Morales-Medina, William R.

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    Assessing the effect of pretreatments on different tropical non-edible photosynthetic feedstock for biogas production
    (2017) Morales-Medina, William R.; Ríos-Hernández, Luis A.; College of Arts and Sciences - Sciences; Rodríguez Minguela, Carlos; Cafaro, Matías J.; Department of Biology
    With petroleum reserves estimated to scarce in less than 55 years, scientists had been emphatic in the search for alternative fuels. Biogas is considered a potential substitute fuel due to its high calorific value, renewability and zero emissions capacity. This gas is easily obtained through a biological process called anaerobic digestion in which a community of fermentative bacteria degrades organic matter into acetate, CO2 and H+ and subsequently methanogenic Archaea convert these products in to CH4. One of the mayor challenges for large scale biogas production using plants as feedstock is the land space needed to grow this biomass. To overcome this problem, we proposed the use of marine biomass that can be grown in the ocean, eliminating the requirement of land space, and climbing vines which can grow vertically, reducing horizontal land space. Previous research in our laboratory assessed the feedstock potential of these biomasses for biogas production and observed that not all the biomass was degraded at the time that methane production ceased. This lead us to hypothesized that some components of our feedstock’s are recalcitrant under our condition, therefore biomass pretreatments are needed to increase methane productivity in the reactors. To test this hypothesis, we started by submitting 10 different biomasses: 2 seagrasses (Syringodium sp and Thalassia sp), 4 marine algae (Sargassum sp, Dictyota sp, Cladophora sp and Acanthophora sp) and 4 climbing vines (Epipremnum sp, Dioscorea sp, Cissus sp and Jasminum sp), to a simple liquid extraction treatment where we separate the water soluble and the non-soluble fraction and test them as individual feedstock for biogas production. Two replicas of this experiment were performed and results showed that climbing vines were in general the most productive feedstock and that more methane was obtained from the untreated biomass than the solid fraction. A prokaryotic diversity analysis using DGGE showed that more methanogens are present in climbing vines reactors. The acetoclastic methanogen M. barkeri was detected in all reactors of both replicas. In the case of bacteria, similar diversity was noticed in all reactors. A further analysis showed that some of the bacterial species present in our reactors are identical to microbial communities in reactors enriched with short chain fatty acids. To determine if more complex pretreatment can enhance methanogenesis in our reactors, we selected two biomasses of each type (climbing vine, seagrasses and algae) and submit them to an oxidative, hot water, and biological enzymatic treatment. Climbing vines still produced more methane than marine biomasses with the exception of Acanthophora sp that was the third most productive feedstock in the hot water treatment experiment, surpassed only by untreated and hot water treated Epipremnum sp. Because of the little difference on methane productivity between treated and untreated feedstock, using non-treated biomass seem to be more cost effective. Our data also showed that each treatment have a different effect on each biomass suggesting that generalization of the effect of a specific treatment is scientifically unsound. Difference in methane production is apparently caused by the chemical composition of each biomass and not due to the prokaryotic diversity that is present in the reactors. An energetic analysis of the methane productivity of climbing vines, our best feedstock, in relationship with their relative growth rate and growth characteristic suggests that Epipremnum sp is the best feedstock for high scale biogas production with the capacity of sustaining the energetic yearly consumption of average refrigeration by digesting less than 7,200 fully developed plants, under our reactor conditions.