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. 2021 Jan 20;3:100082. doi: 10.1016/j.cscee.2021.100082

Fate of disposable face masks in high-solids anaerobic digestion: Experimental observations and review of potential environmental implications

Felipe Pereira de Albuquerque a, Mayank Dhadwal a, Wafa Dastyar a, Seyed Mohammad Mirsoleimani Azizi a, Ibrahim Karidio b, Hamid Zaman a,b, Bipro Ranjan Dhar a,∗
PMCID: PMC7816893  PMID: 38620309

Abstract

Face masks became a part of our daily life amid the global COVID-19 (SARS-CoV-2) pandemic. Most of the face masks are made for single-use and primarily disposed of in garbage bins with other non-recyclable wastes. To date, little is known about how disposable face masks in municipal solid waste (MSW) would interfere with high-solids anaerobic digestion (HSAD) in waste management facilities. Here, we first report preliminary results from a lab experiment conducted with the organic fraction of municipal solid waste (OFMSW) amended with used disposable face masks. The lab-scale HSAD systems were operated with percolate recirculation comparable to commercial HSAD systems typically used for full-scale processing of OFMSW. The results suggested that the presence of face masks in OFMSW could negatively affect methane productivity and kinetics. In the digesters amended with face masks, total cumulative methane production decreased by up to 18%, along with a 12–29% decrease in maximum methane production rates than the control digester (without face masks). Moreover, lag phases increased by 7–14%. The results also suggested that the type of polymeric materials used in face masks would be more critical than their total number/loading in the digester, which warrants further investigation. The visual inspection of digestate showed that the face masks were mostly undegraded after 40 days of operation. Much remains unknown about how the undegraded face masks will affect the digestate management practices, such as composting, land application, and landfilling. However, the review of existing literature suggested that they can be a potential source of plastic and microplastic pollution and amplify transmission of antibiotic resistance genes to the ecosystem. In summary, this study underscores the importance of developing safe and reliable disposal guidelines and management plans for single-use face masks.

Keywords: COVID-19, SARS-CoV-2, Disposable face mask, Organic fraction of municipal solid waste, High-solids anaerobic digestion, Waste management

Graphical abstract

Image 1

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1. Introduction

As the COVID-19 (SARS-CoV-2) pandemic continues, the efforts to stop the spread of the virus have prompted the municipal solid waste (MSW) generation rate in cities around the globe [[1], [2], [3]]. Particularly, MSW characteristics and volume are considerably influenced by face masks, gloves, and other personal protective equipment (PPE) [[4], [5], [6]]. Notably, the recommendation/requirement of using face masks in public or crowded spaces has triggered the massive use of face masks (mostly non-medical) worldwide [7,8]. The global demand for disposable face masks (which should not be worn longer than a few hours) was estimated at 129 billion per month to protect public health amid COVID-19 [9]. The disposable or single-use face masks are primarily disposed of in the waste bin [10] with other non-recyclable items. In municipalities without residential source separated organics (SSO) program, face masks can be commingled with landfill items and organic wastes. After removing landfill items from commingled waste, organic fraction of solid waste (OFMSW) can be collected for biological treatment (e.g., composting, anaerobic digestion) [11]. Thus, depending on the sorting process (e.g., manual separation, trommel, etc.) used for screening out of landfill items, face masks may end up in OFMSW. It is still unknown how face masks in OFMSW would affect the biological processes in waste management facilities.

High-solids anaerobic digestion (HSAD) of OFMSW, which is also known as dry anaerobic digestion, is an increasingly adopted practice by many cities and municipalities in North American and European countries [12]. For instance, a full-scale HSAD facility with an annual capacity of processing 40,000 tonnes of OFMSW has been integrated into the City of Edmonton’s Waste Management Centre (Edmonton, Alberta, Canada). In addition to handling heterogeneous feedstock like OFMSW, lignocellulose biomass, etc. HSAD alleviates the requirement of dewatering of residuals (i.e., digestate), which is typical for low-solids/wet-type anaerobic digestion [13,14]. Moreover, HSAD systems require less heating cost due to low water content in feedstock and/or smaller digester footprints [15]. A majority of full-scale commercial HSAD systems are based on modular system design combining digester tanks for solids and percolate storage tanks [[16], [17], [18]]. The recirculation of percolate (also called leachate) between the digester and percolate storage tanks provides homogenization of organics and nutrients, which is critical because high-solids digester is typically operated without any active mixing [16,18,19]. Due to high heterogeneity and solids content in OFMSW, HSAD is used instead of low-solids/wet-type anaerobic digestion [12,16]. However, based on the authors’ knowledge, there has been almost no research looking at how face masks can potentially affect HSAD performance amid the COVID-19 pandemic.

Single-use face masks are manufactured from various polymeric materials such as polypropylene, polyurethane, polyacrylonitrile, polyethylene, polystyrene, etc. [[20], [21], [22]]. A few studies already identified disposable face masks as emerging sources of microplastics contamination of our ecosystem [[20], [21], [22]]. There have been shreds of evidence that microplastics compounds can adversely affect methanogenic communities in anaerobic digesters [[23], [24], [25]]. However, based on the authors’ knowledge, the potential impact of face masks in OFMSW on HSAD performance and operation is still unknown. Although the use of face masks is critical to slow down the spread of the COVID-19 outbreak, it is desirable to estimate the potential environmental risks associated with the uncontrolled disposal of single-use face masks. Consequently, in this study, we investigated the impact of disposable face masks on the performance of lab-scale HSAD systems with percolate recirculation. Furthermore, we briefly reviewed existing literature to provide an outlook on the possible environmental consequences of undegraded face masks in the digestate of the HSAD process. We acknowledge that there have been concerns regarding the potential spread of the virus through inappropriate handling of infected face masks and medical waste [3,26]. It should be noted that these aspects were not considered within the scope of this preliminary study.

2. Materials and methods

2.1. Experiments

OFMSW and solid inoculum were obtained from the Edmonton Waste Management Centre (EWMC), operated by the City of Edmonton, Alberta, Canada. Dewatering centrate from anaerobic digester sludge collected from the Gold Bar Wastewater Treatment Plant (Edmonton, Alberta, Canada) was used as the percolate (liquid inoculum) in this study. After sampling, OFMSW and inoculums (solid and liquid) were stored in plastic containers in a cold room at 4 ​°C. As the City of Edmonton currently does not have a source separated organics (SSO) program, OFMSW was a blend of food waste, lawn trimmings, paper, wood, and few fragments of plastics and metals (Fig. 1 a). Based on visual inspection, large pieces of inorganic materials were removed, and the remaining feedstock was manually homogenized before loading to the digesters. The average characteristics of OFMSW were as follows: moisture content (MC): 72.03 ​± ​3.31%; total solids (TS): 28.95 ​± ​3.31%, and volatile solids (VS): 21.03 ​± ​2.91%, respectively. Dewatered biosolids (digested sewage sludge) was used as the solid inoculum in this study (Fig. 1b). The average characteristics of biosolids (solid inoculum) were as follows: MC: 76.87 ​± ​0.86%; TS: 23.56 ​± ​0.86%, and VS: 12.84 ​± ​0.47%. The average characteristics of percolate (liquid inoculum) were: MC: 94.17 ​± ​0.14%; TS: 5.83 ​± ​0.14%; VS: 0.04 ​± ​0.00%; total chemical oxygen demand (TCOD): 35,238 ​± ​2069 ​mg/L; soluble chemical oxygen demand (SCOD): 30,396 ​± ​148 ​mg/L; total ammonia nitrogen (TAN): 2002 ​± ​58 ​mg/L; alkalinity: 7957 ​± ​303 ​mg/L as CaCO3; pH: 6.98 ​± ​0.01. Both solid and liquid inoculum were acclimated at ~37 ​°C for about two weeks before reactors’ start-up.

Fig. 1.

Fig. 1

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Photographs of (a) organic fraction of municipal solid waste (OFMSW), (b) biosolids, (c) lab-scale HSAD systems consisting of the digester and percolate tanks, (d) interior of the digester tank with stainless mesh plate in the base separating its upper and bottom sections, and (e) disposable face masks mixed with OFMSW and biosolids for loading in digester tanks.

The experimental set-up used three identical lab-scale HSAD systems (Fig. 1c). Each system was composed of a digester tank and a percolate storage tank [27,28]. The digester tank was cylindrical, with a diameter of 20 ​cm and a height of 50 ​cm. It was manufactured with polycarbonate and sub-divided into two sections by a stainless mesh plate (1 ​mm thickness and ~2 ​mm holes diameter) (Fig. 1d). With approximately 1.5 ​L of volume, the bottom section of the tank had the purpose of collecting the leachate (percolate). The upper section (~14.5 ​L) carried the OFMSW plus the biosolids. To maintain an operating temperature of 36 ​± ​2 ​°C, the upper section’s exterior wall was wrapped with heating tubes connected to a circulating water bath. The tubes were then covered with bubble-wrap insulators to minimize heat loss during operation. The percolate tank was an anaerobic glass reactor with a mechanical mixer equipped with an electrical motor, which was operated at 300 ​rpm during operation. The percolate tanks were partially submerged in a water bath to maintain a temperature of 37 ​± ​2 ​°C. The gas outlets of both tanks were connected to an individual absorption trap bottle containing a 3 ​M NaOH solution with a thymolphthalein pH-indicator to capture acidic gases from the biogas (e.g., CO2, H2S, etc.) [[27], [28], [29]]. The resulting methane volume was measured with wet-tip gas-flow meters (ISES-Canada, Vaughan, ON, Canada). For experiments, digester tanks were loaded with a 4 ​kg mixture of OFMSW and biosolids. The food to microorganisms (F/M) ratio of the mixture was 2 (kg VSOFMSW/kg VSBiosolids). Thus, the mixture contained 2.2 ​kg of OFMSW and 1.8 ​kg of biosolids. Before loading feedstock to the digester, OFMSW and biosolids were thoroughly mixed. The first reactor served as the control (No face mask). For the test reactors T1 and T2, the mixture of OFMSW and biosolids were amended with 4 and 20 used disposable face masks, respectively (Fig. 1e). The number of face masks added in reactor T1 was based on an average MSW generation rate of 578 ​kg/capita/year among 17 developed countries [30]. Considering 40% of MSW is OFMSW, the daily OFMSW generation rate was estimated at 0.6 ​kg/capita/day. Based on the assumption that each person uses one disposable face mask per day [3], 4 face masks were added for 2.2 ​kg OFMSW. For reactor T2, the number of face masks was increased to 20 to represent OFMSW generated from sources other than residential, such as offices, schools, and other institutions.

The disposable face masks were collected from a separate waste bin dedicated to our lab members’ disposal of used face masks. The percolate tanks of all systems were filled with 2.5 ​L of liquid inoculum (i.e., percolate). During operation, percolate was recirculated between the digester and percolate storage tanks for 1.5 ​h/d (rate of 100 ​mL/min) for the first 27 days of operation. It should be noted that the operating conditions (F/M ratio and percolate recirculation rate) were selected based on the optimum operating conditions obtained from our preliminary experiments (results not shown). Following the loading process, all reactors were sealed and purged with ultra-pure nitrogen gas to ensure anaerobic conditions. The methane production from reactors was monitored regularly.

2.2. Analytical methods and kinetic analysis

The MC, TS, VS, and alkalinity were measured according to Standard Methods [31]. HACH reagent kits (HACH, Loveland, CO, USA) were used to determine COD and TAN concentrations. The methanogenesis process kinetics were evaluated with the modified Gompertz kinetic model, as described in the literature [32]. The modified Gompertz kinetic model was used to estimate the best-fit values of the maximum methane production rate (R, L/d) and lag phase time (λ, d). The experimentally measured total cumulative methane production was used as the maximum methane production (Vm, L) for the estimation of kinetic parameters (R and λ) [32].

3. Results and discussion

3.1. Degradability of disposable face masks and their impact on HSAD

Fig. 2 shows the daily and cumulative methane production from the control and test digesters. An immediate methane production (Control: 5–14 ​L, T1: 4–14 ​L, T2: 4–9 ​L) was observed from all reactors during the first two days of operation (see Fig. 2a). As suggested in the literature, such high methane production at the beginning of start-up period could have resulted from the utilization of readily biodegradable organics available in the feedstock [28]. From day 3, the lag phase (daily CH4 production <1 ​L/d) was noticeable in all reactors, which continued until day 10, then the exponential phase began. It was apparent that the daily methane production for the test digesters (T1 and T2 loaded with face masks) was considerably lower than the control (without face mask) during most of the period of this exponential phase. This observation indicates that the presence of face masks in the feedstock adversely affected the methanogenesis kinetics.

Fig. 2.

Fig. 2

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(a) Daily, and (b) cumulative methane production from the control and test digesters.

The control digester showed a maximum daily methane production of 13 ​L on day 24, while both test digesters showed the maximum daily methane production on day 25 (T1: 10 ​L, T2: 13 ​L) (see Fig. 2a). The cumulative methane production from the control on day 24 was 116 ​L, which was considerably higher than the cumulative methane from test digesters (T1: 69 ​L, T2: 83 ​L) (see Fig. 2b). After reaching the maximum daily methane production, methane production from all digesters started to decline. However, this pattern is quite typical for the batch high-solids anaerobic digestion [27,28]. Unlike the exponential phase, the methane productions from the test digesters loaded with face masks were slightly higher than the control during this entire declining phase (see Fig. 2a). Nonetheless, after 40 days of batch operation, the total cumulative methane generation for the control was 9–18% higher than the test digesters (216.20 vs. 178.14 and 197.4 ​L) (see Fig. 2b).

It should be noted that full-scale HSAD digesters are typically operated for a retention time of 28 days [17,18], while we continued this experiment for 40 days as face masks loaded digesters were producing more methane during the declining phase. After 28 days, the difference in the cumulative methane production between the control and test digesters were more prominent (157 vs. 104 and 126 ​L). Thus, the full-scale facilities operated <30 days residence time may see a considerable decline in methane productivity due to the presence of face masks in OFMSW. Interestingly, the digester loaded with 20 face masks (T2) showed appreciably higher methane production than the digester loaded with 4 face masks (T1). Thus, the results suggest that the characteristics of face masks (e.g., polymer types) would be more critical than their total number/loading.

Table 1 summarizes the best-fit values of maximum methane production rates (R) and lag phases (λ) estimated using the modified Gompertz model. The estimated R value for the control was 12–29% higher than the test conditions. The estimated λ values for the test digesters were 7–14% higher than the control. The R value for T2 (20 face masks) was considerably higher than T1 (4 face masks), while λ values showed an opposite trend. Thus, the higher methane production rate in T2 ultimately led to higher total cumulative methane production.

Table 1.

The estimated methanogenesis kinetic parameters.

Conditions Maximum methane production, Vm (L)a Maximum methane production rate, R (L/d) Standard error for R Lag phase, λ (d) Standard error for λ
Control (No face mask) 214.83 11.44 0.70 13.05 0.62
T1 (4 face masks) 175.24 8.10 0.58 13.98 0.83
T1 (20 face masks) 194.34 10.12 0.57 14.89 0.58
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a

Experimental total cumulative methane production.

Previous reports suggested that degradation of disposable face masks can release microplastic fibers [21,22]. As identified as an emerging contaminant in sewage sludge, research on the fate and degradation of microplastics in anaerobic digestion has already received considerable attention [23,33,34]. These studies reported the inhibitory effects of microplastics on methanogenesis kinetics. The leaching of toxic compounds from microplastics has been identified as an inhibition mechanism. For instance, polyvinyl chloride can leach toxic bisphenol-A (BPA) during anaerobic digestion that can inhibit methanogens [24]. However, there have also been reports on the positive impact of microplastics on the anaerobic digestion process when their concentration was within certain limits. For instance, polyamide microplastics could enhance fermentation and methanogenesis kinetics in anaerobic digestion by leaching caprolactam, which could promote activities of key enzymes associated with these biochemical steps [35]. During biological/chemical degradation, polymeric materials used in face masks can release various toxic compounds, such as polybrominated biphenyl ether, phthalate, nonylphenol, triclosan, etc. [21]. Based on the previous reports, some of these compounds (e.g., triclosan) could inhibit the anaerobic digestion process [36]. In contrast, others (e.g., nonylphenol, phthalate, polybrominated biphenyl ether) can be degraded in anaerobic digesters depending on the operating conditions (e.g., temperature) [[37], [38], [39]]. These previous findings suggest that the potential effects and anaerobic degradability of microplastics or compounds leached from microplastics would depend on their type (e.g., chemical structure). Thus, these previous reports indirectly support our notion of the higher significance of polymer types used in face masks rather than their total loading in the digester. Different polymeric materials are used for disposable face masks [21,22], and various types of polymers can lead to a different impact on anaerobic digestion (discussed earlier). This could possibly explain our experimental observation (i.e., higher methane production from T2 than T1), which warrants further investigation.

In addition to inhibition by microplastics or leaching of toxic compounds, face masks can also create barriers to percolate distribution in lab-scale digester tanks. Despite a more prolonged lag phase, the digester loaded with 20 face masks (T2) provided higher methane production than the one loaded with 4 face masks (T1). Moreover, the digesters loaded with face masks did not show any signs of additional clogging with lower solid inoculum washout, as compared to the control. Hence, the possibility or effect of localized mass transfer limitations can be ruled out. We also envision that the clogging of full-scale digester by face masks would be quite unlikely or may not be realized depending on the design of the recirculation system.

Fig. 3 shows the photographs of digestate from the test digesters loaded with face masks. For both test digesters, face masks remained undegraded. It should be noted that such a visual inspection does not rule out the possibility of leaching any toxic compounds during digestion. Nonetheless, it was evident that 40 days of mesophilic anaerobic digestion was inadequate to provide complete disintegration/hydrolysis of face masks. To acquire insights on the environmental impacts that may result from face masks in digestate, we looked into the existing literature.

Fig. 3.

Fig. 3

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Photographs showing undegraded face masks in the digestate collected after 40 days of operation of test reactors: (a) T1, and (b) T2.

3.2. Possible environmental consequences

Depending on the waste management facilities, the management of digestate from anaerobic digestion may include landfilling or lime stabilization/composting followed by land application. Aerobic composting of digestate is used by many waste management facilities. During the composting process, the temperature of the compost pile can reach up to 70 ​°C [40,41]. Recently, thermophilic and hyperthermophile (>90 ​°C) composting processes showed effectiveness in degrading plastics [42,43]. However, some microplastics may have high thermal stability [44], and may not be degraded effectively without any thermochemical conversion processes like gasification, pyrolysis, etc. [20]. Lime stabilization of digestate is often practiced for removing pathogens before land application [43,45]. It has been reported that lime application to digestate can disintegrate plastics into microplastics [43,45]. The land application of digestate is considered one of the major routes for transmitting antimicrobial resistance genes (ARGs) to the environment [46], while microplastics can adsorb antibiotics and also serve as potential carriers for ARGs [47,48]. Thus, microplastics released from face masks in digestate may amplify the transmission of ARGs to our ecosystem via the land application. Also, sending the digestate to the landfill may not provide a suitable solution [22]. Microplastics released from landfilled plastic waste can also serve as potential carriers for ARGs and stimulate the propagation of ARGs to the landfill leachate [49]. Ultimately, it can provide a gateway for the transmission of ARGs from landfills to the groundwater [49]. Therefore, future research should examine the relationships between face masks in OFMSW on the possible transmission of ARGs to our ecosystem. Fig. 4 provides a conceptual representation of how face masks in digestate can increase transmission of ARGs to the environment via landfilling and land application.

Fig. 4.

Fig. 4

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A conceptual representation of how face masks in digestate can amplify transmission of ARGs to the environment through landfilling and land application of digestate.

Based on these, we cannot afford to overlook such significant environmental consequences of the unregulated disposal of used face masks or disposal of digestate having undegraded face masks. Thermochemical valorization of face masks through pyrolysis has already been proposed in the literature [20]. Thus, thermochemical conversion processes (e.g., pyrolysis, gasification) may provide an environmentally benign solution to divert digestate having face masks from the landfill or land application. However, to date, only a few waste management facilities in the world have full-scale MSW-based thermochemical processes [50]. Thus, more research is encouraged to address the environmental consequences of face masks in digestate. Although there has been significant progress towards COVID-19 vaccine developments, face masks will not go away immediately. Face masks will be remained as a part of our life for at least several more months. In addition to addressing immediate concerns regarding disposable face masks, this is an opportunity to develop innovative technologies for sustainable plastic waste management, an existing challenge for waste management industries.

4. Conclusions

Most of the municipalities in Canada and other countries recommended their citizen to keep used face masks out of recycling bins. Therefore, disposing of them in garbage bins with other non-recyclable wastes became a common practice. For municipalities without a residential source separated organics (SSO) program, face masks will potentially end up in OFMSW used for biological waste treatment like high-solids anaerobic digestion. Our results suggested that face masks in OFMSW could negatively impact high-solids anaerobic digestion, while more research is encouraged to understand the underlying mechanisms. The available literature indicates that undegraded face masks in digestate may pose significant threats to our ecosystem, like microplastic pollution and enhanced transmission of antibiotic resistance genes. Therefore, a better disposal and management plan (e.g., pre-processing of OFMSW and/or post-processing of digestate) for single-use face masks should be developed. Although our crucial priority these days is to safeguard public health and the economy, this is an opportunity to improve our waste management practices and build a resilient waste management infrastructure from the challenges and lessons learned amid the COVID-19 pandemic.

Declaration of competing interest

The authors declare that there is no conflict of interest regarding the publication of this research manuscript.

Acknowledgements

This study was supported by the Natural Sciences and Engineering Research Council of Canada (Discovery Grant). The authors sincerely acknowledge Jennifer Chiang (Edmonton Waste Management Centre, Edmonton, Alberta) and Hok Nam Joey Ting (Dr. Dhar’s Lab) for their kind assistance with the sampling of OFMSW and biosolids.

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