February 25, 2008 | General

Evaluating Digester Design For Electricity Generation (Costa Rica)

BioCycle February 2008, Vol. 49, No. 2, p. 51
Low-tech digesters alleviate environmental degradation and produce high-quality biogas, but technological improvements are needed.
Joaquìn Vìquez, Stephanie Lansing and Helen Martìnez

IN DEVELOPING countries, electricity generation is achieved through fossil fuel consumption or hydroelectric dams, both of which have a high environmental impact, including greenhouse gas emissions and ecosystem alteration. In many developing countries, the price of electricity is relatively high, making it inaccessible to portions of the population. In Costa Rica, the average household electricity price is $0.13 kW h-1.
In addition, the discharging of wastewater into water supply areas has a large environmental impact. Due to the inability of many governments in developing countries to enforce environmental laws and the high cost of implementing wastewater treatment plants, wastewater from animal production facilities passes largely untreated into water supply areas.
Considering both issues are key for sustainable development, anaerobic digestion of livestock manure has been presented as part of the solution. During anaerobic digestion, biogas, containing a mixture of methane and carbon dioxide, is produced.
An added benefit of anaerobic digestion of livestock manure is a reduction in the negative impacts of wastewater on air and water quality.
Conventional anaerobic digestion using complete-mix or plug-flow technologies has not been shown to be practical or economical for developing countries. Studies have shown that the Taiwanese model-biodigesters can produce biogas with methane concentrations above 60 percent, with temperatures at or below the mesophilic range (25 to 35°C or 77°F to 95°F) (Lansing et al., 2007).
The Taiwanese-model biodigester is a simple flow-through reactor consisting of a double tubular polyethylene bag and PVC piping. The construction, materials and labor costs of an 8-meter long (26-foot) Taiwanese-model digester, with a volume of 10 m3 (353 ft3), in Costa Rica is approximately $200.
A 10 m3 digester can treat waste from 20 pigs, offering up to 3 m3 (106 ft3) of biogas per day with methane content above 60 percent. The digester requires little lifetime maintenance. There are approximately 1,000 of these digesters in Costa Rica, 5,000 in Colombia and around 20,000 in Vietnam.
In Costa Rica, at EARTH University, an international private, nonprofit undergraduate university specializing in the study of sustainable agriculture, a research team evaluated the possibilities of using this low-cost technology for electricity generation. The produced biogas would supply energy to the university’s dairy and swine farm and at the same time be a learning component for students, government, private organizations and the community.
This case study evaluated an electricity generation plant using biogas from a Taiwanese-model biodigester as the fuel source. The study was conducted at EARTH University, located in the humid tropics of Costa Rica, at an elevation of 50 m (164 ft), with an average temperature of 25°C to 30°C (77 °F to 86 °F).
The electric generator received biogas from two biodigesters: a 102 m3 (3602 ft3) digester receiving flushed cow manure from a milking facility and a 76 m3 (2684 ft3) digester receiving flushed manure from a swine facility. The dairy farm digester treated 2200 L (77.7 ft3) of wastewater and the swine facility treated 4460 L (157.5 ft3) of wastewater daily. The hydraulic retention time at the dairy farm digester was 46 days and 16 days at the swine facility.
The biogas produced was transported via PVC pipeline through an absorption tower with a triazine complex to remove hydrogen sulfide. Following the cleaning process, the biogas was compressed at 5 psi and sent to a 40 kW, electric, internal combustion Cummins Power Generation generator.
Multiple inflow and outflow samples from the digester were collected during an eight week period from both digesters (July to October 2007), and analyzed for the following using Standard Methods: temperature, conductivity, dissolved oxygen (DO), pH, chemical oxygen demand (COD), total solids (TS), volatile solids (VS), turbidity, ammonia, total Kjeldahl nitrogen (TKN), phosphates (PO4), total phosphorus (TP) and potassium (K).
In addition, biogas production was measured daily using an American Diagram gas meter (model AC-250), and biogas quality was analyzed using an IR-30M methane meter and a Z-900 hydrogen sulfide meter (Environmental Sensors Co.). The electricity production and biogas usage data were taken directly from the Cummins generator.
Finally, a simple economic analysis (cash flow and internal rate of return) was conducted using a 20-year generator lifetime. The economic input to the system was the capital cost of the system, and the economic output was the income generated from the savings in the electric bill.
This case study demonstrated that low-tech digesters, with technical supervision for the installation and proper design and measurements, can produce high quality biogas and reduce the organic loading in the wastewater at levels comparable to high-tech digesters, but at a lower price.
The dairy farm and swine facility produced biogas with a methane concentration of 62.3 percent ± 0.7 (n = 15) and 76.6 percent ± 1.4 (n = 16), respectively, and had hydrogen sulfide concentrations of 246.2 ppm ± 29.6 (n = 6) and 324.3 ppm ± 7.8 (n = 10), both of which are less than 0.05 percent of the produced biogas. Both digesters produced on average 0.25 m3 of biogas per m3 of digester, with a total of 44.5 m3 of biogas produced per day.
The EARTH University farm, which includes the dairy farm and swine facilities, has an electricity demand of 12.9 ± 1.05 kW h1 during the eight hour work day. Considering that the demand of the farm is only 32 percent of the generator potential (40 kW), there is an efficiency of only 7.0 percent, with 2.2 m3 of biogas being used to generate each kW h1 of electricity. If the generator were used at the full 40 kW capacity, running at the manufacture-stated efficiency rate of 35 percent, only 0.44 m3 of biogas would be used to generate each kW h1. At full capacity, the produced biogas from the EARTH University digesters would provide 40 kW h1 for 2.53 hours a day.
The concentration of organic matter in the wastewater was significantly reduced during the digestion process. At the dairy farm, there was a 78 percent decrease in COD and a 63 percent decreased in TS. At the swine facility, there was a 91 percent decrease in COD and a 64 percent decreased in TS (Table 1). This decrease in organic loading will reduce the environmental impact of the wastewater on aquatic life in the receiving waters.
The impact of wastewater on the rivers in Costa Rica has resulted in new laws and regulations governing the handling and disposal of manure, resulting in fines of $0.22 for every kilogram of COD discharged above the 500 mg L-1 limit. The effluent from the digester at the swine facilities fall below the legal limit without any further treatment. The dairy farm, however, is above the legal limit due to a high COD levels in digester influent. Currently, the effluent from both digesters undergo further treatment in tertiary lagoons, which results in the effluent waters being well below all legal limits.
During the anaerobic treatment process, nutrients are converted from the organic form to the dissolved form, which increases the fertilizer value of the digester effluent.
The EARTH University electricity generation project had a capital cost of $60,000, which included the cost of both biodigesters (including piping and roofing) ($12,000), the electric equipment ($43,000), and the hydrogen sulfide absorption tower ($5000). Cummins Power Generation donated the electric generator to EARTH University.
Considering only the income from the savings in the electricity bill, all the installation costs would be recovered in 14 years, with an internal rate of return of 5.44 percent for a 20 year period. If the generator were run at full capacity and the biogas production was doubled using codigestion (Lansing et al., 2008), the capital costs would be recovered in 5 years with an internal rate of return of 22 percent.
With almost no maintenance or internal control, this system produces high quality biogas, reduces over 50 percent of the organic matter in the wastewater, and increases the fertilizer quality of the manure (effluent).
EARTH University’s dairy farm has 36 milking cows that are kept in the milking parlor for only 2 hours a day, thus the digester receives only a fraction of their daily manure. The swine facility has 80 pigs that have an average weight of 50 kg and are kept in the corrals 100 percent of the time. The size of this farm is relatively small, but the amount of biogas offered is sufficient to supply for the farm with its energy needs.
Although the system is likely still inaccessible for small farmers in developing countries, with cheaper engines and codigestion, this system can become more accessible, especially for medium to large-scale farmers, in a way that could contribute to Costa Rica’s electricity production.
J. Vìquez is based in Costa Rica and can be e-mailed at:; S. Lansing is at The Ohio State University’s Department of Food, Agricultural and Biological Engineering and can be contacted at:; H. Martìnez can be contacted in Guatemala at
Botero, R., Preston, T.R., 1987. Biodigestor de bajo costo para la producción de combustible y fertilizante a partir de excretas. Manual para su instalación, operación y utilización. Centro para la Investigación en Sistemas Sostenibles de Producción Agropecuaria (CIPAV), Cali, Colombia.
Lansing, Stephanie, Raúl Botero and Jay F. Martin. 2007. In Press. “Wastewater treatment and biogas production in small-scale agricultural digesters.” Bioresource Technology. doi:10.1016/j.biortech.2007.09.090
Lansing, Stephanie, Tatiana Nogueira da Silva, Ederson Dias da Silva Raúl Botero Botero and Jay F. Martin. 2008. Methane production in small-scale digesters using differing mixtures of swine manure and used cooking oil. Abstract accepted for the 2008 American Society of Agricultural and Bio. Eng. Intl. Conference Proceedings, Providence, RI. June 29-July 3, 2008.

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