Biofilms are colonies of bacteria attached to the surface at a solid-fluid interface. Bacteria in biofilm produce exopolysaccharides (EPS) that form a gel-like matrix in which the bacteria are embedded. Biofilms have numerous consequences in industrial and medical settings, both positive (bioreactors, digestion) and negative (blocking, as corrosive damage of materials/devices, food contamination, clinical infection). The use of antibiotics or mechanical clearing can be effective at removing biofilms, but such treatments are not always effective or appropriate in all situations. Recently, non-thermal atmospheric plasma treatments have been proposed as an alternative (or complementary) form of treatment, that can target sites of infection with minimal damage to the surroundings (e.g. host cells in a clinical setting). These plasmas generate a multitude of chemical species, most of which are very short lived, that can infiltrate and diffuse into the biofilm killing the bacteria within. The aim of this thesis is to develop a multi-dimensional mathematical model to investigate the effect of a non- thermal plasma on biofilms in time and space and to identify key factors that determine effectiveness of the treatment. Most of the chemical products of cold plasmas are too short lived, or too reactive, to be effective in killing the biofilms, it is the longer live species, e.g. ozone, hydrogen peroxide, acid species, that penetrated the biofilm and do the most damage. However, the EPS in biofilms is an effective barrier against ozone and hydrogen peroxide. No published biofilm model combines multi-dimensional growth with a detailed description of EPS production, hence a new mathematical model is developed and applied to simulating plasma treatment. The thesis is split broadly into two parts. The first part presents a new biofilm model framework that simulates growth in response to any number of substrates (e.g. nutrient, oxygen). The model combines an Individual based model (IbM) description of bacteria (individuals or clusters) and substrates are described as a continuum. Novel features of the framework are the assumption that EPS forms a continuum over the domain and the explicit consideration of cellular energy (ATP). Simulations of this model demonstrate the contrast between biofilm grown with topical nutrient sources (forming irregular, bumpy biofilm) and basal nutrient source with topical oxygen such as biofilm grown on agar (forming regular spatially uniform biofilms). The former is in broad agreement with experiments whilst the latter, to our knowledge, has been the subject of very little experimental study. The second part extends the modelling framework to consider the effect of the plasma species. The simulations demonstrate that penetration is a key factor in their effectiveness, for which EPS plays a key role in preventing spread within and beyond the plasma treated zone. The simulations provide estimates of the timescale of equilibration of the main plasma species, predict the effect of combining these species and demonstrate how the constituents of the biofilm can change following treatment. A number of recommended suggestions for future theoretical and experimental study are discussed in the conclusions.

<p> This thesis focuses on the thermal plasma treatment of non-point source pollutants accumulating in stormwater ponds. Stormwater ponds are constructed as a part of urban non-point source pollution control systems. Pollutants from various sources are collected in the stormwater ponds as sediments. In this work, stormwater sediments were first separated by a filter with an opening of 208μm. The filtered sludge-water was subjected to pulsed arc electrohydraulic discharge (PAED) treatment while the solid part (i.e., wet sludge and dried PAED treated sludge) was subjected to thermal plasma treatment under non-DC transferred and partial transferred operation modes. The results from the PAED sludge-water treatment show that the reduction of TOC in sludge-water was approximately 80% and was greater than 90%, respectively, after 5 minutes and 2 hours of PAED treatment. The accumulated gaseous concentrations of CxHy, CO, C02, S02, H2S and NO emission from sludge-water treatment were 8.2, 3.1, 1.9, 0.32, 0.29 and 0.07 mg/L, respectively, after 2 hours of PAED treatment. The concentrations of volatile elements in sediments such as S, Br, Cl and K decreased approximately 80, 90, 30 and 20% respectively. The solid-phase carbon was observed to be approximately completely removed after treatment. Based on the above results, it can be concluded that PAED successfully degraded organic compounds into C02, CO and CxHy, and converted sulfur and nitrate compounds into S02, HzS and NO. </p> <p> Thermal plasma wet-sludge treatments showed that a reduction of TOC was approximately 52% with argon plasma gas and air flow rates (in the reaction zone) of 24 and 2.4 L/min, respectively. Based on SEM images, wet sludge was melted under partial transferred mode. Thirteen elements with concentration relationships of 0 > Si > Al > Ca > S >Fe> K > Mg > Na > Cu > C > Ti > Cl were quantified by the X-ray energy dispersion technique. The elemental weight percentages of Si, K, Fe and 0 increased with increasing reaction zone air flow rate, while Ca and Cu decreased with increasing air flow rate. Thirty two elements were quantified by Neutron Activation Analyses (NAA) but only 27 elements were above the detection limits. Major elements (concentration> 1000 ppm) with relative concentrations of Ca > Al >Fe> K > Mg > Na > Ti > Cl; minor elements (100 - 1000 ppm) with relative concentrations of Mn > Ba > Sr > Zn; and trace elements(< 100 ppm) with relative concentration were Mo > V > Cr > Br >La> As > Sc > Th> As > Co > Dy > W > Sb > Eu; were determined. Concentrations of Zn, La and Co were enriched 90, 50 and 30% on average respectively, while concentrations of Br, W and As decreased by 80, 50 and 20% on average respectively. The chemical compositions in sludge were quite different after thermal plasma treatment. The average percentages of sand (Si02) and calcite (CaC03) decreased 35 and 10% respectively, while compounds such as KAlSi08, Fe304, NaCl and CaS04 were formed after thermal plasma treatment. Gaseous hydrocarbons, H2S, CO and NO were emitted continuously during the thermal plasma treatment of sludge. Higher reduction of organics and sulfur compounds and suppression of NOx formation were observed in the thermal plasma treatment of wet sludge. The integrated system consisting of PAED sludge-water treatment and thermal plasma wet sludge treatment under partial transferred mode may provide a potential for stormwater pond sediment treatment control. </p> / Thesis / Doctor of Philosophy (PhD)