Thermal Treatment

In contrast to incineration, some thermal treatment methods can use the high water content of medical waste to advantage. Water can provide an effective heat transfer medium, to help distribute heat throughout the mass of the waste.

One problem with water as a heat transfer medium is that the temperature at which water boils at normal atmospheric pressure is not sufficiently high to kill some of the hardier microorganisms (spore-forming species, for example). One common solution is to carry out the treatment in a pressure chamber. As the pressure is raised, the boiling point of water increases. At a pressure twice as high as normal atmospheric pressure, the boiling point of water increases by about 36 degrees F, to 240 degrees F (i.e. by about 20 degrees C, to 120 degrees C), which is sufficient to kill most organisms of concern. Systems using steam under pressure are called autoclaves, and are among the most common alternatives to incineration for medical waste treatment.

Another thermal treatment system that takes advantage of the properties of water uses microwaves as the energy source. In a microwave system, the waste is subjected to high intensity radio waves, tuned to a frequency that is readily absorbed by water molecules. It is an efficient way to deliver the energy where it is most needed for sterilization purposes. The other side of that coin is that microwave heating will be inefficient if the waste is too dry. Microwaves will penetrate bulk materials to some extent, but the heating will proceed more efficiently if the waste is shredded and mixed in the chamber during the process (for much the same reason that many kitchen microwave ovens use a rotating platform).

An advantage to both autoclaves and microwave systems is the fact that air does not have to move through the systems while they operate. Emission of volatiles only occurs during loading and unloading, and can be minimized with proper design and operation.

Autoclaves and microwave systems are effective, but the necessary equipment is somewhat expensive (pressure chambers and microwave generators, respectively). In contrast, dry heat systems use less demanding equipment, but typically require higher temperatures and longer exposure times to ensure that the heat supplied by the system penetrates to the center of the waste. Rather than directing the heat into the mass of the waste, evaporating water carries a substantial quantity of the heat away. On the other hand, the drying of the waste has some advantages, including substantial weight and volume reduction and easier handling of the residue.

Since dry heat systems do not involve combustion, unwanted reactions such as dioxin formation are not an issue. But if air moves through the system, it can carry volatiles and pathogens. The exhaust stream is typically filtered before release, but the potential for release always exists.

One disadvantage with all of these systems, stemming from the fact that they operate at substantially lower temperatures than incinerators, is that they require a certain minimum contact time to ensure that all pathogens have been destroyed. Higher temperatures are required to process large quantities of waste in a shorter time. To obtain a higher throughput while avoiding the problems associated with ordinary combustion, some large scale systems use advanced heating methods to create very high temperatures with a minimum of air passing through the system. One method to produce the desired temperature uses a plasma arc—an electric discharge producing intense heat in the absence of combustion. Other types of heating, such as induction, may also be used. In any case, the heat is sufficiently high to cause the organic molecules in the waste to break down to simpler compounds, even though no combustion is occurring. (This kind of heat breakdown with minimal oxygen present is generally called “pyrolysis”.) Unfortunately, those simpler compounds include a significant proportion of gases (including carbon monoxide), which are somewhat harder to handle than solid residue. Since they must flow out of the pyrolysis chamber as the reaction proceeds, the advantage of not having to flow combustion air through the system is somewhat nullified. The offgases are burned in an oxidation chamber. The volume of air that must be treated is somewhat less, but all the contaminants present in an incinerator exhaust stream are there as well, and must be filtered out or they will be emitted from the system.