October 25, 2010 | General

Anaerobic Digestion Of Pig Manure (Spain)

BioCycle October 2010, Vol. 51, No. 10, p. 41
Two facilities in Toledo region, constructed to service small hog farms, yield heat, electricity and fertilizer product.
Nora Goldstein

SPAIN has a high number of pig farms, with about 25 million pigs bred in the country. Most of the farms are concentrated in the regions of Catelonia, Castilla La Mancha, Castilla Le&oagrave;n or Murcia. This concentration results in water pollution, due to excessive nutrients being land applied. “Pig manure is about 95 percent water and five percent solids, and it was too expensive to ship it long distances out of the region,” says J. Fernando Suárez Mejido, General Manager of Valpuren Bañuelo S.L. and Valpuren Comatur S.L., subsidiary companies of SENER, a multinational private engineering and technology firm based in Bilbao, Spain.
In the early 2000s, the Spanish government tried to solve this problem by establishing special tariffs for cogeneration of electricity to make it feasible to construct solids drying facilities for pig manure. “The Spanish government added a surplus for the produced power – for example, converting natural gas into electric power with gas engines – if a certain percentage of the produced heat from the engines is used for something reasonable,” explains Bernhard Schulz, CEO of Entec Biogas gmbh is Austria. “As pig manure is a problem in Spain – producing pigs is for the famous ham only but there is no farming activity around and therefore no demand/possibility of using the manure as fertilizer – the heat produced from the engines is used for drying the manure in order to reduce the quantity and the problem.”
Companies began selling pig manure drying processes that utilized natural gas. “About 30 drying plants were built in Spain during the early 2000s,” adds Suárez Mejido. “All the processes have to burn natural gas, which isn’t renewable.” However, the Central Government of Spain’s special tariffs also include biogas from anaerobic digestion. Directly drying the manure causes tremendous problems, both technically and in terms of odor generation, adds Schulz. “Therefore SENER developed the Valpuren process, which includes the anaerobic digestion of the manure to solve these problems.”
The Valpuren process, which SENER developed with a Catelonia engineering firm, includes anaerobic digestion of the liquid pig manure, solids separation and production of a solid fertilizer via drying of the digested manure. In 2009, SENER installed two Valpuren plants, one in Polan and one in Consuegra, both in the Toledo region of Castilla La Mancha. SENER utilized the Entec anaerobic digestion technology at the facilities. Each plant has the capacity to process 120,000 cubic meters (m3)/year of pig manure, or about 330 m3/day.

Polan Tour
BioCycle toured the Polan anaerobic digestion plant in May 2010. Manure from 30 farms is delivered to the plant. The farmers created and own a society to manage the manure. The society collects the manure and transports most of it to the treatment plant. (On occasion, when excess nutrients are not an issue, it is utilized directly for crops.) The local government has subsidized this society, which has one transport tank and one truck. This society also owns five percent of the treatment plant.
Samples are taken of each load to test for chemical oxygen demand, percent solids and nitrogen content. The facility has two 3,000 m3 receiving/storage tanks and two 3,000 m3 complete stirred tank reactors (CSTR). The two CSTR digesters are loaded continuously (about 165 m3/day each). Retention time in the digesters averages 18 days. The tanks are insulated; heat from the gas engines is used to maintain the digester temperatures at about 37°C.
The biogas generated is about 70 percent methane. After conditioning to remove sulfur, it is combusted in one of two Rolls Royce 8.2 MW gas engines (total of 16.5 MW). The biogas accounts for up to 1.5 MW of electricity, with natural gas comprising 15 MW. The current tariff for digester gas is 103 Euro/MWh.
Digested solids are separated with a centrifuge and conveyed to the drying plant. The liquid fraction is neutralized with sulfuric acid, then evaporated. Steam from the evaporation process and heat from the exhaust gas of the CHP unit are used in the drying plant to produce a granulate, which is sold as fertilizer. Water condensed from the evaporation is used in the cooling towers for cooling the warmed water from the engines. The fertilizer has an NPK content of 7.5, 5.5 and 4.5 respectively, with an organic matter content of 65 percent. About 5,000 metric tons/year of fertilizer are produced.
Polan is adding a small amount of slaughterhouse sludge as a codigestion substrate. “We meter it in very slowly,” says Suárez Mejido. “We have tried cow manure, but there is too much straw. We are working with the University of Lerida and GIRO, a Catalan government research center, on research into a manure codigestion technology to generate biogas and electricity.”
While the capacity of the digester is completely used, the pig manure has very low biogas methane potential (BMP). “We are able to produce only one-quarter of the biogas that could potentially be produced with alternative products with higher BMP,” explains Suárez Mejido. “But the present legislation does not allow us to use more than 10 percent of other products for digestion, because the special electricity tariff is only for pig manure and not for other products or manures.”

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