Kinetics Study of UV-ZnO Photocatalytic Process for Post Treatment of Composting Leachate

Economic and industrial growth of the recent decades in most countries and the changing pattern of life, cause a dramatic increase in the production of industrial and urban waste. Considering population growth and economic, cultural and industrial changes, it`s necessary to have a proper management plan to prevent environmental damage and save the cost of waste management. One of the issues associated with urban waste management is controlling and treatment of leachate. Leachate treatments methods are classified into three groups include: leachate transmission; biodegradation techniques and physiochemical treatment methods. Due to its reliability, simplicity and high cost-effectiveness, biological processes are the most conventional methods in leachate treatments. However, due to high organic load and presence of refractory contaminants in leachate, biological treatment alone cannot remove all of the organic matters from leachate. Therefore, to meet discharge standards, additional treatment is required to remove the remaining materials from biologically treated leachate. Photocatalytic processes have been proven to be an appropriate technology for final treatment of these types of wastes. In this study the capability of UV-ZnO photocatalytic process as a post treatment method for composting leachate was examined at laboratory scale and in batch mode. The effect of some factors such as initial pH, catalyst concentration, light intensity and reaction time on the removal of organic load and color of leachate were investigated. Biological pre-treated leachate samples were collected from the effluent of leachate treatment facility of a composting plant in north of Iran. The leachate samples have been collected in 20 L plastic containers transported to the laboratory and immediately stored in refrigerator at 4˚C to minimize any changes in its physical, chemical, and biological properties until the experiments were carried out. A Plexiglas column with 110 mm inner diameter and 300 mm height were used to conduct experiments. UVC lamps (as the source of irradiation) with different power levels were placed inside a quartz tube (external diameter 0.026m) mounted at the axial centre of the reactor. In each experiment, a specified amount of nano particle was added to leachate into the reactor, at ambient temperature and under a specified amount of UVC radiation. In order to prevent the settling of nano particles, air was continuously injected into the column through a diffuser at the bottom of the reactor. Samples were taken periodically from the reactor for analysis. Prior to analysis, the liquid samples were centrifuged at 3000 rpm for 10 min to remove all suspended particles. In order to prevent reflection and scattering of UV radiation in the environment, the reactor was covered with a thick layer of aluminum foil. Based on the results of experiments, after 120 minutes of radiation with 32 W UVC lamps in pH 11 and in the presence of 1 gr/L of slurry nanoparticles (ZnO), maximum COD and color removal were achieved to be 60% and 68%, respectively. In kinetic studies of batch UV-ZnO photocatalytic process, a pseudo-first order model with reaction rate constant of 0.38 hr-1 was found to fit well (R2= 0.99) with the experimental results.