Water disinfection with UVC and/or chemical inactivation.Mechanistic differences, implications and consequences

Abstract

The chemical inactivation of Escherichia coli employing a commercial mixture of peracetic acid (PAA) was studied. For this purpose, experiments were carried out using dilutions of the unmodified mixture, and also the same mixture but altered with hydrogen peroxide (HP) previously inhibited. Also, these results were compared to those obtained before employing HP alone. It was found that the mixture is much more efficient than HP and PAA acting separately. Furthermore, it was found that PAA without HP is much more efficient than HP alone. A plausible explanation is presented. The homolysis of PAA would give rise to a chain reaction that generates a significant number of highly oxidizing radicals. An attacking scheme to bacteria in two stages is proposed, where the initial step, mainly caused by PAA, is very fast and eliminates some specific components of the bacteria that would otherwise inhibit the parallel action of HP. Thereafter, the emergence of a potentiating synergetic action of the second oxidant seems to be immediately unveiled. Human society requires water for drinking, sanitation, cleaning, production of food and energy, and support of commercial and industrial activities. Water in nature can contain a variety of contaminants such as minerals, salts, heavy metals, organic compounds, radioactive residues and living materials, for example parasites, fungi, and bacteria (US EPA, 2003). In rural and urban areas of low-income countries, millions of the most vulnerable people lack access to improvedwater, sanitation and hygiene (WASH) services. Unsafe water from all sources contributes significantly to the global burden of disease, principally through thewaterborne transmission of gastrointestinal infections such as cholera, typhoid, hepatitis, and a wide range of agents that cause diarrhea and even death. Thus, cheap and effectivewater treatment systems that can be used at different scales, from single-point water sources to small-community water supplies, can make a valuable contribution to reducing the burden of disease by improving access to safe water (Ahmed et al., 2011). Microbiological contamination is a widespread problem and water is one of the most important vehicles for disseminating this type of pollution, contributing to the dispersion of bacteria, yeasts, fungi, spores, etc. Part of this contamination is the product of an uncontrolled discharge of biological wastes or the usage of domestic sewage systems without the corresponding treatment. Typically, these problems are very often solved with chlorine (or its derivatives) disinfection, an old, low cost water treatment technology that is very efficient and has an extensive use. Alongside these advantages, it is well-known the existence of an important drawback resulting from the toxicity of some of the chlorine disinfection by-products (DBPs) produced by the interaction of chlorine and chlorine derivatives with organic substances either naturally existing in water, or resulting from improperly treated industrial or sanitary wastes (McDonnel and Russell, 1999). Some of these DBPs have been already included in the existing lists of substances having mutagenic or carcinogenic properties. During the last years, organizations of different origin have insisted in the need for a gradual substitution of chlorine for water disinfection and requested for more research efforts aimed at developing efficient alternatives having reasonable costs (Ahmed et al., 2010). Global reduction of chemical deposition into the environment is necessary. Addressing these problems calls out for a tremendous amount of research to be conducted to identify robust new methods of purifying water at lower cost and with less energy, while at the same time minimizing the use of chemicals and impact on the environment. In the latest advances in water purification and disinfection, mainly in the oxidation of toxic organic compounds, persistent and cumulative, are used the new technologies of advanced oxidation processes (AOPs), which are methods that involved chemical or photochemical generation and use of species transitional powerful as the hydroxyl radical (HO?). This work contains a comprehensive, albeit reduced, report on some of the processes in use, the kinetic modeling that accompany several of them and the theories behind those proposals, especially when they have been developed by us, in relation to technologies for water disinfection. Five disinfection methods were compared: (i) UV disinfection, (ii) hydrogen peroxide disinfection, (iii) peracetic acid disinfection (iv) peracetic acid+UV disinfection and (v) Hydrogen peroxide+UV disinfection. The main target of the study was trying to understand and interpret the differences that exist between the different procedures. In addition, we were searching for quantitative information in order to get an idea, as approximate as possible, about operating conditions and final results, with the aim of being able to distinguish among them, which might be the most efficient, economical and environmentally friendly method.