Hospital waste disposal technologies

The hospital waste group includes infectious waste, used sharp objects, pharmaceutical waste, waste containing hazardous chemicals and preparations, waste with a heavy metal content. Medical waste is classified as hazardous on the basis of one or a combination of properties - content of infectious microorganisms, cytotoxicity or Geno toxicity, mutagenicity, reproductive toxicity, toxic chemicals content, etc.

Improper storage and disposal of these types of waste poses risks to the environment and human health. The main processes for the treatment of medical waste are 5 – mechanical, chemical, radiation, biological and thermal.

Mechanical processes

Mechanical methods are used as a supplement to the four main processes for treating hospital waste. Mechanical processes include shredding, grinding, stirring and compacting. In general, these processes are applied to improve the degree of heat exchange, steam penetration or contact with the chemical disinfectant. Mechanical processes are also applied to reduce the volume of treated waste or to eliminate physical hazards when treating sharp objects.

Unless shredders, stirrers and other mechanical equipment are part of a closed treatment system, they must not be used before disinfection of incoming waste. If this is not observed, workers are at risk of exposure to pathogens in aerosols released during mechanical destruction.

If mechanical processes are part of a closed system, the technology must be designed in such a way that the air in and out of the mechanical process is disinfected before it is released into the working or the environment.

Chemical technologies

Chemical disinfectants are used in chemical processes to destroy pathogens in waste. The effectiveness of microbial inactivation depends on the type of chemical disinfectant, its concentration, the possibility of exposure to all surfaces, the contact time, but can also be affected by temperature, pH, water hardness and the amount of organic in the waste. The most widely applied technologies are based on chlorine treatment, using either dissolved chlorine dioxide or sodium hypochlorite.

Other technologies use quaternary ammonium salts, lime slurry or calcium oxide. In recent years, ozone treatment has developed. Older technologies used peroxyacetic acid, iodophores and formaldehyde. A specific case of chemical treatment is alkaline hydrolysis, in which bases are used for the decomposition of waste in heated stainless steel tanks. The reduction of the volume of waste depends on the type and degree of shredding, grinding and stirring.

Radiation and biological methods

It is known that some types of radiation destroy pathogens. Since ionizing radiation using electronic rays and cobalt-60 are used for sterilization of medical instruments, the same concept has also been tested for the treatment of hospital waste. The effectiveness of microbial inactivation depends on the dose absorbed by the waste. UV-C radiation is also applied, but as an auxiliary technology for the treatment of medical waste.

The concept of biological methods for the treatment of waste refers to the natural decomposition of organic matter. Some biological treatment systems use enzymes to accelerate the destruction of organic waste containing pathogens.

Thermal methods

These processes use heat to destroy pathogens in waste. The process is applied in most treatment facilities worldwide. This category can be divided into low temperature and high temperature processes. There are clear differences between the ongoing thermochemical reactions, the physical changes that waste undergoes and emissions from low temperature and high temperature processes.

Low temperature processes use thermal energy at temperatures high enough to destroy microorganisms, but insufficient to lead to the incineration or pyrolysis of waste. Low temperature processes take place at temperatures between 100°C and 180°C.

They can take place in both wet and dry environments. Wet thermal treatment involves the use of steam to disinfect waste. The technologies using this process are autoclave systems, hybrid autoclave systems and continuous steam treatment systems.

The autoclave consists of a metal chamber designed to withstand high pressures, with a stable door and a system of pipes and valves through which steam passes. Some autoclaves are designed with a fur coat around the camera, with steam being brought into both the space between the casing and the camera, and into the camera itself. Heating the camera on the outside reduces condensation along the inner walls of the chamber and allows the use of steam at a lower temperature.

Since the construction of autoclaves without a steam jacket (retsorts) is cheaper, they are applied on a large scale. In an autoclave can be treated medical waste such as needles, materials contaminated with blood and bodily fluids, bandages, gauze, bed linen. Volatile and semi-volatile organic substances, chemotherapy waste, mercury and other hazardous chemical waste cannot be disposed of.

Autoclaves can treat from 20 l to over 20,000 l of medical waste per cycle. Their capacity ranges from 1 kg/h to 2700 kg/h, including charging time, treatment and waste removal itself.

Microwave treatment is also a wet low temperature process. In microwave systems, disinfection is carried out by the action of wet heat (hot water and steam generated by microwave energy). Microwave equipment operates at a microwave frequency of 2450 MHz. Serial microwave technologies are usually small facilities, while continuous microwave treatment technologies are larger in size.

The waste that is treated microwave is identical to those that are disposed of in an autoclave – sharp objects, materials contaminated with blood and bodily fluids, bandages, gauze and bed linen. Needles and other sharp metal objects must be placed in impenetrable containers.

Containers must not be closed hermetically in order for steam to penetrate. Volatile and semi-volatile organic compounds, mercury and other hazardous chemicals should not be treated microwave. The capacities of these systems range from 30 to 210 kg/h.

A technology similar to microwave treatment is electrothermal deactivation (TLD). ETC uses low-frequency radio waves at 64 MHz to obtain a strong electric field, resulting in heating medical waste and destroying pathogens.

Steam-based technologies that do not involve subsequent size reductions and microwave technologies that do not involve shredding waste achieve a minimal reduction in volume (mainly by removing air and melting certain plastics).

In the case of hybrid autoclaves, continuous steam treatment systems and microwave equipment, which include shredding and stirring, can be achieved from a 60% to an 80% reduction in waste volume, depending on the type of shredding used. In technologies based on steam treatment, which also include a drying cycle, a weight reduction of 15-20% can be achieved.

Dry thermal processes use hot air without adding water or steam, and operate at temperatures lower than those of combustion. In dry thermal systems, waste is heated by conductivity, convection and/or thermal radiation from infrared or resistance heaters. Another technology uses the heat from friction between waste as a source of heating. These installations are usually small-scale. High temperature thermal processes take place at more than 180°C.

Combustion is a process of high temperature dry oxidation that reduces organic and combustible waste to inorganic, non-combustible matter and leads to a significant reduction in the volume and weight of waste. High temperature thermal processes occur at temperatures from 200°C to above 1000°C.

These include chemical and physical decomposition of organic matter through combustion, pyrolysis or gasification processes. A disadvantage of these technologies is the generation of emissions and ash with dangerous properties. The incineration of medical waste results in the release of gaseous emissions consisting of steam, carbon dioxide, nitrogen oxides, a variety of volatile substances (metals, halogen acids, incomplete combustion products) and dust.

A number of waste incineration technologies are known, but two chamber incinerators are most often used for the disposal of hospital waste. They consist of a primary combustion chamber, a secondary combustion chamber and a pollution control system. It is also desirable to have a waste charging system preventing the temperature from lowering in the primary chamber and an ash collection system.

The waste is loaded into the primary combustion chamber with a auger. The temperature in the primary chamber is above 850°C, supported by several gas burners. The vapour received in the primary chamber shall pass to the secondary combustion chamber, which is equipped with one or more burners. The secondary chamber shall be designed to tolerate high turbulence, higher temperatures (1100°C for medical waste) and a minimum of 2 s during waste downtime.

Waste gases from the secondary combustion chamber pass through a treatment system, which may include a gas cooling system, wet or dry scrubber, bag filter, cyclone, electrofilter, catalytic oxidiser, etc. After purification, gases are released into the atmosphere through a chimney.

In incinerators can be disposed of a number of hospital waste – sharp objects, materials contaminated with blood and bodily fluids, bandages, gauze, bed linen. Thermal chemical waste, pharmaceutical, halogenated (PVC) and cytotoxic waste are treated in high temperature facilities (>1200°C).

The incineration of hospital waste is appropriate only if the caloric value of the waste exceeds 8370 kJ/kg. Although plastics may exceed 16 740 kJ/kg, some medical waste has a high moisture content and has a much lower calorific value, which proves that in order to achieve a higher total caloric value, medical waste must be mixed.

Incinerated waste must contain more than 60 % combustible components and a combustible fraction of less than 5 %. The moisture content must be less than 30 %. The capacity of incinerators varies from 10 kg/h to 100 t/d.