EMERGY FROM COMBINED WASTE STREAMS IN A BEEF PRODUCTION AGROECOSYSTEM
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Wastewater from hydraulic fracturing (HF) and manure from beef cattle production are two of the largest waste streams in the Texas Panhandle. The objectives of this research were to 1) evaluate the potential of generating bio-methane (CH4), a renewable natural gas, from the combination of HF wastewater and beef manure through anaerobic digestion (AD), 2) simulate the annual amount of energy that can be produced from these two waste streams, and 3) evaluate the environmental impacts and sustainability of biogas through an emergy analysis. The research included a laboratory study and the development of two dynamic systems models. Substrate combinations of manure mixed with produced and flowback water (PFW) were evaluated at four moisture contents (MC; 65, 70, 80, and 90%). Substrate combinations mixed with well water (WW), and a 50/50 mixture of WW and PFW were evaluated at MC of 80 and 90%. Manure was harvested from the WTAMU Research Feedlot. The PFW was collected from a HF operation in the Texas Panhandle. Regression analyses were used to predict bio-methane production based on MC for each water type. The regression model statement for WW was y ̂ = -2,176 + 59.3 x, which indicated that CH4 volume (ml) increased with increasing MC. Conversely, CH4 volume decreased with increasing MC for the 50/50 mixture (y ̂ = 2.94x109e-0.199x) and PFW (y ̂ = 343,662e-0.118x). Regression model statements from the biogas data were used in the energy estimation model, which simulated feedyard manure production and the amount of diesel and electricity generated from AD. The optimum MC for PFW was 70%, with simulated results of 59,800 L of fuel and 139,200 kWh of electricity. These results were for raw methane and did not account for cleaning/upgrading gains or losses of CH4. The results from the biogas experiment and the feedlot energy model provided inputs for the emergy model. The emergy model was used with traditional emergy analysis methods to determine the relative sustainability and environmental impact of the transformation of energy from dry-lot beef cattle manure and PFW at the WTAMU Nance Ranch. Three options were evaluated: no AD, AD with WW, and AD with PFW. Several emergy indices were used to compare the three options. The percent renewable index simulated by the model was 0.49 for no AD, 0.50 for AD with PFW, and 0.33 for AD with WW. The environmental sustainability index was 7.25 for no AD, 31.62, for AD with PFW, and 6.14 for AD with WW. These results suggested that AD with PFW was the best long-term option from a sustainability perspective.