College of Agriculture & Natural Resources
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Item INCREASING THE SUSTAINABILITY OF PSYCHROPHILIC SMALL-SCALE ANAEROBIC DIGESTERS(2015) Witarsa, Freddy; Lansing, Stephanie; Environmental Science and Technology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The research was aimed at increasing the energy production efficiency of small-scale anaerobic digesters in temperate climates while quantifying their environmental impacts. Biochemical methane potential tests were used to quantify methane (CH4) production from separated and unseparated manure during psychrophilic digestion, and compare CH4 production when pre-incubated alternative inocula (wetland sediment (WS), landfill leachate (LL), mesophilic digestate (MD)) were used. Methanogenic and Archaeal communities were analyzed using T-RFLP and qPCR. At 24 ºC, unseparated manure produced significantly higher (40%) quantity of CH4 than separated manure due to higher volatile solids (VS) content, but differences were insignificant at digestion times of ≤16 days. At lower digestion times, farmers could digest liquid, separated manure without sacrificing CH4 production, but at longer digestion times, the VS in unseparated manure has the time necessary for CH4 conversion. The alternative inocula studies showed that LL inoculum after incubation for 91 days at 25 ºC produced significantly higher quantity (≥20%) of CH4 than MD and WS during digestion at the same temperature, and was not significantly different in CH4 quantity than MD that was incubated and digested at 35 ºC (202 ± 4 L/kg VS). Methanosarcinaceae was dominant in the LL reactor, while the other reactors were abundant in Methanosaetaceae, indicating that inoculum rich in Methanosarcinaceae may be beneficial for starting digestion at lower mesophilic temperature ranges. Longer incubation time generally reduced the inoculum amount needed for batch digestion and prevention of volatile fatty acids accumulation. In batch systems with long digestion time (90 days), MD inoculum from well-established digesters, 35% inoculum to substrate ratio, and 35 ºC operation temperature are recommended for highest CH4 production per unit of digester volume. Additionally, life cycle assessments (LCA) were conducted to compare the sustainability of an unheated Chinese fixed-dome digester with a heated and insulated small-scale plug-flow digester in the US. The LCA showed that the US plug-flow digester was more sustainable than the Chinese fixed-dome system only in climate change category, but contributed negatively towards 17 impact categories. Digester heating and heating infrastructure were the main contributors towards the negative impacts observed in the US plug-flow digester.Item COUPLING ANAEROBIC DIGESTION TECHNOLOGY AND FORAGE RADISH COVER CROPPING TO OPTIMIZE METHANE PRODUCTION OF DAIRY MANURE-BASED DIGESTION(2015) Belle, Ashley Juanika; Lansing, Stephanie; Environmental Science and Technology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Anaerobic digestion technology was coupled with a new forage radish cover cropping system in order to increase biogas production of a dairy manure digester. Specifically, this research investigated forage radish as a renewable source of energy in terms of methane (CH4) production, the effect of radish co-digestion on hydrogen sulfide (H2S) production, and the relationship between H2S production and methanogenesis limitations. Optimal substrate co-digestion ratios and inoculum to substrate ratios (ISR) were determined in the laboratory with biochemical methane potential assays (300 mL) and pilot-scale complete mix batch digesters (850 L) were constructed and operated to determine energy production potential at the farm-scale level. Laboratory results showed that forage radish had 1.5-fold higher CH4 potential than dairy manure on a volatile solids basis, with increasing the radish content of the co-digestion mixture significantly increasing CH4 production. Initial H2S production also increased as the radish content increased, but the sulfur-containing compounds were rapidly utilized, resulting in all treatments having similar H2S concentrations (0.10-0.14%) and higher CH4 content in the biogas (48-70% CH4) over time. The 100% radish digester had the highest specific CH4 yield (372 ± 12 L CH4/kg VS). The co-digestion mixture containing 40% radish had a lower specific CH4 yield (345 ± 2 L CH4/kg VS), but also showed significantly less H2S production at start-up and high quality biogas (58% CH4). Utilizing 40% radish as substrate, decreasing the ISR below 50% (wet weight) resulted in unstable digestion conditions with decreased CH4 production and an accumulation of butyric and valeric acids. Pilot-scale experiments revealed that radish co-digestion increased CH4 production by 39% and lowered the H2S concentration in the biogas (0.20%) beyond that of manure-only digestion (0.34% - 0.40%), although cumulative H2S production in the radish + manure digesters was higher than manure-only. Extrapolated to a farm-scale (200 cows) continuous mixed digester, co-digesting with a 13% radish mixture could generate 3150 m3 CH4/month, providing a farmer additional revenue up to $3125/month in electricity sales. These results suggest that dairy farmers could utilize forage radish, a substrate that does not compete with food production, to increase CH4 production of manure digesters.