SUBJECTS disinfection

In order to ensure the destruction of pathogens, the process of chlorination must achieve certain control of at least one factor and, preferably two, to compensate for fluctuations that occur. For this reason, some authorities on the subject stress the fact that the type and concentration of the chlorine residual must be controlled to ensure adequate disinfection. Only this way, they claim, can chlorination adequately take into account variations in temperature, pH, chlorine demand and types of organisms in the water. While possible to increase minimum contact times, it is difficult to do so. Five to ten minutes is normally all the time available with the type of pressure systems normally used for small water supplies. Many experts feel that satisfactory chlorine residual alone can provide adequate control for disinfection. In their opinion, superchlorination-dechlorination does the best job. Briefly, what is this technique and how does it operate ... 

Unrestricted use of reclaimed wastewater for drinking water, however, requires careful examination. While practically a complete barrier to viruses, bacteria, and other toxic entities that must be kept out of a potable supply, RO membranes could pose serious problems should any defect develop in their separation mechanism. Given the purity and clarity of RO-treated wastewaters, however, it might be advantageous to use RO and then subject the product to well-established disinfection procedures. 

It is only very recently that organic componnds synthesized by humans have begun to exert a selection pressure upon natural populations, with the consequent emergence of resistant strains. Pesticides are a prime example and will be the principal subject of the present section. It should be mentioned, however, that other types of biocides (e.g., antibiotics and disinfectants) can produce a similar response in microbial populations that are exposed to them. 

This book examines comprehensively the chlorine industry and its effects on the environment. It covers not only the history of chlorine production, but also looks at its products, their effects on the global environment and the international legislation which controls their use, release and disposal. Individual chapters are dedicated to subjects such as end use processes, water disinfection and metallurgy, environmental release of organic chlorine compounds, polychlorinated biphenyls, legal instruments and the future of the chlorine industry. 

The use of chlorine for disinfecting wastewaters or drinking water has become widespread in this century. Recently, however, the increasing presence and variety of aquatic contaminants has raised the question of the chemical fate of these contaminants when subjected to aqueous chlorination [36]. In fact, the production of organochlorine compounds in chlorinated water, including mutagenic and carcinogenic substances, is well established [37,38], A number of alternatives to chlorination are used in many parts of the world, but the risks associated with their by-products are even less well established [39],... 

Insecticides. Whereas fleas can be effectively dealt with by disinfection of clothes and living quarters, lice and mites require the topical application of insecticides to the infested subject. 

Bromate has been classified as a human carcinogen by both the I/VRC (International Agency for the Research on Cancer) and the USEPA (United States Environmental Protection Agency) and is known to be toxic to fish and other aquatic life [11, 12]. Bromate could be produced in aquatic systems upon the oxidation of aqueous bromide. Controlled ozonation has been considered as an effective disinfectant tool in aquatic systems [13] but when sea water is subjected to ozonation, oxy-bromide ozonation by-products (OBP) are produced and these are important both in terms of their disinfection ability and also in relation to their potential toxicity. When seawater is oxidized, aqueous bromide (Br-) is initially converted to hypobro-mite (OBr ) which can then either be reduced back to bromide or oxidized further to bromate (Br03-) which is known to be toxic to fish and other aquatic life and classified as a human carcinogen. There has been thus a considerable interest in bromate analysis so that trace analysis of bromate in water has received considerable attention in recent years. 

Considerable information of a general nature is available for uncontaminated water subject to the production of disinfection byproducts. The mutagens produced by drinking water chlorination appear to be numerous, but they exist either at low levels or are of low potency. For both the unresolved mixtures and for the few mutagenic compounds thus far identified, activity is readily reduced or destroyed by treatment with alkali or 4-nitrothiophenol and may be removed by GAC treatment. From water sources subject both to mutagen formation via disinfection and to periodic contamination by toxic chemicals, experimental full-scale GAC treatment systems have provided mutagen-free water. 

In industrially developed and many newly developing countries, the manufacturing and registration of sanitary or disinfection chemicals has been subject to tough regulations for some years by the various national authorities. 

The earliest example of the semiconductor photocatalysis application as a method of disinfection was published by Matsunaga et al. (1985). This work reveals that Ti02 particles were effective in the inactivation of bacteria, such as Lactobacillus acidophilus, Saccharomyces cerevisiae, and Escherichia coli. To date more than 200 studies are related with this subject and at least three reviews were dedicated to photocatalytic disinfection (Blake et al. 1999 Srinivasan and Somasundaram 2003 Carp et al. 2004). Some general conclusions on Ti02 disinfection are reported below and the literature will be discussed more specifically throughout the chapter. 

Disinfection of municipal water contaminated with coliforms and fecal streptococci was the subject of a study by Patermarkis and Fountoukidis [31]. Disinfection was achieved using titanium electrodes and direct current. The polarity was alternated every minute to eliminate titanium oxide buildup. No additives or supporting electrolytes were used in this room-temperatures process. At a current density of 2.5 mA/cm and an applied voltage of 45 V, no microbial activity was detected after 30 min of operation. Noncontaminated, electrochemically treated water possessed a residual disinfection capacitiy addition of treated water to a contaminated sample destroyed the microbial life in the sample. 

It is now very well estabhshed that DOM is the major source of trihalomethanes and other disinfection by-products in disinfected water. In fact, the measurement of THMFP is now a routine monitoring task in the water treatment industry, and suppliers in the US are required to advise consumers of the concentrations of trihalomethanes and other disinfection by-products in drinking water. Efforts to remove DOM from waters before they are chlorinated have driven much of the research that has led to advances in membrane-based methods of isolation of DOM from water (see the discussion of UF, NF, etc., in Section 5.10.4.2.2). Nikolaou and Lekkas (2001) have recently reviewed many aspects of the reactions of DOM with chlorine and other disinfectants. They review the relationships between reactivity of DOM (i.e., formation of disinfection by-products) and the chemical properties of DOM and several types of fractions of DOM. They also discuss the formation and potentially adverse effects of several classes of disinfection by-products. Urbansky and Magnuson (2002) have reviewed the subject of disinfection by-products, including a brief discussion of DOM. Both of these reviews are recommended for further up-to-date details on the role of DOM in the formation of disinfection by-products. 

The poly(vinyl alcohol)-iodine charge transfer complexes have been the subject of many investigations " Water-soluble iodine complexes with ix)ly(l-vinyl-pyrrolidone) are commercially available as disinfecting agents PoIy(N-vinyl car-bazole) polyamides and poly(ethylene oxide) are also found to form molecular addition complexes with iodine and iodine compounds. An account of the different types of molecular complexes formed with various polymers has been published ... 

Chemical disinfection, on the other hand, is not as effective in killing organisms as thermal disinfection, but has several advantages It is simple to use, thereby ensuring greater user compliance. Lens life is longer with chemical disinfection as lenses are not subjected to daily heat treatment. The method results in fewer deposit problems as surface debris left on the lens surface due to improper cleaning is not baked by heat. 

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