Combination between Composting and Vermicomposting of OFMSW: A Sicilian Case Study

Abstract

In nature, earthworms process enormous quantities of plant debris, ingesting and converting them into vermicast, the final product of their digestion process. Vermicast is widely recognized as an organic fertilizer suitable for plants, usually obtained from the transformation of animal manure. Indeed, controlled vermicomposting of the organic fraction from municipal solid waste (OFMSW) has yielded contradictory results, limiting the extension of laboratory results to larger-scale initiatives. This study aims to analyze a combined composting–vermicomposting process using Eisenia fetida (also known as Californian red worm) for the treatment of OFMSW (containing different impurities, such as heavy metals), mixed with a suitable substrate for earthworms, consisting of a blend of animal bedding and pre-selected food scraps (SOM—pre-selected organic material). Different fractions from a municipal composting plant were tested for the biological process: raw OFMSW, pre-composted (PC, organic matrix that has completed the thermophilic biodegradation phase), and compost (C). Laboratory tests involved five different applications with varying mixing proportions, partly aided by the addition of OM. The physical–chemical parameters (e.g., pH, temperature, moisture) and worm growth rate in the different treatments were measured and compared. The results showed that the process improved the quality of the final product, especially for the selected matrices, and revealed a significant reduction in the carbon-to-nitrogen ratio (lower than 14 in all tests) when vermicomposting was applied to the mixed matrices and SOM. Worms increased during the process in net weight and growth rate, even if there was an accumulation of heavy metals in the “worm tea”. Worm mass reached a little over double the initial value in all tests (from 200 g to more than 500 g in the SOM test), except for an increase of only 87% in the OFMSW test, while heavy metal content in the solid matrix was reduced in all tests compared to the starting content.

1. Introduction

The production of solid waste (SW) inevitably tends to increase with population growth and economic and social development [1,2,3]. Solid waste includes municipal (MSW), industrial, agricultural waste, and sewage sludge; there is a projected progressive increase in the production of such waste [4]. Unfortunately, conventional waste disposal methods (such as landfills and incinerators) have more significant environmental impacts compared to treatments based on biological processes. These biological processes, especially those focusing on material recovery strategies, are generally more efficient than thermal transformation processes. Specifically, treatment methods based on biotechnological processes offer many advantages, including high speed, ease of control, low costs, high environmental and health acceptability, and potential applicability at different scales [5].

Composting is the most widely used and studied biological process for managing the organic fraction of MSW [6]. On the other hand, vermicomposting is inspired by a similar biological (aerobic) process operated by the combined activity of worms and bacteria, rather than merely aerobic bacteria [7].

Composting is a process of aerobic biodegradation of organic waste carried out by microorganisms in which organic waste is converted into mineral compounds such as CO₂, H₂O, and NH₄, as well as organic compounds rich in humus (a complex of essential organic substances for plant nutrition that result from the decomposition; it is strongly adsorbent, and it has a blackish brown color). These humus-rich compounds are suitable for improving the physical properties of the soil [8,9]. On the other hand, vermicomposting is a biological process similar to the composting process, but it is not exclusively bacterial, and nutrients contained in organic waste are transformed into a stable, available, and nutrient-rich product for plant growth [10].

In vermicomposting, the oxidation and biological stabilization of organic waste depend on the interaction between worms and microorganisms in the mesophilic transformation process. Specifically, the activity of worms increases and enhances the organic matrix intended for microbial consumption [11,12,13]. The final vermicompost is a stabilized and homogeneous material with high nutrient content and low toxicity, offering several advantages, such as excellent cation exchange capacity and a higher nutrient retention capacity [14,15].

Worms are the animals that process the most substantial amounts of soil in nature [16]. They consume dead plant debris, animal excrement, and soil, thereby contributing to positive soil transformations and promoting the mineralization of dead plant matter [17]. However, despite the variety and abundance of worms in nature, the controlled use of worms for treating organic waste has so far been largely limited to the vermiculture of animal manure and, to a lesser extent, domestic food waste [18].

In the management of MSW, there are other more recalcitrant vegetable fractions such as leaf waste, weeds, crop residues, and toxic elements derived from imperfect upstream separation. For this reason, vermicomposting applied to OFMSW has shown many limitations and contradictory results, restricting its widespread application. Even if vermicomposting is a cleaner process than composting, as it requires much less energy due to lower temperatures during the process and does not require periodic turning of piles or forced aeration thanks to the activity of worms, there are doubts about long-term results and the real negative quality influences due to the presence of certain pollutants in municipal waste (even if selected) that currently confine the application of vermicomposting mainly to the transformation of waste from zootechnics, bovine manure, and selected food waste at a mono-residential level. In this context, for example, it cannot be ignored that the presence of potential toxic elements, such as heavy metals, in the waste can threaten soil quality and the worm’s growth. Nevertheless, the biochemical transformations of heavy metals during the vermicomposting process are still controversial, despite the current compost standard setting limits on total concentrations of some toxic metals. Several studies have reported increases in the total concentrations of heavy metals during vermicomposting [19,20,21], while other studies have obtained a completely opposite behavior [22]. Probably, the different observed development patterns are closely related to site-specific bioaccumulation mechanisms and the evolution of substance reduction in terms of biological adaptation [23,24]. Therefore, the activity of worms can influence the solubility and bioavailability of heavy metals, modifying the speciation of the vermicast [25,26]. Different authors have reported that earthworms secrete a mucus-like substance under metal stress that aids in reducing the bioavailability of metals and bioaccumulating them on the surface of their tissues [27,28].

In this scenario, the scientific interest in combining composting and vermicomposting processes is highly relevant. In this optimized transformation pathway, a wide range of organic waste, including those resistant to decomposition, can potentially be decomposed without compromising process efficiency. Thus, the combination of composting with the vermicomposting process could be suitable for the biodegradation of solid waste from municipal selective collection, regardless of its quality [8,29]. Indeed, the excessive use of chemical fertilizers has generated several environmental problems that can be solved by the use of biofertilizers, such as municipal solid waste composts and vermicomposts, which are natural, beneficial, and ecologically friendly [30].

There are important factors that can affect the process of vermicomposting, among which pre-composting is included. Pre-composting is usually preferred to eliminate anaerobic conditions and remove any volatile gases potentially toxic to the earthworms [31].

Considering the large volume of OFMSW produced and the need for proper management and transformation of these waste materials into reusable and environmentally friendly products, this study aims to evaluate the combined process of composting–vermicomposting for the management of OFMSW, potentially complementing the process with the addition of preselected organic matrices. Due to legislative difficulties in the introduction of OFMSW as a suitable substrate for the vermicomposting process, the study also aimed to show the advantages of the biological process carried out by not only microorganisms but also earthworms. These advantages are not referred to only in terms of the time required but also in terms of the quality of the final product. The experimentation was based on laboratory batch tests using a pilot vermicomposting system, in which the physical–chemical parameters, worm growth rate, and their growth characteristics (e.g., accumulation of metals in tissues, production of worm tea) were monitored and measured during the transformation processes of real organic matrices derived from a composting plant placed in the province of Catania (Italy).

2. Materials and Methods

2.1. Composting Plant

The composting plant from which the matrices subject to study have been sampled is located within the territory of the Province of Catania (Sicily), covering an area of approximately 25,500 square meters.

The receptive capacity of the facility is currently about 150,000 tons per year of OFMSW, with a processing line that includes the following:

(1)

Weighing and reception;

(2)

Unloading and storage of lignocellulosic waste;

(3)

Wood shredding and storage of shredded wood;

(4)

Unloading and storage of organic waste;

(5)

Waste mixing;

(6)

Accelerated and intermediate bio-oxidation;

(7)

Stabilization, maturation, and storage of raw compost;

(8)

Final screening;

(9)

Storage of finished compost.

Within the reception and preparation area, a preliminary operation of mixing different wastes with shredded wood is planned in order to improve the characteristics of the mixture in terms of composition and moisture for the development of biochemical processes and the quality of the final product. Following mixing, the material is transported into the bio-cells, where the composting process begins. The impermeable flooring is equipped with a leachate collection system. The aeration system can be activated in both the “supply” and “exhaust” modes; aeration ensures control of temperature and oxygen content. After the bio-oxidation phase, which generally lasts an average of 14 days, the material is transferred to the stabilization–maturation section, lasting at least 75 days. Once the maturation phase is completed, the product undergoes a final refinement operation, consisting of screening to remove coarse materials.

The samples subject to the experimental tests conducted at the Laboratory of Environmental Sanitary Engineering of the University of Enna involved the collection of three samples of approximately 30 kg each:

• OFMSW, collected before the mixing phase with bulking agents (shredded wood).
• PRE-COMPOST, collected after the accelerated bio-oxidation phase.
• COMPOST, stabilized fraction collected after the maturation phase before screening.

In order to facilitate a comparative experimental investigation with regular matrices, a sample of selected organic material from household waste was concurrently prepared. This sample represents the “blank” reference, free from impurities. In fact, it consisted of properly differentiated food scraps to prevent the possible presence of non-organic contaminants. Within that SOM was also included an amount of rabbit manure and vegetable waste used before the start of the tests to ensure worm acclimatization. The analytical characterization of each (initial) sample is presented in

Table 1. Initial characterization of organic substrates.

Parameters	UM	OFMSW	PC	C	SOM
Organic content (O.C.)	[g/kgdm]	323.52	284.87	268.47	321.76
Nitrogen content (N)	[g/kgdm]	10.97	12.89	13.84	7.54
Phosphorus content (P)	[g/kgdm]	0.525	0.809	0.881	0.389
O.C./N	--	29.5	22.1	19.4	42.7
N/P	--	20.9	15.9	15.7	19.4
pH	--	4.0	6.0	6.0	5.5
Moisture	%	85	85	85	95

.

2.2. Experimental Setup

The experimental study involved the use of “prefabricated” vermicomposters provided by the company CONITALO, a leader in the vermiculture sector since 1979 (https://www.conitalo.it/, accessed on 16 June 2024). Each composting unit consisted of a lower compartment for leachate accumulation and three additional compartments (of which only 2 were utilized for the purposes of the proposed study) for placing the composting matrices. The bio-treatment compartment (the upper than leachate accumulation compartment) essentially comprised a bedding system in which the worms could be introduced within a thin layer of organic material that would be consumed over time. The organic matrices to be tested were added to the upper platforms, which were accessible to the worms residing in the lower layers. It was important that the various layers of organic material in each compartment were not too thick (maximum size 4–5 cm) to allow the worms to colonize and degrade the entire depth.

Five tests were planned using five different composting units, studying the four matrices described in the preceding paragraph (OFMSW, PC, compost, and SOM). Additionally, a “mixed” test was conducted by combining compost and selected organic material (consisting of food scraps from household waste). This was performed to ensure additional organic support in case the matrix composted in the actual composting facility was excessively depleted of substrate.

Each vermicomposter was initiated with 3 kg of matrix/bedding for the start of the trial.

The layout of the vermicomposters and the experimental setup are illustrated in Figure 1, while

Table 2. Percentage composition of the different organic substrates in vermicomposters.

Vermicomposters Layout	TEST	OFMSW	PC	C	SOM
Upper compartment	C1	100%	-	-	-
	C2	-	100%	-	-
	C3	-	-	100%	-
	C4	-	-	50%	50%
	C5	-	-	-	100%
Lower compartment	C1	-	-	-	100%
	C2	-	-	-	100%
	C3	-	-	-	100%
	C4	-	-	-	100%
	C5	-	-	-	100%

provides the percentage compositions of the matrices arranged in each compartment for the 5 tests.

2.3. Analytical Methods

Measurements of humidity, pH, and temperature were carried out using insertion probes directly on the solid matrix. The nitrogen content was evaluated using a standardized method that involved an initial preparation phase, including the grinding of the organic matrices to be analyzed and a preliminary assessment of moisture (in an oven for 24 h at 105 °C). Starting from the initial moisture value, the necessary aliquot of the ground matrix was determined to be mixed with deionized water, ensuring the correct dry matter-to-water ratio (0.5 g of dry matter and 500 mL of water or 1 g DM with 1 L of water). The subsequent phase included initial mechanical agitation aimed at breaking up larger-sized granules and creating a homogeneous suspension, followed by gentle agitation of the samples (on a magnetic stirrer) for 30 min. At the end of the agitation phase, an aliquot of the slurry needed for total nitrogen analysis was collected for cuvette testing and then spectrophotometrically analyzed (using the chromotropic acid method).

The phosphorus content was evaluated using ICP (Inductively Coupled Plasma) spectrometry. Sample preparation involved initial grinding or pestle–mortar pulverization to render the solid matrix homogeneous and destructured. Subsequently, a volume of 6 mL of nitric acid and 3 mL of hydrochloric acid were added to 1 g of wet sample to prepare for sample digestion at 145 °C for 2 h. After filtration (0.45 microns), a 50 mL sample was subjected to spectrometric analysis.

Regarding the content of organic matter in the samples and vermicasts of each test, two measurements were conducted: organic carbon (O.C.) content and organic matter content using the Walkley-Black method [32]. This method involved the determination of O.C. by titration using initially potassium bichromate and sulphuric acid (to oxidize organic carbon) and subsequently iron sulfate heptahydrate as a titrating agent. O.C. is a measurable component of soil organic matter (O.M.). Specifically, O.C. (<O.M.) is the “available” measurable component of soil organic matter that constitutes a small portion of the soil but plays a fundamental role in its physical, chemical, and biological functions. The organic carbon content of soils varies with soil type, as well as changes in land use and, to an even greater extent, with different agricultural practices (e.g., organic fertilization in regions where livestock farming is prevalent). For this reason, C/N ratios have been referenced to the organic carbon content.

Lastly, heavy metal content was assessed using ICP spectrometry, which concerned both the analysis of solid matrices and worms. Sample preparation was the same as for the above-mentioned determination of phosphorus content.

2.4. Analysis of Earthworms’ Growth

The worms, in particular the Eisenia fetida species [33], were provided by the same company that supplied the vermicomposters. The provided worms were young and kept in the mother culture in 200 g packages. Initially, the worm biomass was taken and acclimated in mixed cultures of rabbit manure and selected organic material before being placed in the vermicomposters.

Specifically, before the vermicomposting experiment, rabbit manure and vegetable waste were mixed in a 2:1 (w/w) ratio for the first 10 days and a 1:1 (w/w) ratio in the following 10 days. To ensure better aeration and humidity control while simultaneously removing any toxic gases, daily manual mixing of the bedding was performed. The prepared matrix was placed in the lower compartment of each vermicomposter. The worms in the lower tray were free to move to the upper tray, where, on day 0 (t = 0 days), the matrix to be investigated was allocated, as described in the previous paragraph.

The initial inoculation was carried out with 200 g of pre-acclimated worms placed in the lower compartment of each vermicomposter. The measurement of worm growth was comprehensively assessed only at the end of the experimentation (t = 180 days) when watermelon scraps were placed inside the vermicomposters in a separate compartment below the lid and specifically in the upper compartment than the one in which the organic matrices subject to testing were initially placed. This was performed daily to separate the worms attracted to the specific type of organic matter (rich in water and sugars).

On the other hand, intermediate analyses at 15 and 90 days were conducted on 5 statistically significant samples taken from the center and the 4 corners of both trays. The worms present in the samples were weighed and reported as a proportion of the total mass. Specifically, the intermediate analyses involved the sampling of 5 homogenized aliquots (50 g) from each tray.

To proceed with the physical–chemical analysis (see Section 2.3), the samples were initially air-dried at room temperature, crushed, and sieved to 0.5 mm. The worms, manually removed beforehand, were weighed again after being washed with deionized water to remove adhering material. They were then reintroduced into their specific vermicomposter.

For the assessment of metal bioaccumulation, approximately 50 g of worms from each significant compartment (C1–C5) were randomly selected and kept in the dark for 24 h. This was necessary to allow the worms to release their intestinal content, which could affect accurate measurements, especially if they had ingested a substantial amount of organic material relative to their weight. Subsequently, after washing them with ultrapure water, freeze-drying was carried out for the analysis of heavy metals (only for day 180) [34].

3. Results

3.1. Changes in pH, Temperature, Moisture, and Compostable Volume

Figure 2 shows measurements of process control parameters as well as the volume changes for each vermicomposter (C1–C5). All the reactors were maintained at room temperature within a shaded and ventilated storage area. As a result, the temperature of the compostable mass followed normal seasonal fluctuations, stabilizing around 25 ± 3 °C after sixty days.

Humidity was monitored through continuous and appropriate “moisturizing” interventions. Specifically, the goal was to maintain humidity around 70 ± 10% so as not to hinder the action of the worms and the necessary aerobic conditions for their development.

Only for samples C4 and C5, it was necessary to introduce an appropriate amount of coconut fibers in the initial phase to reduce the excessive initial humidity (>90%), which is characteristic of selected organic material accumulations.

Regarding pH values, it was observed that the vermicompost in each test reached an almost neutral value, although starting from different values among them. The initial values were notably acidic for the compost and selected organic material (C1 and C5) and slightly acidic for all the others.

Possible contributions to pH variations could be attributed to the differential production of CO₂, NO₃–N, and organic acids during the process [35]. In general, the increase in pH of vermicompost, compared to other composting processes, is due to greater loss of organic matter and the higher accumulation of various mineral salts in their available forms (e.g., phosphate, ammonium, potassium) in vermicompost [35,36].

As the transformation process progressed, the mass of material in the vermicomposter gradually increased in density at the expense of a significant reduction in volume.

The most substantial volume reduction was observed in test C1 concerning the OFMSW (over 65%), while for tests C2 and C3, it remained at lower values (37–40%).

The last two tests, conducted with carefully selected organic matter, recorded smaller volume reductions and higher densities. It is important to note that in this case, after the first two weeks, the volume of the material to be transformed was even greater due to the addition of coconut fibers, with an amount of about 250 g uniformly distributed in each of the two vermireactors.

3.2. Changes in Available Organic Matter and C/N Ratio

Figure 3 shows the trend of the organic component, nutrients, and the O.C./N (N/P) ratio during the experimentation for the five vermicomposting tests. As highlighted earlier, a loss of organic matter was observed. In general, the most significant loss of organic mass (O.M. and O.C.) was observed for the initially more “biodegradable” matrices, in the order C5–C1 (~8.5%) and C4 (~6.5%). The matrices related to pre-compost and compost from the actual facility, having undergone prior bacterial transformation in the composting plant, showed a lower loss of organic components (maximum 1–3%). In a complementary manner but with more significant percentage variation, the increase in total nitrogen (N) was notably recorded in the vermicompost obtained from tests C5, C4, and C1 (198.6%, 138.5%, and 128.5%, respectively) compared to tests C2 and C3 (106.3% and 70.8%, respectively).

A similar trend was observed for phosphorus (P), although due to its low initial concentration, the % increase seems more pronounced (concentration values were even 8–9 times higher than the initial ones). It is worth noting that while the increase in phosphorus follows a linear trend, nitrogen significantly increased in the second half of the experimental period once the stationary phase had stabilized.

Regarding the development of vermicast, it is evident that the reduction in organic matter is correlated with the reduction in the dry mass of the organic waste discussed in the previous paragraph. This is the result of the varying yields of decomposition and mineralization of the starting matrices and the transformations into vermicompost with different and specific concentrations of enriched nitrogen [22].

Furthermore, the carbon-to-nitrogen ratio (O.C./N ratio), where the organic component refers to the bioavailable portion related to the organic carbon, is one of the most common indicators used to estimate compost maturity and suitability for field applications [20]. Vermicomposting reduced the O.C./N ratio by 69.1% for C5, 60.8% for C4, 59.9% for C1, 52.1% for C3, and 43.5% for C2.

These values are generally higher than those observed in the literature for biological transformation through composting [34]. This confirms the simultaneous and opposite trends of O.C. and nitrogen. In absolute terms, the O.C./N ratio was found to be below 14 in all tests, with lower values observed for matrices from FORSU and the composting plant (≤11). Slightly higher values were observed for matrices containing selected organic matter, still below 12 for C4 and ≤13.5 for C5.

In this context, it is worth noting that Suthar (2010) [20] had previously reported that a C/N ratio < 20 indicates acceptable maturity in finished compost, but a ratio of 15 is preferred for agricultural applications. In general, an O.C./N ratio below 12 indicated that the vermicompost possessed favorable properties for potential field applications.

3.3. Biomass Growth

Figure 4 illustrates the growth trend of the worm mass in the vermicomposters. At the end of the experiment, the total worm mass increased in all tests. As expected, the most significant increase was achieved for C5, where the matrix obtained from selected organic material was investigated. In this case, the worm mass nearly tripled.

For the intermediate matrices, C2, C3, and C4, the increase in worm mass reached a little over double the initial value. In the case of C1, however, the worm mass increased by only 87% (less than double).

Certainly, the growth results are directly linked to the availability of nutrients and organic matter, but above all to the initial growth environment. In this regard, intermediate measurements taken after 2 weeks highlighted a significant biomass loss due to the unsuitable conditions of the starting matrix.

Specifically, the worms “cultivated” in the lower compartment (optimal bedding), identical for all tests, found themselves transforming a very different matrix in the upper compartment. Some matrices, moreover, were very “complex” and unsuitable for the ordinary growth scenario; consequently, the majority of the worms “died”.

Mortality was notable for C1 and C2 (65% and 60%) and significant for C3 (50%). On the contrary, no mortality was recorded in C5. In C4, where compost and selected organic material were mixed, mortality was contained to less than 50%.

The cause of this initial “stabilization” was likely due to the particularly acidic and putrescible environment of the matrix (more pronounced in C1, as shown in Figure 3c), which was profoundly different from the degradable matrix obtained from the mixture of organic and bedding litter manure (lower compartment).

It is important to highlight that the surviving biomass rapidly adapted, generating new worm biomass adapted to the harsh growth conditions, as shown by the increasing values on the 90th day. The final result allowed defining an active matrix composed of both adult worms and worms “in the development phase” (which in some tests reached up to 50% of the total) that still contributed to the transformation of the vermicast, albeit with growth outcomes lower than those obtained in other experiments based on matrices different from municipal organic waste.

Finally, it is possible that the growth and the need for “initial adaptation” were influenced by the bioaccumulation of heavy metals in their tissues, as confirmed by the studies [23,34] or the specific physiology of the metal [37,38].

In particular, as discussed in the subsequent paragraph, earthworms can potentially accumulate a certain amount of toxic substances, such as metals, in their tissues and inevitably introduce these toxic substances into terrestrial food chains.

3.4. Heavy Metal Bioavailability

Figure 5 presents the content of four metals of interest (Cu, Zn, Ni, and Cd) in the vermicast and leachate from the five tests carried out. The selection of the investigated metals was based on the characterization of the initial matrices, which, due to their origin (urban solid waste), contained significant concentrations of some heavy metals compared to others. This circumstance was partly attributed to the significant impurities in the starting matrix (batteries, plastics and microplastics, cans, etc.).

It is important to highlight that the concentration of heavy metals in the initial matrices was higher in the untreated raw matrix (OFMSW, C1) compared to the processed matrices (pre-compost, C2, and compost, C3). Lower content was recorded in matrices containing pure selected material (C5) or combined material (C4). However, in all tests and for all the analyzed metals, the initial values were reduced during the vermicomposting process. Following are some noteworthy observations from Figure 5. In neither the starting nor final matrices did values exceed the limits set by Legislative Decree 75/2010.

First and foremost, it should be highlighted that the bioaccumulation of metals, evaluated also in the tissue of the worms, was found to be significantly lower than that observed by other researchers [34].

Probably, this can be attributed to the fact that the worms required a significant adaptation period to the working conditions, with quite notable initial mortality rates. Furthermore, the net growth observed at the end of the six-month observation period was lower compared to that observed in the transformation of animal manure. For this reason, a portion of the heavy metals was “concentrated” in the vermicast and eluate liquid (partially produced by the biomass) rather than in the worms themselves.

Cd has shown the most significant reduction in concentration in the final vermicast, exceeding 50% for C1 and C2, nearly 70% for C3, and surpassing 70% for C4. The reference matrix for C5, pure selected organic material, exhibited a reduction of approximately 90% in Cd. With the exception of Cd, all other heavy metals exhibited a general reduction trend mainly concentrated in the second period of the test (between days 90 and 180). As anticipated, the concentrations of heavy metals in the vermicast obtained from pure organic material (C5) were negligible.

The observations made in the solid matrix were confirmed by the concentrations of metals obtained in the liquid collected from each composting unit. Figure 6 shows the volumes and metal concentrations in the liquid samples of each test.

It is observed that the volume of “leachate” produced by the “domestic” organic matrices was significantly higher than that collected from tests conducted with raw matrices (OFMSW in C1) and semi-processed matrices (pre-compost and compost, C2 and C3).

This is likely due to the different consistency/density of the analyzed matrices and the developed vermicast (Figure 7); in particular, this aspect influenced the starting value of FC (field capacity) and, therefore, percolation. On the other hand, the values of different initial moisture, discussed earlier, support the volumetric differences in collection.

In any case, as reported in Figure 6b, the metal concentrations are significantly high in C1, C2, and C3; they are much lower in C4 and negligible in C5. This result suggests that the potential use of “worm tea” as a fertilizer obtained from OFMSW matrices (raw or processed) must be seriously evaluated/paid attention to if the purity of the matrices is compromised by suboptimal waste separation.

On the other hand, for the same reason, in the studied case, the liquid collected in the vermicomposting trays has been referred to as “leachate” since the dominant formation process primarily involves elution processes rather than biological formation due to the earthworms’ metabolism.

Conversely, the liquid collected from tests conducted with selected organic material from household environments, mixed (C4) or pure (C5), was found to be in line with the research conducted in the literature [39,40,41,42].

Finally, it should be emphasized that in this experimental work, bioaccumulation in the earthworm tissues was found to be secondary to the leaching processes described above. In general, it was observed (Figure 8) that the most bioaccumulable metal was Cd, followed by Zn. Conversely, Cu and Ni were predominantly found in the leachate rather than in the earthworm tissues.

Overall, the tests confirmed what was observed by [34], wherein earthworms contribute to reducing heavy metal concentrations through the bioaccumulation of varying quantities in their tissues, especially for Cu and Zn.

3.5. Future Aspects: Phyto-Growth Tests in Controlled Environment

The total concentrations of heavy metals are important for assessing potential environmental risks, especially the fraction still available in vermicast and potentially extractable, which represents a potential phytotoxic aspect for plants [43].

As previously observed, compared to the initial compost and pre-composted material, the concentrations of extractable metals in the final product have decreased significantly. In general, the proportions of available fractions to the total are almost coincident with the trends in the concentrations of metals contained in the collected liquid phase. The residual fraction can be considered “unavailable” as it is concentrated in the earthworm’s tissue as a result of its own activity. Other studies have also found different metal availabilities, which, however, always decrease during the vermicomposting and maturation period [15,26,35,44].

The decrease in metal content in vermicompost occurs for at least two reasons. Firstly, vermicompost processed by earthworms generally has higher levels of humic acid fractions, which promote a stronger adsorption effect on metal–humus formation, leading to the creation of stable complexes, especially for Cu and Zn. Secondly, in line with the bioaccumulation in the tissues of earthworms, the earthworm’s epithelial layer and body fluid may absorb metals during their passage through organic waste [23,44].

In conclusion, in order to assess the potential risk of transferring residual heavy metals from vermicast to plants, a series of “germination” tests should be designed to study this specific aspect. This topic is not included among the objectives of this paper; however, an experimental setup has already been prepared that will aim to test the vermicast obtained from vermicomposters C1-C5 in the future based on the observation of growth rates and the bioaccumulation of specific metals on the roots, leaves, or fruit of different species. The species analyzed will be differentiated by their ability to accumulate metals at the root level (e.g., castor bean) or on the fruit/leaves (e.g., tobacco).

4. Discussion

The study confirmed that it is possible to obtain vermicompost from different organic fractions of municipal waste, whether raw or pre-treated. In general, achievable vermicompost could potentially have better nutritive properties compared to compost and initial blends, thanks to enhanced mineralization and humification. The initial adaptation phase of worms to relatively harsh growth conditions, which can result in their mortality, is certainly crucial in this context.

On the other hand, the transformed matrices must be appropriately characterized on a case-by-case basis for their metal content. While the highest concentrations of heavy metals were still below the threshold values permitted for compost, this is not “assumed” for the leachate/worm tea obtained.

In particular, the availability of heavy metals in vermicompost is lower compared to the starting matrices and the compost obtained from a facility. The worms facilitate their transformation into an unavailable fraction through the bioaccumulation and stabilization associated with humus.

Therefore, vermicomposting technology could enhance nutrient quality compared to traditional compost (making the end product potentially a more effective soil amendment) and has the potential to mitigate the environmental risk of heavy metals from the disposal of solid organic waste. In any case, future research should assess the long-term impacts of field application.