(Invited) Advanced Oxidation Process of Micropollutants Removal from Wastewater through Plasma Based Technology

Abstract

During the last century, global population growth, combined with raising living standards resulted in the large-scale production of new compounds such as pharmaceuticals, pesticides, plasticizers and many other chemicals. Many of them are more and more often present in drinking and technological water all over the world. Since 1970, these relative new class of organic contaminants, also referred to as “micropollutants”, have been systematically detected at very low concentrations (µg/l – ng/l). Among them, pesticides have endocrine disruptive, carcinogenic or bioaccumulative properties and are therefore considered hazardous even in trace concentrations of sub-ppm level. Since municipal wastewater treatment plants (MWWTPs) are generally recognized as the major access point of micropollutants into the aquatic environment, the ongoing detection of trace organics demonstrated its incapability in completely eliminating micropollutants. On industrial level, removal of micropollutants is a challenging task due to their high chemical stability, low concentration, and very long lifetime. At Ghent University, the Research Unit Plasma Technology (RUPT), has developed its expertise in novel plasma assisted water treatment for more than 10 years. RUPT successfully applied advanced water treatment by a combination of a pulsed powered direct barrier discharge over a moving water film with absorption on active carbon material [1,2].RUPT studied plasma assisted water treatment of several pesticides: atrazine (ATR), alachlor (ALA), dichlorvos (DVOS), diuron (DIU), pentachlorophenol (PCF), and isoproturon, since they are good representatives of persistent and hazardous micropollution in water. A novel approach combining activated carbon adsorption, non-thermal plasma treatment and ozonation was proposed for the removal of persistent pollutants from water. The study aims an examination of micropollutants removal in view of optimal micropollutant elimination. Prior to optimization of the plasma reactor system, the generation of different active species (O3, H2O2, NO3 - and NO2 -) has been studied [3]. It is shown that presence of N2 in the plasma forming gas is unfavorable and leads to very low efficiency of the treatment due to formation of NOx being OH scavenger. The kinetics of micropollutants decomposition by non-thermal electrical discharge was determined through GC-MS and HPLC methods as a function of input power, duty cycle and type of the used gas. Oxidation by-products for all studied pesticides were examined. Pathways of pesticides destruction were compared with ozonation process showing deeper plasma oxidation with lower selectivity of oxidation that is beneficial for industrial applications. In order to study possible industrial scale up of the process of the plasma treatment the efficiency of conventional UV, O3 and H2O2 based water treatment with a relatively new plasma based process, in terms of removal and energy efficiency was investigated. The experimental study on a synthetic water matrix spiked with four different micropollutants: atrazine (ATZ), alachlor (ALA), bisphenol A (BPA) and 1,7-α-ethinylestradiol (EE2) was performed [4]. For the different processes examined in this study, ozonation was more effective compared to UV based techniques in terms of energy efficiency. Although the energy efficiency of plasma-ozonation falls between the energy cost of O3 and UV-based AOPs, the removal kinetics generally proceed faster compared to other AOPs, achieving complete elimination (> 99.8 % removal) of the target compounds within short treatment time. Moreover, the results suggest that improvement in the mass-transfer in the plasma-ozonation setup permits to further decrease the energy cost of this process up to electrical energy per order (EE/O) values already close to the accepted value in industrial applications.