From waste-to-worth: energy, emissions, and nutrient implications of manure processing pathways

Four manure processing pathways are evaluated to provide a system-level understanding of their impacts on different sustainability indicators. In particular we look at how solid-liquid separation (SLS), anaerobic digestion (AD), and AD+SLS affect depletion of fossil fuels (DFF), nutrient balances, global warming potential (GWP), and ammonia emissions when compared to the base-case (BC) pathway of direct land application. Lifecycle sustainability assessment techniques are applied to develop inventory data and model a Wisconsin dairy farm. For the BC, net GWP is 101.2 kg CO2-eq, DFFS is 106.1 MJ, ammonia emissions are 2.62 kg, and nitrogen availability is 2.45 kg per ton of excreted manure. Net GWP is reduced in all pathways compared to BC by 19% for SLS, 48% for AD, and 47% for AD+SLS. DFF is reduced by 43% for AD and 40% for AD+SLS, but increased by 13% for SLS. Ammonia emissions are increased in all pathways by 2% for SLS, 40% for AD, and 44% for AD+SLS. Nitrogen availability remains the same in SLS but decreases in AD and AD+SLS due to higher ammonia volatilization, which could be reduced by injecting manure. Ratios of fossil energy (FERAD) and energy return on investment (EROIAD) of 3.7 and 0.98-1.8 are determined for AD pathways, compared to FER of 0.29 and EROI of 0.27 for grid electricity. When allocating results to specific outputs, variability can be reduced by applying system subdivision and allocation. Sensitivity analyses highlight the importance of reducing emissions during manure storage and the influence of changes in fertilizer and sand bedding. (C) 2014 Society of Chemical Industry and John Wiley & Sons, Ltd