Microbubble-enhanced DBD plasma reactor for the treatment of organic suspensions

Atmospheric pressure plasmas have been applied to a multitude of liquid-based applications such as wound healing, combustion, surface cleaning and small-scale chemical processing. For most of the applications, a plasma jet placed above the liquid is favoured, that is typically supplied with argon or helium. For large-scale processes such as wastewater or biomass pretreatment, localised plasma jets become uneconomical and inefficient. In the work presented in this thesis, a novel microbubbleenhanced DBD plasma reactor is presented which allows efficient transfer of highly reactive species from the plasma to a liquid medium. This novel design allows the discharge to be formed in the immediate vicinity of the gas-liquid interface enabling effective mass transfer of short-lived species. The reactor was characterised in the first instance to evaluate and optimize its performance by varying all the input parameters. These studies include understanding the effect of the bubble size, input gas composition and input power supply. The reactive species generated in the gas phase were measured using absorption spectroscopy, and the liquid phase concentrations of the reactive species were measured using a variety of chemical probes. By modelling the reactor, a more in-depth analysis was carried out by tracking the short-lived species over a high time resolution. This led to improvements in efficiency by selecting the optimum operating conditions depending on the plasma chemistry required. As one of the key reactive species is ozone, being able to quantify this concentration in the liquid phase with both precision and accuracy is essential. This led to the development of a selective chemical probe and measurements made with this new probe were compared against existing measurement techniques. The reactor was then applied to several applications including pretreatment of lignocellulosic biomass, pretreatment of anaerobic digester feed and treatment of final wastewater effluent. Treatment of lignocellulosic biomass is often required to break apart the structure to ease the conversion of biomass feedstock to a fuel, usually facilitated by enzymes or bacteria. By treating a suspension of biomass within the reactor, the reactive species transferred to the solution attack the structure increasing the yield of either biogas or ethanol depending on the process applied. Limitations of this approach to pretreatment were identified and future improvements were suggested. When treating a solution contaminated with E. coli, the oxidative species were found to be highly effective in inactivation of the bacteria. The presence of organic matter such as humic acid can significantly reduce the inactivation rate as they consumed the reactive species with competing reactions.