Waste mismanagement represents a global issue [1
], and solid waste management (SWM) is still an environmental, economic, and social issue in the 21st century [3
]. Zero waste is a utopia, and solutions should be found in the short term in both developed and developing countries [4
]. Global responses must be identified to reduce greenhouse gas emissions due to SWM [5
] since, in 2018, 2010 million tons of municipal solid waste (MSW) were generated all around the world, estimating that these quantities might increase to 3400 million tons by 2050 [6
]. Out of 17 Sustainable Development Goals (SDGs) of the 2030 Agenda for Sustainable Development, at least 12 SDGs have a direct link to SWM [7
]. Therefore, SWM must be a priority for policy-makers, scholars, engineers, and international associations [8
After collection, MSW organic waste should be valorized, while recyclable materials recovered [9
]. However, the remaining waste (composed of unrecyclable residues), generally and virtually consisting of about 20–25 wt% of the total, would go to final disposal sites (i.e., sanitary landfills) [10
]. These mixed and unrecoverable waste amounts could be used for energy recovery, although it is not still a widespread methodology in the developing world [11
]. Therefore, in developing regions, the aim is to reduce these garbage fractions or recover them by other waste-treatment technologies [12
], better if appropriated for the local context. A waste fraction abundant in mixed and unrecyclable portions, and usually not valorized, is disposable baby diapers. In developing countries, they are typically disposed of in sanitary landfills or in open dumpsites [13
], although these residues have potentialities to be recovered [15
Disposable diapers contain synthetic polymers (i.e., superabsorbent polymer) and natural ones. The main component is cellulose pulp (on average 35 wt% of the diaper) [16
]. In order to be recovered, used baby diapers (UBDs) must be collected separately at the source and transported to dedicated facilities that allow selecting plastics and the biodegradable fractions [17
]. Therefore, separate collection and appropriate treatment technologies can support the increase of the recycling rate and the reduction of waste disposed of into sanitary landfills. In developed countries, these practices can be implemented, while developing countries suffer a lack of technologies and reliable solutions for their application [18
In 2016, Bolivia generated approximately 2 million tons of MSW per year, the equivalent of 5400 tons per day, with 70% exclusively coming from the 10 megacities of the country [20
], which also happens in other Latin America’s countries like Colombia [21
], Ecuador [22
], and Mexico [23
]. In addition, the demand for disposable baby diapers has increased over the years, showing an increase in sales volumes from 17.18 million in 2012 to 26.80 million in 2016. In Bolivia, most of the used diapers are disposed of in open dumps, which finally fall into water bodies [24
The objective of the research presented in this article was to suggest a solution appropriate for the context and able to recycle disposable UBDs. The implementation of innovative recycling options can support the reduction of uncontrolled disposal of mixed waste and the introduction of a new economy based on waste-recycling options. Therefore, the aim of the paper is to present the findings of the research conducted in Bolivia based on the objective already mentioned and discuss possible future applications that can be employed in the poor areas of the global South.
Other studies have been conducted at the international level in order to find solutions to the UBD disposal. Some scholars suggested using the superabsorbent polymer recovered from waste baby diapers for increasing water retention in soil and improving irrigation management [25
]. Other researchers explored the applicability of shredded waste diapers as an innovative viscosity-modifying admixture for cement grouts and concrete [26
]. More conventional studies analyzed the biological treatment of UBDs after selective collection. For example, a full-scale composting of door-to-door collected organic waste with a 3 wt% of compostable diapers was conducted, finding good results in terms of compost production [16
]. Other studies reported that edible mushrooms mixed with UBDs and gardening waste can be implemented as an attractive treatment alternative for urban organic waste [28
]. Similarly, other authors conducted research for assessing the feasibility of composting UBDs along with grass and dry and fresh leaves, providing positive results [29
]. Finally, other solutions described in the scientific literature might be the microwave pyrolysis, which has a great prospect in transforming UBDs into liquid oil and char products [30
]; or the use of recycled pulp and superabsorbent polymer as materials for new paper diapers in order to introduce closed-loop recycling options [31
According to these studies, there are potentialities for transforming disposable UBD waste stream into value-added products. However, a number of factors influence the recycling viability, including cost-effective sorting and separation, public perceptions, and sustainable recycle market outlets [32
]. A recent review reported that recycling, biodegradation, and pyrolysis are promising, providing a future research direction to enhance the efficiency of these processes in UBD recycling [33
]. Nevertheless, it can be underlined that UBD vermicomposting has been cited only once, and it has not been reported within scientific studies or published within the scientific literature. In addition, no details have been provided regarding developing countries and rural areas: scientific literature lacks contribution to this topic.
The hypothesis behind this research was that the use of Californian red earthworms and the mix with cow dung can be employed for treating the biodegradable fraction of disposable UBDs. Other studies were carried out with the same methodology for the treatment of kitchen waste [34
] and non-recyclable paper waste [35
]. However, in the scientific literature, there are no similar studies implemented for the treatment of UBDs with the method already mentioned. Therefore, this paper provided the first results in this framework, underlining the importance of its application in low-to-middle-income areas. It represents progress in the scientific literature, providing new knowledge and new approaches that can support the application of sustainable options toward the circular economy.
For these reasons, the possible application for disposable used diapers composting introduced in this research involved the use of Californian red earthworms with the aim to improve the biodegradability of the waste flow mentioned (i.e., UBDs and cow dung). Other studies suggested that earthworms (Eisenia fetida
) can rapidly convert organic fractions into compost, reducing the pathogens to safe levels, and also ingest the heavy metals by mixing waste with other components (i.e., cow dung) [36
]. Eisenia fetida
is available in Bolivian local markets, and it has been used by other authors in order to achieve better performances if compared with other earthworm breeds [38
]. Therefore, in the current study, the treatment of disposable diapers was implemented, evaluating the degradation of the biomass as a function of the co-composting of cow dung, the adjunct of earthworms, and the addition of activated bacteria (AB). The use of AB was evaluated since it has been demonstrated that they facilitate the proliferation of indigenous microorganisms, thus accelerating the composting process [40
]. The aim was to evaluate the efficiency of the composting process in the best available conditions. Experimental trials were conducted for demonstrating that vermicomposting of disposable UBDs can be a viable recycling option after selective collection and manual pre-treatment in developing regions.
This research follows a previous study conducted in the same Bolivian rural area for treating animal manure with Californian red earthworms. The objective of the previous research was the evaluation of the availability of the treatment components within the context, the viability of its use, and the reliability of the process [41
]. Therefore, the research presented in this paper represents the second step toward the application of the vermicomposting process for the treatment of waste organic fractions. The results of UBD composting assessment can be of interest to local and international experts for finding a solution for the final disposal of used diapers and to international scholars involved in improving SWM in low-middle income countries.
2. Materials and Methods
2.1. Study Area
The research was carried out in the community of Carmen Pampa, Municipality of Coroico (Nor Yungas Province, Department of La Paz, Bolivia). The study area is located at an altitude of 1850 m a.s.l. at 16°15′31″ South latitude and 67°41′35″ West longitude (Figure 1
). This area has precipitation above 2000 mm per year, reporting an average relative humidity of about 71%. The annual temperature ranges between 17 and 24 °C, with an average temperature of 18.2 °C.
Currently, the city of Coroico has no MSW management plan. Therefore, information about the amount of MSW generated was not available. MSW is simply collected and disposed of in open dumps. According to the 2012 census, about 1813 children with less than 4 years of age live in the city, with a formal population of about 19,397 inhabitants. It has been estimated that each child from 0 to 30 months, on average, uses 5 diapers in 24h. It can be estimated that about 50% of the children reported to be from 0 to 4 years old are less than 30 months old. Therefore, about 4533 used diapers can be potentially generated per day. Considering that each UBD weights about 500 g, it can be estimated that the city of Coroico potentially produces about 2.3 tons of disposable UBDs per day, equal to about 827 tons per year. These waste amounts are dumped in the environment.
2.2. Experimental Trials
The aim of the research was to provide a possible solution to the open-dumping of UBDs. To assess the feasibility of the composting process, the research was divided into 5 experimental trials according to the combinations of AB, cow dung, and Californian red earthworms. The list of the combinations used for the investigation, conducted fourfold, are reported in Table 1
The experimental units are identified by the letter Ti, where i represents the identification number. The third treatment (T3) refers to the treatment with all the components, while T2 is the only trial without the application of earthworms. Overall, the analysis was conducted in 5 months, considering the collection of the diapers, its preparation, treatment, and the final assessment of the products.
The UBDs were collected from the center of Coroico and Carmen Pampa. About 20 families were involved. In total, 500 diapers were collected in one week (about 5 diapers per family per day), from Monday to Friday. This amount was used for all experimental trials, which was equivalent to approximately 250 kg of the sample (0.5 kg per UBD).
2.3. Preparation of the Sample
2.3.1. Diaper Selection
The scheme of the analysis conducted is represented in Figure 2
. The samples were prepared starting with the UBDs. After collection, the diapers were opened, and the plastic part was separated from the organic fraction (fecal residues and cellulosic polymers). The organic fraction will be here defined as “substrate.” During the preparation of the UBDs, safety precautions and biosecurity measures were adopted in order to reduce the risk exposure of the volunteers who supported the research. Rubber gloves, masks, caps, and chinstraps were used to protect the hands, head, mouth, and nose. While the plastic waste was removed from the diapers, the organic fractions were collected and segregated in specific containers. The plastic bands were mixed with sand and placed in PET bottles to obtain bricks used for the stabilization of the slopes of the garden outside the research area. It represents the only way available locally for plastic recycling, although other solutions should be found in the future. The time required for the separation of the substrate and the plastic materials takes approximately 1 to 2 min for each UBD.
2.3.2. Mixing and Preparation of the Trials
For the preparation of the trials, each sample was mixed according to the combination reported in Table 1
. For T2
, and T5
, 50 wt% of the substrate was mixed with the same quantity of cow manure (about 15 kg). Therefore, the amounts of UBDs and cow dung corresponded to the proportion of 1:1. At the same time, for T1
, only about 30 kg of the substrate was used, representing 100% of the sample. Then, the AB were added to the respective trials (T1
The cow dung was collected from local farms, provided by farmers of the near community of Carmen Pampa. The cow dung was used for the trials after 30 days from its generation. Similarly, before treatment, the substrate was left for 5 days in open-air conditions, protected by the light and the environmental agents. During this time period, the samples were prepared, and the treatment time required for the trials was reduced, while the experimental condition was maintained in order to assess the statistical significance of the variables considered. In addition, the precomposting period of one week has been suggested by other authors to be ideal for an effective vermicomposting [42
The AB were made up of phototrophic bacteria, yeast, lactic acid-producing bacteria, and fermentation fungi. These were purchased in the local market. To activate the microorganisms, 2.5 L of molasses were added to 2.5 L of AB. The container was hermetically sealed for a period of 15 days. After activation, the AB were mixed with the samples. T1, T2, and T3 incorporated globally 5 L of the prepared solution of the AB, while the T4 and T5 were mixed with tap water (5 L). About 1 L of solution or tap water was added to every 5–6 kg of the sample.
2.3.3. Location of the Samples
After mixing, the samples were all located in 20 wooden boxes built specifically for the research. The wooden boxes had dimensions of 30 × 20 × 15 cm (length × width × depth). About 2.5 kg of the sample was added to each wooden box. The boxes were located in a covered and controlled environment, maintaining a constant 80%-moisture content, and guaranteeing an average temperature above 21 °C. The environmental temperature was measured every morning and night, while the moisture content was measured by a hygrometer. Irrigation was carried out daily by means of a manual watering can. Approximately 0.2L per experimental unit was added, which was equal to 80 mL of water per kg of biomass per day.
Finally, in T1
, and T5
, 50 Californian red earthworms were placed per box. Afterward, the boxes were covered with cardboard to maintain humidity, reduce light, and avoid an attack from external vectors. Therefore, about 20 earthworms were added per kg of biomass. Figure 3
reports the photos related to the steps of the preparation of the experimental trials. The trials were evaluated for a maximum period of 5 weeks, depending on the time required for the process. For every sample, the treatment ended when a final odorless, black, and homogenous matter was obtained.
2.4. Variables Considered
Four parameters were evaluated to compare the processes:
The acidity was measured with a pH-meter before and after the treatment. The initial and final pH were recorded, and the variation was calculated. The acidity was measured for three different parts of the sample. The average was registered and considered as the pH of reference of the entire replicate of the experimental trial.
The time required to decompose each experimental unit was chosen in terms of the treatment time (days, 24 h) required for generating compost. The compost produced should be homogeneous in size, odor, and form. Therefore, the changes were evaluated by visualization. For evaluating the earthworm reproduction, the number of earthworms before and after the treatment was counted for the sample that contained earthworms and that managed to produce compost. The number of reproduced earthworms was calculated considering the total sum of the young earthworms (2–3 cm long) and adults (7–10 cm in length). The amount of compost produced was weighed at the end of the process. The variation between the starting point and the endpoint was measured for counting the mass loss after treatment.
Finally, a chemical analysis of the compost produced was implemented to compare it with other biological and chemical fertilizers. In particular, the analysis was conducted by an external consultant at the Bolivian Institute of Science and Technology, who is specialized in chemical analysis. Nitrogen, phosphorous, potassium, calcium, and magnesium compounds were evaluated in terms of percentage in weight. The results were compared with other data available in the literature related to chemical concentrations.
2.5. Statistical Analysis
The results are presented in terms of average plus the standard deviation. The analysis of variance (ANOVA) method was used for assessing the statistical significance of the mass loss, earthworms’ growth rate, pH, and treatment time among the trials and pre- and post-treatment conditions. The significance level was considered acceptable for an error equal to 5% (p-value < 0.05).
The research presented in this paper provides the first attempt to find a solution for implementing UBD recycling in developing countries. Though some limitations are detectable due to the scarce resource availability in the area, preliminary results were obtained in order to provide some understanding of the application of the vermicomposting treatment for reducing environmental impact and providing solutions to MSW open dumping.
The research demonstrated that vermicomposting can be implemented for treating disposable UBDs mixed with cow dung within 60 days of treatment time. The major outcome of the research is that the substrate without cow dung did not manage to decompose the substrate since, after 15 days of treatment, it was submerged by algae and fungi. Cow dung combined with Californian red earthworms allowed obtaining good results in terms of compost production, showing an average increase in the number of earthworms of about 56.5 ± 7.0 without AB. The composting treatment time was 32.3 ± 0.9 days with cow dung and Californian red earthworms, with about 55–60 days globally required for biomass decomposition in the environmental conditions described.
These findings can have potential application for developing regions with poor technical and financial resources in order to reduce the open dumping of UBDs. The treatment of disposable UBDs required selective collection systems in place, pre-treatment solutions, and treatment options viable for developing regions. These requirements are challenging in rural areas. The current research demonstrated that the biodegradable fraction of waste UBDs can be recovered producing compost with chemical characteristics optimal for its application as a natural fertilizer. Future perspectives are related to more investigations for the application of sustainable mechanical pre-selection, environmental assessment, and costs analysis in order to provide more details on the potentialities of the UBD co-composting process. Although future investigations are still required, this paper represents the first attempt to provide data related to this technique within the scientific literature.
At a global level, SWM is challenging. Appropriate technologies and approaches are required in developing countries since the growth rate of the population and the lack of economic resources represent a reason for concern. The research conducted in Carmen Pampa is an example of the requirement to also investigate possible innovative recycling solutions in poor areas with the support of international alliances and the application of appropriate technologies. In conclusion, the results presented in this paper allow supporting the achievement of the targets provided by the SDGs and introduced by the United Nations. The study can be considered a novel contribution to boosting the circular economy and the global call for action toward sustainable development.