Management of medical and veterinary waste - legal regulations, threats to people and the environment and methods of disposal

Medical waste arises in various healthcare facilities including: hospitals, rehabilitation sanatoria, addiction treatment centres, addict rehabilitation centres, nursing and care establishments, medical and educational establishments, care and treatment establishments, spa hospitals, spa sanatoria, hospices, outpatient clinics, health care centres, health centers, health clinics, medical points, physician's offices - medical practice. The number of people using home treatment, aesthetic medicine offices and tattoo parlours rises from year to year, hence the problem of potentially hazardous waste affects an increasingly larger population, and is not just about the qualified clinical staff.

Veterinary refuse arises from the examination and treatment of animals in veterinary practice, as well as from animal research and testing. Medical and veterinary waste affects not only human health but also the natural environment. Consequently, there is a growing public interest in the issue of healthcare waste and its management system.

Considering their nature and origin, medical and veterinary wastes should be regarded as separate and specific waste categories, which should be covered by special prescriptive regulations in the field of control and management. According to the National Waste Management Plan 2014 [Official Gazette of the Republic of Poland 2010, No. 101, item 1183], approximately 35.5 thousand Mg of healthcare waste were generated in Poland, of which 29.4 thousand Mg was hazardous medical waste. At the same time, the forecast contained in the above document provided for the annual production of healthcare waste at the level of 28.0 – 29.5 thousand Mg. However, in the next Plan for 2022 [Official Gazette of the Republic of Poland 2016, item 784], the estimate of the annual generation of medical and veterinary refuse was at 45–47 thousand Mg. The latter waste group poses an epidemiological threat and has to be rigorously contained. The main hazard results from the likelihood of environmental contamination with infective pathogens, such as bacteria, mycobacteria, viruses, fungi and parasites. Healthcare waste can be a source of infection with diseases such as polio, typhoid, cholera, leprosy, anthrax, plague, smallpox, dysentery, rabies, tularemia, fever, glanders, tuberculosis or HIV. Healthcare wastes with hazardous infectious properties constitute, as mentioned above, only about 20% of wastes arising from facilities providing medical and veterinary services. Among them, numerous wastes can be classified as non-hazardous, provided that they are not mixed with infectious refuse [Chmielewski et al. 2020].

The management of medical and veterinary waste in this country is unsatisfactory. It may happen that the waste ends up in a municipal landfill or is burnt in the hospital's boiler room without any safety precautions. Appropriate treatment and disposal of medical waste is currently a substantial pitfall, recognized by the sanitary-, epidemiological and environmental protection services. That is why procurement of the knowledge on waste sources and disposal methods is indispensable for solving the problems of proper management of medical and veterinary waste.

The overriding legal act regulating waste management in Poland was the Waste Act of April 27, 2001 [Journal of Laws 2001, No. 62, item 628]. The Act was amended in 2005, among others, to regulate the matter of medical waste handling - the Act of 29 July, 2005 [Journal of Laws 2005, No. 175, item 1458], amending the Act on Waste and amending certain other acts. On February 1, 2007, an announcement was issued by the Marshal of the Polish Parliament [Journal of Laws 2007, No. 39, item 251], concerning the publication of a consolidated text of the Waste Act, setting out rules which cover waste management procedures in a way that ensures protection of human life and health as well as environmental protection.

On September 14, 2010, the Marshal of the Polish Parliament issued a notice on the publication of a consolidated text on waste [Journal of Laws 2010, No. 185, item 124] Act of April 27, 2001 [Journal of Laws of 2010, No. 139, item 940]. The above Acts define waste as: ‘any substance or object of the category specified in Annex 1 to the Act, which the holder discards or intends or is required to discard’. The definition of medical waste is provided by the Regulation of the Minister of Health of July 30, 2010, on the detailed way of medical waste management [Journal of Laws of 2010, No. 139, item 940], pursuant to Art. 7, Par. 4, Act of April 27, 2001 on Waste [Journal of Laws 2001, No. 62, item 628]. Accordingly, medical waste is termed as ‘waste arising from the provision of health services and from medical research and testing’. Pursuant to the Act, the definition of veterinary waste stands: ‘veterinary waste is waste arising from the examination and treatment of animals or providing veterinary services, as well as from animal research and testing’. The Regulation also outlines a detailed way of medical waste management. The Act on Waste of December 14 2012 [Journal of Laws of 2013, item 21], defines measures relating to the protection of the environment and human life and health, which prevent and reduce the negative effect on the environment and human health resulting from waste generation and improper management, limit the consequences of resource use and improve the resource use efficiency. The above-mentioned acts define the rules of waste management and, above all, inform about how to prevent waste generation, minimise its amount, remove from places where it is produced and define measures of safe waste recuperation or safe and environmentally sound disposal, in accordance with the principle of sustainable development. The basic waste categorisation is provided in the Regulation of the Minister of Climate of 2.01.2020 on the waste catalogue [Journal of Laws of 2020, item 10]. The Regulation defines the waste catalogue complete with a list of hazardous waste, where medical and veterinary wastes were assigned to group 18 of the waste catalogue and considered as hazardous to the environment.

The main mission of hospitals is to provide healthcare and to protect human health. Nevertheless, a majority of hospital incinerators is the source of emission of the most potent toxic compounds - dioxins, which threaten both humans and the environment. The amount of pollutant emissions depends primarily on the type and properties of the waste incinerated. The furnace design and the parameters of the waste disposal process also have a significant influence.

Permissible values for substances encountered in flue gases from thermal waste treatment are defined in the Regulation on emission standards for installations (https://www.nik.gov.pl/aktualnosci/nik-o-odpadach-medycznych.html). The toxicity of dust containing heavy metals, such as arsenic, lead, cadmium, nickel or mercury, depends on the dust grain size and chemical composition. Adverse effects on living organisms are associated with mutagenic and carcinogenic properties of dusts, and with their potential to penetrate and be assimilated by the respiratory system into the body; this applies primarily to heavy metals, sulphur and nitrogen compounds as well as to dangerous hydrocarbons. Heavy metals, such as chromium, cobalt, nickel, copper, thallium, vanadium are mainly found in slag and dust. Others, like mercury, arsenic, selenium and cadmium, which may be released in the form of vapours, pose a serious threat to the health and development of organisms [Journal of Laws of 2017, item 1119].

Dioxins and furans (PCDDs/Fs) are among the most potent low-molecular-weight poisons, harmful to the environment, which arise during the operation of medical and veterinary waste incinerators. ‘Dioxins’ is a commonly used generic name for the sum of PCDDs (polychlorinated dibenzo-para-dioxins) and PCDDs/Fs (polychlorinated dibenzofurans), a group of organic chemical compounds that are derivatives of oxantrene. Some PCB congeners (polychlorinated biphenyls) in view of their toxicity, have also been included in dioxins. ‘Dioxins’ is a group of 75 congeners, i.e. derivatives having a similar structure, including: polychlorinated dibenzo-p-dioxins (PCDDs), and a group of 135 congeners of polychlorinated dibenzofurans (PCDFs), of which 17 are of toxicological concern. The most potent toxic isomer among PCDDs is TCDD or 2,3,7,8-tetrachlorodibenzodioxin. Chemically, dioxins are polychlorinated aromatic hydrocarbons containing carbon, hydrogen and chlorine atoms deprived of colour, odourless and tasteless, thus organoleptically imperceptible. These compounds have lipophilic properties, are poorly soluble in water (<1 μg/dm³ at 20°C), and their deleterious effects appear at low concentrations, of the order of pg. Dioxins, which have a very low volatility and melting point between 88°C and 332°C, are resistant to oxidizing agents and acids. They are disintegrated by ultraviolet radiation and degraded during combustion at high temperatures (over 800°C) in the presence of oxygen. Dioxins can be attached to dusts that are often generated during the combustion process, consequently, their decomposition takes place only at temperatures over 1000°C. A precondition for a properly conducted process of healthcare waste burning is that the suitable temperature, over 1000°C, is continuously maintained in the combustion chamber [Oleniacz 1999]. Thermal waste transformation is a two-stage process. In the first stage, organic substances contained in the waste are gasified at 700 – 800°C. In the second stage, the final burning of organic components takes place at 1050 – 1300°C along with a thermal breakdown of toxic chemical compounds, including dioxins and furans (this situation applies to technologically modern waste incinerators). In Poland, apart from modern incinerators, there are also those that do not meet the emission standards currently in force. There is a direct correlation between the chemical composition of the fuel burned, the presence of catalysts (e.g. CuCl₂) and inhibitors (e.g. SO₂) of dioxin formation and the amount of dioxins emitted. Cooling of hot exhausts favours the recombination of radicals and the formation of thermodynamically stable molecules of CO₂ or HCl. Chemicals, which are formed de-novo during cooling of exhaust gases include dioxins [Journal of Laws of 1975, No. 35, item 189]. Healthcare waste, such as gloves, syringes, containers or various tools are made of polyvinyl chloride and contain about 60% of chlorine, which allows for the formation of dioxins during incineration. During that process, even traces of chlorine in healthcare waste contribute to the formation of chlorinated biphenyls, chlorinated dibenzodioxins and dibenzofurans and other dangerous chlorinated aromatic compounds. A significant amount of the latter gets adsorbed onto the ash, thus residues from waste combustion, owing to the content of toxic substances, must be disposed of in hazardous waste landfills [Chmielewski et al. 2020a].

Since dioxins and dibenzofurans vary in toxicity, a toxicity equivalency factor (TEF) has been introduced that expresses the toxicity of a given carcinogen relative to that of TCDD, which is a reference compound.

The main routes of release for PCDDs and PCDDs/Fs are by emission into the air. The above compounds are sorbed on solid surfaces (e.g. on bottom sediment particles) and are easily accumulated in the soil, where they may remain for many years. From the soil, dioxins are taken up by plants, which are then consumed by animals and, ultimately, they bioaccumulate in animal tissues. PCDDs, PCDFs and PCBs were found to be particularly persistent in the marine environment, where the biological half-life of PCDDs/Fs was estimated to be between 25 and 275 years.

Human exposure to dioxins is largely through the inhaled air (8%), via skin (2%), and, first of all, through the consumption of contaminated food and water (90%). Dioxins are found mainly in fish and fish products, in milk and dairy products as well as in meat and eggs.

Dioxins slowly accumulate in the body over time, reaching particularly high levels in the liver and adipose tissue. They are transported to tissues and internal organs by lipids and plasma lipoproteins. Dioxins are very persistent in human tissues: the estimated half-life of TCDD is about 10 years. Dioxins affect the synthesis of nucleic acids, their replication process, and the synthesis of carbohydrates and lipids [Journal of Laws of 2003, No. 8, item 104]. They interfere with pro- and antioxidant equilibrium, while TCDD is the source of cation-radical. Exposure to dioxins may be a risk factor for the development of bone diseases. Dioxins, due to their structural similarity to steroid hormones, exert hormone-like effects mimicking sex hormones. They are classified as endocrine disruptors, i.e. exogenous substances, which are capable of stimulating or inhibiting the endocrine system, and function at the receptor level, influencing the synthesis, metabolism, secretion, transport and excretion of hormones from the body. They interfere with the feedback processes in the hypothalamic-pituitary-peripheral gland axis. The antiestrogenic activity of PCDD and PCB proceeds through binding to a specific receptor for aryl hydrocarbons (AhR). The mechanism of their toxic action is connected with the induction of xenobiotic metabolizing enzymes (monooxygenases) through binding to the Ah-receptor. The xenobiotic-protein receptor complex thus formed is then transferred to the cell nucleus, where it may affect the genetic material [Chmielewski et al. 2020a].

Dioxins can cause foetal damage, associated with mutagenic impact on the development of gonads during foetal life and disorders of the foetus endocrine processes. Dioxins can accumulate in adipose tissue since the early stages of foetal life, initially, owing to the ability of lipophilic substances to cross the blood-placenta barrier, and later, during neonatal, infant and early-infant periods - via breast milk. A correlation was observed between the content of those xenobiotics in the blood serum and the risk of miscarriage. Newborns have been found to have a low birth weight and reduced head circumference, body length and muscle tone.

It was also shown that dioxins affect male fertility, cause ovarian cysts and neurodegenerative disorders, impair learning and memorizing abilities, and induce teratogenic effects in the immune system [Roszczyńska 2013]. Dioxins may elicit such effects as thymus involution, increase in corticosteroid levels and changes in the composition of plasma proteins, manifested by the increased concentration of α- and β-globulins and a delayed immune response. One of the symptoms of significant exposure to dioxins in humans is chloracne, i.e. chlorine acne. In addition to skin eruptions, the exposure symptoms could include the induction of microsomal enzymes, breathing disorders, conjunctivitis, hepatomegaly, brown nails and hirsutism. The high blood dioxin levels often contribute to the development of soft tissue sarcoma, hematopoetic and lymphatic malignancies, and could lead to a higher incidence of breast cancer in women. Cumulative exposure to dioxins and mercury may increase the risk of insulin resistance and the development of type 2 diabetes. These compounds also adversely affect the transport and metabolism of retinoids. Another disorder resulting from the body's load of dioxins is the increase in the total blood cholesterol, resulting from liver damage. Exposure of people to dioxins increases the risk of cardiovascular diseases, and leads to the increase in blood pressure, cardiomyopathy and chronic arthritis [Górski 2005].

Dioxin concentrations in exhaust gases of technically advanced incinerators, with a high quality performance control, are below 0.1 ng TEQ/m3. Such concentration values are not problematic from the stand point of public health and the environment, though it should be remembered that dioxins are very stable chemicals, resistant to physical, chemical and microbiological degradation.

In exhausts, in addition to dioxins, acid gases are emitted, including sulphur and nitrogen oxides, which acidify the precipitation, and are of concern from the health effects standpoint. Toxic effects of acid gases include disorders of respiratory-, digestive- and nervous systems, while nitrogen oxides are known to have mutagenic and carcinogenic activity. The above gases significantly deteriorate aerosanitary conditions, and belong to the group of risk factors responsible for smog episodes. HCl and HF are highly acidic components of exhausts having suffocative and corrosive effects [Journal of Laws of 2016, item 1819].

In Poland, the average annual production of healthcare waste is around 44 thousand tonnes (about 133,000 tonnes of medical waste arised from about 40,000 entities in 2011–2013). As much as 90% of that amount is hazardous, mainly infectious waste, which, owing to the presence of bacteria, toxins and chemicals, requires a strict observation of all the safety procedures during handling, storage, transportation and disposal. Most healthcare waste, up to 1/5, was produced in total in the Śląskie and Mazowieckie Voivodships (provinces) [https://www.nik.gov.pl/aktualnosci/nik-o-waste-medycznych.html].

The main stream of medical waste is generated in hospitals, nursing and care establishments, outpatient centres, health centers and outpatient clinics. Healthcare waste arises also from households, most often as medical supplies and expired drugs. All activity related to waste management must comply with the requirements defined in the Act on Waste, as well as with the requirements set out for specific waste categories, including healthcare waste. In accordance with the regulations in force, the recovery of healthcare waste is prohibited, the only option is waste containment.

The Regulation of the Minister of Health on detailed procedures of healthcare waste handling [Journal of Laws of 2017, item 1975] clearly defines the principles that should be followed by those responsible for proper management of medical waste in healthcare facilities. In particular, the Regulation covers the procedures for handling healthcare waste at the site of generation, including:

‘1) handling of medical waste resulting from the provision of health services on premises;

Pursuant to the Regulation, waste is stored in containers or bags, at the place where it arises, for no longer than 72 hours. The exception is waste known as ‘highly infectious medical waste’, which is stored in the place of its generation for no longer than 24 hours. The latter category encompasses the refuse that is known or suspected to contain biological pathogens that fall into the A category. The definition of this category is referred to in the European Agreement concerning the international carriage of dangerous goods by road [Journal of Laws of 2017, No. 719, item 1919; Journal of Laws 1975 No. 35 item 189], it includes infectious materials (microorganisms), which by coming in contact are liable to cause a permanent impairment, life threatening or fatal illness in humans and animals. The waste should be stored in double, red bags or containers (applies to sharp items) - inner packaging and outer packaging in the form of a red container, marked with a clear biological material caution sign and a label ‘Infectious substance’.

Pursuant to the Regulation, containers and bags must be marked with easily visible labels identifying, among others: waste code; name of waste producer; producent's REGON (National Business Registry Number); number of the registration book of the waste producer in the register of entities performing medical activity and the name of the registry authority (if applicable); the date and time of opening (commencing of use); the date and time of closing the container or bag. There is also an obligation to place a thermometer to control the temperature in a stationary refrigeration facility as well as in a medical waste storage room and portable refrigeration unit. Healthcare waste storage room shall be technically secured against the spread of stored waste, including the collection of likely leakeages. If medical refuse is stored in a clearly marked, sealed collection bins or containers, it is permissible not to divide the storage room into separate boxes for individual waste types. The Regulation specifies hygiene requirements for people who leave healthcare waste storage room and/or stationary refrigeration facility. The requirement that refuse bags be opaque has been waived. An obligation has been introduced that the rooms used for disinfection, washing and storing of the means of internal carriage of medical waste and reusable containers be ventilated. A detailed procedure for segregating, collection, transporting and storage of healthcare waste has also been developed.

Healthcare waste management standards for healthcare facilities and laboratories have been defined in the legal provisions specified in laws and regulations, including: Act of on Waste of December 14 2012 [Journal of Laws 2013, item 21], Regulation of the Minister of Health of July 13 2010 on the detailed way of medical waste management [Journal of Laws of 2010, 139, item 940], Regulation of the Minister of Health of December 23 2002 on acceptable ways and conditions for the disposal of medical and veterinary waste [Journal of Laws of 2003, item 8, item 104] and Regulation of the Minister of the Environment of December 9 2014 on the waste catalogue [Journal of Laws of 2014, item 1923].

Facilities which carry out medical activity also generate municipal waste. The majority of healthcare facilities, in most cases, commonly manage all types of waste generated in their area [Roszczyńska 2013]. Healthcare waste generated in these facilities, or in other medical entities, is selectively collected on premises, and segregated into categories of infectious, special and other waste [Górski 2005].

Ensuring correct containment and disposal of medical and veterinary refuse requires that the following conditions are met: –

infection hazard eliminated,

Waste containment and disposal is a non-recovery process, even if the recovery of material or energy is a secondary effect of such a process [Journal of Laws of 2013, item 21]. On November 8, 2016, the Regulation of the Minister of Health of October 21, 2016 was published, relating to the requirements and methods of containment and disposal of medical and veterinary waste [Journal of Laws of 2016, item 1819]. The Regulation covers the whole group of medical and veterinary waste, introducing order into the area of waste containment and disposal, while, basically, maintains the requirements enforced by the Act on Waste of December 14 2012 [Journal of Laws of 2013, item 21, as amended, 4]. Under the Act, the recovery from medical and veterinary waste is prohibited, and the duty is imposed to dispose of medical and veterinary wastes, both infectious and non-infectious, by thermal transformation on land (‘D10’ symbol), hereinafter referred to as the ‘D10 process’. Within the above context, it should be remembered that the use of the so-called ‘principle of proximity’ is important, according to which the waste shall be disposed of at or near the point of generation, e.g. in a voivodeship where it was generated. One exception is the case, when there is no installation in a given voivodship, capable of neutralizing a particular waste type, or installations have no spare capacity available. In that case, the law permits to dispose of refuse in the nearest installation, even if it is located in another voivodeship.

Another method of containment of non-infectious healthcare waste is the physico-chemical treatment, excluding autoclaving, thermal disinfection, microwave treatment, provided that the techniques used in these treatments ensure that waste disposal does not pose a threat to the environment and human health and life (‘D9’), hereinafter called the ‘D9 process’. Non-infectious waste, other than hazardous, is to be disposed of in a landfill designed for waste other than non-hazardous and neutral (D5), hereinafter referred to as the ‘D5 process’.

Under the Act [Journal of Laws of 2016, item 1819], monitoring of the incineration of medical and veterinary infectious waste includes continuous measurement of the following parameters in the combustion chamber: temperature of gases generated during combustion, oxygen concentration in combustion gases, exhaust gas pressure and water vapour content in flue gas. The monitoring also involves registration of parameters indicating the effectiveness of the disposal method used, appropriate and characteristic of the process, and of the type of device or installation to conduct the process, inspection of containers or bags, in which medical and veterinary waste will be disposed of. In case of infectious medical waste and infectious veterinary waste, the control of the effectiveness of the waste disposal process should be based on tests of waste resulting from the disposal processes in terms of loss of infectious properties as well as control of temperature at the waste storage locations.

Under the law, it is permissible, on occasions, to recover certain categories of the waste. The exceptions are specified in the Regulation of the Minister of Health of July 24 2015 on the categories of medical and veterinary waste, the recovery of which is permissible [Journal of Laws of 2015, item 1116]. This applies to treatment and surgical instruments, material and plaster dressings, bedding, disposable clothing, diapers, or some chemical reagents.

Unlike the other categories, the Act regulates liability for infectious waste as well. The producer of infectious medical or veterinary waste may be exempted from liability for proper waste collection or treatment only when the refuse is actually disposed of in the incinerator. He/she shall then obtain a special certificate of destruction evidencing that the waste has been subject to appropriate processes. For all other types of waste, exemption from liability applies as soon as waste is transferred to the subsequent waste managing entity. The intent of the above legal arrangement is to facilitate the attribution of culpability in the event of any irregularity.

The methods of collecting medical and veterinary waste, its storage, physico-chemical and biological properties, toxicity, consistency, quantity, transporting options, investment costs and the availability of technologies are the main factors determining the choice of waste disposal technology. Solid hazardous refuse shall be fragmented (pre-treatment of waste for disposal), and treated using chemical, physical, radiation and microwave disinfection processes. Infectious waste is disposed of using the processes of combustion, smelting, decontamination, disinfection, pasteurization, thermal sterilization and macrowave radiation. Chemical waste is disposed of by neutralization, precipitation, sedimentation, filtration, sorption, combustion and encapsulation. The processes of autoclaving, microwave radiation, thermal disinfection or other types of physico-chemical treatment shall obtain, for each type of device, a positive opinion of the Chief Sanitary Inspector, or of a unit authorized by the Inspector. Disposal of the products processed at a designated site constitutes the ultimate stage of hazardous waste conversion [Roszczyńska 2013].

The most common method of refuse disposal is incineration, this also applies to medical and veterinary waste. The waste treatment methods may be classified according to the type of products obtained during the thermal treatment of medical and veterinary waste. There are methods based on the process of waste thermal decomposition without external oxygen (pyrolysis), leading to waste carbonization and the formation of a carbonaceous residue (solid residue enriched in free carbon). The by-product of waste degassing is the combustible pyrolytic gas containing, predominantly: hydrogen, methane, ethane and their homologues, carbon monoxide and dioxide, as well as other compounds, such as hydrogen sulphide, ammonia, hydrogen chloride, hydrogen fluoride and some volatile organic compounds (including the so-called pyrolytic oil). Considering the temperatures applied in the process, three types of pyrolysis may be distinguished. The first one consists in a low-temperature pyrolysis, or the so-called low-temperature degasification, which takes place in the temperature range of 500 – 600°C, and yields large amounts of soot and oil and low amounts of gases. The second type is a medium-temperature pyrolysis, proceeding in the temperature range of 1000 to 1200°C, while the third type – is a high temperature pyrolysis, also known as pyrofusion, or a high temperature combustion, which occurs in the temperature range between 1400 and 1650°C. The products of the latter process include gas and slag discharged in a liquid form.

Thermal carbonization of medical waste is carried out in pyrolytic reactors (mostly retorts) on a periodic basis, while the pyrolytic gas formed in this process is usually cleaned and burnt in a thermo-fusing unit, the exhaust gases from which are not subject to treatment.

Other methods consist in waste incineration, and are based on the process of direct waste combustion or combustion of gaseous products of gasification (partial oxidation, with the addition of e.g. steam), which is carried out for a period extended enough, so that the solid residue no longer contains any combustible ingredients.

Incineration of medical waste occurs in a single- or multistage systems, based on one or more chambers (reaction zones). Direct burning of medical waste with an excess air is rarely applied, however, if the waste is introduced directly into the combustion chamber, then an additional post-combustion chamber is used, which extends the residence time of flue gases at high temperatures of the order of 850°C ÷ 1200°C to a minimum of two seconds, with a minimum of 6% oxygen in the exhaust gas. The post-combustion chamber provides a certain amount of oxygen, which increases the oxidation efficiency of the resulting products of incomplete combustion. If the installation is not equipped with an automatic control of the amount of air fed to the combustion chamber, there may occur a sudden increase in the amount of incomplete combustion products (mainly carbon monoxide and hydrocarbons). In the two-stage systems, the actual combustion chamber is the post-combustion chamber, because in the pre-chamber there occur the processes of waste degassing and gasification, while gaseous products of the latter processes are subject to burning in the post-combustion chamber [Oleniacz 1999].

Waste combustion in high temperatures results in reactions between the substances incinerated, what produces new chemical compounds, which may be even more harmful than the original waste. In the post-combustion chamber, sulphur compounds are oxidized to SO₂, nitrogen compounds are converted to N₂ and NO, which oxidizes to NO₂ at lower temperatures. The presence of nitrogen in the combustion air accounts for the formation of nitrogen oxides in the oxidation process with molecular oxygen, or a hydroxyl radical. The process takes place directly at high temperatures. The observed drop in the NO concentration in the exhaust gases from post-combustion chamber is directly related to the decrease in the amount of NO generated from nitrogen contained in the waste, while HCl enters the flue gases virtually unchanged.

Several types of furnaces have been used for pre-combustion or gasification of medical waste, including pusher furnace, shaft furnace, grate furnace (with sloping or flat grate), or fluid bed or rotary furnaces. Very often, the multi-chamber stoves have been used, in which the pre-combustion, mixing and post-combustion chambers are connected by a combustion train to create one chain. Medical waste is only rarely incinerated in commercial incinerators for hazardous refuse, which use rotary kilns with a post-combustion chamber [Oleniacz 1999].

The major threat resulting from the combustion processes is the emission of dioxins and furans. Dioxins emission are derived from waste incineration when organic substances are burned in the presence of chlorine compounds. Flue gases contain also acid gases, including sulphur dioxide, nitrogen oxides, chlorides, hydrogen fluoride, in addition to heavy metals and organic compounds. The presence of chlorine accounts for a higher concentrations of some heavy metals in flue gases. In the combustion chambers, there are formed volatile chlorides of metals, including: Cu, Pb, Cd and Zn, which escape in the form of vapours together with exhaust gases. Therefore, each combustion installation should be equipped with an efficient, multistage system for cleaning impurities, in both solid and gas phase, in order to comply with the emission standards applicable to hazardous waste incineration plants [Zarzycki & Wielgosiński 2003].

In thermal treatment of medical and veterinary waste, both the short-term- and average load of waste installations are mainly responsible for the composition of exhaust gases and the amount of pollutant emissions. The amount of gaseous pollutants emitted to the air can be limited by reducing the stream of toxic components introduced along with the waste to the installation, by minimizing the amount of incomplete combustion products in the fluid bed reactor, or by a more effective oxidation of the products in the post-combustion chamber as well as by optimizing the existing exhaust purification method, or its extension with another working node using the wet method.

The composition of exhaust gases entering the atmosphere may also be regulated by the method of waste feeding into the combustion or gasification chambers. It is conditioned by the physical state of the waste incinerated and the operation rhythm of the installation. Pumpable liquid waste can be fed continuously and evenly. Feeding takes place through special burners and spray nozzles, used on a large scale in the systems with liquid injection. Loading of fragmented solid refuse and sludge is steadily carried out pneumatically or mechanically (which does not cause significant differences in the course of the flue gas composition), through screw feeders, scraper feeders and bucket feeders. The above type of feeding is used in fluid bed and shelf furnaces.

Combustion process reduces the volume of waste by 90%, which means that for every 1,000 kg of waste 100 kg is sent to landfill. Thus the method helps to reduce the cost of waste storing in the landfills. The amount of pollutants emitted to the air should not exceed the permissible values. Emission standards in the form of permissible concentrations in dry exhausts are assumed in Poland at a similar level as that in EU with regard to the values applicable to the process of hazardous waste incineration [Nadziakiewicz et al. 2007].

Traditional combustion belongs to the most expensive options of waste disposal, moreover, it is environmentally unfriendly due to gaseous, solid and liquid combustion products. In addition, the presence of plastics in the waste contributes to the emission of heavy metals, used as stabilizing or colouring additives, including: cadmium, chromium and lead, as well as of polychlorinated organic compounds [Chen et al., 2013]. There exist more advanced and entirely safe alternative methods for the disposal of healthcare waste, which do not pose any threat to people and the environment, because they do not contain any pathogenic microorganisms.

Steam sterilization is one of such methods of healthcare waste treatment, carried out with the use of an autoclave (steam sterilizer). Two types of autoclaves, vacuum and gravity, have currently been in use. In the vacuum type, the air is removed from the chamber before the steam is introduced, in the gravitational type - the air is removed by the steam itself. The autoclaving process is conducted in pressure chambers using saturated steam at 130–190°C and a pressure of usually 100–500 kpa. This method is particularly effective in relation to pathogenic microorganisms, and is applied in medical centres to sterilize certain types of waste, including: low-radioactive, organic solvents, laboratory reagents, chemotherapeutic and pharmacological or pathological waste. As compared to other system of thermal waste transformation, the method of steam sterilization has more advantages. Firstly, the cost of construction and exploitation of steam sterilization is much lower, and secondly and most importantly, there is no emission of harmful substances to the atmosphere from installations for steam sterilization. This process is highly effective in neutralizing microorganisms, provided that the reaction time and temperature is sufficient to kill their spores (at minimum 121°C). The disadvantages of the above described system include: the possibility of damaging the grinding mechanism, odour formation (due to chemical compounds added to the waste loads) and a high energy consumption. The effect of autoclaving processes is waste reduction up to 75%, while the remaining 25% is the municipal waste [Kim et al. 2017]. It is noteworthy to add that the thermal (dry) disinfection is a two-stage process. In the first stage, pre-fragmentation of the waste takes place (up to about 25 mm in diameter), while in the second the waste is heated in a special device at 110 to 140°C. The process is not time-consuming, since it takes only 20 minutes, and the resulting secondary waste is then compacted, and the exhaust gas filtered. As a result of thermal disinfection, the volume of waste is 80% reduced, and there is 20–35% loss of its weight. This type of disinfection is particularly useful when disposing of infectious waste and sharp items, although there are contraindications in the case of pathological waste or low-radioactive substances [Chmielewski et al. 2020].

Microwave sterilization is a relatively new technology using steam heated by means of microwaves. The majority of microorganisms is destroyed with the wave frequency of 2450 MHz and 12.24 wavelength. The first stage of microwave sterilization consists of cleaning the medical waste with steam heated to 110°C, then the air is pulled out by the filter system. The waste is transferred to the waste collection chamber for about 20 to 30 minutes, then it is mechanically crushed and, after this preparation, passed to the previously heated chamber in order to undergo microwave radiation for at least 30 minutes. In the final stage, the waste is kept at about 95°C for complete disinfection [Chmielewski et al. 2020].

The microwave sterilization method reduces the waste volume by about 80%, while its weight remains unchanged so that the remaining 20% ends up in a municipal landfill. This method involves no emissions of harmful substances because the system is closed. On the other hand, microwave technology can no be used for liquid blood and hazardous chemicals [Zhou et al. 2018].

The above described methods of waste disposal may be applied on condition that a positive opinion for each type of device is issued by the Chief Sanitary Inspector, or by a unit designated by him.

Hydroclaving is another method, which may be used for the treatment of medical and veterinary waste. The hydroclave uses hydrolysis of waste organic components with the operation of steam and heat. Inside the hydroclave there is a mixing and shredding rotating mechanism. The introduction of steam is combined with immediate heating (up to 152.7°C) of mixed waste and their dehydration (up to 40%). The condensed steam is then returned to the process, and the loading volume does not affect the efficiency of the sterilization process. The total duration of the operation is about 1 hour, resulting in a 50% reduction of waste weight and about 80% reduction in waste volume. The sterilization process (combined steam operation, pressure and mixing of waste) takes approximately 20 minutes. This technology can be used to neutralize infectious waste as well as anatomical waste and, in particular, animal waste [Chmielewski et al. 2020].

Burying the healthcare waste and, especially, pathological waste on land is possible only in the event of exceptional incidences, such as natural disasters, catastrophes or armed conflicts, when the producer of the waste indicated does not have the possibility of using any other disposal methods. The latter method can be relatively safe, given that the following conditions are fulfilled: the access to the burial site is restricted, the burial site must be lined with an impermeable layer to prevent leakage to waters, and the input of chemicals to the waste should not exceed 1 kg, in order to prevent any hazardous effect to the environment. The methods of healthcare waste treatment include also: chemical disinfection using aldehydes, chlorine compounds or phenolic compounds, waste mineralization by mixing waste, e.g. pharmaceuticals with cement, and sterilization with mineralization, which consists in crushing, sterilization and subsequent treatment with calcium oxide and sodium or potassium silicate and stabilize the whole mass with cement. The main advantage of the latter method is a low investment and operating cost, as well as the opportunity to perform the treatment operation on the spot, without the risk of fumes and leakages [Bernhardt et al. 2015].

Threats that may occur during the management of healthcare waste due to their biological and chemical properties require that all the rules and procedures in the health and veterinary units, research units, laboratories and pharmacological facilities are strictly observed. The leftovers from home treatment are also included in the above waste group. The healthcare waste is considered hazardous and poses a threat of infection to the surroundings, thus it needs to be isolated at the site of generation, and then appropriately treated. In Poland, waste incineration is the most common method of waste treatment. The method raises considerable concerns among the society in view of the threat to the health and the environment due to the emission of gases, in spite of the fact that the method is the safest one. Over the last decade, there has been a significant increase in applications of alternative methods of healthcare waste treatment, including autoclaving, thermal disinfection or microwave sterilization [Marszelewski 2014].