A set S of vertices in a graph G is a total dominating set if every vertex of G is adjacent to some vertex in S. The minimum cardinality of a total dominating set of G is the total domination number of G. A graph is total domination vertex removal stable if the removal of an arbitrary vertex leaves the total domination number unchanged. On the other hand, a graph is total domination vertex removal changing if the removal of an arbitrary vertex changes the total domination number. In this paper, we study total domination vertex removal changing and stable graphs.

The objective of this dissertation was to provide a controlled comparison of identical continuous flow BNR processes both with and without prefermentation in order to provide a stronger, more quantitative, technical basis for design engineers to evaluate the potential benefits of prefermentation to EBPR in treating domestic wastewater. In addition, the even less understood effect of prefermentation on denitrification kinetics and anoxic phosphorus (P) uptake was studied and quantified. Other aspects of BNR performance, which might change due to use of prefermentation, will also be addressed, including anaerobic stabilization. Potential benefits to BNR processes derived from prefermentation are compared and contrasted with the more well-known benefits of primary clarification. Finally, some biokinetic parameters necessary to successfully model both the activated sludge systems and the prefermenter were determined and compared for the prefermented versus the non-prefermented system. Important findings developed during the course of this dissertation regarding the impact of prefermentation upon the performance of activated sludge treatment systems are summarized below: • For a septic COD-limited (TCOD:TP < 40:1) wastewater, prefermentation was found to enhance EPBR by 27.7% at a statistical significance level of alpha=0.05 (95% confidence level). • For septic P-limited (TCOD:TP > 40:1) wastewaters, prefermentation was not found to improve EBPR at a statistical significance level of alpha=0.05 (95% confidence level). • The increased anaerobic P release and aerobic P uptakes due to prefermentation correlated with greater PHA formation and glycogen consumption during anaerobiosis of prefermented influent. • Improvements in biological P removal of septic, non-P limited wastewater occurred even when all additional VFA production exceeded VFA requirements using typical design criteria (e.g. 6 g VFA per 1 g P removal). • Prefermentation increased RBCOD content by an average of 28.8% and VFA content by an average of 18.8%, even for a septic domestic wastewater. • Prefermentation increased specific anoxic denitrification rates for both COD-limited (14.6%) and P-limited (5.4%) influent wastewaters. This increase was statistically significant at alpha=0.05 for COD-limited wastewater, but not for P-limited wastewater.

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Biomass gasification is a promising technology for conversion of various biomass feedstocks to producer gas for subsequent production of fuels and chemicals. A dual fluidised bed (DFB) steam gasifier is used in the present research to produce the producer gas for Fischer-Tropsch (FT) liquid fuel synthesis. However, NH3 and H2S gases in the producer gas remain an issue to be resolved because they are poisonous to the catalysts employed in the FT reactor. To remove NH3 and H2S, two methods were investigated in this research: (1) primary measures which were employed in the DFB steam gasifier including process optimisation and application of bed materials for catalytic NH3 decomposition and H2S adsorption; and (2) secondary measures or downstream cleaning methods after the gasifier. The combination of the primary measures and the secondary measures is an effective way to remove the NH3 and H2S in the producer gas from gasification process. Studies on the primary measures were divided into two parts. In the first part, in situ reduction of NH3 and H2S in biomass producer gas from the DFB steam gasifier was performed. The primary measures consisted of optimisation of operation conditions and application of bed materials. The main operation conditions in the DFB steam gasifier studied were gasification temperature, steam to fuel (S/F) ratio, and mean gas residence time (f). The bed materials tested include silica sand, iron sand (ilmenite), and calcined olivine sand. For the second part of the primary measures, an influence of the lignite to fuel (L/F) ratio on NH3 and H2S concentrations and conversions in co-gasification of blended lignite and wood pellets in the DFB steam gasifier was investigated. Experiments were performed in the DFB steam gasifier at 800C with blended lignite and radiata pine wood with the L/F ratio ranging from 0% to 100%. It was found that all of the studied parameters including gasification temperature, S/F ratio, f, bed material, and L/F ratio significantly influenced the NH3 and H2S concentrations and conversions in the producer gas. For the secondary measures, a novel hot catalytic reactor and adsorber was developed in the present research for the simultaneous removal of NH3 and H2S. In a hot gas reactor operated at 500-800C and under atmospheric pressure, titanomagnetite was tested for NH3 and H2S removal by hot catalytic NH3 decomposition and H2S adsorption reactions. Titanomagnetite was tested with three different gas streams including 2,000 ppmv NH3 in Ar, 2,000 ppmv NH3 and 230 ppmv H2S in Ar, and 2,000 ppmv NH3 and 230 ppmv H2S in simulated biomass producer gas. From the experimental results, it was discovered that ferrite (α-Fe) readily formed by the H2 reduction of titanomagnetite has shown almost complete NH3 decomposition (100%) in Ar gas at 700 and 800C. The presence of H2S in the gas mixture of NH3 and Ar slightly reduced the catalytic activity for NH3 decomposition at 700 and 800C (>96%) and H2S adsorption of more than 98% could be achieved at the same temperature range. However, in the test with simulated biomass producer gas, 60% NH3 decomposition and 9% H2S adsorption were obtained at 800C, whereas 40% NH3 decomposition and 80% H2S adsorption were obtained at 500C. The decrease of NH3 decomposition and H2S adsorption at 800C in simulated biomass producer gas could be due to the high content of H2 (45 vol%) in the feed gas that favours the reverse reactions of NH3 decomposition and H2S adsorption, the increased surface coverage of the active α-Fe phase by adsorbed hydrogen, and the competition of α-Fe for the reverse water-gas shift reaction. Besides, it was discovered that the temperature significantly affected the removal of NH3 and H2S in simulated biomass producer gas and thus it needs to be optimised.

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