Microwave radiation is used to treat wastewater sludge and as a heat source to treat medical waste. Microwave treatment units can be either on-site installations or in mobile treatment vehicles. The processing usually includes front-end shredding of the waste, both to increase the efficacy of the microwave treatment and to reduce the volume of the end waste for disposal. If the waste is dry, water is added and the wet waste is introduced to the microwave chamber.
It basically works like a souped-up version of your kitchen’s microwave oven. Typical operation is at 2450 Hz. While an autoclave provides heat from outside the waste, like a conventional kitchen oven, the microwave unit transmits energy as microwaves and that energy turns into heat inside the wet waste.
Microwave disinfection works only when there is water in the waste. because the radiation directly works on the water, not the solid components of the waste. For this reason, treatment units are often supplied with a humidifier. Processing time is determined by the manufacturer and experience of the operators, but somewhere about 20 minutes per batch is typical. Mechanical treatment is often positioned upstream of the microwave in order to make sure the waste pieces are small. Smaller pieces enhance the heating action as microwaves are able to penetrate to where infectious microbes are. With enough power the water is converted to steam and makes all of the waste around 100°C. The entire process takes place within a single vessel. Bacteriological and virological tests are periodically conducted to ensure the process is effective. A common bacteriological test examines eradication of Bacillus subtilis. 99.99% reduction of spores is considered a benchmark.
Treatment of medical waste through exposure to microwaves is less expensive than incineration although it will not chemically alter hazardous materials that way incineration can. This method is not recommended by the EPA for the treatment of pathological waste. Microwave treatment can also melt syringes, but it is rarely used for this purpose. Put your syringes in a sharps container and have them processed through mechanical destruction.
Microwave treatment is sometimes referred to as irradiation. However, it should not be confused with irradiation with gamma rays (from radioactive elements) or electrons.
Irradiation disinfects waste by exposing it to gamma rays that are fatal to bacteria. A radioactive isotope of cobalt is employed. This is basically the same radiation source used for radiation treatment of cancer. In cancer treatment, radiation is intended to kill the malignant cells. In irradiation for sterilization of equipment or treatment of waste, the radiation is intended to kill pathogens. Some irradiation treatment systems use electron beams. Both gamma rays and electron beams can penetrate plastic bags used for waste collection, so the waste does not need to be removed from the bag before treatment.
Irradiation does not change the appearance of the waste so process designers often install mechanical grinding or shredding upstream. This also makes the waste pieces smaller, which tends to enhance the efficacy of the treatment.
Unlike some other treatment methods, irradiation requires a dedicated place – there are no mobile treatment modules that use radiation. It is fairly expensive to build an irradiation facility and operating precautions must be taken to protect workers from radiation. These are among the reasons this method is not widely used, especially when heat treatment methods are typically just as effective. The efficiency of irradiation as a sterilization process depends to a large extent on the total energy delivered, but even then waste surfaces facing the radiation source get more sterile than the waste on the shaded side. Odd-shaped waste pieces may not get adequate exposure to the radiation, if contaminated surfaces face away from the cobalt source. Heat treatment, by contrast, brings every piece of waste to an adequate temperature for sterilization if done correctly.
Ultraviolet (UV) radiation is also employed in treatment of wastewater. UV used for wastewater is at a lower frequency and less lethal than gamma radiation. When UV is used for disinfection, the radiation in the UV-C spectrum which is germicidal is employed. UV-C disinfection systems exploit the effects of this radiation on the DNA and RNA, breaking the helical chain and preventing the replication of the genetic code: The affected microorganism is then inactivated and unable to reproduce.
Although it is rarely used, vitrification can be an effective treatment for medical waste. The solid waste is mixed in when glass is formed (vitrification means production of glass). The high temperatures kill pathogens and some combustible material may burn or pyrolyze, resulting in an off-gas. Remaining material is encapsulated in glass, which has a very low diffusivity. There is little danger of hazardous materials leaching out of glass in significant quantities. The vitrified waste can therefore be put in a landfill with confidence, although if there are radioactive materials in the waste, that landfill may have to be one that is certified to accept radioactive waste. Vitrification is often mentioned as a long-term solution for radioactive waste produced at nuclear reactors; this waste is generally more radioactive than any medical waste.
Plasma treatment has been developed and proposed as an alternative to incineration. However, despite some interest in using it for medical waste, it has not found widespread use.
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Ozone is rarely used for medical waste but it is often used for water treatment.
Ozone generators take in air and put out air that is slightly enriched with ozone. The concentration of ozone is low (about 1.5 percent ozone by weight), but ozone is such a powerful oxidizer that even low concntrations are effective. Capital costs tend to be high; energy costs are typically 8 kWh/lb of air treated.
Ozone generation works better with dry air. Water makes the ozonated air corrosive. For this reason, a drying unit is installed upstream of the ozone generator.
Ozone has been used for decades for disinfection of municipal water supplies. These installations are bigger than what a medical waste treatment process will normally require. Will ozone be a good choice for your medical waste treatment? You could try it in a pilot plant. One great thing about ozone is that it is so strong. It has a higher oxidation potential than chlorine compounds used to destroy organic materials and disable microorganisms.
Advantages of ozone treatment are that the oxidant is generated at the point of use and no storage of hazardous materials is needed. The system can be quickly switched off. Ozone treatment is not as sensitive to the pH of the waste or the temperature as some treatment technologies. More on ozone treatment.
Mechanical and Biological Treatment of Medical Waste
