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Anaerobic digestion of organic wastes: the impact of operating conditions on hydrolysis efficiency and microbial community composition




Griffin, Laura Paige, author
De Long, Susan K., advisor
Sharvelle, Sybil, committee member
Stromberger, Mary, committee member

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Anaerobic digestion (AD) is an environmentally sustainable technology to manage organic waste (e.g., food, yard, agricultural, industrial wastes). Economic profitability, however, remains a key barrier to widespread implementation of AD for the conversion of specific feedstocks (e.g., manure, the organic fraction of municipal solid waste (OFMSW), and agricultural residue) to energy. Specifically, high capital and operating costs and reactor instability have continually deterred the use of AD. In order to develop AD systems that are highly efficient and more cost-effective, it is necessary to optimize the microbial activity that mediates the digestion process. Multi-stage AD systems are promising technologies because they allow for separate process optimization of each stage and can enable processing of high-solids content waste. Leachate is recycled through the system, which reduces heating and pumping costs, as well as conserving water. The leachate recycle, however, leads to an increase in ammonia and salinity concentrations. At this time, the impact of reactor conditions (ammonia and salinity concentrations) on hydrolysis is not well understood. As hydrolysis is one rate-limiting step of the process in the conversion of refractory wastes (e.g., lignocellulosic materials), optimization of hydrolysis has the potential to radically improve the economic profitability of AD. The specific objectives of this research were to: 1) determine the effects of operating conditions on hydrolysis efficiency for a variety of solid wastes (manure, food waste, and agricultural residue); 2) determine hydrolysis kinetic parameters as a function of the operating conditions; and 3) identify characteristics of microbial communities that perform well under elevated ammonia and salinity concentrations. To this end, small-scale batch reactors were used to determine hydrolysis efficiency and kinetic rates. Initially, the AD sludge inoculum was exposed directly to the high ammonia and salinity concentrations (1, 2.5, 5 g Total Ammonia Nitrogen (TAN)/L and 3.9, 7.9, 11.8 g sodium/L) as would occur in a reactor treating organic waste with leachate recycle. Results demonstrated a need to acclimate, or adapt, the microorganisms to high concentrations, as methane generation was significantly inhibited with high concentrations. Thus, the organisms were acclimated for two to four months to these testing conditions. The batch studies were repeated, and results demonstrated substantial improvement in hydrolysis efficiency and methane generation. However, although differences in kinetic rates were not statistically significant, general trends in hydrolysis rates suggested that hydrolysis efficiency decreases with increased ammonia and salinity concentrations for a variety of feedstocks (i.e., manure, food waste, agricultural residue). Additionally, results demonstrated that acclimation was necessary to achieve optimal hydrolysis rates. Furthermore, microbial community composition changes in the inocula post-acclimation indicated that reactor inoculation could help improve tolerance to elevated levels of ammonia and salinity to minimize reactor start-up times and improve economic viability.


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