A dissertation submitted to the Faculty of Engineering and the Built Environment, University
of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of
Doctor of Philosophy in Engineering, October 2017 / The dwindling fossil reserves coupled with environmental pollution necessitate the search for
clean and sustainable energy resources. Biohydrogen is emerging as a suitable alternative to
fossil fuels and has received considerable attention in recent years due to its economic, social,
and environmental benefits. However, the industrial application of biohydrogen has been
hindered by low yield. Therefore, development of novel techniques to enhance the yield is of
immense importance towards large-scale production of biohydrogen.
Thus, this research effort explored various options to enhance the yield of biohydrogen
during dark fermentation process. Some options explored included (i) the utilization of
feedstocks from the agricultural, industrial and municipal sectors, (ii) parametric optimization
of biohydrogen production, (iii) investigation of biohydrogen production using metal ions and
nitrogen gas sparging, and (iv) assessing the feasibility of biohydrogen scale-up study to pave
the way for pilot-scale development. Solid biowaste feedstocks consisting of apple, bread,
brewery residue, cabbage, corn-cob, mango, mealie-pap, pear, potato, and sugarcane were
investigated for dark fermentative biohydrogen production using anaerobic mixed sludge.
The experimental results showed that substrates which are rich in carbohydrates are suitable for dark fermentative biohydrogen-producing bacteria. Consequently, a maximum
biohydrogen fraction of 43.98, 40.32 and 38.12% with a corresponding cumulative
biohydrogen yield of 278.36, 238.32 and 215.69 mL H2/g total volatile solids (TVS) was
obtained using potato, cabbage, and brewery wastes, respectively. Based on these results,
potato waste was chosen as a suitable substrate for subsequent biohydrogen production
studies.
Parametric optimization was carried out on biohydrogen production via dark fermentation
using potato waste as the substrate. Effects of operating variables such as pH, temperature, fermentation time, and substrate concentration were investigated via response surface
methodology (RSM) approach using a two-level-four factor (24) central composite design
(CCD). The obtained predictive model (statistical model) was used to explain the main and
interaction effects of the considered variables on biohydrogen production. In addition, the
model was employed in the optimization of the operating conditions. Consequently, a secondorder
polynomial regression with a coefficient of determination (R2) of 0.99 was obtained and
used in the explanation and optimization of operating variables. The optimum operating
conditions for biohydrogen production were 39.56 g/L, 5.56, 37.87 oC and 82.58 h for potato
waste concentration, pH, temperature and fermentation time, respectively, with a
corresponding biohydrogen yield of 68.54 mL H2/g TVS. These results were then validated
experimentally and a high biohydrogen yield of 79.43 mL H2/g TVS indicating a 15.9%
increase was obtained. Furthermore, the optimized fermentation conditions were applied in
the scale-up study of biohydrogen production that employed anaerobic mixed bacteria
(sludge) which was immobilized in calcium alginate beads. A biohydrogen fraction of
56.38% with a concomitant yield of 298.11 mL H2/g TVS was achieved from the scale-up
study.
The research also investigated the influence of metal ions (Fe2+, Ca2+, Mg2+ and Ni2+) on
biohydrogen production from suspended and immobilized cells of anaerobic mixed sludge
using the established optimal operating conditions. A maximum biohydrogen fraction of
45.21% and a corresponding yield of 292.8 mL H2/g TVS was achieved in fermentation using
Fe2+ (1000 mg/L) and immobilized cells. The yield was 1.3 times higher than that of
suspended cultures. The effect of nitrogen gas sparging on biohydrogen conversion efficiency
(via suspended and immobilized cells) was studied as well. Cell immobilization and nitrogen
gas sparging were effective for biohydrogen production enhancement. A maximum
biohydrogen fraction of 56.98% corresponding to a biohydrogen yield of 294.83 mL H2/g
TVS was obtained in a batch process using nitrogen gas sparging with immobilized cultures.
The yield was 1.8 and 2.5 times higher than that of nitrogen gas sparged and non-sparged
suspended cell system, respectively.
Understanding the functional role of microorganisms that actively participate in dark
fermentation process could provide in-depth information for the metabolic enhancement of
biohydrogen-producing pathways. Therefore, the microbial composition in the fermentation
medium of the optimal substrate (potato waste) was examined using PCR-based 16S rRNA
approach. Microbial inventory analysis confirmed the presence of Clostridium species which
are the dominant biohydrogen-producing bacteria.
The results obtained from this research demonstrated the potential of producing biohydrogen
using South African solid biowaste effluents. These feedstocks are advantageous in
biohydrogen production because they are highly accessible, rich in nutritional content, and
cause huge environmental concerns. Furthermore, optimization techniques using these
feedstocks will play a pivotal role towards large-scale production of biohydrogen by
increasing throughput and reducing the substrate costs which accounts for approximately
60% of the overall costs. The findings from this research also provide a solid basis for further
scale-up and techno-economic studies. Such studies are necessary to evaluate the
competitiveness of this technology with the traditional processes of hydrogen production. In
summary, the findings from this research effort have been communicated to researchers in the
area of biohydrogen process development in the form of peer-reviewed international
scientific publications and conference proceedings, and could provide a platform for
developing an economic biohydrogen scaled-up process. / CK2018
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/25544 |
| Date | January 2017 |
| Creators | Sekoai, Patrick Thabang |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (xix, 202 leaves), application/pdf, application/pdf |
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