Tissue Digestion

In Tissue Digestion (alkaline hydrolysis), all tissues and carcasses are placed in a pressure vessel and the vessel is sealed before the process begins. Tissue Digesters are loaded from the top through an opening that is equal to the diameter of digestion vessel. For small- and medium-sized digesters, an internal basket is preloaded and then lifted into the vessel with an integral hoist. For large-sized vessels, suitable for multiple equine or bovine carcasses, whole carcasses are lowered onto a tray in the vessel from an overhead crane (customer or BioSAFE Engineering provided). Thus, Tissue Digesters are easy to load.
For the digestion process, the tissue/carcass load is weighed automatically by load cells on which the vessel is mounted, alkali and water are added in precise proportion to the weight of tissue, and the system is heated to a specified operating temperature and held there for a specified time. Agitation in the system, necessary for appropriate exposure of all material to the alkali solution, is achieved by constant circulation of the digestion fluid.
The carcasses are solubilized (liquefied) within the first 30 minutes of the digestion cycle and hydrolysis of proteins, fats, and nucleic acids begins immediately on the solubilized material. Alkaline hydrolysis destroys all known pathogens. It is the only process demonstrated to destroy the infectivity of tissues contaminated with the agents causing Transmissible Spongiform Encephalopathies (Mad Cow disease, CJD, etc.).
Alkaline hydrolysis is a true batch process. The Tissue Digester is sealed during operation. At the end of the process, the liquid effluent is a sterile, EPA neutral solution of amino acids, small peptides, sugars and soaps that may be released to a sanitary sewer. Since KOH is used as the alkali, the liquid effluent is an excellent fertilizer that may be broadcast or injected into the soil.
Carcass volume and weight reduction exceed 97%.
After processing, both the effluent and all internal parts of the digestion system are sterile. All wetted parts of the Tissue Digester system are 316L stainless steel. The vessel is insulated for greater efficiency and operator safety. The shell of the Tissue Digester is also stainless steel and all exposed electrical parts are NEMA 4 compliant, facilitating external cleanup and making the whole system hosable, if necessary.
The only residue remaining after digestion is the inorganic material (pure calcium phosphate) of bones and teeth. This may be landfilled or used as sterile bone meal fertilizer. Materials that may enter the system with the carcasses, such as gloves, catheters, and instruments, are also sterilized and may be treated as ordinary waste.
In a Tissue Digester, most toxins associated with the carcasses are destroyed, no new toxins (such as dioxins from incompletely oxidized plastics) are produced, and heavy metals are not concentrated; rather, they are diluted by the release of the nearly 75% of the animal that is water.
The Tissue Digester automatically cools the fluid in the vessel to a temperature at which it is safe to release it to a sanitary sewer. The operator is not exposed to hot materials.
A Tissue Digester operates at between $0.03 to $0.06 per pound of tissue processed, a fraction of the cost of operating an incinerator. A Tissue Digester is designed to last much longer than an incinerator, with less maintenance.
There is no smokestack, there are no air emissions, and there are no applicable EPA air quality regulations.


Incinerators come in many forms. Pathological incinerators, with secondary gas retention chambers of 2 seconds retention time (no scrubbers currently required) are discussed here.

The basic pathological incinerator is a stuff and burn system, with a side-fed door. This type of system is difficult to feed, particularly if large animals are to be processed. Carcasses have to be dissected or butchered to provide pieces small enough to fit through the feed door and these pieces may be heavy, difficult, and potentially dangerous to handle. Pathologic incinerators are available with a top load, large door but this adds considerably to the cost and is usually still not adequately sized for whole equine, bovine, or porcine carcasses. Some units are available with a top-fed ram that loads tissues into the side door of the burn chamber. Again, however, large animals must be quartered or further cut up before loading into the hopper of the ram.

An incinerator is not a true batch system because air goes in and air flows out constantly during the processing cycle which can violate the concept of a “contained area”. In addition, most incinerators must be operated by adding more tissue as the burn progresses in order to meet the throughput requirements of the institution.

Ram feed systems become contaminated and remain that way, they are not easily cleanable.

Incineration also achieves significant volume reduction, up to 97%, but toxins and heavy metals are concentrated in the incinerator ash, yielding a residue that is usually classified as toxic waste. Toxins, including dioxins, halogenated hydrocarbons, and mercury vapor are produced by the combustion process and are released through the stack into the atmosphere.

Destruction of pathogens by incineration is only as good as the least burned material remaining in the bottom of the combustion chamber. Unburned “material” is often found in the ashes as they are raked out and must be returned to the chamber, putting the operator at risk. It has been shown that TSE agents are NOT destroyed by conventional incineration or by heating to as much as 600°C under controlled conditions. The need for a reliable, simple method for destruction of TSE agents is imperative as more contaminated wildlife, ungulates, and zoo animals are turning up in the US.
The cost of operating a pathologic incinerator ranges from $0.025 to $0.75 per pound, depending on the age of the unit, natural gas costs, and the frequent repair and maintenance needed on these systems.
Operators are exposed to dust, heat, and toxic gases while running an incinerator.
There is a smokestack with visible output, toxic emissions are produced, and incinerators are subject to increasingly stringent clean air regulations.

Incineration has the ability to contaminate not only the air we breathe, but also ground water, soil, livestock, and Humans through the emissions emitted from the smokestack.

Tissue Disposal System Summary

A Tissue Digester system is complete on installation. No preprocessing or post-processing equipment must be added for convenience of operation or to meet clean air regulations.

A Tissue Digester is a non-polluting, environmentally-friendly, more efficient method of waste decontamination and tissue disposal as compared to incineration.
Loading mid-size to large animal carcasses is ergonomically-friendly since the carcasses can be loaded from the top, in their entirety, un-sectioned.

For BSL3 and BSL4 bio-containment applications, the Tissue Digester is far more adaptable to work-flow considerations in laboratory design thus making a Tissue Digester system more desirable:

  • Horizontal or Vertical Designs for ease of loading various size carcasses from small rodents, primates, and large animals
  • Double-door pass-through configurations allow infectious material to be processed from the BSL3 or BSL4 containment area.
  • Biocontainment Seals provided on Tissue Digester systems prevent cross-contamination on double door units and provide a true bio-barrier as compared to pass-through incinerator models.
  • The Tissue Digester is a sealed, closed-loop system while the incinerator is not.
  • Alkaline Hydrolysis provides a higher level of efficacy since exposure of the infectious waste to time and temperature can be controlled precisely.
  • A guaranteed higher level of efficacy can be attained using Alkaline Hydrolysis since the process utilizes both thermal and chemical sterilization principles.

The Tissue Digester is much less costly to own and operate over the long term when maintenance and yearly air pollution permits are considered in the yearly operating costs.

Alkaline Hydrolysis eliminates the potential to expose the attendants to pathogens, toxins, and other types of hazardous waste as compared to incineration.

While the installation of a Tissue Digester may be slightly more expensive than a basic pathologic incinerator of comparable capacity, the environmental “greening” aspects of Alkaline Hydrolysis technology and the ability to provide a higher level of efficacy far out-weigh the old, pollution-generating, inexact methods of the past.