Utilization and Modification of Municipal Sewage Treatment Plant Sludge and Wastewater Streams for the Production of Lipid-Based Biofuels

Sunday, November 7, 2010
Hall 1 (Salt Palace Convention Center)
Andro Mondala, Chemical Engineering, Mississippi State University, Mississippi State, MS

Municipal wastewater treatment plant sludge and influent wastewater streams present an abundant source of lipids that could be extracted, refined, and converted to biofuels such as biodiesel and green diesel. Initial studies on primary and secondary or activated sludge have shown that fatty acid methyl esters (FAMEs) can be produced via in situ transesterification to maximum yields of 14.5 % and 2.5 % by weight of primary and secondary sludge, respectively. The biodiesel produced from primary sludge were thought to originate from free oils, grease, and fatty acid whereas those obtained from activated sludge were converted from phospholipids in the cell membranes of bacteria comprising the microbiota in activated sludge. In order to improve lipid yields and quality in terms of fatty acid composition, two options were considered. The first option was to inactivate the indigenous microflora in the wastewater using ozone followed by inoculation of the disinfected wastewater with oleaginous microorganisms, which are known to accumulate up to 80 % of its biomass as lipids. In this study, ozone was shown to be very effective in inactivating a majority of the naturally-occurring microflora in the wastewater by up to six log units. Ozone was also able to oxidize recalcitrant materials in the wastewater, improve its biodegradability, and increase dissolve oxygen content. This will be beneficial for the cultivation of oleaginous microorganisms in the wastewater in order to produce an activated sludge containing known oil-producing microbial strains such as Rhodotorula glutinis and Rhodococcus opacus. The second study, which is the focus of this paper is the attempt to trigger lipid accumulation in the activated sludge microbiota through cultivation in a high carbon-to-nitrogen (C:N) ratio medium. Activated sludge bioreactors were dosed with lignocellulose sugars using batch and semi-batch fermentation modes at varying levels of C:N ratio and substrate loading. Biomass production, lipid yields, fatty acid composition, and residual sugars were determined using conventional gravimetric and instrumental methods. Kinetic modeling of the fermentation data was conducted for process design and scale-up. In order to understand the dynamics within the activated sludge microbiota, variations in the microbial composition of the activated sludge were determined using sequence analysis of 16S rRNA gene clone libraries. These data will be useful in identifying and isolating potential lipid producers within the sludge microbiota. These technologies, which are being tested for possible integration in conventional wastewater treatment plant infrastructures could then generate biofuels that can be used to power energy intensive treatment processes such as disinfection and tertiary treatment.

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