Three investigations were conducted on small-scale digesters in Costa Rica to optimize their ability to treat animal wastewater and produce renewable energy. The first study investigated variability in biogas quality and water pollutant reductions in seven digesters located at small-scale farms in Costa Rica. The second study investigated the production of electricity using combined biogas from swine and dairy digesters. The final study investigated increases in biogas production by co-digesting swine manure and used cooking grease in 12 field-scale digesters. This study assessed methane production, wastewater pollutant reductions, and fertilizer utility.
The digester variability study revealed that the digesters maintain high reductions in organic matter (84.1 %), produce biogas with a high percentage of methane (66.3 %), and produce a fertilizer that has 78.3 % more NH4-N. In the electricity production study, the dairy and swine digesters produced 27.5 and 6.0 m3/day of biogas, with 62.6 and 76.4 % methane, respectively, and reduced organic matter by 86.1 and 91.9 %, respectively. Eighty-two percent of the farm’s 2-hour peak electricity demand (25.8 kW/day) was met. The $21,000 capital cost of the digester project will be recovered in 10 years through electricity savings and reductions in wastewater fines. If the generator were more appropriately sized for the farm, the capital recovery time would have been 7.6 years.
Co-digestion experiments were conducted on 12 field-scale digesters using three replications of four treatment groups: the control (0%) and 2.5, 5, and 10 % used cooking grease (by volume) combined with swine manure. The 2.5 % treatment increased the methane production by 111%. No additional benefits were seen by increasing the grease concentration beyond 2.5 % due to the lower quality of biogas produced. No adverse effects were observed from co-digesting with 2.5 % grease in terms of organic matter removal, pathogen reduction, and grease removal. There was less phosphorus reduction with co-digestion, resulting in lower N:P ratios in the grease treatment groups.
These investigations proved that small-scale digesters are ideal systems for rural farmers to treat livestock wastewater and obtain renewable energy from their waste products. The digesters provide energy, which results in the elimination of firewood collection or propane purchase for cooking, improve the water quality of the effluent, and produce a liquid fertilizer. In addition, these systems are cost-effective and require little maintenance. The technology should be spread to other rural farmers in tropical areas. Larger installations can be used for electricity production, and mixing different types of manure can increase electricity output. Adding grease to the influent is a simple way that energy production can be doubled without affecting other digester benefits.