An atmospheric microwave plasma-based distributed system for medical waste treatment

Abstract

Inadequate handling of infectious medical waste may promote the spread of the virus through secondary transmission during the transfer process. Microwave plasma, an ease-of-use, device-compact, and pollution-free technology, enables the on-site disposal of medical waste, thereby preventing secondary transmission. We developed atmospheric-pressure air-based microwave plasma torches with lengths exceeding 30 cm to rapidly treat various medical wastes in situ with nonhazardous exhaust gas. The gas compositions and temperatures throughout the medical waste treatment process were monitored by gas analyzers and thermocouples in real time. The main organic elements in medical waste and their residues were analyzed by an organic elemental analyzer. The results showed that (i) the weight reduction ratio of medical waste achieved a maximum value of 94%; (ii) a water–waste ratio of 30% was beneficial for enhancing the microwave plasma treatment effect for medical wastes; and (iii) substantial treatment effectiveness was achievable under a high feeding temperature (≥ 600 °C) and a high gas flow rate (≥ 40 L/min). Based on these results, we built a miniaturized and distributed pilot prototype for microwave plasma torch-based on-site medical waste treatment. This innovation could fill the gap in the field of small-scale medical waste treatment facilities and alleviate the existing issue of handling medical waste on-site.

Graphical abstract

Introduction

Since 2019, the COVID-19 pandemic has affected more than 200 countries and led to a substantially increased demand for single-use medical waste that intensifies pressure on an already out-of-control global medical waste problem (Ahmadi et al. 2020; Govindan et al. 2020; Kargar et al. 2020; Peng et al. 2021; Sarkodie and Owusu 2021; Paladhi et al. 2022). Failure to properly handle the infectious medical waste generated by health facilities and households could exacerbate the spread of COVID-19 through secondary transmission and aggravate potential health risks (Jeevanandam et al. 2019; Pal et al. 2020; Aljabali et al. 2021; Sarkodie and Owusu 2021).

Current traditional centralized waste treatment methods (e.g., open burning, incineration, and sanitary landfills) are prone to secondary transmission of medical waste during transfer (Jie et al. 2021a, b; Sarkodie and Owusu 2021; Jie et al. 2022). These methods are becoming increasingly apparent as public awareness of environmental protection and resource recycling grows due to their potential risks, particularly to groundwater and soil (Ma et al. 2017; Wang et al. 2020a, b, c; Chakraborty and Saha 2022). Thus, more safe, sustainable, and effective medical waste management strategies must be implemented to reduce treatment waiting time and personnel contact with medical waste by rationally planning the collection, transportation, and disposal of medical waste (Wong et al. 2019; Yu et al. 2020; Tirkolaee et al. 2021; Dam 2022). Additionally, accelerating research and development and promoting the use of small, mobile medical waste treatment equipment is crucial for reducing and even eliminating the burden of medical waste treatment.

Consequently, novel methods used in small and mobile medical waste devices, such as microwave heating and thermal (arc) plasma treatment techniques, have emerged in recent years (Mazzoni and Janajreh 2017; Samal 2017; Sanlisoy and Carpinlioglu 2017; Li et al. 2020a, b; Jie et al. 2021a, b; Li et al. 2021; Bhatt et al. 2022; Huang et al. 2022; Pancholi et al. 2022a). Microwave heating is a green and environmentally friendly method and a substitute for traditional heating in batch treatments of various materials (Li et al. 2021; Wang et al. 2022). Compared to traditional heating methods, the microwave heating technique allows uniform volume heating, a timely response, and a fast heating rate (Li et al. 2020a, b, 2021), etc. Microwave heating increases the effective reaction area and the reaction rate and thus shortens the treatment time, which improves the yields of some value-added byproducts (Li et al. 2021; Chen et al. 2022). Furthermore, to obtain much higher heating rates and shorter treatment times, plasma techniques are more suitable than microwave heating because plasma can instantly generate high temperatures, and these high temperatures can be controlled with millisecond-level response times. Recently, low-temperature plasma (dielectric barrier discharge and corona discharge) has been implemented in wastewater treatment, and arc plasma has been implemented in various solid waste treatments (Dam 2022, Gao et al. 2022; Pancholi et al. 2022b). However, these kinds of low-temperature discharge plasma have difficulty effectively “burning” solid medical waste due to its low gas temperature of ~ 300 K. Medical waste can roughly be divided into plastics and glass. Thermal plasmas have proven to be an effective means of harmlessly treating and vitrifying plastic products into glassy vitreous (Pancholi et al. 2022b). However, a single arc plasma torch requires an excess of 50 kW of electric power, which occupies a large volume in the equipment (Jie et al. 2022). Furthermore, the considerable heat loss under this high power inevitably induces electrode ablation, necessitating an additional cooling system for cooling the electrodes and the regular replacement of the electrodes. Contrary to arc plasmas, the microwave plasma torch can be ignited without an electrode in a length of ~ 30 cm under a low incident microwave power, typically from ~ 1 to ~ 3 kW (Jie et al. 2021a, b, 2023). Additionally, our recent research has proven that microwave plasma can environmentally treat solid wastes and vitrify the residual ash into slag (Jie et al. 2022, 2023). These findings indicate that less space and cost consumption are achievable in an integrated medical waste treatment facility by adopting microwave plasma technology. The microwave plasma obtains instantaneous high-temperature characteristics. Although the gas temperature of the microwave plasma torch, which ranges from 2000 to 9000 K, is lower than that of arc plasma (Huang et al. 2020; Jie et al. 2021a, b), a high gas temperature on the order of magnitude of thousands of degrees is sufficient for treating medical waste. The interior temperature of the reaction chamber can be readily controlled during medical waste treatment by varying the number of microwave plasma torches. Therefore, to address medical waste safely, effectively, promptly, and locally, we first introduce microwave plasma technology into the on-site treatment of medical waste and build a complete system of microwave plasma treatment for medical waste.

In this study, we conducted experiments using atmospheric pressure air microwave plasma to treat various medical wastes (surgical masks, protective garments, infusion bags, and mixed medical wastes) under several water-to-medical waste mass ratios (hereafter referred to as the water–waste ratio), feeding temperatures, and inlet gas flow rates in a thermolysis chamber. The characteristics of the plasma treatment process and the treatment effectiveness of medical waste by microwave plasma are studied. Finally, the prospects of future medical waste treatment by microwave plasma torches are determined based on the principle of resource recovery.

Experiment

Experimental setup

A diagram of the distributed medical waste treatment system based on microwave plasma technology is shown in Fig. 1. The microwave plasma-based medical waste treatment system included a thermal decomposition (thermolysis) reaction chamber, a recombination reaction chamber, eight microwave plasma modules, a gas analyzer, and other auxiliary devices.

Fig. 1.

Atmospheric pressure microwave plasma-based distributed medical waste treatment system

The thermolysis chamber gasified medical waste, while the recombination chamber further pyrolyzed and burned the gas produced in the thermolysis chamber. The functions of the thermolysis reaction chamber and the recombination reaction chamber were realized primarily by microwave plasma torches. The thermolysis reaction chamber was equipped with three microwave plasma modules, and the recombination reaction chamber was equipped with five microwave plasma modules. To sustain the high-temperature environment, ceramic fibers adhered to the inner walls of the reaction chambers. The cavity was cylindrical and composed of stainless steel 304; the lower portion served as the thermolysis reaction chamber, and the upper portion served as the recombination reaction chamber. A metal pipe lined with ceramic fiber connected the chambers. Due to the electromagnetic shielding effects of the stainless-steel cavity and the electrons in the plasma, the microwave radiation leakage was less than 1.0 mW/cm², making the system safe for human exposure.

The atmospheric microwave plasma module consisted of a microwave power source (Wepex 1000B, Megmeet, China), a tapered rectangular waveguide (WR-340), a 2450-MHz magnetron (2M248K, TOSHIBA, Japan) that is widely used in household microwave ovens, a mass flow controller (MFC), and an air pump. The microwave power output was set to 1.5 kW in the experiments. The magnetron generated a 2450-MHz microwave electromagnetic field, which entered the resonant cavity of the tapered rectangular waveguide and established a stable standing wave inside the cavity (Zhang et al. 2019; Jie et al. 2021a, b). The center of the quartz tube, with an inner diameter of 28 mm and a length of 8 cm, was located precisely at the high electric field of the standing wave peak, i.e., λ/4 away from the short end surface of the waveguide. The waveguide was compressed near the quartz discharge tube to increase the density of the microwave energy, hence enhancing the electric field intensity at the quartz tube center and enabling easier excitation of the microwave plasma. The vortex generator converted the directionally fed gas into a vortex gas flow into the quartz tube, which maintained and stabilized the torch-like morphology of the plasma and protected the quartz tube from high-temperature damage (Jie et al. 2021a, b). The plasma torch presented a stable, torch-like morphology and a long torch length of approximately 30 cm.

The portable gas analyzer (M60x, Afriso, Germany) offered a real-time gas analysis for the concentrations of CO, CO₂, and O₂ in the exhaust gas terminal. Sensors for carbon monoxide and oxygen were integrated into the gas analyzer. The range of the O₂ and CO sensors in volume fractions varied from 0 to 25% and 1%, respectively. The accuracies of the O₂ and CO sensors were 0.2% and 0.0001%, respectively.

The auxiliary devices contained a medical waste entrance, a screw discharger, pressure gauges, thermocouples, a flow controller, MFCs, and other devices, which were placed into action orderly under the supervision of a programmable logic controller (PLC, S7-200 SMART, Siemens, China).

Materials and methodology

The initial temperature of the recombination reaction chamber for the experiments was 1100 °C, and the ambient temperature was 25 °C. The waste entered the thermolysis chamber via the medical waste entrance and fell to the bottom of the thermolysis reaction chamber. Under the impact of microwave plasma torches, waste thermal decomposition generates combustible gas. After entering the recombination reaction chamber, the gas further decomposed and combined with air from the gas pipe (not shown in Fig. 1) for combustion. The exhaust gas was vented to the atmosphere or the subsequent processing unit via the gas outlet; the screw discharger transported the slag that fell from the metal mesh at the bottom of the thermolysis chamber to the collecting box. The micronegative pressure state of the thermolysis reaction chamber and the recombination reaction chamber was accomplished mostly through flow controller management during experiments. The total gas flow rate at the outlet of the recombination chamber was approximately 150 m³/h.

To properly verify the actual effects of the microwave plasma-based distributed medical waste treatment system and to investigate its industrial value, a series of microwave plasma processing medical waste experiments were conducted. In addition, by considering the accessibility and universality characteristics of materials, we chose surgical masks (Guangzhou Quantum Laser Intelligent Equipment Co., Ltd., China), protective garments (Guangzhou Quanhai Electronics Co., Ltd., China), and infusion bags (Jiangxi Hongda Medical Equipment Group Ltd., China) as the experimental objects. The basic properties of the chosen medical wastes are shown in Table 1. These medicinal materials are mostly composed of polypropylene (PP), polyvinyl chloride (PVC), and polyethylene (PE). The organic content and weight loss rates of various materials in Table 1 were derived through experiments, as described in the “Elemental analyses” section.

Table 1.

Basic properties of various medical waste materials

Material type	Major compositions	Organic content	Weight reduction ratio
Mask	PP (≥ 90 wt%) + PET + Al/Zn (~ 4 wt%)	98.5 wt%	92 wt%
Protective garments	PP (≥ 80 wt%)	85.5 wt%	84.8 wt%
Infusion bag	PVC (≥ 50 wt%) + ABS (~ 10 wt%) + PE + stainless steel (~ 0.2 wt%)	62.5 wt%	94 wt%
Mixed medical wastes	PP (≥ 55 wt%) + PVC (≥ 20 wt%) + PE	82.2 wt%	90.8 wt%

By considering the real world, the experiments were divided into four groups based on various materials, water–waste ratios, feeding temperatures, and inlet gas flow rates in the thermolysis chamber. The conditions of the four experimental groups are shown in Table 2. The anhydrous mass of each feed was 500 g. Simultaneously, to eliminate the randomness of the experimental results and determine the experimental principles for treating medical waste with microwave plasma, each experiment was conducted three times.

Table 2.

Specific conditions of the experimental groups under varied variables

Group	Material	Water–waste ratio	Feeding temperature	Gas flow rate
1	Masks, protective garments, infusion bags, and mixed medical wastes (mass ratio of masks, protective garments, and infusion bags was 1:1:1)	0%	500 °C	20 L/min
2	Mixed medical wastes	0 ~ 40%	500 °C	20 L/min
3	Mixed medical wastes	0%	400 ~ 600 °C	20 L/min
4	Mixed medical wastes	0%	500 °C	10 ~ 40 L/min

Results and discussion

Principles of microwave plasma treatment

The chosen medical wastes, masks, protective garments, and infusion bags include PP, PVC, PE, and high-density PE as the primary organic components. The fundamental principle of microwave plasma treatment of medical waste involves utilizing the active particles in the microwave plasma and forming a high-temperature field to interact with medical wastes. The heat levels of microwave plasma and energetic particles act on medical waste, resulting in various physicochemical reactions, including plasma drying, gasification, pyrolysis, combustion, and vitrification. Since PP, PE, and PVC are highly similar in composition and structure, this section focuses on PE to illustrate the sorts of reactions that occur during plasma treatment of medical waste.

Plasma drying occurs under the strong heat-transfer effects of the plasma torch when moisture attached to the surface of medical waste evaporates rapidly. Some water molecules decompose into hydrogen and oxygen under the action of microwave plasma. The related reaction reads

1

Plasma gasification indicates that the chemical bonds of organic components in medical waste are bombarded by high-energy particles of microwave plasma without oxygen or hypoxia to generate low-carbon hydrocarbons and small molecular compounds, including H₂, CO, and CH₄. The main reactions are as follows in Eqs. (2)–(6):

2

$$\begin{array}{cc}{2\mathrm{C}+\mathrm{O}}_2\to 2\mathrm{CO}& \mathrm{\Delta }\mathrm{H}=222\mathrm{MJ}/\mathrm{kmol}\end{array}$$ 3

$$\begin{array}{cc}{\mathrm{C}+\mathrm{CO}}_2\to 2\mathrm{CO}& \mathrm{\Delta }\mathrm{H}=-172\mathrm{MJ}/\mathrm{kmol}\end{array}$$ 4

$$\begin{array}{cc}{\mathrm{CO}+\mathrm{H}}_2\mathrm{O}\to {\mathrm{CO}}_2{+\mathrm{H}}_2& \mathrm{\Delta }\mathrm{H}=-41\mathrm{MJ}/\mathrm{kmol}\end{array}$$ 5

$$\begin{array}{cc}{\mathrm{CH}}_4{+\mathrm{H}}_2\mathrm{O}\to {\mathrm{CO}+3\mathrm{H}}_2& \mathrm{\Delta }\mathrm{H}=-227\mathrm{MJ}/\mathrm{kmol}\end{array}$$ 6

Additionally, some other side products are involved in the above process. For example, hydrogen chloride (HCl) forms in the plasma treatment due to the chlorine (Cl) element introduced by PVC. The air microwave plasma also contains a large amount of heavy particles, such as N₂, O₂, ${\text{N}}_2^{+}$, ${\text{O}}_2^{+}$, ${\text{O}}_2^{-}$, ${\text{N}}_2^{\ast }$, and ${\text{O}}_2^{\ast }$ (Saifutdinov et al. 2019). These high-energy heavy particles in plasmas may lead to the formation of NO_X, HCN, and NH₃, which can be eliminated by the subsequent tray scrubber and three-way catalytic converter (Xiao et al. 2007).

Plasma pyrolysis indicates that in the absence of oxygen, the low-carbon hydrocarbons generated from medical waste are further treated by a microwave plasma torch to generate flammable, small molecular compounds.

In plasma-assisted combustion, CO₂ and H₂O are generated from the oxidation of syngas. The main reactions are as follows:

$$\begin{array}{cc}{\mathrm{C}}_{\mathrm{x}}{\mathrm{H}}_{\mathrm{y}}+(\mathrm{x}+\frac{\mathrm{y}}{4}){\mathrm{O}}_2\to {\mathrm{xCO}}_2+\frac{\mathrm{y}}{2}{\mathrm{H}}_2\mathrm{O}& \mathrm{\Delta }\mathrm{H}>0\end{array}$$ 7

$$\begin{array}{cc}{\mathrm{CH}}_4{+2\mathrm{O}}_2\to {\mathrm{CO}}_2{+2\mathrm{H}}_2\mathrm{O}& \mathrm{\Delta }\mathrm{H}=782\mathrm{MJ}/\mathrm{kmol}\end{array}$$ 8

$$\begin{array}{cc}{2\mathrm{H}}_2{+\mathrm{O}}_2\to {2\mathrm{H}}_2\mathrm{O}& \mathrm{\Delta }\mathrm{H}=484\mathrm{MJ}/\mathrm{kmol}\end{array}$$ 9

$$\begin{array}{cc}{2\mathrm{CO}+\mathrm{O}}_2\to {2\mathrm{CO}}_2& \mathrm{\Delta }\mathrm{H}=568\mathrm{MJ}/\mathrm{kmol}\end{array}$$ 10

Through the above physicochemical reactions, most of the organic components in medical waste become gaseous substances. The above equations illustrate the main reactions that may occur when treating medical waste with microwave plasma. Usually, medical waste can decompose into small hydrocarbons or monolithic carbon (Kohno et al. 1998; Huang et al. 2011); these processes are called hydrogasification and carbonization, respectively, which are displayed as follows:

$$\begin{array}{c}{\mathrm{C}}_6{\mathrm{H}}_5{-\mathrm{C}\mathrm{H}}_3+\mathrm{e}\to {\mathrm{C}}_6{\mathrm{H}}_5·{+\mathrm{C}\mathrm{H}}_3·+\mathrm{e}\to \mathrm{P}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s}\\ \to {\mathrm{C}}_6{\mathrm{H}}_4{-\mathrm{C}\mathrm{H}}_3+\mathrm{H}\to \mathrm{P}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s}\end{array}$$ 11

$${\text{C}}_6{\text{H}}_5{\text{-CH}}_3{+\text{O}}^{+}{\text{, O}}_2^{+}{\text{, N}}^{+}{\text{, or N}}_2^{+}\to {\text{C}}_6{\text{H}}_5{{\text{-CH}}_3}^{+}{\text{+ O, O}}_3{\text{, N, or N}}_2$$ 12

$${\text{C}}_6{\text{H}}_5{-{\text{CH}}_3}^{+}+\text{e}\to {\text{C}}_6{\text{H}}_5·{\text{+ CH}}_3·\to \text{Products}$$ 13

$${\text{C}}_6{\text{H}}_5{\text{-CH}}_3\text{+ O}\to {\text{C}}_6{\text{H}}_5{\text{-CH}}_2\text{O + H}\to \text{Products}$$ 14

$${\mathrm{C}}_6{\mathrm{H}}_5-{\mathrm{CH}}_3+·\mathrm{OH}\to {\mathrm{C}}_6{\mathrm{H}}_5-{\mathrm{CH}}_2·+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Products}$$ 15

The charge transfer reactions shown in Eq. (12) of toluene with ions and the recombination shown in Eq. (13) of toluene ions and electrons are significant. However, the ion densities are 4 to 5 orders of magnitude smaller than the radical densities. The energetic electron impact dissociations of Eq. (11) are thus the most likely pathways for chemical reactions (Kohno et al. 1998). The “products” mentioned in Eqs. (11)–(15) refer to the final products of CO₂ and H₂O.

Plasma vitrification is the melting of the inorganic components of the preprocess reaction residues at the high temperature of plasma to form a stable amorphous slag, thereby preventing the diffusion of hazardous compounds (Jie et al. 2022).

Experimental process analysis

By taking mixed medical waste as the experimental subject, we maintained the feeding temperature at 500 °C and the gas carrying capacity at 20 L/min; we did not implement additional water. Each mixed medical waste treatment process took approximately 40 min. Figure 2 shows the characteristics of the plasma treatment process for the mixed medical wastes. These characteristics included (i) the temperature changes in the thermolysis reaction chamber and recombination reaction chamber during the microwave plasma treatment of medical waste and (ii) the oxygen, carbon dioxide, and carbon monoxide concentration changes at the outlet. The measurement positions of the thermocouples in the two chambers in the experiment are worth noting. The actual measured temperatures were essentially the temperature at the upper center of the chamber, not the gas temperature near the plasma torch.

Fig. 2.

Variations in temperatures and gas concentrations with time during medical waste treatment

According to its characteristics, the entire treatment process was divided into five stages: (I) preheating, (II) pyrolysis, (III) plasma-assisted combustion, (IV) deceleration treatment, and (V) recovery.

The duration of stage I was a physical heat absorption process lasting for approximately 60 s. Due to the recent addition of medical waste (at room temperature) to the thermolysis reaction chamber, it was necessary for the waste to absorb heat from the chamber to boost its temperature, resulting in a minor decrease in the chamber temperature.

Stage II was a short pyrolysis process characterized by an instant increase in CO concentration, while oxygen and CO₂ levels remained stable. In the thermolysis reaction chamber, medical waste was attacked with high-energy particles (especially electrons, ions, and free radicals) and subjected to high temperatures, leading to the pyrolysis of waste and the formation of synthesis gases, such as CO; however, the concentrations were insufficient for combustion. Plasma collision reactions (Eqs. (11)–(15)) and endothermic reactions (Eqs. (2)–(6)) mainly occurred in stage II. The duration of this stage was very short at approximately 15 s. Due to the short duration of this process, even though it is a heat absorption process, the chamber temperatures only minimally decreased.

In step III, plasma-assisted combustion occurred. The thermolysis chamber reached the stoichiometric amount of CO required for combustion, and thus, the waste in the thermolysis reaction chamber and previously generated synthesis gases were ignited. In this stage, the exothermic reactions corresponding to Eqs. (7)–(10) proceeded with the assistance of plasma torches. Therefore, the temperatures of the thermolysis reaction chamber and recombination chamber increased rapidly, the O₂ and CO contents decreased rapidly, and the CO₂ content increased rapidly. This stage lasted approximately 7 min.

The duration of stage IV was long at approximately 10 min. Since most medical wastes were treated in stage III, the waste treatment speed slowed, and the release of combustible gas from the thermolysis reaction chamber decreased gradually. As a result, the temperature of the thermolysis reaction chamber continued to increase, while the temperature of the recombination reaction chamber and the amount of CO₂ started to decrease, and the amount of oxygen increased steadily. Additionally, with the decrease in temperature in the recombination chamber, the contents of CO and other combustible gases began to increase and fluctuate in the lower concentration range.

In stage V, most of the medical waste was treated. Stage V was a recovery period for the microwave plasma-based distributed medical waste treatment system to return to the initial state to prepare for the following feedstock. The process of this stage was slow and lasted for a long time (~ 21 min, which was half of the total time). Note that the treatment of medical waste was mainly completed in stages III and IV, and the CO emission concentration was maintained at a very low level with the highest concentration below 30 ppm or 37.5 mg/m³.

Elemental analyses

To more thoroughly understand and evaluate the effects of the experiment, estimates of the principle elemental compositions of the medical wastes and residues remaining after plasma treatment were necessary. Thus, an organic elemental analyzer (Vario EL III, Elementar Analysensysteme GmbH, Germany) was employed to determine the principle elemental contents of the masks, protective garments, infusion bags, mixed medical waste, and their corresponding slags after treatment. Figure 3 depicts the elemental analyses of medical wastes and slags. Masks, protective garments, and infusion bags included mostly carbon, hydrogen, and oxygen; the quantities of nitrogen and sulfur were less than 0.1 wt%. Among these elements, the contents of C in various materials were the highest, and the contents of C in masks, protective garments, and infusion bags were 84.7 wt%, 68.25 wt%, and 50.685 wt%, respectively; the contents of C in the corresponding slags to these components were 58.085 wt%, 10.18 wt%, and 53.925 wt%, respectively.

Fig. 3.

Elemental analyses of medical wastes and slag. Codenames A1, A2, B1, B2, C1, C2, D1, and D2 indicate masks, mask slags, protective garments, protective garment slags, infusion bags, infusion bag slags, mixed medical wastes, and mixed medical waste slags, respectively

The mass change characteristics of the experiment are shown in Table 1. The initial mass for each material before the plasma treatment was 500 g. According to Table 1 and Fig. 3, the removal ratios of organic matter in masks, protective garments, and infusion bags were 94.51 wt%, 97.71 wt%, and 93.62 wt%, respectively. In addition, the predicted weight of the slags of the mixed medical wastes should have been 48.7 g based on the experimental slag weights of the masks, protective garments, and infusion bags and the mass mixing ratio (1:1:1) of these 3 components. The actual experimentally measured weight of the slag of the mixed medical wastes was 46 g, with a small negative deviation of 5.5% relative to the predicted weight of 48.7 g. Moreover, the predicted C content in the slag of mixed medical waste was 32.29 wt%, whereas the actual C content in the slag of mixed medical waste was 31.975 wt%, with an error of 1% between these measurements. These small errors of 5.5 and 1% between the predicted values and actual values demonstrated that the treatment effects of microwave plasma were identical to those of single or mixed materials. That is, there was no synergy between the reactions of mixed medical wastes during plasma treatment to enhance the treatment effect. Under the impact of microwave plasma, the organic content in the experimental materials was essentially entirely decomposed, and the processed slag consisted mostly of inorganic powder and biochar particles. Furthermore, the slags generated by the plasma treatment can be vitrified utilizing microwave plasma with increased power, which is not in the scope of this paper (Jie et al. 2022).

Analysis of the treatment effectiveness

To evaluate the treatment effectiveness levels of the medical wastes by the microwave plasma-based distributed medical waste treatment system under varied conditions, the weight reduction ratio (e.g., the ratio of slag mass to the feeding weight), treatment time, and increasing temperature (the difference between the highest temperature of the thermolysis reaction chamber and feeding temperature in each experiment) were chosen. In addition, it was believed that these data would show the potential rules in each experimental group and serve as guides for the practical application of microwave plasma treatment for medical waste.

Influence of the waste composition on the treatment effectiveness

For each experimental group, the weight reduction ratio, treatment time, and increasing temperature were determined. Figure 4a, b, and c depict the variations in the weight reduction ratio, treatment time, and temperature increase in the experimental groups of different materials, respectively.

Fig. 4.

Microwave plasma treatment characteristics of various medical wastes: a weight reduction ratio, b treatment time, and c increasing temperature

Figure 4 shows that the microwave plasma-treated infusion bags exhibited the best results among the four kinds of medical wastes; the highest value of the weight reduction ratio was 94%, and the lowest value of treatment time was ~ 22 min per 500 g. This result occurred mainly because the infusion bags had the maximum concentrations and calorific values of organic matter according to Fig. 3. Microwave plasma treatment for protective garments took the most treatment time and resulted in the lowest weight reduction ratio. This phenomenon occurred due to the lowest organic content of protective clothing, making it more difficult to treat protective garments than masks and infusion bags.

Influence of water addition on the treatment effectiveness

The associations between the weight reduction ratio and the water–waste ratio for experimental groups with varied water–waste ratios are depicted in Fig. 5a. As shown in Fig. 5a, the weight reduction ratios of the mixed medical waste under various water additions are all higher than 90%, and a water–waste ratio of 30% can result in the highest weight reduction ratio of ~ 92%. Figure 5b demonstrates that when the water–waste ratio ranges from 0 to 40%, the treatment time remains nearly constant with fluctuations of less than 3 min; however, the increasing temperature initially decreases and then tends to remain flat with the change in the water–waste ratio. Moreover, when the water–waste ratio equals 30%, the treatment time is the shortest. This phenomenon occurs due to the decomposition of a certain amount of water under the action of high-energy particles of microwave plasma to assist and enhance the thermolysis and recombination reactions. This finding indicates that a water–waste ratio of 30% is beneficial for enhancing the microwave plasma treatment effect for medical wastes to increase the weight reduction ratio with a relatively short treatment time.

Fig. 5.

Microwave plasma treatment characteristics of mixed medical waste under various water–waste ratios: a weight reduction ratio and b treatment time and increasing temperature

Influence of the feeding temperature on the treatment effectiveness

For the experimental groups with different feeding temperatures, Fig. 6a shows that the weight reduction ratio first decreases and then increases rapidly with increasing feeding temperature and reaches the lowest value at 550 °C. Additionally, Fig. 6b shows that the treatment time and increasing temperature decrease rapidly with the change in the feeding temperature. The higher the temperature, the faster the waste thermolysis rate; the longer the treatment time, the more sufficient the waste thermolysis. When the feeding temperature changes in a low-temperature range (~ 550 °C), the treatment time is dominant; thus, the weight reduction ratio decreases with increasing feeding temperature. When the feeding temperature changes in a high-temperature range (~ 550 °C), the high-temperature environment of the thermolysis reaction chamber is dominant; thus, the weight reduction ratio increases rapidly with increasing feeding temperature. According to the analysis of Fig. 6, to improve the treatment capacity and effect of the microwave plasma-based distributed medical waste treatment system for treating medical waste, the theoretical feeding temperature can be set at a higher temperature (≥ 600 °C).

Fig. 6.

Microwave plasma treatment characteristics of mixed medical waste under various feeding temperatures: a weight reduction ratio and b treatment time and increasing temperature

Influence of the gas flow rate on the treatment effectiveness

For the experimental groups with different gas flow rates, the curves of the weight reduction ratio, treatment time, and increasing temperature under various gas flow rates are depicted in Fig. 7. This figure shows that the weight reduction ratio and increasing temperature first increase and then stabilize with the changing gas flow rate, and the treatment time decreases with the increase in the gas flow rate. This phenomenon occurs because the type and quality of each feed remain unchanged; thus, the content of organic matter in the material is basically unchanged. The experimental group shows that with the increase in the gas flow rate, the oxygen content in the thermolysis reaction chamber increases, and the combustion reaction becomes more intense; these phenomena accelerate the waste treatment efficiency and improve the waste treatment effect.

Fig. 7.

Microwave plasma treatment characteristics of mixed medical waste under various gas flow rates: a weight reduction ratio and b treatment time and increasing temperature

Exhaust product analysis

The thermolysis, recombination, and combustion of medical waste by microwave plasma can produce flue gas and a small amount of waste residue. In the experiment, mixed medical waste is taken as the experimental object to detect the flue gas and residues. The various gas concentrations in the flue gas are shown in Table 3. There are clear requirements for flue gas blackness and the concentrations of particulate emissions, SO₂, CO, NO_x, and HCl in environmental detection. In this experiment, the particulate emission concentration is measured by an electronic balance (QUINTIX125D-1CN), the concentrations of SO₂, NO_x, and CO are measured by an automatic smoke detector (Laoying 3012H), the concentration of HCl is detected by an ultraviolet–visible spectrophotometer (UV-3200), the emission concentration of nonmethane total hydrocarbons is detected by a gas chromatograph (GC-2014), and the flue gas blackness is detected by a Ringelmann flue gas concentration diagram (LG30). The test results show that the emission concentrations of SO₂, NO_x, and HCl are below the lower limit of detection. The emission concentrations of particulate matter, CO, and nonmethane total hydrocarbons are 1.23 ± 0.07 mg/m³, 1.7 ± 1.7 mg/m³, and 2.05 ± 0.05 mg/m³, respectively; these values are far below the corresponding Chinese national standards of GB 39707–2020. The flue gas emission is colorless and odorless. According to a comparison of the Ringelmann smoke chart and the exhaust, the Ringelmann number is less than 1. These results show that microwave plasma has a considerable treatment effect on medical waste and realizes the emission of harmless flue gas.

Table 3.

Exhaust product detection results

Items	Unit	Detection result	Instrument name	Instrument type
Particulate emission concentration	mg/m3	1.23 ± 0.07	Electronic balance	QUINTIX125D-1CN
SO2 emission concentration	mg/m3	Undetected	Automatic smoke detector	Laoying 3012H
NOx emission concentration	mg/m3	Undetected	Automatic smoke detector	Laoying 3012H
CO emission concentration	mg/m3	1.7 ± 1.7	Automatic smoke detector	Laoying 3012H
HCl emission concentration	mg/m3	Undetected	Ultraviolet–visible spectrophotometer	UV-3200
Emission concentration of nonmethane total hydrocarbons	mg/m3	2.05 ± 0.05	Gas chromatograph	GC-2014
Blackness of smoke plumes	Level	< 1	Ringelmann smoke chart	LG30
Thermal reduction rate	%	4.0	Electronic balance	JM-A 10002

In addition, the thermal reduction rate is the most reliable criterion for identifying whether a waste treatment is complete, and it can be used to predict the treatment’s final condition in traditional waste treatments. Usually, the thermal reduction rate is controlled at 3 ~ 5% in traditional industrial treatments of wastes. In this study, the thermal reduction rate is used to evaluate the treatment thoroughness of medical waste by a microwave plasma torch. The thermal reduction rate is calculated by Eq. (16).

$$\text{Thermal reduction rate =}\frac{\text{Dried mass A}-\text{Burned mass B}}{\text{Dried mass A}}\times 100\%$$ 16

where the dried mass A denotes the residual mass of the medical waste residue after drying at 110 °C for 2 h and the burned mass B denotes the residual mass of the medical waste residue after being burned at 600 °C for 3 h. In our study, the waste residues are weighed by an electronic balance (JM-A 10,002); the thermal reduction rate of the medical waste treated by the microwave plasma torch is calculated to be 4%, meeting the recent industry demands.

Conclusions, outlook, and future aspects

Microwave plasma technology for medical waste on-site treatment is an attractive process with potential applications soon. Based on microwave plasma technology, we have pioneered the use of microwave plasma as a novel and very effective pollutant on-site treatment approach for medical waste. An atmospheric microwave plasma-based distributed medical waste treatment pilot prototype was built to treat distributed medical waste on-site to avoid the secondary spread of the virus. This system was also utilized to conduct research on the treatment of various medical wastes under various conditions. The following results were reached:

• (i)The weight reduction ratio of the medical waste was more than 90%, and the highest ratio of 94% was reached under microwave plasma treatment
• (ii)The shortest microwave plasma treatment time of 22 min per 500 g with a weight reduction ratio of 94% was reached for the infusion bags
• (iii)A water–waste ratio of 30% could be beneficial for enhancing the microwave plasma treatment effect for medical wastes to gain a higher weight reduction ratio and a relatively short treatment time
• (iv)A great treatment effectiveness could be achieved under a high feeding temperature (≥ 600 °C) and gas flow rate (≥ 40 L/min)

The results also show that environmentally friendly microwave plasma technology has a considerable treatment effect on medical waste without any harmful emission of flue gas. This result indicates that only electricity-based microwave plasma technology holds great potential for future distributed on-site medical waste treatment.

This study does not address the determination of biomedical samples, which is a drawback. Due to the high temperature (10³ K) and high energy of the particles in microwave plasma, biohazards (bacteria, viruses, etc.) must be eliminated in plasma environments. Therefore, sterile medical supplies were chosen to illustrate the therapy process under microwave plasmas, and the study of biomedical materials was not addressed in this work. However, biocide efficacy is a critical factor that could be studied further in the future using biological sensors and other nanotechnology-based innovative sensing technologies (Aljabali et al. 2021).

Considering that the centralized treatment of domestic waste in remote areas will significantly increase the transportation and management costs of waste treatment, this system is perfectly suited for the on-site treatment of high-risk medical waste and for domestic waste in remote areas.

To reduce carbon emissions and contribute to carbon neutralization, the constructed system could be upgraded to the structure depicted in Fig. 8 to achieve the potential for resource utilization. For example, the synthesis gas generated from medical waste can be collected by nitrogen plasma treatment in the prior bottom chamber to achieve secondary energy utilization and power generation.

Fig. 8.

Resource recovery of medical waste treatment by microwave plasma torches

In addition, the microwave plasma waste treatment system is applicable to isolated villages, resorts, ships, and other locations for treating local domestic waste trash. The residual ash produced during microwave plasma treatment can be filtered and classified according to its toxicity, allowing nonhazardous inorganic elements to be utilized as fertilizer and hazardous metals to be vitrified into glass, as shown in Fig. 8 (Jie et al. 2022). The generated glassy slags can be buried on-site or used as a building material. Eventually, the recycling of resources can be realized.

Author contribution

All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Ziyao Jie, Daolu Xia, and Cheng Liu. The first draft of the manuscript was written by Daolu Xia and revised by Ziyao Jie. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 51176098), the Beijing Science & Technology Planning Project (Grant No. Z191100002019014), and the China Postdoctoral Science Foundation (Grant No. 2019M660639).

Data availability

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

Declarations

Ethics approval

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Competing interests

The authors declare no competing interests.