Theses and dissertations (Applied Sciences)
Permanent URI for this collectionhttp://ir-dev.dut.ac.za/handle/10321/6
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Item Enhancement of biohydrogen production from the aquatic weed Pistia stratiotes through a dark fermentation process(2019) Mthethwa, Nonsikelelo Precios; Pillai, Santhosh Kumar Kuttan; Bux, Faizal; Kiambi, Sammy LewisAquatic weeds are well known for their fast growth rate and high carbohydrate content that can be easily hydrolysed into fermentable sugars. This study was aimed at the utilization of an indigenous aquatic weed, Pistia stratiotes for biohydrogen production through the dark fermentation process. Characterization of the biomass, effect of pre–treatment methods on biomass hydrolysis, effect of reactor operational conditions and type of inoculum on enhancing hydrogen production potential of P. stratiotes was assessed. Physical and chemical pre–treatments were employed on P. stratiotes biomass to increase digestibility and to achieve high conversion rates of fermentable sugars. The highest sugar yield of 139± 0.8 mg/g was obtained when the oven dried biomass was subjected to H2SO4 (2.5%) pre– treatment followed by autoclaving at 121°C for 30 min. Biohydrogen production under different operational conditions was thereafter optimized using One–factor–at–a–time (OFAT) batch experiments in 120 mL serum bottles. A maximum hydrogen yield (HY) of 2.46 ± 0.14 mol-H2/mol-glucose (3.51 ± 0.20 mg-H2/g-dry weight) and 2.75 ± 0.07 mL h-1 hydrogen production rate was observed under optimized conditions (pH 5.5, Temp 35°C, S/X: 1.0 g-COD/g-VSS and HRT 8 h). The organic mass balance (92 – 96%) and electron– equivalent balance (92 – 98%) further indicated the reliability of the obtained fermentation data. Assessment of microbial activity was achieved using molecular techniques such as quantitative polymerase chain reaction (qPCR) targeting both 16s rRNA (of Clostridium spp., Bacillus spp., and Enterobacter spp.) and the functional hydrogenase gene (hydA). The highest gene activity of hydrogenase was noted at pH of 5.5 with 2.53×104 copies/ng-DNA compared to low pH: 4.5 (6.95 × 103 copies/ng-DNA) and high pH: 8.5 (7.77×103 copies/ng- DNA). A similar trend was also observed for the species containing a highly active hydrogenase (i.e. Clostridium spp., Bacillus spp., and Enterobacter spp.). During the optimum reactor conditions, three hydrogen producing bacterial strains Bacillus cereus and Enterobacter cloacae were successfully isolated. These isolates were used as inoculums for the pure culture studies and achieved HYs of 2.2, 1.10 and 1.97 mol-H2/mol-glucose respectively under optimized fermentation conditions. However, the thermally treated mixed culture displayed a marginally higher HY (2.46 mol-H2/mol-glucose) compared to the pure culture used alone. Furthermore, the cost estimation indicated a potential and economically feasible for biotransformation of P. stratiotes to hydrogen energy. In conclusion, the results from this study has revealed the potential of employing P. stratiotes biomass for biohydrogen production. The results also indicated the importance of employing suitable pre–treatment methods, operating conditions as well as inoculum types for enhanced hydrogen production using P. stratiotes.Item Evaluation of biohydrogen production potential of sugarcane bagasse using activated sludge in a dark fermentation process(2016) Reddy, Karen; Bux, Faizal; Kuttun Pillai, Sheena Kumari; Gupta, Sanjay KumarAnaerobic dark fermentation is an efficient biological process to produce hydrogen from waste material. In South Africa, this technology has not been explored adequately to extract energy from biological wastes. Within the KwaZulu Natal region of South Africa, the sugar industry is a prominent venture that produces mass quantities of sugarcane bagasse amongst other waste products. This by-product can be an ideal source of substrate for biohydrogen generation. In this study, sugarcane bagasse was used as the main substrate for biohydrogen production by anaerobic fermentation using sewage sludge as the inoculum. Different pre-treatment methods were employed to maximize the release of fermentable sugars from the lignocellulosic biomass. Among the different pre-treatment methods employed, the maximum sugar yield (294.4 mg/g) was achieved with 0.25% H2SO4 for 60 minutes at 121°C. Prior to inoculation, the sewage sludge was also subjected to thermal pre-treatment to eliminate methanogens. Thermal pre-treatment of inoculum sludge for 30 min was effective in eliminating methanogens. Fluorescence in situ hybridization was used to positively identify the hydrogen producing bacteria present before and after treatment. The pre-treated substrate and inoculum was integrated into a dark fermentation process to further optimize the effect of pH, substrate to biomass, iron and magnetite nanoparticles on hydrogen production. The maximum hydrogen production (1.2 mol/mol glucose) was achieved at a pH range of 5-6, a substrate to biomass ratio of 3.5, and iron and magnetite nanoparticle concentration of 200 mg/L. Microbial analysis using quantitative polymerase chain reaction has confirmed the dominance of Clostridium spp. in the reactor. The highest hydrogenase gene activity (number of copies of hydrogenase gene expression/ng DNA) was recorded in the reactor supplemented with magnetite nanoparticles with lowest being in the raw sludge. There was a direct positive correlation between the hydrogenase gene copy number and the hydrogen yield obtained at different reactor conditions. Scanning electron microscopy was a useful to visually analyse the interaction of microorganisms with activated sludge. This study highlights the significance of anaerobic microorganisms from waste sludge being able to utilize agricultural waste material to produce biohydrogen which could be further scaled up for continuous hydrogen production. In addition, statistical tools used to predict the possible sugar (Design of experiments) and hydrogen yields (Gompertz model) produced would be helpful in saving time during full-scale operation of biohydrogen producing reactors.