DEVELOPMENT OF ADVANCED TECHNOLOGY OF CHLORINE-FREE WASTEWATER DISINFECTION

Global water demand has increased with urbanization, industrialization and agricultural development. The incidence of chronic drought and water quality issues has also increased. These factors have led to water scarcity, which negatively impacts water-intensive industries financially as well as communities, many of which are rural and disenfranchised.
In addition to bacteria, contaminants such as viruses, pesticides and pharmaceuticals can impact water quality. Decontamination methods that are environmentally friendly have become an area of interest for wastewater treatment facilities. To meet these growing requests, a better understanding of decontamination, pollution prevention and water reuse is urgently needed.
We report about new approach that uses electrochemical (EC) and photochemical (PC) processes to induce oxidative stress in foodborne bacteria.
Electrochemistry
We propose low-voltage EC processes because they generally use power at levels one order of magnitude lower than the high-voltage processes. For example, electrolyzed oxidizing water is generated through the electrolysis of sodium chloride solutions and products of these reactions have been shown to kill bacteria in poultry wastewater [1]. Micro-Tracers R&D has identified similar low-voltage, electrolytic processes that are completely chlorine-free [2].
In the absence of chlorine, the primary killing function of low-voltage EC disinfection is provided by short-lived products of oxygen reduction [3, 4]. When molecular oxygen (O2) undergoes sequential, univalent reductions, the resulting molecules and ions are known as Reactive Oxygen Species (ROS) [5]. At ambient pressure and temperature, the first two reactive intermediates, superoxide (O2●-) and hydrogen peroxide (H2O2), can be produced by reducing O2 [6] and during direct metal autoxidation [7].
Metal ions in solution, especially copper (Cu+) and iron (Fe2+), further reduce these relatively stable pro-oxidants and form highly-reactive hydroxyl radicals (OH●). To perpetuate the Fenton reaction, metal ions must be continuously reduced (e.g. by O2●-, ascorbic acid, cathodic reduction and/or photo-reduction). Major advantages of the approach are: the in-situ generation of H2O2, the release of metal ions at the electrode surface, the regeneration of reduced metal ions and the use of copper which is registered as an antimicrobial material by the U.S. Environmental Protection Agency in 2008.
In addition to stainless steel electrodes, electrodes made of copper are proposed because they have been shown to be effective at killing bacteria [8, 9]. Findings from the research will also provide insight into potential synergistic combinations of oxidants while minimizing energy consumption and corrosion on conductive pipelines. In addition to measuring the biocidal effect of low-voltage EC processes, photochemical and combined electro-photochemical processes will be investigated.
Photochemistry
Another common alternative to chlorinated water disinfection uses non-thermal, light energy. Photodisinfection devices expose contaminated water to solar radiation or to radiation from an electric light bulb. Typically, units with light bulbs maintain a consistent radiation contact time by controlling the continuous flow of contaminated water. While the addition of photosensitive compounds has become common for water disinfection [10], some lesser-known additives have not yet been investigated.
In general, water turbidity reduces the efficiency of photochemical (PC) processes. However, introducing photosensitive compounds (i.e. photosensitizers,) into contaminated water can improve the biocidal effect. Absorption of light by Ps results in the excitation of molecules from ground state to excited state. After the initial excitation to a singlet state, a Ps can lose its excitation energy by emitting light or dissipating energy in the form of heat. Alternatively, excited Ps can interact with neighboring molecules. In the case of excited Ps interacting with dissolved O2, Photodynamic Reactions catalyze the production of O2●-, H2O2, OH● [11] and 1O2 [11].
Overall, 1O2 is toxic in biological systems; it leads to cell death through oxidation of a variety of biomolecules, such as membrane-bound mitochondrial proteins, DNA, and lipids [12]. Its generation through Ps that target specific biomolecules has therefore been proposed as an alternative to antimicrobial drugs, insecticides and herbicides, in wastewater treatment and blood sterilization, and in photodynamic therapy of various forms of cancer [13], among others.
While traditional Ps include cyclo-aromatic hydrocarbons and dyes (e.g. fluorescein, Rose Bengal, methylene blue), these compounds are not known for their photochemical stability. This decomposition of unstable dyes can limit the duration of their practical application [14]. However, thiophene -based dyes with higher photo-stability present an opportunity to improve phototoxicity.
The research will measure 1O2 generated from heterogeneous phase photosensitive dyes with improved lightfastness. While these unique PC processes may also improve the disinfection performance, the use of visible radiation versus UV radiation will keep operating and capital costs low. Micro-Tracers R&D has found both photochemical generation of 1O2 and electrochemical generation of H2O2 and OH● to be suitable for killing bacteria in poultry wastewater [14]. Each individual component can achieve a reduction in bacterial counts separately, however we are certian the collective biocidal effect will be many times greater.

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