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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.
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