Microwave Enhanced Advanced Oxidation in the Treatment of Dairy Manure

Microwave heating is a dielectric heating process, in which heat is generated via the interaction of dielectric materials with electromagnetic radiation. The dielectric constant is a measure of the material’s capacity to retard microwave energy as it passes through, while the loss factor is a measure of the material’s capacity to dissipate the energy. The materials with high loss factor are easily heated through microwave irradiation. Energy dissipation mechanism in the materials is via ionic conduction and dipolar rotation. Generally, the dielectric properties of a material are related to temperature, moisture content, density and material geometry. Microwave heating has the advantages of higher heating rates, no direct contact between the heating source and heating material, selective heating, reduced equipment size and better process control than those of conventional heating. Moreover, both DNA and bacterial cellular membranes can be damaged by microwave irradiation. Such an effect on the treatment process, besides heating, is referred to as the athermal effect (Hong et al. 2006). Microwave heating has been applied in various processes and manufacturing industries, such as food process, wood drying and waste treatment process (Jones, et al., 2002). For the sewage wastewater treatment industry, there are many applications of microwave heating on sludge treatment for reducing volume, improving dewaterability, enhancing digestibility, enhancing nutrient release, pathogen destruction, and stabilizing heavy metal (Menendez, et al., 2002; Liao, et al., 2005; Wojciechowaska 2005; Hong et al., 2006; Eskicioglu et al., 2007; Hsieh et al., 2007; Yu, et al., 2010). It has also been reported that the overall treatment efficiency could be increased with a combination of microwave heating with chemicals (thermo-chemical) for the treatment of sludge (Chan, et al., 2007; Qiao, et al., 2008). The common chemicals used on thermo-chemical process are acid, base and oxidants. The microwave enhanced advanced oxidation process (MW/H2O2-AOP) uses microwave irradiation in combination with hydrogen peroxide to generate hydroxyl radicals to react with organic compounds. As a result, the structure of the solid materials is altered, and nutrients can be released into solution in the process. During this treatment, the suspended solids (SS) content of the slurries is also reduced. Its mechanism is assumed to be similar to the wet-air oxidation process wherein the first process involves the breakdown of large particulate organic matters, such as carbohydrate and proteins into smaller and more soluble organic components, and the second process involves the further oxidation or gasification of some of the resulting