The research aim was to increase energy production efficiency and reduce the environmental impacts of waste-to-energy technologies, specifically anaerobic digestion (AD) of dairy manure (DM) and combustion of poultry litter (PL). The first objective was co-digestion of DM with gummy vitamin waste (GVW) to increase methane (CH4) yield. The GVW co-digestion treatments significantly increased CH4 yield by 126% - 151% compared to DM-only treatment and significantly decreased the H2S concentration in the biogas by 66% - 83% compared to DM-only. The second objective was understanding the effect of hydrogen sulfide (H2S) scrubber management, operation, and maintenance parameters on H2S removal efficiency. Even though the capital and operating costs for the two H2S scrubbing systems in this study were low (< $1500/year), they showed ineffective performance due to insufficient air injection, substitution of proprietary iron oxide-based H2S adsorbents for cheaper alternatives, and the lack of dedicated operators. The third objective was adsorption of H2S using Fe-impregnated biochar as a substitute for activated carbon (AC). Fe-impregnation of biochar led to a 4.3-fold increase in the H2S adsorption capacity compared to AC. When compared to unimpregnated biochars, Fe-impregnation led to an average 3.2-fold increase in the H2S adsorption capacity. The fourth objective was in-situ use of biochar in AD to remove H2S. In-situ biochar addition at the highest dose (1.82 g biochar/g manure total solids (TS)) resulted in an average H2S removal efficiency of 91.2%. Biochar particle size had no significant effect on H2S reduction. In-situ addition of Fe-impregnated biochar resulted in an average H2S removal efficiency of 98.5%. The fifth objective was a life cycle assessment (LCA) of a PL fluidized bed combustion (FBC) system. The LCA assessment showed that heating poultry houses using heat obtained from the combustion of PL in the FBC system had 32% lower climate change potential (CCP) compared to use of propane for heating poultry houses. However, analyzing the FBC system under a net positive electrical output scenario resulted in 66% less impact on CCP and a 48 – 98% reduction in environmental impacts compared to the previous scenario with net electricity input.