the longest continuously operating UV disinfection plant installed in Ft. Benton, MT, in the early 1970’s (Craik et al., 2000).  In fact, through 1987, UV radiation and ozonation remained popular in Europe while only five plants had utilized this method in the U.S. (Hall, 2000). 

 A century later, UV radiation is used in many applications from decontaminating meat to degrading dangerous chemicals found in industrial wastewater.  UV is also used in room air sanitizers, air duct disinfection systems, photochemical reactors, and phototherapy equipment.  This wide range of use has spurned more research on UV radiation to determine what other areas might benefit from it.  For example, considerable amounts of research have been done to determine the removal rates of Arsenic (III), Atrazine, and 2-naphthalenesulfonate when UV radiation is paired with a catalyst like Ti0₂ or an electron acceptor such as Fe (II).  UV can also act on its own in the photodegradation of chlorinated volatile organic compounds (VOCs) (Feiyan et al., 2002).  Much research is also being performed to determine the intensity of light and duration of exposure that a particular water or wastewater needs in order to be appropriately disinfected.  As one would expect, turbid water requires higher doses of UV to achieve the same level of disinfection as less turbid water.  This is due to the fact that colloids and particles in suspension absorb the light waves and shield microbes from the crippling effects of the UV rays.  The absorption of light also decreases the penetration distance into solution further reducing disinfection efficiency.

 The Department of Treasury issued the first drinking water standard in 1914, which called for two or less bacterial coliforms per 100mL of treated water.  Further standards for drinking water followed, but were unenforceable until the Safe Drinking Water Act of 1974 gave power to the government to ensure the drinking water was indeed safe for ingestion (Hall, 2000). Present drinking water standards initiated by the United States Environmental Protection Agency (U.S. EPA) call for 3-log (99.9%) inactivation of Giardia lamblia cysts and 4-log (99.99%) inactivation of all viruses (EPA-815-R-99-015).   

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