The breakdown on biodigesters
Alternative for the disposal of TSE-infected animals, other animal waste gains popularity
In just six hours, a giant pressure cooker-like machine can turn a 1,200-pound cow carcass into a pathogen-free, aqueous solution of small peptides, amino acids, sugars, and soaps and a pile of powdered bone.
The machine, called a biodigester or tissue digester, has become popular with veterinary colleges and laboratories as a cheaper, more environmentally friendly alternative to incinerating animal carcasses.
Biodigesters use alkaline hydrolysis to decompose animal carcasses and other potential hazardous wastes rapidly. Gordon Kaye and Peter Weber, medical researchers at Albany Medical College, developed the first biodigester in 1992, as an inexpensive way to dispose of laboratory animal remains containing low-level radioactivity.
The subsequent discovery that biodigesters destroy prions, coupled with a decline in the use of incinerators, has buoyed their popularity. Incineration can destroy prions if the heat is high enough, according to experts, but many incinerators at veterinary colleges and laboratories have been decommissioned because they are aging and no longer meet Environmental Protection Agency standards.
Joseph H. Wilson, the president of WR2, the Indiana-based company that makes the biodigesters, said the company has sold more than 30 of the machines, which range in price, depending on their size. Machines designed to dispose of rodents can cost as little as $25,000, whereas machines that digest several large animals at once can cost more than $1 million. The company's clients include the veterinary colleges at University of Florida, Colorado State University, the University of Minnesota, Texas A&M, and the Department of Agriculture. Several other veterinary colleges have biodigesters on order or have expressed interest in getting one, Wilson said.
Though the machines are expensive to purchase, the cost of operating them is about a third of the cost of using a commercial incinerator, said Dr. Robert Shull, the director of the Wisconsin Veterinary Diagnostic Laboratory, which houses the USDA's new mobile biodigester. The biodigester arrived at the laboratory, which is part of the National Animal Health Laboratory Network, last fall and became fully operational in mid-January.
For Dr. Shull's lab the cost of digesting animal carcasses is about 25 cents per pound versus 75 cents per pound for incineration. The biodigester may provide substantial savings for the state, which will likely cull and test hundreds of deer this year as it tries to stop the spread of chronic wasting disease.
Biodigestion also is a safe way to dispose of animals infected with other foreign animal diseases, such as bovine tuberculosis and brucellosis, Dr. Shull said.
Dr. Linn A. Wilbur, the USDA area veterinarian-in-charge of Wisconsin, said the $900,000 biodigester in Wisconsin, which is built into an 18-wheeler, is a regional resource that the USDA could use to dispose of animal carcasses in an animal disease outbreak anywhere in the Midwest.
"If we have an emergency, we can take it out and move it to wherever we need it," Dr. Wilbur said.
In addition to being cost-effective, the biodigestion is less harmful to the environment and the health of staff than other methods, according to Robert L. Hockman, the associate director for facilities at the University of Florida College of Veterinary Medicine. He explained that staff members responsible for animal disposal are not exposed to the fumes and ash created during incineration; in fact, with the biodigester, they have no direct contact with the remains.
"It's much better for the environment, much cheaper (than incineration), and it's better for the people who work with it," Hockman said.
The process begins by lowering a carcass into a large pressure cooker-like machine, using a crane, then adding caustic liquid and heating the carcass. After several hours, the carcass is reduced to a sterile, aqueous solution that can be sent through a sanitary sewer to a sewage treatment facility. The only solid byproducts are the bones and teeth, which are reduced to mineral ash, and crumble when touched. The solid remains are biologically inactive and can be sent to a landfill, Dr. Shull said.
At Colorado State, the liquid waste is cooled until it becomes gooey and is put to use as fertilizer. Dr. Terry Nett, the associate dean for research at the veterinary college, explained that, because of the high nitrogen content of the liquid waste, the local sewage treatment facility had been charging the university extra for its disposal, so the university found an alternative.
"As it turns out, this is excellent material to add to compost piles," Dr. Nett said.
Proponents of biodigesters say they expect the machines will replace incineration at most research and diagnostic facilities.
"I think eventually, everybody will be going to this system or something similar," Hockman said.
Bridget M. Kuehn