The main task of a regulated treatment system is to render the waste noninfectious. The technical means to disinfect medical waste has existed for a long time. Incineration was once the method of choice for dealing with medical waste, and many hospitals burned their waste on site. But it gradually became clear that, while protecting the public from infection, hospitals using onsite incinerators were exposing the public to emissions that included mercury, dioxins, and other highly toxic substances. In 1996, medical waste incinerators were listed as the largest source of dioxin and in 1997, as the third largest source of mercury to the environment.
Pressure from public interest groups and new, more stringent EPA standards on emissions from medical waste incinerators in 1997 brought about the closure of several thousand onsite medical waste incinerators at healthcare facilities. Fewer than 100 such installations nationwide are still operating. Currently, most healthcare facilities either ship their waste to large, centralized incinerators, or use technologies other than incineration.
Currently, available treatment technologies rely on two basic approaches to sterilization. Infectious organisms can be killed by subjecting them to excessive heat, or by bringing them into contact with chemical agents.
The most popular options include:
- steam autoclaves
- microwave systems
- dry heat and hot air systems
- plasma arc
- chemical agents:
- chlorine compounds (including hypochlorite, chlorine dioxide)
- other disinfectants (peracetic acid, glutaraldehyde, etc.-- typically used for small batches)
Some systems use combinations of these treatments. Incinerators, for example, use both heat and a chemical reaction (oxidation by atmospheric oxygen). Another example is a system that operates at a relatively moderate temperature, which would otherwise leave the waste intact, but that uses alkali to liquefy the waste.
There is an additional important consideration relevant to the treatment of infectious waste. Whatever the lethal agent is, it can only be effective if it is applied in sufficient strength throughout the entire bulk of the waste. Either the treatment must be applied for a time sufficient to allow the agent to penetrate to the interior of the waste mass, or the waste must be shredded or ground up to bring the interior to the surface.
Shredding or grinding the waste also has the advantage that it renders any recognizable body parts unrecognizable, as required in some states before disposal. It can also help reduce the volume of the waste. The disadvantage of including a shredding or grinding system is the cost and the additional maintenance required. Breaking up the waste before it has been rendered uninfectious also involves the risk of disseminating the pathogens, so shredding or grinding operations must be carried out in equipment specifically designed for medical waste processing.
The following lists a few of the most significant points of comparison among the available treatment options. A more extensive discussion can be found in Health Care Without Harm publication "Non-incineration medical waste treatment technologies."
Incineration is unquestionably effective but is associated with serious air quality concerns. Because atmospheric oxygen is used as the reagent, a large volume of air must constantly pass through the system. Unless the exhaust air passes through a control device, all substances that are volatile at the operating temperature of the system will be emitted with the exhaust stream. Incinerators emit dioxin, mercury, other heavy metals that can impact human health.
Incinerators are also inherently inefficient from an energy standpoint, particularly when dealing with wastes with high water content. To maintain combustion temperatures, many pounds of fuel must be burned to destroy each pound of waste. Much of that energy is spent simply to boil off the water so that the organic portion of the waste will burn. This has historically been less of a consideration for medical waste, since processing costs are high in any case, but will undoubtedly become more of an issue as the cost of fuel (typically natural gas) increases. Moreover, the burning of large quantities of fuel entails the generation of excessive greenhouse gases (primarily carbon dioxide) relative to the amount of waste material destroyed.
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 36F, to 240F (i.e. by about 20C, to 120C), 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 the 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.
The obvious disadvantage of chemical treatment systems is that they consume chemicals. In addition, even if they are effective in rendering the waste noninfectious, the products of the chemical reactions they undergo are present in the waste, and may pose problems of their own. However, chemical treatment systems are convenient, and may be suitable in some situations, particularly when small quantities of waste are involved.
One of the most common constituents of chemical treatment systems is chlorine, either in the form of sodium hypochlorite solution (common bleach), or as the more powerful (and correspondingly more hazardous) gas, chlorine dioxide. These compounds are relatively cheap and effective. However, in the course of reacting with organic compounds, they tend to form objectionable byproducts such as chloroform and other persistent toxins.
The chlorine compounds work by "oxidizing" (stripping electrons from) organic compounds, including the constituents of pathogenic microorganisms. The original "oxidizer" is, of course, atmospheric oxygen. Although it is, in fact, a fairly powerful oxidizing agent, ordinary oxygen is not harmful to -- is in fact essential for the survival of -- many organisms, including most of the pathogens in medical waste. However, when oxygen (O2) is converted to ozone (03), a much stronger oxidizer, it becomes toxic to most life forms. Ozone can readily be generated by passing an electric arc through ordinary oxygen gas. When used in a medical waste treatment system, ozone acts as an effective sterilizer, without the tendency to generate the types of by-products found with chlorine compounds. The major problem encountered with ozone systems is the need to avoid exposure to anyone in the vicinity of the treatment system, since ozone is highly injurious to lungs.
Alkaline agents are also used in medical waste treatment, either in highly corrosive form (sodium hydroxide, or lye), or in a somewhat milder form (calcium oxide, or quicklime). Alkali tends to hydrolyze (decompose) proteins, among other effects. Apart from the cost of the reagents, the major disadvantage is the risk of contact, since alkaline solutions damage skin and lungs.
Disinfectants like glutaraldehyde and peracetic acid are also used for small-scale medical waste treatment. More information on these materials can be found on the Healthcare Environmental Resource Center.
After it has been rendered noninfectious, most medical waste can be disposed of as if it were ordinary solid waste. However, there are some important exceptions:
- Waste that must be managed as hazardous waste must be disposed of in compliance with RCRA regulations
- Some states require that recognizably anatomical waste must be rendered unrecognizble before being disposed of in a solid waste landfill.
Some states allow cremation or interment (burial) of certain kinds of untreated waste. "Interment" presumably refers to burial in a manner that follows standard mortuary practices, rather than disposal in a landfill.