Animal Wastes on the Farm

Because animal wastes consist of high solid levels (e. g., 20-100 g volatile solids [VS]/L for swine waste [2%-10% VS]), anaerobic treatment of animal waste offers many advantages over aerobic treatment, such as low electricity requirements (no O2 addition is required), greater than 50% solids reduction, value-added biogas production, nutrient conservation, and odor reduction. However, post-treatment of anaerobic effluent may be necessary to convert, remove, and possibly recover nutrients. The liquid fraction of the treated animal waste from the digester must be recovered and applied to agricultural lands and this may cause odor, pathogen distribution in the environment (Collick et al. 2006), nitrogen pollution of ground water (Kross et al. 1993), nitrogen and phosphorus run off (Gerard-Marchant et al. 2005), and dissipation of antibiotic-resistant genes into the environment (Chee-Sanford et al. 2001; Aminov et al. 2002; Angenent et al. 2008).

Despite the need for further treatment to prevent environmental problems, the increasing size of confined animal feeding operations (CAFOs), encroaching urbanization, greenhouse gas emissions, heightened awareness of local air pollution (including odors and ammonia), and contamination of water have created strong incentives to develop long-term environmen­tally sound manure management practices and systems that include anaerobic digesters. The process of anaerobic digestion is not new and has been utilized for decades to manage animal wastes. More recently the opportunity to capture methane to develop a combined heat and power (CHP) system has attracted much attention in developed countries. The heat and elec­tricity generation on the farm (cogeneration) has led to increasing numbers of anaerobic digesters.

With the further development of net metering (i. e., selling to the electrical grid) for farm anaerobic digesters in many localities, the economic benefits of installing digesters are enhanced. However, researchers still look for other biogas utilization systems besides CHP because, from the perspective of simplicity of operation and thermodynamic efficiency, the most desirable and least expensive option is to use biogas in a boiler or other combustion process for heat generation. To benefit from this, however, a proximate demand for this heat must be available. An alternative option is to upgrade biogas (50%-60% methane) to pipeline natural gas quality (95%-98% methane). To upgrade biogas, a system that includes pres­surization and removal of contaminants (particularly hydrogen sulfide) and CO2 must be procured. Thus, this option is limited to large CAFOs, a community digester receiving wastes from numerous farms, or a gas pipeline connection fed by a number of farms.

The economic assessment of farm-based anaerobic digesters is complex because each farm digester system is specifically designed for that site. For example, the variations within New York State include farm systems that do not generate electricity, farms with microturbines for CHP, and farms with internal combustion engines. One study for the New York State Energy Research and Development Authority (NYSERDA) showed that the net predicted annual benefit per cow for five farms with digesters varied from -$106 to +$299 (Gooch et al. 2007). The negative or low benefit per cow was on farms with only dairy manure digestion while the farm showing a positive benefit was receiving food wastes (co-digestion), which increased biogas production and was paid a “tipping fee” to accept the wastes.