Chemical disinfection, used routinely in health care to kill microorganisms on medical equipment and on floors and walls, is now being extended to the treatment of health-care waste. Chemicals are added to waste to kill or inactivate the pathogens it contains; this treatment usually results in disinfection rather than sterilization. Chemical disinfection is most suitable for treating liquid waste such as blood, urine, stools, or hospital sewage. However, solid—and even highly hazardous— health-care wastes, including microbiological cultures, sharps, etc., may also be disinfected chemically, with the following limitations:
• Shredding and/or milling of waste is usually necessary before disinfection; the shredder is often the weak point in the treatment chain, being subject to frequent mechanical failure or breakdown.
• Powerful disinfectants are required, which are themselves also hazardous and should be used only by well trained and adequately protected personnel.
• Disinfection efficiency depends on operational conditions.
• Only the surface of intact solid waste will be disinfected.
Human body parts and animal carcasses should not normally be disinfected chemically. If alternative facilities for disposal are not readily available, however, they may be shredded and then subjected to chemical disinfection. In planning the use of chemical disinfection, requirements for the eventual disposal of the residues should be carefully considered; improper disposal could give rise to serious environmental problems.
Microbial resistance to disinfectants has been investigated and it is possible to list the major groups of microorganisms from most to least resistant as follows: bacterial spores—mycobacteria—hydrophilic viruses—lipophilic viruses—vegetative fungi and fungal spores—vegetative bacteria. A disinfectant known to be effective against a particular group of microorganisms will also be effective against all the groups that are less resistant. Most parasites, such as Giardia and Cryptosporidium spp., are significantly resistant to disinfection and are usually rated between the mycobacteria and the viruses.
The effectiveness of disinfection is estimated from the survival rates of indicator organisms in standard microbiological tests.
At present, chemical disinfection of health-care waste is limited in industrialized countries. However, it is an attractive option for developing countries, particularly for treating highly infectious physiological fluids, such as patients’ stools in case of cholera outbreaks.
Chemical disinfection is usually carried out on hospital premises. Recently, however, commercial, self-contained, and fully automatic systems have been developed for health-care waste treatment and are being operated in industrial zones. The disinfected waste may be disposed of as non-risk health-care waste, but the chemical disinfectants may create serious environmental problems in case of leakage or after disposal.
Chemical disinfection of hospital sewage requires less powerful—and
The speed and efficiency of chemical disinfection will depend on operational conditions, including the following:
• the kind of chemical used;
• the amount of chemical used;
• the contact time between disinfectant and waste;
• the extent of contact between disinfectant and waste;
• the organic load of the waste;
• operating temperature, humidity, pH, etc.
Shredding of waste before disinfection
Shredding of solid health-care waste before disinfection is essential for the following reasons:
• to increase the extent of contact between waste and disinfectant by
increasing the surface area and eliminating any enclosed spaces;
• to render any body parts unrecognizable to avoid any adverse visual
impact on disposal;
• to reduce the volume of waste.
Water is usually added during shredding; it prevents excessive warming and facilitates subsequent contact with the disinfectant. Excess water may have to be treated, e.g. by chemical disinfection.
Rotating-blade shredders are used most commonly, and consist of blades attached to two wheels that rotate in opposite directions. The presence of an excessive proportion of sharps in waste may cause deterioration of the shredder.
Shredding of waste before disinfection plus subsequent compacting can reduce the original waste volume by 60–90%.
Types of chemical disinfectants
The aim of disinfection is to eliminate microorganisms or at least reduce their numbers to a “satisfactory” level. Some disinfectants are effective in killing or inactivating specific types of microorganisms and others are effective against all types. It is therefore essential to know the identity of the target microorganisms to be destroyed. However, selection of disinfectants depends not only on their effectiveness, but also on their corrosiveness and other hazards related to their handling.
The types of chemicals used for disinfection of health-care waste are mostly aldehydes, chlorine compounds, ammonium salts, and phenolic compounds. The use of ethylene oxide is no longer recommended for waste treatment because of the significant hazards related to its handling. However, it has been used in the past and may still be in use in some places.
The use of ozone (O3) for disinfection of waste is currently being investigated. This disinfectant is strong and relatively safe.
Most of the disinfectants described here are stable for at least 5 years and—with the exception of sodium hypochlorite—remain effective for 6–12 months after opening of the container.
Powerful disinfectants are often hazardous and toxic; many are harmful to skin and mucous membranes. Users should therefore wear protective clothes, including gloves and protective eye glasses or goggles. Disinfectants are also aggressive to certain building materials and should be handled and stored accordingly.
Small amounts of disinfectants can be discharged into sewers without pretreatment, provided that there is an adequate sewage-treatment process; large amounts of disinfectants should never be discharged into sewers. No disinfectants should be discharged into natural water bodies.