Validation of the Circular Composter Prototype in the Production of Compost Obtained from Domestic Organic Solid Waste (DOSW) for Use in Food Production

1. Introduction

The generation of DOSW constitutes one of the main environmental challenges associated with population growth, urbanization, and rising consumption patterns worldwide. The organic fraction is estimated to represent between 40% and 60% of the municipal solid waste generated in numerous countries, so its inadequate disposal in landfills and open-air dumps promotes greenhouse gas emissions, leachate production, the proliferation of pathogenic organisms, and the loss of nutrients and carbon that could otherwise be reincorporated into productive systems [1,2]. In response to this issue, the principles of the circular economy promote the transformation of DOSW into value-added resources through technologies that close carbon and nutrient cycles, simultaneously reducing pressure on natural resources, mitigating greenhouse gas emissions, and converting these materials into viable inputs for agriculture [3,4,5].

Among biological valorization strategies, aerobic composting has become established as one of the most efficient, economically viable, and environmentally sustainable technologies for treating organic waste. During this process, the metabolic activity of bacteria, fungi, and actinomycetes transforms biodegradable organic matter into a stable, hygienized material rich in humic substances that improves the physical, chemical, and biological properties of the soil [6,7]. In addition to substantially reducing waste volume, composting promotes nutrient recycling, increases water retention capacity, improves soil structure, and stimulates microbiological activity, contributing to the development of more sustainable agricultural systems [1]. However, composting efficiency depends on the interaction of multiple factors, such as the initial substrate composition, carbon/nitrogen (C/N) ratio, moisture, pH, temperature, oxygen availability, and the composition of microbial communities, which determine the rate of organic matter degradation, the formation of humic substances, and the quality of the resulting compost [7,8].

Recently, interest has grown in composting systems for the utilization of organic waste generated in households, educational institutions, and small communities. These systems reduce costs associated with waste transportation, decrease the burden on landfills, and promote local nutrient recycling. Nevertheless, obtaining mature, high-quality compost remains a challenge, since food and household waste tends to have high moisture content, low porosity, and rapid compaction, which restrict oxygen diffusion, promote anaerobic zones, and reduce the activity of the aerobic microorganisms responsible for organic matter degradation [1,6]. In light of this, lignocellulosic structuring materials and the use of microbial inoculants represent promising strategies for accelerating biodegradation without compromising the stability, safety, or agronomic quality of the final product [4,8].

Among the available structuring materials, rice husk represents a particularly attractive alternative due to its high availability as an agricultural by-product, low cost, fibrous structure, and high silica content, characteristics that increase substrate porosity and improve conditions for the development of aerobic microbial communities. [9] showed that incorporating rice husk during the composting of sewage sludge significantly decreased moisture content, reduced carbon and nitrogen losses, and yielded compost with a more favorable C/N ratio and a higher degree of maturity. Similarly, [10] observed that different structuring agents, including rice husk, favored the production of compost classified as high quality according to fertility and cleanliness indices, in addition to reducing phytotoxicity and keeping heavy metal concentrations within the limits established by international regulations.

[11] demonstrated that the joint valorization of food waste and rice husk through co-pyrolysis produced a biochar with high organic carbon content, capable of significantly enhancing crop growth, improving nutrient use efficiency, and reducing heavy metal uptake by plants. These results show that rice husk constitutes a strategic resource for transforming organic waste into high-value-added products and highlight the potential of agro-industrial residues as key components in technologies oriented toward the circular economy and sustainable agriculture. Although this study used a thermochemical process rather than composting, its findings demonstrate the functional value of rice husk as an input for the production of organic amendments.

Additionally, inoculation with beneficial microorganisms constitutes a viable alternative for accelerating biodegradation processes. Recent studies highlight that the use of specialized microbial consortia can increase metabolic efficiency during the decomposition of organic waste, improve organic matter stabilization, promote the degradation of complex compounds, and contribute to the control of undesirable microorganisms, thereby increasing the quality of the resulting compost [4,12]. Among these microorganisms, lactic acid bacteria have attracted growing interest due to their ability to produce antimicrobial metabolites, modify the physicochemical conditions of the substrate, and favor microbial communities associated with efficient fermentation and organic matter degradation processes. Although their potential has been widely recognized in biotechnological processes, information on their effect during the composting of DOSW remains limited, particularly in combination with structuring materials.

At the same time, the design of the composting system represents a fundamental component for ensuring adequate oxygenation and homogenization of the material. Rotating horizontal composters offer operational advantages by facilitating periodic turning, improving moisture and oxygen distribution, and reducing compaction of the mixture, conditions that favor microbial activity and process stability. Despite these advantages, the scientific evidence available on this type of system, particularly when combined with lactic acid bacteria and structuring materials for the treatment of DOSW, as well as their combined effect on the physicochemical and microbiological quality of the compost and its agronomic performance as an organic amendment, remains limited.

In this regard, it is pertinent to evaluate technological alternatives that optimize the composting process in low-cost systems, simultaneously contributing to sustainable waste management and the production of high-quality organic fertilizers.

Therefore, the objective of this research was to evaluate the effect of substrate composition and inoculation with lactic acid bacteria on the dynamics of the composting process, the physicochemical and microbiological quality of the compost, and its effect on lettuce growth, using a horizontal composting system for DOSW. It was hypothesized that incorporating rice husk as a structuring material and lactic acid bacteria as a microbial inoculant improves the composting dynamics, increases the physicochemical and microbiological quality of the compost obtained from DOSW, and favors its agronomic performance as an organic amendment.

2. Materials and Methods

2.1. Study Area

The research was conducted at the Faculty of Agricultural Sciences of the University of Cauca, Popayán, Colombia, during 2025. The study was carried out under the environmental conditions typical of the Colombian Andean region, using an experimental horizontal composting system designed for the utilization of DOSW.

2.2. Material Collection and Preparation

The DOSW used as raw material came from the cafeteria of the Faculty of Agricultural Sciences of the University of Cauca. Prior to the start of the experiment, the personnel responsible for handling the waste received training to ensure proper source separation and to prevent the incorporation of inorganic materials or other waste that could affect the composting process. The waste was then collected, sorted, and used as the base substrate for preparing the experimental mixtures.

Rice husk acquired from an agricultural supply store in the city of Popayán was used as the structuring material. This material was incorporated to improve the porosity of the mixture, promote aeration, and help adjust the C/N ratio during the composting process.

The microbial inoculant consisted of lactic acid bacteria obtained through the fermentation of rice water, wheat bran, raw milk, and molasses, following the protocol described by [13]. The resulting product was kept refrigerated until its application during the experimental process.

2.3. Experimental Design

A completely randomized design was used with four treatments and three replicates, for a total of twelve experimental units, represented by independent horizontal composters.

The treatments evaluated were:

T0: 100% DOSW (control).

T1: 50% DOSW + 50% rice husk + lactic acid bacteria.

T2: 75% DOSW + 25% rice husk + lactic acid bacteria.

T3: 100% DOSW + lactic acid bacteria.

The proportions were established to evaluate the combined effect of the structuring material and microbial inoculation on the composting process and the final compost quality.

2.4. Composting Process Management

The mixtures corresponding to each treatment were placed in previously conditioned horizontal composters. Throughout the experimental process, each experimental unit was turned daily to maintain adequate aeration conditions and homogenize the decomposing material.

Treatments that included inoculation received daily applications of lactic acid bacteria by spraying, ensuring uniform distribution over the surface of the material and adjusting the moisture content beforehand when necessary.

2.5. Variables Evaluated During Composting

Process monitoring was carried out through evaluations performed twice a week from the establishment of the experiment until mature compost was obtained.

The variables monitored were: temperature, moisture, hydrogen potential (pH), color, and odor.

• Temperature: was determined using a geothermometer inserted into the middle zone of the mixture.
• Moisture: was evaluated using the empirical hand-squeeze test method.
• pH: was determined using litmus paper indicator strips.
• Color and odor: were evaluated using a three-category ordinal scale, in which dark brown color and an earthy odor were considered indicators of compost maturity.

Composting lasted 103 days, during which the variables were monitored to evaluate the progression of the DOSW degradation process under the different experimental treatments.

2.6. Physicochemical and Microbiological Characterization of the Compost

At the end of the composting process, a representative sample of each treatment was obtained for characterization at the Multilab Agroanalítica laboratory S.A.S.

Physicochemical properties were determined following the routine analytical procedures used for the evaluation of organic fertilizers.

Microbiological quality was assessed according to the Colombian Technical Standard (NTC) 5167, through the quantification of fungi and yeasts, facultative mesophilic microorganisms, and enterobacteria, as well as the qualitative determination of Salmonella spp., in order to verify the microbiological safety of the resulting compost.

2.7. Soil Microbiological Analysis

To determine the populations of functional microbial groups, soil samples were sieved through a 2 mm mesh, and soil moisture content was determined. After correcting for dry weight, serial dilutions were performed on each soil sample up to dilutions of 10⁶ and 10⁷. From these last two dilutions, duplicate plating was carried out on Petri dishes with Nutrient Agar (NA) for bacterial counts, Potato Dextrose Agar (PDA) for mold and yeast counts, Ashby’s Agar (ASHBY) for counts of biological nitrogen fixers, SRS Agar for counts of phosphate solubilizers, and a culture medium for actinomycete counts.

2.8. Agronomic Evaluation

The agronomic potential of the compost obtained was evaluated through a bioassay in lettuce (Lactuca sativa L.), comparing the crop’s response across the different composting treatments. The growth variables established in the experimental design were recorded to determine the effect of the compost on plant development.

2.9. Statistical Analysis

The data obtained were organized into a database for statistical processing. The assumptions of normality and homogeneity of variance were first verified. An analysis of variance (ANOVA) was then performed to identify differences between treatments, and when significant effects were detected (p < 0.05), means were compared using Tukey’s test at a 5% probability level.

3. Results

3.1. Composting Process Dynamics

Temperature was one of the main indicators of microbiological activity during the composting process. Statistical analysis showed significant differences between treatments (P < 0.05) on evaluation days 1, 40, and 99. On the first day, the control treatment (T0) recorded the highest temperature (38 °C), followed by T1 (35 °C), T2 (34 °C), and T3 (22 °C). Thereafter, thermal differences between treatments progressively decreased, without reaching temperatures characteristic of a thermophilic phase (>45 °C) [14].

The absence of a well-defined thermophilic phase suggests that the system’s conditions limited the development of highly thermophilic microbial populations. Treatments that incorporated rice husk (T1 and T2) exhibited lower initial temperatures, a behavior attributed to the lower moisture availability caused by this material’s low water retention capacity. This condition would have reduced the metabolic activity of the microorganisms responsible for organic matter degradation, partially delaying heat release during the early stages of the process.

From the perspective of the biological process, these results indicate that the initial substrate composition played a determining role in the thermal dynamics of composting [15]. Although rice husk improved the physical structure of the mixture and promoted aeration, its high proportion in T1 likely reduced the moisture available for microbial activity. In contrast, the control treatment showed a faster thermal increase due to the greater availability of easily biodegradable waste, although this condition also increased the risk of compaction and the formation of zones with low oxygenation.

Moisture behavior was consistent with the composition of the mixtures. According to the monitoring conducted during the trial, treatments with a higher proportion of rice husk showed lower moisture levels during much of the process, while treatments composed solely of DOSW retained greater water availability. This response demonstrates the dual role of rice husk as a structuring agent: on one hand, it increases porosity and facilitates gas exchange; on the other, it can reduce moisture retention when incorporated in high proportions, slowing the degradation of organic matter.

pH monitoring showed variations during the composting process and significant differences between treatments, evaluated through analysis of variance and comparison of means. The evolution of pH indicates the occurrence of the biochemical processes characteristic of composting, in which the initial formation of organic acids during the degradation of easily fermentable compounds is followed by a gradual increase associated with protein mineralization and the release of nitrogenous compounds [16]. This behavior is consistent with the microbial succession observed in aerobic composting systems and constitutes an indicator of material stabilization.

Qualitative indicators also allowed evaluation of the process’s progress. Regarding odor, treatments T1 and T2 began with a neutral odor, experienced a short period of unpleasant odor, and ended again with a neutral odor. In contrast, T0 exhibited an unpleasant odor from the outset, while T3 maintained a neutral odor until day seven and subsequently developed unpleasant odors until approximately day 40. The persistence of odors in treatments without rice husk suggests temporary conditions of poor aeration, favoring anaerobic processes and the formation of volatile compounds such as acetic acid, hydrogen sulfide, and methane.

Color behavior reinforced this interpretation. Treatments T0, T2, and T3 rapidly reached the dark brown color category, characteristic of materials with a higher degree of humification, while T1 showed a slower transition and ended with a light brown color. This behavior is attributed to the high proportion of rice husk (50%), a material that degrades more slowly and has lower moisture content, which partially delayed the organic matter transformation process.

Taken together, the results show that composting dynamics were determined by the balance between aeration and moisture availability. The incorporation of rice husk improved the physical structure of the mixture and reduced the persistence of unpleasant odors, but at high proportions it partially limited microbial activity by reducing available moisture. In contrast, treatments with a higher proportion of organic waste showed a more intense initial thermal response, though with a greater likelihood of developing localized anaerobic conditions. These findings indicate that optimizing horizontal composting of DOSW depends on an appropriate ratio between structuring material and biodegradable fraction, allowing favorable aeration, moisture, and microbial activity conditions to be maintained simultaneously throughout the process.

3.2. Physicochemical Quality of the Compost

Physicochemical characterization allowed the quality of the products obtained in the four treatments to be evaluated, along with their compliance with the requirements established by the NTC 5167 for organic fertilizers and soil conditioners. Overall, the composts obtained met several of the parameters required by the standard, particularly regarding oxidizable organic carbon content, density, and moisture retention capacity. However, limitations were identified related to moisture content, phosphorus, and, in one treatment, pH value.

Moisture content exceeded the maximum limit established by NTC 5167 (35%) in all treatments. The recorded values were 37.43% (T0), 58.04% (T1), 49.82% (T2), and 63.85% (T3), with the latter showing the highest residual moisture. These results indicate that, although the composting process allowed for the stabilization of the organic matter, the final product still retained a significant amount of water, likely due to the high initial moisture content of the domestic organic waste and the environmental conditions during the process. From an agronomic standpoint, high moisture does not necessarily reduce the nutritional quality of the compost; however, it can reduce its stability during storage, increase transportation costs, and promote the development of opportunistic microorganisms if the product remains stored for extended periods.

Compost pH ranged from 7.53 to 9.37. Treatments T0 (8.96), T1 (7.53), and T2 (8.08) remained within the range accepted by NTC 5167 (>4 and <9), while T3 showed a pH of 9.37, slightly above the established limit. This behavior is consistent with the dynamics observed during the composting process, where treatments made exclusively with DOSW tended to develop higher pH values due to the mineralization of nitrogenous compounds and the release of ammonium during the final stages of degradation. The incorporation of rice husk in T1 and T2 likely helped maintain a better carbon/nitrogen balance, buffering the pH increase and favoring a final product closer to neutrality.

Electrical conductivity showed wide variation between treatments, with values of 15.75 mS/cm (T0), 3.75 mS/cm (T1), 6.42 mS/cm (T2), and 19.69 mS/cm (T3). This behavior reflects differences in the concentration of soluble salts generated during organic matter mineralization. The high value observed in T3 suggests greater release of mineral nutrients during the decomposition process, while the incorporation of rice husk in T1 and T2 likely produced a dilution effect on the salts present in the mixture. Although NTC 5167 does not establish a limit for this parameter, high conductivity values can restrict the direct use of compost in crops sensitive to salinity, so its application should be adjusted according to soil and crop characteristics.

Moisture retention was above 100% in all treatments, with values ranging from 100.70% to 120.40%, demonstrating a high capacity of the compost to store water. This property represents an important agronomic advantage, as it favors water availability for plants and helps improve soil physical properties, especially in agricultural systems established on soils with low water retention capacity.

Oxidizable organic carbon content exceeded the minimum required by NTC 5167 (15%) in all treatments, reaching values between 30.10% and 34.50%. These results indicate an adequate degree of organic matter stabilization and confirm the compost’s potential as a carbon source for the soil. The incorporation of stable organic matter promotes aggregate formation, increases cation exchange capacity, and improves soil biological activity, all fundamental aspects for maintaining fertility in sustainable agricultural systems.

Bulk density ranged from 0.29 to 0.43 g cm⁻³, values below the maximum allowed by NTC 5167 (0.60 g cm⁻³). This behavior reflects the production of a light, highly porous material, a characteristic attributable both to organic matter degradation and to the incorporation of rice husk as a structuring material. Low density facilitates handling, application, and mixing with soil, in addition to promoting aeration of the root zone after incorporation.

Regarding macronutrients, total nitrogen content showed significant differences between treatments. T0 and T3 recorded concentrations of 3.93% and 2.98%, respectively, above the minimum value established by NTC 5167 (1%), while T1 showed 0.95% and T2 showed 1.22%. The nitrogen reduction observed in treatments with a higher proportion of rice husk can be attributed to the increased C/N ratio caused by this lignocellulosic material, promoting temporary nitrogen immobilization during the decomposition process. In contrast, treatments composed mainly of DOSW retained greater availability of this nutrient.

Phosphorus content was below the minimum value established by NTC 5167 (1%) in all four treatments, with concentrations between 0.12% and 0.79%. Potassium, on the other hand, showed a differential response, reaching values above the minimum required only in T2 (1.87%) and T3 (9.13%), while T0 (0.81%) and T1 (0.81%) remained below the regulatory limit. These results indicate that the nutritional quality of the compost was determined by the initial composition of the mixtures. In particular, the high potassium content observed in T3 suggests a higher concentration of waste rich in this element and greater efficiency in its conservation during the composting process.

Micronutrients also showed differences between treatments. T0 recorded the highest concentrations of iron (13,895.08 mg kg⁻¹), zinc (377.12 mg kg⁻¹), and copper (19.81 mg kg⁻¹), while T1 and T2 showed considerably lower concentrations. These differences reflect the heterogeneous composition of the DOSW used as raw material and the influence of the structuring material on the final concentration of mineral elements (Table 1).

Overall, the physicochemical characterization demonstrates that the compost obtained through the horizontal composting system had properties compatible with its use as an organic amendment, although with some limitations regarding moisture content, phosphorus, and pH in certain treatments. Among the materials evaluated, T2 showed the best balance between physicochemical stability and compliance with quality criteria, suggesting that this proportion simultaneously optimizes organic matter degradation, nutrient conservation, and the physical properties of the compost. This combination therefore constitutes the alternative with the greatest potential for the valorization of DOSW through horizontal composting systems.

3.3. Microbiological Quality of the Compost

The microbiological quality of the compost is a fundamental criterion for determining its suitability as an organic amendment, as it allows simultaneous evaluation of the biological activity of the composting process and the safety of the final product. According to NTC 5167, products intended for agricultural use must report counts of aerobic mesophilic microorganisms and fungi and yeasts when they contain beneficial microorganisms, in addition to ensuring the absence of Salmonella spp. and maintaining enterobacteria concentrations below 1 × 10³ CFU g⁻¹ of final product.

Microbiological analyses revealed differences in microbial load between treatments, particularly in the counts of facultative mesophilic microorganisms and fungi and yeasts. Treatment T3 showed the largest population of mesophilic microorganisms (>30 × 10⁵ CFU g⁻¹), followed by T2, T1, and T0. In the case of fungi and yeasts, treatments inoculated with efficient microorganisms also recorded higher counts than the control treatment (Table 2).

These results indicate that inoculation with lactic acid bacteria favored the establishment of a more abundant microbial community in the final compost. This behavior can be attributed to the incorporation of efficient microorganisms, which include lactic acid bacteria, yeasts, actinomycetes, and fermenting fungi that act synergistically during organic matter decomposition. The higher microbial load observed in the inoculated treatments suggests that the inoculant not only participated in the initial stages of composting but also contributed to maintaining an active microbial community until the final product was obtained.

The predominance of mesophilic microorganisms in T2 and T3 can also be related to the conditions reached during the composting process. In this study, the maximum recorded temperature was approximately 39 °C, a value that remained within the optimal range for the development of mesophilic microorganisms (20–45 °C). Although the microbial populations obtained in the present study were smaller, the results show that the conditions of the horizontal composting system were sufficient to maintain the biological activity needed for the stabilization of the organic material.

The greater abundance of fungi and yeasts in the inoculated treatments constitutes another indicator of the compost’s biological activity. These microorganisms play an important role during the advanced stages of the process, owing to their ability to degrade structural polysaccharides, lignin, and other complex organic compounds that remain after the rapid mineralization of easily biodegradable materials. Consequently, a larger population of these microbial groups can be interpreted as evidence of a functionally diverse community capable of continuing the humification processes until compost maturation.

From a sanitary standpoint, the results were equally favorable. None of the treatments showed enterobacteria growth (0 CFU g⁻¹), a value considerably below the maximum limit allowed by NTC 5167 (<1 × 10³ CFU g⁻¹). Likewise, the absence of Salmonella spp. in 25 g of final product was confirmed in all four samples. These results demonstrate that the compost obtained met the microbiological requirements established by Colombian regulations for its use as an organic fertilizer.

The absence of enterobacteria and Salmonella is particularly important considering that DOSW can contain potentially pathogenic microorganisms associated with food scraps [17]. In this study, the elimination of these sanitary indicators suggests that the conditions reached during composting, together with the competition exerted by the microbiota developed during the process, were sufficient to reduce the microbiological risks of the final product. Although the system did not reach temperatures characteristic of an intense thermophilic phase, the results demonstrate that the process produced a microbiologically safe compost in accordance with the criteria of NTC 5167.

Overall, the microbiological characterization confirms that inoculation with lactic acid bacteria increased the load of beneficial microorganisms without compromising the sanitary quality of the compost. The inoculated treatments, particularly T2 and T3, showed greater microbial activity, while all treatments met the safety requirements demanded for organic products intended for agricultural use. These results indicate that the horizontal composting system evaluated made it possible to obtain a microbiologically stable and safe compost, strengthening its potential as an alternative for the valorization of DOSW.

3.4. Agronomic Response of Compost in Lettuce (Lactuca sativa L.) Cultivation

The agronomic evaluation showed that the composts obtained from DOSW promoted lettuce growth and development; however, the magnitude of the response varied among treatments. Overall, treatment T0 showed the highest values for leaf number, plant height, fresh weight, and yield, while treatments prepared with lactic acid bacteria inoculation showed no increases relative to uninoculated compost.

The average number of leaves ranged from 8.5 to 11 leaves per plant, while plant height varied between 10.5 and 12.3 cm, with the highest values in both cases recorded in treatment T0. These results indicate that the compost provided adequate conditions for the crop’s vegetative growth, likely due to the gradual nutrient supply and improvement of the soil’s physical and biological properties. Several studies have shown that compost application increases the availability of nitrogen, phosphorus, and potassium, improves soil structure, increases water retention capacity, and stimulates rhizosphere microbial activity, promoting foliage development and growth in horticultural crops [1,18,19].

Fresh shoot weight showed greater variation among treatments, with values ranging from 16.5 to 40.0 g per plant, with the highest biomass obtained in treatment T0. This behavior suggests that compost prepared without inoculation promoted greater nutrient availability throughout the crop cycle, favoring biomass accumulation. The progressive release of nutrients during organic matter mineralization is one of the main advantages of organic fertilizers, as it allows for a more stable supply of essential elements and improves plant uptake efficiency [1,20]. In addition, compost incorporation increases soil organic carbon content and stimulates the activity of beneficial microorganisms, processes closely related to greater productivity in leafy crops [19].

Crop yield showed a trend similar to that of the growth variables, reaching a maximum of 4.4 kg m⁻² in treatment T0 and a minimum of 1.7 kg m⁻² in T2. These results demonstrate that compost produced from DOSW has significant agronomic potential as an organic amendment. This is confirmed by authors such as [2] and [5] who reported that the application of stabilized compost improves the yield of horticultural crops by increasing soil fertility, promoting nutrient availability, and improving water use efficiency, while also contributing to the maintenance of soil quality in sustainable agricultural systems.

Although the physicochemical and microbiological characterization showed differences among the composts evaluated, these differences did not translate into a superior agronomic response when lactic acid bacteria were incorporated during the composting process. It is likely that inoculation mainly favored the dynamics of organic matter degradation and stabilization without significantly altering the final nutrient availability for the crop. Similar studies indicate that the effectiveness of microbial inoculants depends on factors such as substrate composition, microorganism survival during the thermophilic phase, the process’s environmental conditions, and interactions with native microbiota, so their effects on agricultural yield are not always consistent [4,8].

Taken together, the results demonstrate that compost prepared from DOSW constitutes a viable alternative for producing organic amendments intended for lettuce cultivation. However, under the experimental conditions evaluated, the incorporation of lactic acid bacteria did not produce agronomic improvements relative to compost prepared without inoculation. These findings suggest that compost performance was determined mainly by its degree of stabilization and the nutrient availability achieved at the end of the composting process, rather than by the microbial inoculation applied during its preparation.

3.5. Effect of Compost on Soil Microbiological and Chemical Properties

The application of the different composts produced changes in the soil’s microbiological and chemical properties, demonstrating that the quality of the material obtained during the composting process influenced not only lettuce growth but also the soil conditions after cultivation. Microbiological analyses showed that compost incorporation increased the abundance of microorganisms present in the soil relative to the initial conditions. In particular, treatments prepared with lactic acid bacteria showed larger populations of beneficial microorganisms, suggesting that compost application favored the establishment of functional microbial communities capable of participating in organic waste decomposition, nutrient cycling, and the formation of stable organic matter.

The greater microbiological activity observed in the inoculated treatments can be explained by the incorporation of microorganisms from the compost itself, as well as by the input of readily usable organic carbon, which constitutes the main energy source for soil microbial communities. This behavior is consistent with the high load of mesophilic microorganisms and fungi recorded during the microbiological characterization of the compost, indicating that part of this microbiota remained viable after soil application and helped strengthen its biological activity (Table 3).

Chemically, the analyses showed increases in organic matter content and in the availability of some macronutrients relative to the soil’s initial conditions. These changes reflect the compost’s ability to reincorporate carbon and nutrients into the soil system, promoting processes related to fertility and the maintenance of agricultural productivity (Table 4).

The increase in organic matter content represents one of the main benefits derived from compost application, as it improves the soil’s structural stability, increases cation exchange capacity, promotes water retention, and constitutes a reservoir of gradually released nutrients. These properties are particularly important in agricultural systems under intensive management, where the decline in organic carbon is one of the main factors associated with soil degradation.

Similarly, compost incorporation helped increase the availability of nitrogen, phosphorus, and potassium in the treatments that showed higher physicochemical quality during the characterization of the final product. This response demonstrates the close relationship between compost’s chemical composition and its ability to supply nutrients to the soil. However, the differences observed among treatments indicate that the initial composition of the mixtures used during composting influenced the amount of nutrients ultimately incorporated into the soil system.

Among the treatments evaluated, T2 showed the most balanced performance, both in terms of the compost’s physicochemical properties and the agronomic response obtained in the crop and the improvements observed in the soil. The combination of DOSW, rice husk, and lactic acid bacteria favored the production of a material with adequate stability, high microbiological activity, and a nutritional composition capable of simultaneously improving the soil’s biological and chemical properties. This result confirms that the use of structuring materials in moderate proportions optimizes the composting process without compromising nutrient availability in the final product.

4. Discussion

The working hypothesis of this study was that incorporating rice husk as a structuring material and lactic acid bacteria as a microbial inoculant would jointly improve composting dynamics and the physicochemical, microbiological, and agronomic quality of compost obtained from DOSW. The results support this hypothesis only partially, revealing that the two interventions operated through distinct and, in some respects, uncoupled mechanisms.

The absence of a thermophilic phase across all treatments departs from the classical composting paradigm, in which intense microbial self-heating is regarded as a prerequisite for both pathogen elimination and material stabilization [6,7]. This outcome is best interpreted as a structural limitation inherent to small-batch, horizontal systems, where the surface-to-volume ratio favors heat dissipation over heat accumulation, rather than as a failure of microbial activity per se [15]. The more relevant interpretation, however, concerns what this implies for how small-scale composting technologies should be evaluated: the fact that microbiological safety criteria were met despite the lack of a thermophilic peak suggests that sanitization in these systems may rely more on microbial competition and antimicrobial activity than on temperature-driven pathogen inactivation. This reframes thermophilicity as one possible pathway to safe compost rather than an indispensable one, a distinction with direct relevance for the design and validation of decentralized, low-infrastructure composting systems, where reaching thermophilic temperatures is often impractical.

The clear structuring benefit of rice husk is consistent with previous work showing that this by-product improves porosity, reduces moisture excess, and enhances compost maturity when combined with high-moisture organic residues [9,10], and with evidence that rice husk functions as a valuable agro-industrial input for organic amendment production more broadly [11]. Interpreted jointly with the nitrogen limitation observed at the highest inclusion level, these findings point to a threshold effect rather than a linear one: structuring material improves physical and sanitary conditions up to a point beyond which its wider C/N ratio begins to constrain nutrient availability through temporary immobilization. This nuance is often absent from studies that treat structuring materials as uniformly beneficial, and it suggests that optimizing horizontal composting of food-derived waste requires identifying an inclusion range rather than simply confirming a positive effect.

The dissociation between microbial inoculation and agronomic outcome is arguably the most conceptually important finding of this study. Those results are consistent with previous cautions that the effectiveness of microbial inoculants in composting is highly context-dependent, shaped by inoculant survival, substrate composition, and competition with native microbiota, such that increased microbial activity does not automatically translate into agronomic benefit [4,8,12]. Interpreted within the broader compost literature, this suggests that inoculation with lactic acid bacteria may act primarily on process-level dynamics, accelerating organic matter turnover and shaping microbial succession, rather than on the pool of nutrients ultimately available to plants, which instead appears to be governed by the proportion of readily biodegradable DOSW in the initial mixture. This distinction has practical implications: producers seeking a fast, stable, low-odor process may benefit from inoculation, whereas those prioritizing nutrient-rich compost for crop production may find inoculation unnecessary. It also nuances the general expectation, based on compost-yield relationships reported elsewhere [1,2,5,18,19,20], that microbial enhancement strategies necessarily improve crop-level performance.

Considered together, physicochemical stability, microbiological safety, and soil-improving capacity did not converge on the same treatment as the highest lettuce yield, which itself is a noteworthy interpretive point: agronomic yield, while informative, is an incomplete proxy for compost quality when evaluated in isolation. This reinforces the value of multi-criteria evaluation frameworks—as opposed to yield-centric assessments alone—for identifying compost formulations that are both agronomically useful and technically sound for broader soil-health objectives.

These findings collectively broaden the interpretive framework for horizontal composting of DOSW in two respects. First, they indicate that thermal performance and microbiological safety can be decoupled in small-scale systems, challenging the assumption that thermophilic conditions are strictly necessary for producing safe compost. Second, they show that substrate structuring and microbial inoculation address different limiting factors in the composting process — physical/structural versus microbial/kinetic — and should therefore be evaluated and optimized independently rather than assumed to act synergistically by default. Future research should investigate design modifications (e.g., insulation, batch size, turning frequency) capable of promoting sustained thermophilic conditions in horizontal systems, systematically test intermediate rice husk proportions to identify the inclusion threshold that balances porosity against nutrient dilution, and explore alternative or combined microbial consortia whose effects on nutrient availability, rather than only on microbial counts, are directly assessed. Complementary studies on gaseous emissions, nutrient release kinetics, and economic feasibility would further contextualize these systems within circular economy and sustainable waste management frameworks.

5. Conclusions

Horizontal composting of domestic organic solid waste proved to be a technically viable, low-cost strategy for producing a stable, homogeneous, and microbiologically safe organic amendment, even in the absence of a defined thermophilic phase. The incorporation of rice husk as a structuring material improved porosity, reduced excess moisture and compaction, and mitigated odor persistence, confirming its usefulness for processing high-moisture organic waste, although excessive inclusion reduced nitrogen availability and delayed maturation. Inoculation with lactic acid bacteria increased microbial abundance in the final compost but did not enhance its nutrient content or improve lettuce growth relative to uninoculated compost, indicating that microbiological activity and agronomic performance are not necessarily correlated in this type of system. Among all treatments, T2 (75% DOSW + 25% rice husk + lactic acid bacteria) showed the best overall balance between physicochemical stability, microbiological quality, and soil-improving potential, positioning it as the most promising formulation for practical application. These results support the viability of horizontal composting combined with agro-industrial structuring materials as an accessible, safe tool for decentralized organic waste management and nutrient recycling in small-scale agricultural and urban contexts.