Anaerobic Co-digestion of Dairy Manure with Potato Waste

In this study, anaerobic co-digestion of dairy manure (DM) and potato waste (PW) was evaluated to determine the best mixing ratio in terms of biogas and methane (CH4) productions. Batch lab-scale anaerobic co-digestion of DM with PW was conducted at two different temperature ranges [mesophilic (MT) (35-37 oC) and thermophilic (TT) (55-57 oC)]. The tests were conducted at five DM:PW mixing ratios (100:0, 90:10, 80:20, 60:40, and 40:60) on a volatile solids (VS) basis. During the 30 days of batch digestion, the highest cumulative biogas and methane (CH4) production (CBP) of 6.4 cubic foot (ft) and 3.3 ft was achieved for the thermophilic co-digestion at the mixing ratio of 80:20 and 2.7 ft and 1.5 ft for the mesophilic co-digestion at the mixing ratio of 90:10, respectively. Based on test results the optimum mixing ratio depends on temperature. The concentrations of volatile fatty acids (VFA) and NH3-N and pH were not concerns for either co-digestion of DM and PW or digestion of DM alone in terms of process stability. Thermophilic temperature proved to be the best temperatures in terms of biogas and methane productions. INTRODUCTION Dairy manure (DM) is a common organic waste from the dairy industry with an estimated production of 22 tons of DM/cow-year (USEPA, 2013a). The methane (CH4) emissions increased by 64.0 percent from 31.7 teragrams of CO2 equivalents (Tg CO2 Eq.) in 1990 to 52.0 Tg CO2 Eq. in 2010 due to the growth in animal production (mainly from dairy and swine manure) (USEPA, 2012). Appropriate treatment of DM is necessary to minimize pollution. In addition to the dairy industry, Idaho is the largest potato producing state in the USA (USDA, 2011). According to National Agricultural Statistics of United States Department of Agriculture (USDA, 2011), approximately 42.9 billion pounds of potatoes were produced in USA where Idaho alone produced 12.8 billion pounds. Among Idaho’s total potato production, 800 million pounds, which was approximately 6% of Idaho’s total potato production, were considered as waste due to shrinkage and losses (USDA, 2011). Additionally, decomposing of PW leads to nutrient leaching to ground and surface waters (Olsen et al., 2001). Finding a treatment method to properly dispose of PW is a challenging task. Anaerobic digestion (AD) has been regarded as a promising technology to treat organic waste (USEPA, 2013b). The microorganisms in the AD process transform organic constituents and nitrogen-containing compounds into biogas in the absence of oxygen. The main components of biogas are CH4 and CO2. Methane can be recovered and diverted to generate heat and electricity. Research has shown that AD of DM brings many benefits including pathogen destruction, odor reduction, reduced lagoon loading, and biogas generation (Rico et al., 2011). There is a limited amount of information on anaerobic co-digestion of DM and PW.