Browsing by Author "Eshton, Bilha"

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    Biochemical oxygen demand (BOD) reaction kinetic and mass transfer in horizontal subsurface flow constructed wetland
    (University of Dar es Salaam, 2006) Eshton, Bilha
    The BOD5 reaction kinetics and mass transfer from three constructed wetlands of different sizes and flow rates planted with Phragmites mauritiamts were studied. The plug flow model was assumed and the concentrations of BOD5 were used to determine the overall removal rate constant of dissolved organic matter. The rate constants were determined at inlet flow rates of2m3/d, 4m3/d, 7.7m3/d, and 44m3/d. Their Corresponding rate constants were found to be 0.06/d,0.1/d, 0.15/d and 0.7/d, respectively. The removal rate constants were observed to increase with increasing flow velocity. This implies that, they obtained rate constants were heavily influenced by mass transfer in the system. Correlation of dimensionless groups namely Reynolds Number, Sherwood Number and Schmidt Number was applied to the obtained rate constants. Plot of the log K against log u yielded a straight line with x = 0.86.The correlation matched the literature values of x for packed beds although the constant x obtained was higher than the literature values for laminar situation in the wetland, i.e. x = 0.67.It was concluded that, the removal rate and thus the efficiency of constructed wetlands for the removal of BOD is highly influenced by the hydrodynamics situation in the wetland. Because of the strong dependence on mass transfer, all measures that increase mass transfer will increase the performance of constructed wetlands.
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    Evaluation of the potential environmental impacts of liquid biofuels production and consumption in Tanzania: the case of jatropha based biodiesel and molasses based bioethanol
    (University of Dar es Salaam, 2012) Eshton, Bilha
    Production and use of liquid biofuels such as biodiesel and bioethanol (or ethanol), are being promoted worldwide with a notion that, they mitigate greenhouse gas (GHGs) emissions, increase energy security, reduce dependence of fossil fuels etc. However, their production requires use of inputs such asfossil fuels and agrochemicals which results into emissions of GHGs and other pollutants that may have negative impacts to environment and human health. Though biofuels are being promoted in Tanzania, but there are no local documented emissions data from their production which are essential for policy makers and bioenergy stakeholders.Thus, this research work has been undertaken with the aim to evaluate the potential environmental impacts of production and use of Jatropha based biodiesel and sugarcane molasses based bioethanol in Tanzania. The study also examined whether production and use of biofuels resultsinto energy loss or gain as well as fossil fuels requirements of in their production chain. Carbon stock change as a result of land use change was also evaluated. In evaluating the environmental impact of liquid biofuels, the life cycle assessment (LCA) approach was used. The study found net positive GHG emissions of biofuels. It was observed that, production and use of biodiesel and bioethanol result into net GHG emissions of about 1000 kg CO2 eq. /t and 533 kg CO2 eq. /t respectively. Other potential environmental impacts observed include Acidification, Eutrophication, Human and Eco-toxicity and Photochemical Oxidant formation. Farming of biocrops in both biofuel systems found to be a major source of environmental emissions followed by use of diesel fuel. The study found production of liquid biofuels in Tanzania results into net energy gain and Jatropha biodiesel consumes more fossil energy than molasses bioethanol. It was also found that, converting miombo woodlands for Jatropha farming and thickets for sugarcane farming result into GHG emissions of 199.3 t CO2 eq./ha and 135.8 t CO2 eq./ha respectively. The lost carbon may be paid back after 35 years of consecutive Jatropha farming and 16 years of sugarcane farming. For the upcoming biofuel policy, the study found the following important aspects to be considered (i) biofuel crops should be cultivated on degraded or wastelands (ii) use of biofuels in farm machinery should be promoted (iii) burning of sugarcane before harvesting should be prohibited (iv) use of by-products from biofuel processing as energy source should be encouraged and (v) biocrops farming by the small scale farmers should be promoted so as to improve rural economic development. It was concluded that production and use of liquid biofuels results to quantifiable emissions to air, water and soil that may have negative impact on environment and human health.Replacing fossil fuels with biofuels reduce GHG emissions, however all other environmental impacts observed may increase. It was also concluded that the results of this study are very useful by policy makers and bioenergy stakeholders. In order to minimize emissions of biofuels, it was recommended that sugarcane burning before harvesting should be avoided; use biomass energy sources in biofuel industries should be encouraged and farming of biocrops on native forests should be avoided. Furthermore, further research such as economic implication of large scale biofuels production in Tanzania should be carried and research and development may be carried out so as to optimize biocrops plantations.