Keywords

1 Introduction

The wastes generated from human activities generally include the heterogeneous disposals from both municipal areas as well as the rural settings. It is generally characterized as municipal wastes, hazardous wastes, biomedical wastes or hospital wastes. The rules governing each category point out at the explicit features leading to specific rules, regulations and protocols for proper treatment, storage and disposal.

The growth of population and proportionate massive outburst in diseases and accidents generate more and more infectious and non-infectious wastes from healthcare facilities. Biomedical wastes (popularly referred by various names as hospital wastes, clinical wastes, healthcare wastes) are produced while the diagnosis, treatment or immunization of humans (or animals) is carried out or when research activities are conducted in these areas. The generated biomedical wastes can be hazardous as well as infectious and contain toxic chemicals and pathogenic organisms which pose contamination risks to both people and the environment. There is a huge irony that in the process of offering healthcare facilities, waste products are generated that are potentially more harmful. The wastes generated from healthcare facilities, research centres and laboratories can be broadly classified into “hazardous-biomedical” wastes which comprise around 25% of the total waste fraction (sharps, infectious, pathological, pharmaceutical, cytotoxic, chemical and radioactive wastes) and 75% “non-hazardous-biomedical” wastes that mainly comprise of domestic waste and garbage [1]. Due to lack of awareness and improper processes of segregation, the generated domestic waste also gets contaminated with the infectious wastes, leading to the generation of huge quantity of potentially infectious wastes. The important parameters of concern for any waste treatment disposal system facility should be the quantity and quality of wastes, the competency of the dedicated disposal facility to manage that much quantity, the treatment efficiency observed, available space for the equipment, effluents and gaseous emissions from the facility, the economy of the operation and most importantly the occupational health and safety mainly for the workers who are continuously exposed in the facility [2]. In a study conducted for biomedical waste management in a hospital in Belgaum, India, the care ensured by personnel for handling the biomedical wastes elucidates the need of systematic handling of infectious and non-infectious wastes [3].

Generally, four methods are employed to treat the hazardous wastes from the healthcare sector. These are: thermal (autoclaving, using microwaving, frictional heat treatment system, dry heat technologies, incinerators), chemical (using either dissolved chlorine dioxide or sodium hypochlorite, glutaraldehyde/quaternary ammonium compound-based treatment, lime slurry or calcium oxide treatment, ozone treatment), irradiative (ionizing radiations, UV-C irradiation) and biological methods (using enzymes to treat organic wastes) [4]. But out of all the methods, the most viable system for the disposal is burning in incinerators [5, 6]. In thermal treatment and disposal of biomedical wastes, the wastes are burned in a supervised condition which converts the waste into ash and gases. However, the process is energy-intensive. Biomedical waste incinerators are operated either using oil or electricity or the combination of both. Multiple hearth type, rotary kiln and controlled air types are the commonly used types of incinerators for disposal of healthcare/infectious wastes. The commonly used incinerators have a refractory lining with primary and secondary chambers for ensuring optimal combustion. The primary chamber has pyrolytic conditions where temperature builds up for a range of about 800 ± 50 °C followed by the secondary chamber that is usually operated in excess air conditions and temperature is around 1050 ± 50 °C as per schedule V of the Bio-Medical Waste (Management and Handling) Rules, 1998 [7]. The volatile compounds are liberated in the first chamber, followed by complete destruction in the second chamber. As per the Bio-Medical Waste (Management and Handling) Rules, it has been suggested to dispose cytotoxic drugs, discarded medicines, human anatomical wastes, animal wastes and other soiled wastes by incineration. As per the report on status and issues on implementation of Bio-Medical Waste (Management an Handling) Rules, Indian hazardous and biomedical waste management system employs around 200 hazardous waste and biomedical waste incinerators, with a major fraction employed in the biomedical waste sector only [8].

In the incineration process, certain wastes must be handled in a careful manner. These complex wastes include pressurized gas vessels, halogenated plastics (e.g. PVC), radioactive wastes, huge quantities of reactive chemical wastes, mercury-containing substances, silver salts, etc. As the process of segregation of wastes is improper, a huge heterogeneous mixture is usually being fed into the incinerator. Even though incineration is well suited for chemical, infectious and pharmaceutical wastes and helps in drastic reduction of weight and volume of wastes; production of ash is a big nuisance. The incineration of biomedical wastes also releases gases like CO, CO2, NO2, dioxins and furans into the environment. The solid residues generated in the system, commonly referred as ash, may contaminate soil as well as groundwater due to the occurrence of toxic metals, organic and inorganic compounds [9]. The bottom ash disposal in a hazardous waste landfill void of any systematic treatment strategies may lead to the contamination of the soil and water due to the production of leachate in due course of time. Fly ash which is very fine and light in weight settles on post-burner devices like the scrubbers. A comprehensive research has been done on the characteristics of fly ash, and it has been added in the list of hazardous waste materials by the European Union with code 190,103 [10]. Bottom ash was added recently in the list, in 2003. In India, special rules and regulations are framed for the proper disposal and management of fly ash. Fly ash is believed to have high content of heavy metals and hence demands for proper environmental management plan [11]. There is also a dire need to evaluate the bottom ash produced in the country. Hazardous Waste Management Report 2008 released by Central Pollution Control Board (CPCB), India, highlights that approximately 0.82 lakh MTA (metric tons per annum) of ash is produced as a result of incinerating 3.28 lakh MTA of hazardous waste [12]. As the population increases and land footprint decreases, the voluminous ash generated is an issue of concern as its disposal pathways are concerned.

This paper focuses on the bottom ash characteristics mainly its particle size distribution, elemental and mineralogical composition. Very limited studies have been done on bottom ash from BMW incinerators. It is inevitable that more research is needed to understand the specific characteristics of bottom ash so that it will aid in developing a secondary raw material for manufacturing building materials. The characterization and evaluation of properties of bottom ash will also help in deciding whether it is to be deemed hazardous, non-hazardous or inert. This will help in ensuring a sustainable environmental management solution for bottom ash disposal.

2 Materials and Methods

Bottom ash generated from BMW management facility in Maharashtra, India, was studied (ash collected at bottom of incinerator as shown in Fig. 1). The incinerator at site is a Thermex model PY-75 installed in 2003. The fuel used in the facility is diesel with waste feed rate of 50 kg/h. The facility handles around 1400 healthcare establishments with 4802 beds. Around 2900 kg/day is supposed to be handled by the facility.

Fig. 1
figure 1

Residues collected at the bottom of biomedical waste incinerator

Full size image

Around 50% of the total waste reaching the facility is supposed to be incinerable in nature. The collection periods of bottom ash ranged over a time span of three months from the facility and were given names as BAS1, BAS2 and BAS3 (sampling was done at an interval of 29–30 days). The type of the waste received at the facility is presented in Table 1. The collection system ensures that the infectious and non-infectious wastes are segregated in different coloured bags. The yellow- and red-coloured bags are fed directly into the incinerator without opening the bags. If mixing of wastes is suspected in other bags (blue and black), then the suspicious bags are fed into the incinerator. The blue bag components are disinfected using autoclaving and hypochlorite solution treatment, and recyclables are sent for recovery. The incinerator ash is packed and sent to hazardous waste landfill site, and the other domestic garbage items like waste from cafeteria are handed over to the municipal waste disposal system.

Table 1 Categories of waste received at the study facility
Full size table

The collected samples of bottom ash weighed about 3 kg was dried at room temperature, and then packed and stored properly. For the study of bottom ash, the following analyses were performed:

  1. 1.

    Bottom ash particle size gradation—Sieve analysis

  2. 2.

    Elemental identification and quantitative compositional information—scanning electron microscope (SEM)

  3. 3.

    Detection of elements—inductively coupled plasma atomic emission spectrometry (ICP-AES)

  4. 4.

    Crystalline study for mineral identification—X-ray powder diffraction

2.1 Bottom Ash Particle Size Gradation—Sieve Analysis

The ash samples for grading were obtained from the incinerator and allowed to get cooled to room temperature. Visible, unburned residues including unshaped metallic objects, melted glass were segregated prior to the gradation study. The samples were oven dried at 105 °C for 24 h and thereafter cooled down to room temperature. 1 kg of each sample was mechanically sieved through set of sieves starting with 4.75 mm sieve at top, 2.36 mm, 1.18 mm, 600 µm, 300 µm, 150 µm, 90 µm and pan at bottom.

2.2 Elemental Identification and Quantitative Compositional Information—Scanning Electron Microscope (SEM)

The sample of bottom ash collected from the incinerator was finely ground, prepared and coated with platinum for around 200 s. The powder was examined in a scanning electron microscope (SEM) fitted with an energy-dispersive X-ray (EDX) prime energy-dispersive analysis system. In the EDX technique, the energy of X-rays produced is collected and measured. It is displayed as a graph of X-ray energy (in keV) versus the frequency of occurrence. The spectrum produced shows a number of characteristic X-ray peaks, associated with the elements present. The instrument used in the study is of Make FEI Model: QUANTA 200 with specifications of operating voltages between 0.7 and 30 kV, with emission current of 100 µA, pressure range in low vacuum and ESEM mode between 0.1 and 40 torr having a resolution of 3.5 nm and magnification range between low 20× to high 50,000×.

2.3 Detection of Elements—Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)

One gram of each ash sample was taken in a beaker. 7.0 mL of HNO3 and 3.0 mL of HF were added. The mixture was gently swirled and kept for approximately 10 min before closing the digestion vessel. The excess HF present in the solution was complexed with a saturated H3BO3 solution to avoid damaging any glassware used during analysis. The sample digestion was carried for 40–45 min in the digester at a maximum temperature of 180 °C. After cooling the samples, the sample was diluted with distilled water and made up to 25 mL. Solution of every sample was filtered to discard any remains as the impurities interfere with the detection process. Metal concentrations in the filtered solutions were subjected to analysis by ICP-AES (Make: SPECTRO Analytical Instruments GmbH, Germany; Model: ARCOS). The ICP spectrometer with radial plasma can analyse the aqueous solutions having high dissolved solid content even up to 30 wt%.

2.4 Crystalline Study for Mineral Identification—X-Ray Powder Diffraction

X-ray powder diffraction (XRD) study helps to analyse the structure of crystalline materials encompassed in the bottom ash matrix. The identification and characterization of the compounds are based on their diffraction pattern. A diffractometer of type X’Pert MPD with a diffraction angle of between 5° and 90° (2θ), 4°/min as rate of scanning, was employed for analysis. The monitoring of crystalline phase and structure was performed using software, Xpert high score.

3 Results and Discussion

3.1 Bottom Ash Particle Size Gradation

The distribution of bottom ash particle size ranges from fine gravel to fine sand with low percentages of gravel and silt–clay-sized particles. Bottom ash on analysis was found to be mainly sand fraction with around 70–90% passing a 4.75 mm sieve and 4–10% passing a 0.075 mm sieve as shown in Fig. 2. Bottom ash is found to be well-graded material although variations may be encountered. In this study, the particle size distribution obtained was in close agreement with studies conducted on ash from a healthcare waste incinerator in Greece [13]. The bottom ash generated in municipal solid waste incinerators from Taiwan distinctively resembles the ash grading in the present study [14].

Fig. 2
figure 2

Sieve analysis of bottom ash

Full size image

3.2 Elemental Identification and Quantitative Compositional Information

The ash samples appeared to be amorphic as shown in Fig. 3.

Fig. 3
figure 3

SEM analysis of bottom ash

Full size image

Subsequent investigation of bottom ash has shown that the particles are composed of Si, Ca, Al, Na, Fe, Mg, Ti, Cu, Zn along with Cl being detected in the samples. This Cl content reveals the burning of polyvinylchloride (PVC) compounds and other chlorinated organics in the incinerator. The presence of elements like Ti associated with others reveals the use of alloys in metallic instruments which are fed as biomedical wastes. The point analysis studies in Greece have also indicated a higher composition of heavy metals like Fe, Cu, Zn and Cr [15].

3.3 Detection and Quantification of Elements

Table 2 shows the elemental distribution in the ash analysed by ICP-AES. The data points out that the collected bottom ash is a heterogeneous mass enriched with various metallic elements of which the main ones were Ca, Al, Mg, Zn, Na and K. The presence of heavy metals like As, Pb, Mn, Cr were also observed. The toxic heavy metals like Cd were below the detection limit in the ash except in one sample. It is believed that the above-mentioned toxic metals and their compounds may have found their way through gaseous emissions and ended up in the fly ash as these are easily volatile in nature. A comprehensive study involving fly ash and bottom ash is required to validate this belief. The bottom ash was found to contain much higher amounts of Zn and Ti. Generally medical instruments made of metal alloys are made of these elements. Hence the presence of these two elements is justifiable [16]. Moreover, Ti has a higher boiling point and has higher chances of ending up in the ash collected at the bottom of BMW incinerator. Also, the higher presence of Ca and Mg are not surprising as the biomedical waste incinerator is also fed with amputated human body parts, tissues, carcasses of dead foetuses. The presence of mercury is found to be very much low and was below detectable limit. This may be an indication that the new regulations [17] laid out by Central Pollution Control Board (CPCB), India, for management of mercury-containing devices in healthcare facilities (HCFs) is in a good pace for implementation. The variations of elements for the three samples are represented in Fig. 4.

Table 2 Elemental composition of bottom ash (g/kg)a
Full size table
Fig. 4
figure 4

ICP-AES analysis of collected ash

Full size image

3.4 Crystalline Study for Mineral Identification

The diffraction study result as given in Fig. 5 reveals that the bottom ash exhibited a highly complex crystalline matrix. The main minerals detected in the study were calcite (CaCO3), halite (NaCl) and anhydrite (CaSO4). The other distinguishing fact is the higher percentage of amorphous mass which is reflected upon in the large background signal and distortions. The bottom ash generated in municipal solid waste incinerators in Taiwan [14] also has the same crystal structures as identified in this study. In the study conducted for the environmentally sound disposal of ash from biomedical waste incinerators by stabilization/solidification, the XRD results of bottom ash have shown a good similarity to the current study [18]. The other important parameter of concern is that there is a huge variation at the mineralogical composition for varying samples. This points out to the fact that the mixture fed into the incinerator varies a lot in its composition in due course of time. There is the presence of rare earth elements detected in the samples, which conveys that a future scope of research lies in identifying and characterizing these rare earth elements. This is in total agreement with some previous studies conducted in China [16].

Fig. 5
figure 5

XRD analysis of bottom ash

Full size image

4 Proposed Alternatives for Sustainable Ash Management

The prevalent method of dumping of bottom ash in landfills is not sustainable. BMW incineration in Italy produced ash with potentially toxic concentrations of Cd, Cr, Cu, Ni, Pb [19]. In the study, it was found that absolute concentrations of these metals in the ash were not sufficient to prove the toxicity. However, the elution/leaching tests pointed out that sufficient care has to be taken before disposing the ash into a landfill.

Thus, to sustainably manage the wastes ash in a landfill site, following methods (either by solidification/stabilization, vitrification or by reusing the waste ash as a secondary construction material) can be practiced:

4.1 Solidification/Stabilization (S/S)

The process of solidification/stabilization (s/s) refers to the technique of using additives or binders such as lime, cement natural or synthetic polymers to immobilize the hazardous substances present in wastes by chemical and/or physical methods. This will ensure that the whole matrix remains intact [17]. This will enable proper encapsulation of the waste material thereby reducing the leachability. The alternative may be found ineffective if the selected additives react and produce soluble substances, and leaching behaviour is observed in the aftermath of the treatment.

4.2 Vitrification Process

Vitrification process is a viable alternative that helps to transform a substance into a glass. Usually, being practiced in radioactive waste disposal method, this technique can be adopted to permanently immobilize the hazardous ash matrix. The bottom ash can be mixed with glass-forming chemicals in a smelter device to form the molten glass. This technique of encapsulation of hazardous components of ash provides a non-leaching, durable material entrapping the contaminants in the matrix. Thus, vitrified ash can be used to produce glass-ceramic materials. Studies had been conducted analysing the properties of glass and glass-ceramic made of municipal solid waste incinerator fly ash [20]. This study presented a positive direction pointing out less leaching out of heavy metal ions from the glass and glass-ceramics.

4.3 Use in Construction Products

There has been a growing trend of utilization of the waste materials in manufacture of construction material worldwide. Fly ash has been in use as an additive in the process of manufacture of cement, concrete and other various construction products. However, the use of bottom ash for the production of construction materials is not common throughout the world. Thus, there is a need to examine and evaluate the potential of bottom ash for such uses. To curtail the ash disposal problem into landfill, work has to be done on utilizing the waste incinerator bottom ash for construction product manufacture [21]. As not much research has been reported on strength and durability properties of materials incorporating bottom ash, it is proposed as a future scope of work to investigate the properties of these materials. It is to be envisaged that the leachability properties, hazardous components in the ash and interaction in wholesome matrix have to be studied before using the bottom ash as a raw material for the construction industry.

4.4 Change in Policy Strategies

As observed in the study, chlorine and associated compounds are a significant component in incinerator residues. The predominant source of chlorine is PVC plastic, which enters the incinerator system en route packaging and from many disposed medical products. The release of chlorinated waste products released from the BMWs is a significant issue. Hospitals and clinics in Austria, Germany and Denmark are reducing the quantity of wastes by switching to reusables, which can be sterilized and used again.

The first step in the proper disposal of wastes in a sustainable manner with less ecological burden is waste minimization and systematic scientific segregation. This methodology outweighs other disposal alternatives in all aspects and particularly from an economic standpoint. All sustainable strategies must include:

  • Reduction at source: This can be initiated by imparting scientific education to the healthcare workers, nurses, doctors, adopting methods like good operating and housekeeping practices, inventory control and necessary technology changes.

  • Segregation: This process of separating the wastes at the point of generation helps in adopting suitable recovery, recycling and disposal techniques. Often it is observed that the domestic or garbage wastes get mixed up with infectious matrix. If not segregated, all healthcare wastes are considered as potentially infectious. Thus, this strategy enables to treat less quanta of infectious waste which usually requires special methods of treatment.

  • Source recovery and recycling: In the well-known hierarchical steps of waste management, recovery and reuse of materials play an important role. If the segregation of infectious and non-infectious wastes is proper, recovery of recyclables can be performed easily.

5 Conclusions

The BMWs generated in an exponential way have a major impact on the health of humans if the wastes remain unattended scientifically. It is essential that an environmentally sound waste management strategy with proper education and awareness for the operators should be adopted. Out of all the disposal methods for complete destruction of wastes and reduction of volume, incineration is widely preferred. The major drawbacks of incineration include generating a new waste stream of ashes (fly ash and bottom ash), furans, dioxins and poisonous emissions. This study reveals some potential insight of contaminants found in bottom ash from BMW incinerator operated in Mumbai, India. The unrestrained and less scientific methods of dumping of the ashes may contaminate the soil as well as surface and underground water.

Elemental analysis indicated low levels of mercury and cobalt. However, the presence of higher levels of toxic metals such as arsenic, chromium and lead is alarming. The recovery of rare earth elements requires a more scientific study incorporating the time and seasonal variation aspects of bottom ash characteristics in India. It should also cover more BMW disposal facilities thus providing a thorough analysis, helping to develop data-driven policies for ash management. A meticulous investigation of physical and chemical properties of bottom ash is required before it is widely accepted as a raw material to be used in production of construction materials or in glass industry. Further studies are needed to analyse the potential of the ash for use as per the alternatives recommended in the study. For the proposed suggestive measures to be effective, a thorough systematic, scientific study on their properties, particularly leachability of the toxic metals and their subsequent long-term bioaccumulation in ecosystem has to be carried out.