Browsing by Author "Boshoff, Sonja"

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    Characterization and fermentation of waste paper sludge for bioethanol production
    (Stellenbosch : Stellenbosch University, 2015-12) Boshoff, Sonja; Gorgens, Johann F.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: The need for renewable energy sources are at an unprecedented high due to the world population and energy demand increasing drastically past the point that the remaining fossil fuels are able to supply. Biomass is a sustainable and renewable source of energy with the potential to mitigate greenhouse gas emissions and to serve as an alternative to fossil fuels when converted into biofuels, such as bioethanol or biodiesel. Paper sludge (PS) is a biomass waste stream from the paper and pulp industry that is often landfilled. By converting PS into bioethanol, landfill can be avoided and an energy stream can be produced to be used at the mill or sold. This study investigated the conversion of PS into ethanol and how the nature of the sludge influenced a high solid loading fermentation process. Paper sludge samples from various paper and pulp mills in South Africa were collected and characterized into categories according to chemical composition and the feed utilized at each mill. Significant variation was observed in the chemical composition between mills, whereas clear correlations were observed within categories utilizing the same feed. Screening for fermentation performance also revealed substantial variation due to the differences in digestibility of the samples Based on characterization and screening data, samples from two categories, namely corrugated recycle mills and virgin pulping mills were chosen for further investigation and optimization. Selecting PS samples with high digestibility to ensure maximum ethanol yield and productivity is a critical requirement for process efficiency. However, the PS samples differed substantially in terms of viscosity. Virgin pulp PS, originating from a chemical pulping process, had a significantly higher water holding capacity and viscosity compared to corrugated recycle PS, originating mainly from recycling and repulping operations. These differences affected the maximum solid loading that could be achieved in fermentations, and inherently, the enzymatic hydrolysis of the material where high viscosity would limit enzyme accessibility to the fibers. Given the viscous nature of virgin pulp PS, solids loadings of between 3 to 9% (w/w) achieved the maximum PS hydrolysis to sugar, whereas for corrugated recycle PS the maximum enzymatic hydrolysis was achieved at substantially greater solids loadings of 15% (w/w) and higher. The optimised process with corrugated recycle PS resulted in an ethanol concentration and yield of 45.5 g/L and 78.2 %, respectively, at a solid loading of 27% (w/w) and an enzyme dosage of 11 FPU/gram dry sludge. The optimised process for the virgin pulp PS required a significantly higher enzyme dosage of 20 FPU/gram dry sludge at a lower solid loading of 18% (w/w), to achieve the optimum ethanol concentration and yield of 34.2 g/L and 66.9% (w/w), respectively. The virgin pulp PS was highly viscous at 18% (w/w) and required high agitation of 1500 rpm that, in turn, had a negative effect on enzyme activity from shear stress of the agitator. This study demonstrated that corrugated recycle PS is more suited for bioethanol production compared to virgin pulp PS, primarily due to water holding capacity, viscosity and shear stress associated with high agitation rates, which had a major influence on high solids loading fermentation processes.