Developing Multiplexed Hybrid Plasma Micro-Reactor for Wastewater Sterilization

Abstract

The industrial revolution has created severe water contamination issues due to the release of toxic chemicals into water channels. This crisis has become worst in underdeveloped countries due to the lack of resources and water management. The textile sector is one of the largest and important industrial sectors consuming a large quantity of water. Consequently, large quantities of highly polluted wastewater containing dyes, toxic materials, and dissolved solids are generated. One of the major pollutants from the textile industry is dyes, being produced 7 x 105 tons annually. The main category of dyes used in the textile sector is Azo dyes containing aromatic amines which are harmful to health. Reactive dyes are the most widely used class of azo dyes which are stable nonbiodegradable organic compounds and are not easily removed by conventional wastewater treatment processes. They cause various health issues such as cancer, skin allergies, and respiratory problems, etc. The economical water treatment technologies are required to resolve water pollution and associated problems. Various Physio-chemical processes such as Coagulation/ flocculation, Adsorption, Membrane Filtration, Ion exchange, Oxidation and Biological process like Aerobic and Anaerobic processes have been used for dyes removal. However, these techniques are associated with some limitations such as either they do not remove dyes completely or they transform them into more toxic compounds. Currently, of all the possible methods for disinfection of water, ozonation is considered to be one of the best techniques as it does not involve the addition of chemicals or produce any harmful by-products. Ozone has a high oxidation potential of 2.07 V. Following fast kinetics, it reacts with contaminants like organic compounds such as aromatics, dyes. Ozone can be generated in non-thermal plasma by air or oxygen discharge. A large number of reactive species such as O3 . ,O.,OH.,NO., N., OONO−, NO2 . - are formed along with ozone in an air discharge. Among these species, OH. radicals have the highest oxidation potential of 2.86 V enabling them to oxidize all kinds of substrates e.g., organic compounds or microorganisms. However, as suggested by kinetic modeling of air discharge, OH. has a very short lifetime of the order of 10-9 s and is decomposed into other products. Similarly, other oxidative species such O. having a shorter life is consumed in the plasma environment to produce a more stable product - ozone. Moreover, in the conventional process, ozone is generated in separate vicinity and is dosed into water. Due to the high reactivity of ozone, it destroys by striking itself and with the container walls. It not only reduces the energy efficiency of the ozone generation process but also the sterilization efficiency of wastewater treatment. An increase in sterilization efficiency could be expected if radicals like OH. and O. could be brought in contact with wastewater along with ozone. However, its economic feasibility with the conventional process could only be compared if air plasma filled bubbles containing ozone and the radicals are dosed in the wastewater simultaneously as they are produced. The transfer of ozone and ions would essentially occur on the bubble/liquid interface. The bubble would be filled with active plasma species converting into ozone. While at the bubble/liquid interface these plasma species and ozone will transfer in wastewater. The efficiency of non-thermal plasmas is limited by their selectivity. The selectivity could be improved by the addition of species that are non-selective and have high oxidation potential. Using photocatalyst will improve the selectivity and provide additional OH for sterilization as well. The research work in this thesis is envisaged to ignite and sustain plasma at the gasliquid interface. Another goal of the dissertation is to integrate plasma with photocatalysis so that the synergic effect of OH radicals could be exploited. The first stage of the thesis is to sustain the plasma in a DBD-Corona hybrid plasma micro-reactor which combines the high energy streamers of the former and homogeneous discharge of the latter. The reactor was optimized for ozone generation using Response Surface Methodology with experimental design -Central Composite Design. A model was developed for analyzing the correlation of parameters, evaluate complex interactions among independent factors, and optimization. Ozone generation was favored at lower values of flow rate and pressure of air, frequency, and higher voltage and electrode length. A novel DBD-Corona hybrid non-thermal plasma micro-reactor was designed and fabricated. The ozone quantification and optimization of the plasma micro-reactor were followed by the degradation of an Azo dye. The efficiency of the hybrid plasma microreactor was investigated and compared for empty channels and packed-bed plasma microreactor (filled with glass beads) for ozone generation. The ozone concentration produced in a packed-bed reactor increased by 24% as compared to an empty channel plasma microreactor. The packed-bed plasma micro-reactor gives 50% higher dye degradation efficiency than the empty channel plasma micro-reactor. The plasma micro-reactor was multiplexed by connecting six individual reactors in parallel, to scale up ozone formation and its application in wastewater treatment. The maximum amount of ozone (480.8 ppm) was generated at the optimized conditions of reactor operating parameters in multiplexed hybrid plasma micro-reactors as compared with 100 ppm of single-channel plasma micro-reactor. The complete degradation of methyl orange was observed in 6 minutes for a 900 ml solution. To improve the selectivity of plasmas, a novel packed-bed Corona-DBD hybrid plasma micro-reactor was coupled with a novel photocatalyst, AgFeO2/CNTs/TiO2. The novel heterojunction of AgFeO2/TiO2/CNTs gave reduced bandgap and charge recombination and facilitated the degradation under visible light irradiations. AgFeO2/TiO2/CNTs provided the additional advantage of the Fenton process, increasing the number density of OH. radicals. The integrated plasma photocatalysis gave 98% degradation of 250 ml solution of methyl orange at a pH 3 with 0.5 g/l of 2.5% AgFeO2/CNTs/TiO2 in 2 minutes.

An advanced oxidation reactor made of Teflon was designed and fabricated to prove the second hypothesis of the thesis, which is simultaneous production and dosing of ozone and other reactive species in-situ in wastewater to degrade targeted substrate. The designed reactor was quantified for ozone. It was observed that methyl orange was degraded at higher rates in the absence of tertiary butanol which showed that OH. radicals produced in the reactor were able to directly react with the synthetic wastewater. This proved the second hypothesis of the thesis-designing a reactor that enables plasma formation at the gas-liquid interface or bringing plasma filled air bubbles inside the liquid to bring OH. radicals in wastewater along with ozone.