The impact of ozone demand on the required dosage in dielectric barrier discharge plasma-ozonation systems : investigation into a simulated surface water

Abstract

The provision of clean and safe water and sanitation (Sustainable Development Goal 6) and the promotion of good health and well-being (Sustainable Development Goal 3) are critical challenges faced globally, exacerbated by rapid population growth and industrialisation leading to increased contamination of freshwater sources. Waterborne diseases, responsible for approximately 1.4 million annual deaths as reported by the World Health Organization (WHO), highlight the urgency of addressing these issues. Traditional water treatment methods such as boiling, chlorination (addition of chorine tablets) or liquid chlorine bleach, ceramic filters, and UV disinfection have limitations, including inadequate inactivation of all microorganisms and the formation of undesirable by-products, necessitating the exploration of advanced oxidation processes (AOPs), specifically ozone treatment. Extensive studies have been done on the generation of ozone using plasma technology. However, depending on the combination of electrode material, feed gas, current (alternating or direct), and type of discharge use (corona, direct barrier discharge), a wide range of ozone concentrations generated from nonthermal plasma has been reported. Consequently, optimizing the ozone dose is essential for effective water disinfection, as residual ozone can pose toxicity risks for human consumption. This research emphasizes the development and optimisation of advanced plasma-ozonation systems, specifically tailored to the properties of reactor materials and the targeted disinfection outcomes. There remains ongoing research that produces conflicting results regarding whether Chemical Oxygen Demand (COD) and Total Suspended Solids (TSS) or COD and Total Organic Carbon (TOC) are the most effective indicators of ozone demand in water bodies for predicting the appropriate ozone dosage required for efficient ozonation. This study investigates the relationship between ozone demand and key water quality parameters, specifically TSS, COD, and TOC. By analysing these relationships, the research aims to predict optimal conditions for treating contaminated water through a Dielectric Barrier Discharge (DBD) plasma ozonation system. Understanding how these parameters influence ozone demand will provide valuable insights into enhancing the efficiency of the ozonation process, enabling better treatment strategies for contaminated water sources. This work ultimately seeks to contribute to the development of effective water treatment protocols that leverage DBD plasma ozonation technology. Utilising Box-Behnken Design (BBD) in Response Surface Methodology (RSM), statistical analyses were conducted to optimise ozone generation in a high-voltage direct current (DC) dielectric barrier discharge (DBD) reactor fed with oxygen. Key operational variables—gas flow rate, applied voltage, and frequency—were assessed. Analysis of variance (ANOVA) revealed that frequency was the only factor with a significant impact on ozone concentration, with the quadratic model yielding a maximum ozone concentration of 1.09 mg/L.3 The influence of optimised ozone doses on simulated water containing glucose, phenol, and humic acid was evaluated by monitoring COD, TOC, TSS. Results indicated no significant reduction in COD and TOC with glucose, underscoring the importance of pH and functional groups for effective ozonation. Ozonation of phenol and HumeGro (a fertilizer consisting of humic acid) samples yielded maximum reductions of 49% and 54% for COD, and 11.6% and 38% for TOC, respectively. Predictive modelling showed that COD and TOC could be effectively analysed using linear or logarithmic transformations, while TSS (93% reduction in HumeGro samples) demonstrated variability that did not conform to linear models. TSS consists of particulate matter with varying characteristics such as size, composition, and density, which can lead to inconsistent responses during ozone treatment. Consistent trends were observed in repeat experiments with surface water. In conclusion, the integration of COD and TOC as indicators, combined with the strategic modelling of ozone doses, underscores the importance of a data-driven approach in optimising the DBD plasma ozonation system. This not only enhances the treatment efficiency but also aligns with broader environmental goals of reducing organic pollutants in water systems. However, several factors require further optimisation, including the filtration of TSS and the reaction time. Additionally, the current setup must undergo scaling up before it can be deemed suitable for point-of-use (POU) implementation.