BioCycle April 2010, Vol. 51, No. 4, p. 55
Case study of a dairy farmer who implemented a complete dairy effluent treatment system using anaerobic digestion to generate biogas for electricity.
Joaquin Viquez Arias

IN Costa Rica, dairy farmers are required to have a permit to run their farm. This permit, known as the Veterinarian Operation Certificate, requires farms to treat dairy effluent before land application. Anaerobic digestión is among the most attractive technologies available, and is adaptable to most dairy farmers in Costa Rica. Use of the biogas for electricity generation is limited to farmers who produce a little over 200 gallons/day of manure.
Dos Pinos Dairy Farmers Cooperative is made up of 1,600 family-owned dairy farms. Over its 60 years of existence, it has provided Costa Rica with a wide range of dairy products, supplying 85 percent of the dairy market in the country. Alejandro Romero Barrientos, a dairy farmer and member of the Dos Pinos Dairy Farmers Cooperative (Cooperativa de Productores de Leche Dos Pinos R.L.), is advised by VIOGAZ. He installed a system on his farm for electricity generation using biogas produced from cow manure in compliance with the country’s environmental requirements.
The farm is located in the province of Alajuela, at an altitude of 260 to 330 feet, with annual precipitation of 160 inches, average temperature of 82°F and 80 percent humidity. Its production system is common in the region; cows remain in the pastures most of the day. The animals are in a rotating system, and are moved to a different pasture daily, giving each lot 30 days of rest before cows return to the original pasture. The cows are milked during the hottest time of day, and then sent to corrals to supplement their feeding (grain, minerals and grass) until their second milking time. Cows then return to the pastures where they spend the rest of the day.
The Romero farm milks an average of 80 cows, with an average body weight of 770 lbs, producing 27 lbs of milk/animal/day. Animals remain in the facilities a little less than 9 hours a day; considering an excretion average of 8 percent of the animal’s body weight per day (24 hrs), the estimated in-barn manure production is 1,900 lbs/day. An estimated 660 gallons/day of water is used for the cleaning and washing process.

Digester Components
Conventional anaerobic digestion using complete-mix or plug-flow technologies has not been shown to be practical or economical for developing countries (see “Evaluating Digester Design For Electricity Generation,” February 2008). Studies have shown that the Taiwanese model digesters can be a feasible and low cost technology for small and medium-sized farms.
On Mr. Romero’s farm, diluted and separated manure (liquid slurry) is processed in a flow-through digester, with influent entering at less than one percent total solids. The digester has no temperature control or agitation/mixing system. The digesters consist of two 66 ft long, tubular (8.2 ft in diameter) PVC membrane bags (40 mils thickness). This type of digester, culturally called a “sausage digester,” is common in this region due to its low cost. The tubular PVC membrane bags are installed in the ground. The digester has 32,500 gallons of effective liquid volume with a 35-day hydraulic retention time (HRT), and 2,700 cu ft of biogas accumulation in the chamber.
To prevent rainfall runoff from entering the digester and increasing the mix volume, gutters were installed in some areas of the facility. Because the animals bring a significant amount of sand in from the pastures, which then is removed from the floors during the washing process, two sand separators were installed ahead of the digester. The separators consist of concrete blocks where water speed is slowed down, letting sand precipitate to the bottom.
Anaerobically treated effluent is stored in a 6,600 gallon concrete tank for land application to the pastures. Fiber solids – about 25 percent (w/w) of total manure (475 lbs/day) – are captured by a screen separator and composted. The piles are aerated manually about two to three times a week for two weeks, and then applied to pastures as an organic fertilizer and soil amendment.

Biogas Power
Hydrogen sulfide (H2S) in the biogas is removed by an iron oxide scrubber. The H2S levels are reduced from 500 ppm to less than 1 ppm. After the H2S has been filtered out, the biogas travels 52 yards in a 1-inch PVC pipe to the engine generator, where a blower is used to increase pressure to about 1 psi. The farm has a Generac 16 kW internal combustion engine, designed for propane and natural gas with modifications for biogas (65 percent methane).
Electricity from the generator is used for the milking process, powering a 5 hp vacuum pump 4 hours/day (about 22 kWh/day). An AC-250 diaphragm flow meter was installed between the blower and the generator. Consumption of 127 cu ft of biogas/hour (25 percent efficiency) has been measured. The farm can annually substitute up to 8,030 kWh.
Due to lack of equipment, biogas production hasn’t been measured, but it’s estimated that this farm may be able to produce between 700 to 900 cu ft/day. The engine generator has been used every day, which means that the digester has been able to provide at least 508 cu ft/day.

Financial Data
Cost of the complete system, including changes done to the infrastructure in the corrals and milking parlors, the sand separator, fiber solid screen separator, anaerobic digesters, effluent tank, H2S scrubber and electric generator, was calculated at $19,275 (Table 1).
This integrated, low cost system also provides other sources of income and savings. For example, it is estimated that the farm will produce 300,000 gallons of effluent and 100 tons of compost annually. Considering its nutrient content, and the value of nitrogen, phosphorus and potassium in the market, both sources of soil amendments are estimated as an income of $3,890 year (offset of chemical fertilizar purchases). The substitution of 8,030 kWh/year and a cost of $0.19 kWh for electricity is an estimated savings of $1,525/year on electricity.
The cost to maintain this system is low. There is no electric equipment such as pumps or mixing/agitation systems; gravity is the main source of water movement through the system. The total cost of maintenance a year, considering labor for the compost and the fiber solid separator area, plus basic maintenance to the generator unit, is $1,560/year. The rule of thumb of needing over 500 cows for a project to be economically feasible may be modified using this type of design and technology.
Based on this savings/income sources, the system can be paid off in five years with an internal rate of return of 20 percent. Finally, and probably most important, the system enables the Romero farm to meet its legal and environmental responsibilities, and stay in the dairy business.

Joaquin Viquez Arias, Agriculture Engineer, is founder and president of VIOGAZ (www.viogaz.com) in Costa Rica. (jviquez@viogaz.com)