BioCycle February 2007, Vol. 48, No. 2, p. 48
This study shows how small-scale digesters can provide methane for household needs and improve fertilizer quality while reducing environmental degradation.
Stephanie Lansing, Raul Botero Botero and Jay Martin

RESEARCH and development in digestion technology has focused on large-scale, capital-intensive systems, but these systems are largely inaccessible to small-scale farmers, especially those in developing countries. Small-scale digesters are inexpensive and easy to build, which makes them an appropriate technology to enhance the environment and livelihoods of farmers in the developing world. Our research team at the Ohio State University is studying the ability of small-scale digesters to treat agricultural wastewater and produce energy, and exploring ways to increase the efficiency of these systems. This research aims to provide small-scale farmers with digesters that produce methane and electricity to meet household needs while reducing disease and environmental degradation caused by poor wastewater management.
Currently, digesters are concentrated in developing countries, with over five million household digesters constructed in China and India alone. Digesters built around the world vary in their design complexity, construction materials, and costs. In developed countries, most digesters are concrete stirred reactors, in which a portion of the produced biogas is used to heat the digester. Many of the digesters located in developing countries are plug-flow digesters, constructed without heating or mixing components. These digesters are adaptable to any tropical climate and require minimal maintenance.
Taiwanese-model digesters are simple, flow-through reactors consisting of a double tubular polyethylene bag, PVC piping, and vinyl hosing to transport the biogas from the digester. The construction, materials, and labor costs of a Taiwanese-model digester can vary from $34 USD in Vietnam to $150 USD in Costa Rica. The digester can be constructed to treat waste from one to 100 pigs or cows and has an estimated life of 20 years.
The wastewater flows through the 10-40 meter long tubular polyethylene bag, where methane-producing microorganisms convert the waste to biogas. The produced biogas has a high methane content and can be used directly as a cooking source, eliminating the need to buy propane or collect firewood for cooking. In addition, wastewater treatment and pathogen destruction occur during the digestion process. The low cost of small-scale digesters and the value-added products they produce result in an increase in household income and a decrease in water pollution and deforestation.
STUDY SITE
This study investigated seven Taiwanese-model digesters located in the Limon Province of Costa Rica, Central America. Four of the digesters were at small-production farms and three digesters were at La Escuela de la Agricultura de la Regi?n Tropical H&uagrave;meda (EARTH University), an international undergraduate university specializing in the study of sustainable agriculture. The digester locations varied in elevation from 50 m to 350m. All digesters used animal wastewater, with the majority using swine manure. The digesters were identical in construction materials, but differed in digester length, wastewater management, wastewater source, and retention times (Table 1). The manure for each digester was washed directly from the stalls into the digesters. The frequency and amount of wastewater used in each digester varied, with an average stall-washing time of 15 minutes, two times a day. The retention time in each digester ranged from 11 to 91 days, depending on the amount of wastewater used to wash the stalls and the volume of the digester.
ANALYTICAL METHODS
Multiple inflow and outflow samples were collected from each of the seven digesters during farm visits from July to October 2005. In total, over 200 water samples were collected and analyzed for the following: pH, temperature, conductivity, biochemical oxygen demand (BOD), chemical oxygen demand (COD), turbidity, total suspended solids (TSS), ammonium (NH4), orthophosphate (PO4), and total kjeldahl nitrogen (TN). An IR-30M methane meter and a Z-900 hydrogen sulfide meter (Environmental Sensors Co.) were used on-site to detect methane and hydrogen sulfide concentrations.
RESULTS AND DISCUSSION
This study revealed that low-tech, plug-flow digesters are able to produce methane and reduce wastewater containments at levels comparable to high-tech, completely stirred digesters favored in developed countries. The Taiwanese-model digesters produced biogas with methane concentrations greater than 60 percent and reduced the organic matter and solids in the wastewater by an average of 85 percent (Table 2). Even with minimal internal control and highly variable influent water quality, these small-scale digesters produced high quality biogas and improved the fertilizer value of the livestock wastewater.
The average methane concentration of the digesters was 66 percent with the highest concentrations at Farm 3 (73 percent), which used swine manure and had the highest solids and organic matter loading rates (167 mg TSS/L-day and 221 mg COD/L-day, respectively). The lowest methane concentrations were found at Farm 2 (62 percent), which used dairy manure and Farm 7 (61 percent), which used swine manure, but had a low solids loading rate (40.6 mg TSS/L-day). Hydrogen sulfide levels at all seven farms were below 0.01 percent.
There were large decreases in solids (86 percent), organic matter (85 percent), and total nitrogen (46 percent) The total nitrogen concentration decreased due to microbial uptake and solids settling in the digester. Solids are not removed from plug-flow digesters, which allow for a more complete digestion of the solids, but results in a high retention time for proper waste treatment and methane production. If the solids in the digester start to harden, the digester bag is manually massaged to loosen the solid mass.
Mineralization occurred during the digestion process, increasing the average ammonium concentration by 44 percent, which increases the usefulness of the digester effluent as an organic fertilizer. The effluent from Farm 6 would be a more suitable organic fertilizer due to the high ammonium content (149 mg/L), while the effluent from Farm 2 would be a less useful fertilizer (19 mg/L), but would have less effect on aquatic life if discharged into nearby waters due to its low organic matter and ammonium levels.
The pH did decrease from an average of 7.3 to 6.6 during digestion due to fatty acid production, but there were not any chemicals added to the digesters to keep the pH circum-neutral. Farm 1 and Farm 3 had higher pH levels in the outflow (7.2 and 6.8, respectively), as well as higher methane production (69 percent and 73 percent, respectively). The optimal pH is 6.4 to 7.6, with maximum methane production occurring above pH 7. These two farms also had high total nitrogen loading rates (286 mg/L and 490 mg/L, respectively), which could have led to higher ammonification rates, thus increasing the pH levels of wastewater and leading to a higher percentage of methane in the biogas.
In order to run a generator fueled with biogas, the methane concentration needs to be greater than 55 percent and the hydrogen sulfide concentration needs to be less than 1 percent. All of the digesters in this study meet these minimum criteria to power a generator. The Taiwanese-model digesters in this study have the potential to be upgraded for electricity generation based on biogas production and the percentage of methane produced, but capital costs to purchase the generator and storage bags might negate the economic value of the digester system and complicate the management and operation of these systems for the average farmer.
Conclusions and Future Studies
The digesters in this study were effective at producing a sustainable energy source and improving the water quality by providing a more useful organic fertilizer, reducing the impact of the wastewater on the receiving waters, and generating methane to meet the farmers’ cooking needs. Organic matter and solids concentrations were consistently reduced in the effluent waters and ammonium concentrations were increased. The low-tech, plug-flow, Taiwanese-model digesters analyzed in this study were able to produce methane at levels comparable to high-tech digesters. The methane production in these digesters creates a number of indirect environmental and societal benefits, including (1) a reduction in deforestation associated with firewood collection, (2) less hours devoted to firewood collection, (3) eliminating the need to purchase propane for cooking, and (4) a reduction in greenhouse gas emissions to the atmosphere.
Currently, there is a Taiwanese-model digester/generator system at EARTH University in Costa Rica that is producing 4 hours of electricity per day. Experimental studies are currently being conducted by our research team to maximize digestion efficiency in order to increase the electricity generation capabilities of these low-tech systems. We are determining what quantity of grease and fats can be added to Taiwanese-model digesters to maximize biogas production without requiring the addition of buffering chemicals to keep the internal digestion environment at an optimal pH for methane production.
Coauthor Stephanie Lansing is at the Ecological Engineering Program at The Ohio State University in Columbus, Ohio. She can be contacted at: lansing.10@osu.edu.