Fullscreen HTML5Med Res (??MB)HTML4

Microwave disinfection of medical waste Hospitals & clinics generate large quantities of potentially dangerous medical waste which must be safely disposed of. Now there is a new method whereby the material is first shredded & then disinfected by exposure to microwave radiation. By JOHN CUSACK & MARK TAITZ* The subject of waste disposal has become of major concern in industrial countries in recent years. Latent dangers, such as possible groundwater pollution, mean that waste should no longer be simply dumped in landfills. And public concern about air pollution has put a question mark against the traditional incineration method of disposal. Very few people are aware of the large quantities of medical waste pro6 SILICON CHIP It is obvious that uncontrolled disposal and open landfill dumping carry a risk of disease transmission. But more than anything else, it has been the spread of AIDS that has focused public concern on infection control and environmental practices within the health care industry. Sterilisation or incineration? The options for treating infectious waste in the past have been sterilisaduced by hospitals, pathology laboration (by autoclaving) or incineration. tories, clinics and doctors' practices. . Waste which has been sterilised can In the United States alone, more than be burnt or shipped to landfills. In the 600,000 tonnes of medical waste are latter case though, it should be thorgenerated every year by the health oughly shredded into tiny fragments care industry. This includes blood and which are unrecognisable as medical blood products, cultures and stocks waste. This is because many municiof infectious agents, "sharps" (neepal landfills now reject even treated dles, razors, etc), as well as pathologi- medical waste when it still recogniscal wastes. By the end of the decade, able. the amount of such waste is expected In Australia, most medical waste to grow significantly. (called "contaminated waste") is dis- ◄ Left: the microwave disinfection process is housed in a steel housing not unlike a standard shipping container. The waste is shredded, sprayed with steam & then irradiated with microwave energy to disinfect it. The end product is unrecognisable as medical waste. k l.r !.. '"' posed of by high temperature incineration or sealed in yellow plastic bags which are trucked to municipal landfills and then immediately buried in designated areas by bulldozer. The first method brings concern about air pollution while the second has concerns about groundwater contamination. Autoclaves are high pressure steam vessels, designed originally for sterilising medical instruments. The process involves heating the waste, usually with saturated steam, to 131 °C and keeping it at this temperature for up to 45 minutes. It must be continuously checked to see that the material being sterilised has actually attained the required temperature, the pressure in the autoclave being about two bar (ie, about two atmospheres). However, the standard small autoclaves on the market are not suitable to treat the large volumes of waste produced by the health care industry. Some large hospitals have their own medical waste incinerators. Initially designed for the disposal of large pathological wastes, they are n0t able to process in an environmentally sound way today's broader medical waste stream, which can include about 15% plastics and 20% non combustible material. Furthermore, due to public concerns about air pollution, the incineration facilities would have to be equipped with special air cleaning systems. It is estimated that in the USA alone, as many as 90% of the 5000 medical waste incinerators currently in operation will have to be shut down as US clean air standards become more stringent. Municipal waste incineration Another option is disposal in municipal waste incineration plants but this creates other problems, although these facilities are usually technically •••• Six microwave generators, operating at 2450MHz, thoroughly disinfect the waste material as it passes beneath them on a conveyor. well equipped for disposing of medical waste. The main problem lies in the transportation and handling of the waste. Between being packaged in the hospital and fed into the furnace, there must be no chance of it going astray, coming into contact with workers, or causing any kind of contamination. Also, municipal solid waste incinerators typically do not operate at temperatures sufficiently high enough to ensure thorough destruction of all micro organisms. The microwave solution ABB Sanitec, Inc recognised that the medical community, arid particularly large regional hospitals, required an on-site treatment facility which is safe, environmentally sound and cost effective. Their response to this need was to develop a system which disinfects the waste by heating it with conventional microwaves. The waste is first shredded into tiny, confetti-like particles and then moistened with high temperature steam. Afterwards, the granulated material is passed beneath a row of microwave generators which thoroughly disinfect it. After processing, the mat~rial is unrecognisable as medical waste and is quite acceptable for dumping in municipal landfills. In 198 7, the first medical disinfection system of this kind was placed in commercial operation at a university hospital in Germany. ABB Sanitec has since continued to develop the technology and units have been delivered to health care institutions and FEBRUARY 1993 7 temperature throughout. Strip charts record the variables necessary to substantiate disinfection, while continuous monitoring ensures that the requirements for thorough treatment are exceeded by a comfortable margin. A lockable port, just below the shredder, provides easy access for introducing standard testing samples. Waste volume reduction The end product is greatly reduced in volume and is unrecognisable as medical waste. It can also be burnt in a high temperature incinerator to produce steam for heating or electricity generation. commercial medical waste disposal companies in Europe and North America. Microwave plant The waste treatment equipment is housed in a steel all-weather shelter measuring 7.2 metres long, 3.3 metres wide and 2.8 metres high. The total weight of the unit is about 11 tonnes. A single electrical mains connection rated at 75kW for the larger unit and just one water hook up are necessary. Enough space must be provided in front of the inlet hopper to allow transportation and handling. At the output end, there must also be a hopper for removing the disinfected material, which is reduced to about 20 percent of its original volume. Since there are no liquid discharges (the small amount of water used is absorbed into the waste residue), a sewer connection is not necessary. The process does not use chemicals or bleaching agents. Fully enclosed and self-contained, the unit can be installed outdoors. Many systems are currently being operated in the loading dock areas of hospitals. 8 SILICON CHIP An automatic hoist lifts the waste container and tips the contents into the hopper at the top of the unit. Before the hopper opens, the air inside it is treated with high temperature steam, then extracted through a high efficiency particulate air (HEPA) filter to emininate any potentially harmful air-borne pathogens. To reduce its volume, the waste is then shredded into tiny particles. The granulated material is automatically conveyed into the treatment chamber, where it is evenly sprayed with high temperature steam. The mixture passes beneath six microwave generators, each with an output of 1.2kW. These heat the mixture to between 95°C and 100°c and maintain it at this temperature for a minimum treatment time of 30 minutes. The microwave generators are standard industrial units with an output frequency of 2450MHz. The microwave generators ensure fast and even heating to the required temperature and uniform disinfection of each granule. Sensors and microprocessors control the entire process, ensluing the proper treatment time and the correct The end product is a granulated solid waste which, since it harbours no risk of infection, is acceptable for dumping in municipal landfills. Even vials, hypodermic needles, syringes and plastic tubing are thoroughly shredded into tiny fragments which are unrecognisable as medical waste. Important for the end disposal is the fact that the waste is reduced by this process to only about one fifth of its original volume. Besides reducing transportation costs, this also has considerable benefits for eventual dumping in landfills or incineration. It is also a reason for using the unit at some hospitals to process waste that is not defined as infectious by present regulations. The heat value of the end product is also very high. At about 20 MJ/kg, it is even higher than that of good firewood. Treated waste is successfully burned in several waste-to-energy plants in the USA and Europe. Low operating costs The total operating costs, covering power, water, labour, maintenance, spare parts and consumables, are low. In America, they have been calculated to be about 9 cents/kg, a low figure compared with the cost of sterilisation or incineration methods. At present, there are more than 30 units in operation in Europe and North America. The two models currently available in Europe are designed for capacities of 150kg/h and 250kg/h. In North America, where the units are also manufactured, only the 250kg/h unit is currently being marketed. The smaller unit will become available during 1993. Acknowledgment Our thanks to Asea Brown Boveri Pty Ltd for permission to reproduce this article which originally appeared in the October 1992 issue of "ABB Review". The authors are employed by Sanitec, Inc, USA. SC