Mechanical processing can be grinding, shredding, and/or compacting Mechanical treatment does not kill pathogens or disinfect equipment, but it can reduce waste volume in preparation of further treatment or disposal. Equipment involved can include crushers and milling machines, Mechanical shattering or splintering of waste can also alter its appearance, which can be useful in lessening the psychological impact of the waste on human observers.
In addition to being able to reduce bulk waste volume, mechanical systems can increase the surface area of the solid pieces before subsequent chemical or heat treatment. Sometimes the waste is mixed with other material (cement or polymers) but doing so can increase the waste mass, so the engineer in charge should take a hard look at whether mixing waste with an outside agent is worth it.
But mechanical systems can also be an operational headache - more things to go wrong and more things to be decontaminated. Further, mashing or shredding of solid waste can generate dust. If this dust becomes airborne, it can be a workplace hazard and a threat to the environment. That’s why mechanical equipment is often kept in a closed room or under a hood, at slightly lower than ambient air pressure.
Chemical disinfection, primarily through the use of chlorine compounds, kills microorganisms in medical waste and can often oxidize hazardous chemical constituents. Chlorine bleach has been used for disinfecting processes for years – you might use it to clean your underwear and kill the e.coli. Chlorine compounds are used in swimming pools to reduce the risk of disease transmission. Ethylene oxide treatment is used to disinfect materials and is sometimes used in treatment of medical waste. Ethylene oxide treatment is used more often to sterilize equipment that will be reused. It is too expensive to use on equipment or waste that will be sent to a landfill - incineration is better. EtO gas infiltrates packages as well as products themselves to kill microorganisms that are left during production or packaging processes.
Unlike heat treatment, the mechanical/chemical disinfection process – which you may think of as similar to a washing machine at the laundromat - is easy to control and can be run in fixed batch sizes and consistent processing times. Liquid medical waste is often best treated with chemical disinfection, that’s if the medical waste is aqueous; organic liquid waste does not mix well with bleach. But almost all liquid medical waste is aqueous.
The EPA identifies chemical disinfection as the most appropriate method to treat liquid medical waste. Treated liquid waste can usually be discharged into the sewer system. Sometimes it goes to a holding tank for testing before discharge.
If the waste is in solid form, the treatment process often has a mechanical cutting or mashing procedure included. The primary function here is to reduce size of the solid waste pieces and increase surface area, to make the chemical treatment more effective. After treatment with the liquid disinfectant, the solids are filtered out and the liquid filtrate goes to the sewer or for more treatment.
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 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 introduced 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. Microwave power causes the temperature of the water to increase. If 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 degree C. The entire process takes place within a single vessel. Bacteriological and virological tests are periodically conducted to ensure the process is effective. Treatment of medical waste through exposure to microwaves is less expensive than incineration. The treated waste can be disposed of in a landfill. 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. By contrast, you may hear about ultraviolet (UV) radiation treatment of wastewater. The radiation in that case is not intended to kill microbes so much as to break down chemicals. UV used for wastewater is a lower frequency and less lethal than gamma radiation. When UV is used for disinfection, the radiation in the UV-C spectrum, which is more germicidal is employed. 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.
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.
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.