Big sums of money are being pumped into biogas/anaerobic digester plants as the world tries to get greener. The concept of producing biogas from manure is nothing new though. For many years small family farms in Asia have stored slurry in sealed containers, using the resultant biogas to cook meals.
Biogas is made up of 50 percent to 70 percent methane, 30 percent to 35 percent carbon dioxide and small amounts of hydrogen sulfide and ammonia. Biogas has a heating value of up to 7 kpm3, depending on the methane content. Digesting hog slurry has the benefit of reducing odors, which is the main reason for its application in the United Kingdom. One of the first anaerobic digesters was built in Scotland in the mid-1970s. The plant consisted of a 13,500-liter digester vessel insulated with 50 millimeters of glass wool to prevent heat loss, as digesters need a temperature of 35° C to work efficiently. The biogas produced was stored in a 7,000-liter gas holder. Some of it was used to fire a boiler that produced hot water required to keep the digester temperature at 35° C. One big advantage of digesters in hot climates is that operating costs are much lower than those in cool climates.
Digesters’ initial attraction was to get rid of manure slurry, reducing the need for lagoons. Municipal sewage plants have long used anaerobic digestion, which served as the base for farm digesters. However, there are some fundamental differences. Sewage sludge has a higher dry-matter content than hog slurry, and problems surfaced because hog slurry was too dilute. So, it was run through a separator to boost the dry matter. Also, sewage sludge has a fairly consistent dry matter, whereas hog slurry is more varied. For example, sow slurry is quite different in dry matter than finishing-pig effluent. Hence, holding tanks were needed so that different slurries could be mixed to produce a more uniform product.
An initial appeal of the early digesters was the supposed low-labor requirement. But that was not the case, as they had to be nursed continually for efficient operation.
Also early on, a digester might suddenly stop producing biogas. That was confusing until investigations found that the in-feed medication used to control a pneumonia outbreak, for example, also was killing the bacteria in the digester.
Things have progressed and plants today are far more sophisticated in design and operation. Many use the biogas to generate electricity via a modified internal-combustion engine. (In the early days the biogas wasn’t scrubbed well enough and the impurities played havoc with engines.)
Ten years ago the economics also were different. Nowadays surplus electricity can be sold to the local power company, which can have a huge influence on profitability. Politics come into play in terms of subsidies and rulemaking. Several Danish plants went broke when a government policy change cut the value paid for electricity.
Many large pig units in Asia have digesters. The liquid fraction coming out of the digester is stored in lagoons in which fish — often tilapia and catfish — are raised. Casualty pigs are cooked using gas from the digester, with that cooked product fed to catfish. This type of integrated system could work in the warmer areas of the United States. In other areas, a digester may still make sense if, for example, there are issues because of odors from slurry lagoons.
In Europe today, anaerobic digestion is viewed as a way of producing green energy in the form of gas and/or electricity versus a way of eliminating slurry. Again, swine slurry is a marginal biogas feedstock because of its low dry matter. Poultry manure is much better, and better yet is food waste and corn silage. Slaughter house waste also is a good substrate.
With regard to the United States, if just 50 percent of food waste was anaerobically digested it would generate enough electricity to power about 2.5 million homes for 12 months. Public authorities and food manufacturers pay to get rid of food and restaurant waste which often ends up in landfills. A biogas plant can better utilize these wastes, converting them into gas or electricity.
Germany is the world leader in biogas technology, and the number of biogas plants there doubled from 1,750 in 2003 to 3,500 in 2006. Initially the German biogas plants just used slurry as a feedstock, but today corn silage is the major substrate.
The Big Dutchman company has built 275 plants in 20 countries and has much expertise in this area. An example of a typical plant is situated on a small farm near Bremen, Germany. The farm had a 600-place hog finisher, which was expanded to 1,000 places. The farm grows 100 acres of corn silage, but the digester needs silage from another 400 acres to keep it working. Fortunately, the German government guarantees the electricity price for the next 20 years (which is good as the plant and associated roads cost a massive $3 million). The plant produces excess gas, which is used to produce hot water that is then piped one-half mile via a government-subsidized underground pipeline to heat the nearby village’s swimming pool.
The German biogas concept is interesting but questionable in how it all adds up when the costs of growing the corn silage — tractor fuel to sow and harvest, fertilizers and sprays, trucking silage to the plant — are all considered.
Certainly this technology has come a long way and is a valuable option to generate green electricity whilst addressing a pork producer’s big headache — slurry.