Eco-Efficiency of Rural Biodigesters: Mono- and Co-Digestion of Agricultural Waste

Abstract

The increasing generation of agricultural waste poses both environmental and economic challenges, particularly in rural areas with limited infrastructure. Anaerobic digestion has emerged as a sustainable alternative, enabling the valorization of waste and the production of biogas and biofertilizer. This study evaluated the economic and environmental gains of mono- and co-digestion of equine manure and vegetable waste using biodigesters of different capacities across four simulated projects—Project 1 (15 m² biodigester with monodigestion), Project 2 (15 m² biodigester with co-digestion), Project 3 (20 m² biodigester with monodigestion), and Project 4 (20 m² biodigester with co-digestion). Economic feasibility was assessed through indicators such as Net Present Value (NPV), Internal Rate of Return (IRR), Modified IRR (MIRR), Profitability Index (PI), Benefit-Cost Ratio (B/C), Discounted Payback Period, sensitivity analysis, and Monte Carlo simulation, adopting a Minimum Attractiveness Rate (MAR) of 6.43% per year. Environmental benefits were estimated based on the annual reduction of CO₂ equivalent emissions. The results showed that all projects were economically viable and had the potential to mitigate up to 36 tons of CO₂eq per year. Additionally, an eco-efficiency indicator (NPV per CO₂eq avoided) was calculated to enable an integrated assessment of economic performance and environmental impact. Projects using 20 m³ biodigesters achieved the best results, with Project 3 being the most eco-efficient (USD256.05/tCO₂eq), while Project 4 yielded the highest absolute return in all economic analysis tools: NPV (USD 9063.81), IRR (25.10%), MIRR (10.95%), PI (USD 1.65), B/C (USD 1.65) and DPP (4.56 years). The integrated analysis underscores the significance of co-digestion and economies of scale in encouraging the adoption of this technology by small rural producers.

1. Introduction

In recent years, agricultural production has increased significantly on a global scale [1], primarily driven by the need to meet the growing demand for food due to population growth and changes in dietary habits [2]. It is estimated that approximately 80% of agricultural activities are carried out by smallholders who play a crucial role in ensuring food security and promoting regional socioeconomic development. In this context, family farming, which combines productive, social, and commercial dimensions within the same area, remains a key element in the global agricultural structure [3].

Although small farms generally have fewer animals and smaller cultivated areas, waste generation in these units is a concern [4]. This issue is mainly associated with the lack of adequate collection systems in rural areas, insufficient technical knowledge regarding the management of organic waste—both animal and plant-based [5]—and limited governmental support [6]. This scenario poses a continuous challenge to sustainability and environmental preservation [2,7], considering that these materials possess high energetic potential and only a fraction of rural waste is properly treated [8].

In small farms, the supply of electricity has been one of the main challenges faced by farmers. Due to their remote location from urban centers, the substantial investments and complex infrastructure required often discourage power companies from providing electricity. Technological alternatives for energy generation can help minimize this problem, making it possible to deliver electricity to isolated areas at acceptable costs [9].

In this context, anaerobic biodigesters stand out primarily as an efficient technology for waste treatment, ensuring proper management and valorization of these residues. Various materials can undergo anaerobic digestion (AD), which can be carried out with a single type of waste, referred to as mono-digestion, or with a combination of two or more wastes, known as co-digestion [10].

Anaerobic co-digestion stands out as an eco-efficient strategy, as it enables the simultaneous treatment of different organic residues in a single process, thereby minimizing pollution while maximizing resource recovery [11]. For small farms, this approach is particularly advantageous, as it provides an effective solution for waste management, increases methane yields, and reduces dependence on external energy sources, since the biogas produced can be utilized for electricity generation, heating, and cooking fuel [12]. Additionally, it supplies biofertilizer that enhances soil fertility while reducing the costs and environmental impacts associated with chemical fertilizers [13,14]. By integrating waste treatment, renewable energy generation, and nutrient recycling, co-digestion offers a practical and cost-effective pathway toward eco-efficient agriculture in rural areas. Moreover, combining waste brings additional process benefits, such as optimizing digestion performance, balancing nutrient availability, enhancing the conversion of organic matter, and stabilizing the system, which ultimately results in higher biogas production and improved overall efficiency [15].

Co-digestion is especially recommended for animal manures, given their heterogeneous nature in form, chemical composition, and degradability [16]. The literature suggests a variety of materials that can be co-digested with animal manure as alternatives for energy production and sustainable waste management. For instance, rice straw is recommended for bovine manure digestion [17], corn residues for swine manure [18], and corn stalks for chicken litter [19].

Within this context, the potential of co-digesting materials such as equine manure and vegetable waste stands out. These residues are common in small rural properties combining horticulture and limited livestock activities, often restricted to two or three horses. Despite the low individual generation scale, these wastes have high energy potential and can be efficiently treated by AD. This demonstrates that even small-scale production units with limited area and resources can adopt biodigesters as a practical and sustainable solution for waste management, promoting environmental, energetic, and economic gains. Thus, this process can be extremely advantageous, since the use of biogas, whether for electricity generation or as cooking fuel, can significantly reduce household costs, making the activity more beneficial overall [20].

Several studies have evaluated the efficiency of anaerobic co-digestion of animal manure and plant residues. Ref. [21] investigated the co-digestion of horse manure with the addition of straw and grass, and corn silage. The authors reported satisfactory results using this mixture, achieving a methane production rate of 43 m³ CH₄·h⁻¹. Ref. [22] investigated methane production through solid-state anaerobic co-digestion of cattle manure with wet grass and observed that the inclusion of up to 45% grass increased methane yield. In an experiment reported in [23], the authors evaluated the effects of including forages at two maturation stages (medium and old) along with swine manure on methane production. The authors tested different inclusion levels of the forages (0, 25, 50, 75, and 100%). They concluded that forage at the medium maturation stage, included at a 25% level, achieved the highest methane yield: 25.2 m³. Ref. [24] carried out an economic analysis of the co-digestion of dairy cattle manure with sweet potato. The authors reported that the addition of sweet potato was highly favorable for electricity generation, particularly at 50% inclusion, which demonstrated a generation potential of 2376.44 kWh/day. However, despite this extensive literature, few studies have focused on the economic feasibility and eco-efficiency of anaerobic co-digestion, particularly in the context of small-scale farms. This remains a critical knowledge gap, as smallholders represent most agricultural producers in many regions and face specific challenges in implementing bioenergy solutions.

Currently, the eco-efficiency analysis has been pointed out as an important approach to evaluate sustainable projects, such as the one evaluated in this research. Therefore, this study aimed to evaluate the economic and environmental benefits of biogas and biofertilizer production through the mono- and co-digestion of equine manure and vegetable waste using biodigesters of different sizes.

2. Materials and Methods

This section is divided into four subsections. The first presents the description of the study area and research design; the second details the components used to build the cash flow of the projects; the third describes the techniques employed for economic feasibility assessment; and finally, the fourth addresses the analysis of the environmental benefits resulting from the use of co-digestion.

2.1. Study Area and Research Design

The study was conducted in the municipality of Dourados, Mato Grosso do Sul, Brazil, located at latitude 22°13′16″ S and longitude 54°48′20″ W. The municipality covers a territorial area of 4086.387 km² and has an economy primarily based on agribusiness, with emphasis on grain cultivation, cattle farming, forest resource extraction, and silviculture. Both large and small rural properties play a significant role in the region’s productive landscape [25].

The area includes approximately 2798 active rural property registrations, comprising 2272 rural properties, 361 agrarian reform settlements, 140 properties on the urban fringe, and 5 located in Indigenous territories. Family farmers are responsible for supplying most local food markets in Dourados, particularly through local farmers’ markets [25].

Given the central role of agribusiness in Dourados, biodigesters may serve as a smart alternative to help producers make use of the organic waste generated on their properties [26], especially small-scale farmers. To contribute to this topic, the present study evaluated four biodigester projects (Figure 1).

Source: Prepared by the authors (2025).

Two biodigester sizes were considered (15 m³ and 20 m³), as described below:

• Project 1—15 m³ biodigester supplied exclusively with equine manure (100%), based on total solids (TS).
• Project 2—15 m³ biodigester supplied with 90% equine manure and 10% vegetable waste, based on TS.
• Project 3—20 m³ biodigester supplied exclusively with equine manure (100%), based on TS.
• Project 4—20 m³ biodigester supplied with 90% equine manure and 10% vegetable waste, based on TS.

The definition of each project’s characteristics was based on the analysis of raw material samples (equine manure and vegetable waste) collected on the property. The materials were individually characterized by determining total solids (TS) (39.98%; 13.03%), volatile solids (VS) (69.10%; 85.04%), and chemical oxygen demand (COD) (654.13 g/L; 311.54 g/L) for equine manure and vegetable waste, respectively.

An initial TS content of 3% was adopted to simulate the formulation of the substrates and the biodigester loading. This choice is made because the optimal range for the anaerobic digestion process lies between 2% and 5% TS, which is considered suitable for maintaining system stability, facilitating substrate input into the digester, and supporting the activity of methanogenic microorganisms [27]. Very low TS levels may limit biogas production by reducing the availability of organic matter, whereas excessively high levels hinder substrate mixing and circulation, thereby compromising process efficiency. Although this adjustment increases water demand, it contributes significantly to operational efficiency by preventing clogging, enhancing microbial access to organic matter, and ultimately improving biogas production performance.

To adjust the proportions of each component (manure and vegetable waste), the original TS concentrations were considered, and water was used for dilution. The substrate loading for the biodigesters was designed for daily feeding. For the 15 m³ biodigester, the daily load was 500 L of substrate, whereas for the 20 m³ biodigester, it was 666.6 L. Thus, within approximately 30 days, each system reached its full capacity. From this point onward, the hydraulic displacement process began, allowing for the release of the first batch of biofertilizer through the biodigester effluent. The calculations related to feeding load, biofertilizer production, and biogas generation were presented every month in order to standardize the estimates and facilitate comparative analysis among the different projects.

Biogas production was estimated based on the TS and COD concentrations of each material, the total amount of waste fed monthly into each biodigester, and the fraction of COD consumed per gram of degraded substrate. The production estimate follows the methodology described below:

Methane production per Kg of waste:

COD (g/L) × 0.70 × 0.35

Biogas production (m³/month):

(Methane production × amount of waste fed into the biodigester per month) ÷ 1000

where 0.70 corresponds to the average fraction of COD effectively consumed during the anaerobic digestion process, and 0.35 represents the amount of methane (CH₄) generated per gram of COD removed [28].

The TS vs. and COD analyses were carried out according to the methodology described by [29].

Table 1. Operational and production parameters of the four projects evaluated.

Biodigester	Biodigester 15 m3		Biodigester 20 m3
Description	Project 1	Project 2	Project 3	Project 4
	100% Equine Manure	90% Equine Manure and 10% Vegetable Waste	100% Equine Manure	90% Equine Manure and 10% Vegetable Waste
Quantity of equine manure to be used in the biodigester (kg/month)	1153.85	1038.47	1538.46	1384.62
Quantity of vegetable waste to be used in the biodigester (kg/month)	-	346.15	-	461.15
Amount of water added for dilution (L/month)	13,846.15	13,615.39	18,461.54	18,153.84
Biogas production (m3/month)	184.92	166.43	246.55	257.13
Equivalent production in LPG cylinders (units/month)	6.16	6.43	8.22	8.57
Biofertilizer production (liters/month)	13,500	13,500	18,000	18,000
Biodigester model	Canadian	Canadian	Canadian	Canadian
Biodigester material	PVC	PVC	PVC	PVC

Note: PVC—polyvinyl chloride. Source: Research data (2025).

details the monthly amount of each material (equine manure, vegetable waste, and water) used in the two biodigester models, as well as biogas production (m³/month), the equivalent production in cooking gas cylinders, and biofertilizer output.

The volume of biofertilizer was estimated as equivalent to 90% of the raw material inputs, considering that, with the TS applied, approximately 10% of the substrate remains as sediment at the bottom of the biodigester. The N, P, and K contents in the biofertilizer were calculated based on the nutrient concentrations found in horse manure (0.90%, 3.74%, and 3.75% of N, P, and K, respectively) [30] and in vegetable residues (2.12%, 8.54%, and 5.24% of N, P, and K, respectively) [31].

After defining the characteristics of each project, budgets were obtained from renewable energy companies to estimate the initial investment cost, revenues, and expenses for the execution of each project. This information was used to formulate the cash flow for each project.

2.2. Cash Flow Formation: Revenue, Costs, and Expenses

Revenues, costs, and expenses were estimated for one year. Revenue estimates were based on biogas production (intended for use as cooking gas) and the generation of biofertilizers, considering the concentrations of nitrogen (N), phosphorus (P), and potassium (K) in their composition. The valuation was based on the market prices of commercial (industrial scale) chemical fertilizers containing these nutrients.

Costs and expenses were determined based on labor (L), maintenance, fuel, fixed capital insurance, rural land tax (ITR), and depreciation. Labor charges included activities such as feeding the biodigester daily, cleaning the equipment, and applying the biofertilizer to crops every 30 days. Labor-related costs were calculated using a surcharge rate of 45.59% [32] applied to the minimum monthly wage of USD 269.18 or BRL 1518.00 (value of the minimum wage in force in the country for the year 2025).

The maintenance cost was estimated using a rate of 2.5% of the initial investment value, following the recommendation by [33]. Fuel expenses were calculated based on the number of hours required to apply the biofertilizer and the average price per liter of diesel fuel. Fixed capital insurance was set at an annual rate of 0.75% according to parameters established by Conab [32].

Although the payment of the ITR is mandatory, its value in the cash flow was listed as USD 0.00, since the producer is exempt from this tax. For depreciation, a residual value was considered for equipment with a lifespan longer than the project’s useful life horizon. Depreciation was calculated using the straight-line method [34].

2.3. Techniques for Economic Feasibility Assessment of the Projects

For the economic feasibility analysis, a useful life of 15 years was assumed for the equipment, with the entire initial investment made in year zero. This stage was structured into four components: (i) definition of the Minimum Attractive Rate (MAR), (ii) application of economic feasibility indicators, (iii) sensitivity analysis, and (iv) Monte Carlo simulation.

The project’s Minimum Attractive Rate (MAR) was estimated using the CAPM (Capital Asset Pricing Model).

Table 2. Estimated Minimum Attractive Rate (MAR) for the rural producer.

$\mathbf{M}\mathbf{A}\mathbf{R}=[{\mathbf{R}}_{\mathbf{f}}+\mathsf{\beta }\mathbf{X}\left({\mathbf{R}}_{\mathbf{M}}-{\mathbf{R}}_{\mathbf{f}}\right)]+{\mathbf{R}\mathbf{I}\mathbf{S}\mathbf{K}}_{\mathbf{B}\mathbf{R}-}\mathbf{I}{\mathbf{N}\mathbf{F}}_{\mathbf{U}\mathbf{S}\mathbf{A}}$
	Parameters	Obtained in: (1)	Value
Rf	Yield Rate of U.S Treasury Bonds	[33]	4.69%
$\mathrm{B}$	Unlevered Beta for the Farming/Agriculture sector	[35]	0.73
RM	Average Return Rate of the MSCI Emerging Markets Index	[36]	7.60%
${\mathrm{R}\mathrm{I}\mathrm{S}\mathrm{K}}_{\mathrm{B}\mathrm{R} }$	Credit Default Swap (CDS)	[37]	1.71%
${\mathrm{I}\mathrm{N}\mathrm{F}}_{\mathrm{U}\mathrm{S}\mathrm{A}}$	Estimated U.S. inflation (2025–2029)	[38]	2.10%
MAR	Ke (Global CAPM/Benchmarking—real rate)		6.43%

NOTE: (1) Accessed on 6 February 2025. Source: [33,35,36,37,38].

presents the estimated cost of capital for the producer, considering as the risk-free rate the 20-year U.S. Treasury Bond yields as the risk-free rate, an unleveraged beta specific to the agriculture sector, and the average return rate of the MSCI Emerging Markets Index, adjusted for inflation. Additionally, Brazil’s Credit Default Swap (CDS) was included, reflecting the country’s perceived sovereign risk in international markets, as well as an estimate of U.S. inflation for the coming years. Based on these parameters, the required rate of return (Ke), adjusted for inflation, was estimated at 6.43% per year.

The economic feasibility assessment was based on six decision-making criteria under the assumption that using a combination of different techniques provides a more robust and comprehensive analysis, as these methods are complementary rather than mutually exclusive [39]. The following techniques were applied: Net Present Value (NPV), Internal Rate of Return (IRR), Modified Internal Rate of Return (MIRR), Profitability Index (PI), Benefit-Cost Ratio (B/C), and Discounted Payback Period (DPP).

The NPV method was used to estimate project profitability by applying a discount rate to future cash flows. According to this criterion, a project is considered economically viable when the NPV is positive and greater than the initial investment, indicating that the projected benefits over time outweigh the costs involved [40]. The Net Present Value (NPV) can be calculated using the following formula:

$$\mathrm{N}\mathrm{P}\mathrm{V}=\sum _{\mathrm{t}=1}^{\mathrm{n}}{\displaystyle \frac{{\mathrm{F}\mathrm{C}}_{\mathrm{t}}}{\left(1+\mathrm{M}\mathrm{A}\mathrm{R}\right)\mathrm{t}}}-{\mathrm{I}}_0+\sum _{\mathrm{t}=1}^{\mathrm{n}}{\displaystyle \frac{{\mathrm{I}}_{\mathrm{t}}}{\left(1+\mathrm{M}\mathrm{A}\mathrm{R}\right)\mathrm{t}}}$$

(1)

where FC_t = cash flow (benefit) in each period, MAR = Minimum Acceptable Rate, I₀ = initial investment at time zero, and I_t = investment value in each subsequent period.

The Internal Rate of Return (IRR) was calculated to determine the percentage return on the invested capital [41]. The acceptance criterion requires the IRR to be higher than the opportunity cost, in this case, the MAR [42]. The IRR is estimated using the following expression:

$$\mathrm{I}\mathrm{R}\mathrm{R}={\mathrm{I}}_0+\sum _{\mathrm{t}=1}^{\mathrm{n}}{\displaystyle \frac{{\mathrm{I}}_{\mathrm{t}}}{\left(1+\mathrm{M}\mathrm{A}\mathrm{R}\right)\mathrm{t}}}-{\mathrm{I}}_0+\sum _{\mathrm{t}=1}^{\mathrm{n}}{\displaystyle \frac{{\mathrm{F}\mathrm{C}}_{\mathrm{t}}}{\left(1+\mathrm{M}\mathrm{A}\mathrm{R}\right)\mathrm{t}}}$$

(2)

where I₀ = investment amount at time zero (project start), I_t = projected investment amounts at each subsequent period, MAR = Minimum Acceptable Rate, and FC_t = projected cash inflows in each period of the project’s lifetime (cash benefits).

The MIRR was used to evaluate the investment’s profitability, accounting for both the investment’s cost and the reinvestment rate of cash flow factors not considered in the IRR calculation [43]. A project is accepted under this criterion when its MIRR exceeds the project’s opportunity cost (MAR). The Modified Internal Rate of Return (MIRR) is determined using the following equation:

$$\mathrm{M}\mathrm{I}\mathrm{R}\mathrm{R}=\sum _{\mathrm{t}\mathrm{j}=\mathrm{o}}^{\mathrm{n}}[Yj/(1+{i)}^{n-j}]/\sum _{\mathrm{j}=0}^{\mathrm{n}}[Cj/(1+{i)}^j]=(1+{MIRR)}^n]$$

(3)

where Yj = Positive cash flow in period j, Cj = Negative cash flow in period j, i = Project discount rate, represented by the minimum required return.

The PI was calculated to assess how much return the project generates for each unit of currency invested. According to this indicator, the project is considered viable when the result is greater than one, indicating value creation above the amount invested [42]. Its calculation is performed using the following equation:

$$\mathrm{P}\mathrm{I}={\displaystyle \frac{\mathrm{N}\mathrm{L}}{\mathrm{N}\mathrm{R}}} \times 100$$

(4)

where PI = Profitability Index, NL = Net Profit, NR = Net Revenue.

The B/C was used to compare the present value of total revenues to the present value of total project costs [42]. A project is deemed economically feasible under this criterion when the ratio exceeds 1, indicating that the projected financial benefits outweigh the costs over time [44]. The B/C ratio is obtained using the following equation:

$$\mathrm{B}/\mathrm{C}={\displaystyle \frac{\mathrm{P}\mathrm{B}\left(\mathrm{M}\mathrm{A}\mathrm{R}\right)}{\mathrm{P}\mathrm{C}\left(\mathrm{M}\mathrm{A}\mathrm{R}\right)}}$$

(5)

where B/C = Benefit-Cost ratio; PB = present value at the MAR (Minimum Attractive Rate), the project discount rate, of the benefit stream; PC = present value at the MAR of the project costs.

Next, the Discounted Payback Period (DPP) was calculated to determine the time required to recover the initial investment [45]. The discounted version was chosen over the simple payback method because it considers the time value of money, thus offering more realistic insights for decision-making. Its calculation was conducted using the following formula:

$$\mathrm{P}\mathrm{a}\mathrm{y}\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}=\mathrm{m}\mathrm{i}\mathrm{n}\left\{\mathrm{j}\right\}\sum _{\mathrm{k}=1}^{\mathrm{j}}{\displaystyle \frac{{\mathrm{F}\mathrm{C}}_{\mathrm{k}}}{\left(1+\mathrm{M}\mathrm{A}\mathrm{R}\right)\mathrm{k}}}\ge {\mathrm{F}\mathrm{C}}_0$$

(6)

where FC_k = project cash flow at time k, MAR = Minimum Acceptable Rate, FC₀ = project cash flow at time zero.

In the next step, sensitivity analysis and Monte Carlo simulation were performed to identify the main risk factors that could affect the economic outcomes of the analyzed projects [46]. The sensitivity analysis examined the impact of predefined variations in selected cash flow elements to assess their influence on economic viability indicators [47]. In this study, five critical variables were considered: (i) cooking gas price, (ii) biofertilizer price, (iii) maintenance cost, (iv) diesel price, and (v) initial investment amount.

Monte Carlo simulation, in turn, was applied to measure the effects of uncertainty in the input parameters on the project’s NPV [48]. A total of 100,000 iterations were performed using triangular distributions to represent the variability of each input variable, allowing for observation of the dispersion and behavior of NPV as the output variable.

2.4. Analysis of the Environmental Benefits Derived from the Use of the Biodigester

The assessment of the environmental benefits of using anaerobic digestion for waste treatment was carried out based on biogas production, the CH₄ concentration in biogas, its global warming potential (GWP), and its density. CH₄ was selected for the analysis as it is the main component of biogas, the gas with the greatest potential for electricity generation, and the one with the highest GWP among the gases involved in the process. Accordingly, the amount of carbon dioxide equivalent (CO₂eq) emissions avoided through methane capture and utilization was estimated, demonstrating its contribution to the mitigation of environmental impacts. The efficiency of CH₄ utilization was estimated at 95%, considering a 5% loss [11]. The parameters adopted were as follows:

• Average CH₄ concentration in biogas: 60% (typical range: 55–65%) [49].
• Global warming potential (GWP) of CH₄: 27.2 times greater than that of CO₂ [50].
• CH₄ density at 0 °C and 1 atm: 0.716 Kg/m³ [50].

Based on this data, the following calculations were performed:

• Volume of CH₄ in biogas (m³): Total biogas production (m³) × CH₄ content (as a decimal) × 0.95.
• Conversion of CH₄ volume to mass (Kg): CH₄ volume × CH₄ density.
• Conversion of CH₄ to CO₂ equivalent (Kg CO₂eq): CH₄ mass × 27.2.

This procedure allowed for the determination of the avoided monthly CO₂eq emissions. By multiplying this value by 12, the estimated annual avoided emissions were obtained, expressed in metric tons of CO₂eq.

3. Results and Discussion

3.1. Initial Investment for Biodigester Implementation

The initial costs were estimated individually for each project based on the biodigester capacity. Projects 1 and 2 required an initial investment of USD 5637.34, while Projects 3 and 4 required USD 6223.48. It can be observed that the investment values are similar within each pair of projects (1 and 2; 3 and 4), with the main distinction being the inclusion of vegetable waste in Projects 2 and 4. However, this addition did not result in significant variations in the total investment value.

Table 3. Description of the initial investment for each project according to capacity and substrate composition.

Biodigester Size	15 m3		20 m3
Projects	Project 1 Project 2		Project 3 Project 4
Description	Total Value (USD)	Share (%)	Total Value (USD)	Share (%)
Biodigester	2019.72	35.8%	2464.80	39.6%
Lagoon	368.83	6.5%	414.58	6.7%
Pressure relief valve	17.56	0.3%	17.56	0.3%
Geotextile fabric (Bidim) (m2)	97.53	1.7%	115.26	1.9%
BGS-2L purifier	51.42	0.9%	62.06	1.0%
Flow meter	63.84	1.1%	77.14	1.2%
BGS-ARM-05 balloon	335.14	5.9%	335.14	5.4%
Biogas pump 220 V AC 15 W	82.46	1.5%	82.46	1.3%
Biogas stove with double burner	82.46	1.5%	82.46	1.3%
Earthmoving	88.57	1.6%	122.89	2.0%
Liquid fertilizer distributor	2355.02	41.8%	2355.02	37.8%
Wheelbarrow	17.56	0.3%	17.56	0.3%
Bare soil	57.24	1.0%	76.57	1.2%
Total investment	5637.34	100%	6223.48	100%

Source: Research data (2025).

presents a breakdown of the components that comprise the implementation costs.

Among the equipment included in the initial investment, the adoption of a biogas-powered stove and a device for liquid fertilizer application stand out. The acquisition of both pieces of equipment was included because the projects considered the use of biogas (as cooking gas) and biofertilizer as sources of revenue resulting from the implementation of biodigesters. This differs from [51,52], who, in assessing the economic feasibility of biogas systems on small farms, considered the use of biofertilizer but did not account for the cost of the equipment needed for its application or for its commercialization.

Therefore, in biogas production projects, when considering the use of biofertilizer—whether for on-farm use or sale—it is important to account for the costs associated with its application or marketing. Omitting these costs compromises the consistency of the economic analysis, as it yields outcomes that do not accurately reflect the actual conditions of the project under evaluation.

It is noteworthy that, for both biodigester sizes, the same liquid fertilizer distribution equipment was used (USD 2355.02), although the amount of biofertilizer produced differs (see Table 1). This is due to the possibility of adjusting the equipment’s efficiency according to demand. The main variation lies in the amount of diesel fuel required for its operation, as well as the labor time needed for fertilizer application, as shown in

Table 4. Cash flow of the four evaluated projects (values in USD).

Description	Biodigester 15 m3
	Project 1			Project 2
	Year 0	Year 1–14	Year 15	Year 0	Year 1–14	Year 15
1. Total Revenue	0.0	2309.38	2309.38	0.0	2496.72	2496.72
1.1 Cooking gas production	0.0	1700.86	1700.86	0.0	1778.54	1778.54
1.2 Biofertilizer production	0.0	608.52	608.52	0.0	718.18	718.18
2. Total costs and Expenses	0.0	1428.81	1428.81	0.0	1611.13	1611.13
2.1 Labor	0.0	960.41	960.41	0.0	1142.73	1142.73
2.2 Maintenance cost	0.0	140.93	140.93	0.0	140.93	140.93
2.3 Diesel oil (fertigation)	0.0	9.50	9.50	0.0	9.50	9.50
2.4 Fixed capital insurance	0.0	42.28	42.28	0.0	42.28	42.28
2.5 Rural Property Tax (ITR)	0.0	0.00	0.00	0.0	0.00	0.00
2.6 Depreciation	0.0	275.69	275.69	0.0	275.69	275.69
3. Operating Profit	0.0	880.56	880.56	0.0	885.59	885.59
4. Depreciation	0.0	275.69	275.69	0.0	275.69	275.69
5. Operating Cash Flow	0.0	1156.26	1156.26	0.0	1161.28	1161.28
6. Investments	5637.34	0.00	0.00	5637.34	0.00	0.00
7. Free Cash Flow	−5637.34	1156.26	1156.26	−5637.34	1161.28	1161.28
Description	Biodigester 20 m3
	Project 3			Project 4
	Year 0	Year 1–14	Year 15	Year 0	Year 1–14	Year 15
1. Total Revenue	0.0	3085.01	3085.01	0.0	3350,66	3350,66
1.1 Cooking gas production	0.0	2273.65	2273.65	0.0	2370.46	2370.46
1.2 Biofertilizer production	0.0	811.36	811.36	0.0	980.19	980.19
2. Total costs and Expenses	0.0	1786.38	1786.38	0.0	2029.27	2029.27
2.1 Labor	0.0	1276.83	1276.83	0.0	1519.72	1519.72
2.2 Maintenance cost	0.0	155.59	155.59	0.0	155.59	155.59
2.3 Diesel oil (fertigation)	0.0	10.13	10.13	0.0	10.13	10.13
2.4 Fixed capital insurance	0.0	46.68	46.68	0.0	46.68	46.68
2.5 ITR	0.0	0.00	0.00	0.0	0.00	0.00
2.6 Depreciation	0.0	297.16	297.16	0.0	297.16	297.16
3. Operating Profit	0.0	1298.63	1298.63	0.0	1321.39	1321.39
4. Depreciation	0.0	297.16	297.16	0.0	297.16	297.16
5. Operating Cash Flow	0.0	1595.79	1595.79	0.0	1618.54	1618.54
6. Investments	6223.48	0.00	0.00	6223.48	0.00	0.00
7. Free Cash Flow	−6223.48	1595.79	1595.79	−6223.48	1618.54	1618.54

Note: ITR = Rural Property Tax. Source: Research data (2025).

.

Table 4 shows that the revenue generated by the biodigesters that included vegetable waste along with equine manure was higher compared to those that used only equine manure. This is because the biodigesters fed with a mixture of equine manure and vegetable waste resulted in greater biogas production, which led to an increase in the number of equivalent cooking gas units (13 kg LPG cylinders) per year: 73.92 in Project 1, 77.16 in Project 2, 98.64 in Project 3, and 102.84 in Project 4.

For all the projects analyzed, the presence of a worker responsible for carrying out routine activities was considered, such as the daily feeding of the biodigester, system cleaning, and the application of biofertilizer to the fields at intervals of approximately 30 days. The corresponding labor costs are presented in Section 2.2. Although in the context of family farming, it is common for these activities to be carried out by family members themselves, thus not representing a direct financial expense, this study chose to include the corresponding remuneration estimate. This decision is based on the premise that, even when performed personally by the farmer, such tasks have an economic value and should, therefore, be recognized as an opportunity cost.

The difference in labor costs between projects is mainly related to the greater volume of biofertilizer produced, which requires more time for handling and application. In the simulated scenarios, we primarily considered the use of family labor, which is common in small rural properties, while also accounting for the possibility of hiring external workers for specific tasks. The cost of hired labor was estimated based on the current rural minimum wage. It should be noted that when family labor is used exclusively, the impact on profitability is minimal, whereas hiring external labor increases operational costs and reduces NPV. Therefore, project profitability is more sensitive to the labor arrangement adopted than to the biofertilizer volume itself, underscoring the importance of support policies for family labor and cooperative management models.

3.2. Economic Evaluation of the Projects

Table 5. Economic feasibility indicators of the four analyzed projects.

Evaluation Technique	Results
	Project 1	Project 2	Project 3	Project 4
NPV	USD 5283.62	USD 5331.05	USD 8848.92	USD 9063.81
IRR	19.00%	19.10%	24.71%	25.10%
MIRR	9.20%	9.23%	10.84%	10.95%
PI	USD 1.94	USD 1.95	USD 2.42	USD 2.46
B/C	USD 1.62	USD 1.55	USD 1.73	USD 1.65
DPP	6.03 years	6.00 years	4.63 years	4.56 years

Source: Research data (2025).

presents the results of the economic evaluation. In this study, traditional techniques were employed, including NPV, IRR, MIRR, PI, Benefit-Cost Ratio (B/C), and DPP, based on a MAR of 6.43%, which was used to assess the feasibility of the four projects.

The results of the evaluation based on the Net Present Value (NPV) indicate that all projects are economically viable, as they present positive values. The Internal Rate of Return (IRR) and Modified Internal Rate of Return (MIRR) indicators reinforced this conclusion, showing returns above the Minimum Acceptable Rate of Return (6.43%) in all cases. The Profitability Index (PI) also demonstrated the projects’ capacity to generate returns above the invested capital, with values greater than one: Project 1 (1.94), Project 2 (1.95), Project 3 (2.42), and Project 4 (2.46).

Regarding the Benefit-Cost Ratio (B/C), all projects exhibited ratios above 1, confirming that the projected benefits outweigh the costs. The values obtained were Project 1—USD1.62, Project 2—USD1.55, Project 3—USD1.73, and Project 4—USD1.65. The Discounted Payback Period analysis also validated the investment feasibility, with payback periods shorter than the 15-year horizon considered for the project’s lifespan. The estimated payback times range from 4.56 years (Project 4) to 6.03 years (Project 1).

The NPV evaluation results show that all four projects are profitable, with positive values obtained for each. The IRR and MIRR indicators confirmed these financial benefits, as all projects exceeded the established Minimum Acceptable Rate of Return (6.43%). The PI metric indicates that all projects can generate wealth for rural producers, as all presented values are above 1: Project 1 (1.94), Project 2 (1.95), Project 3 (2.42), and Project 4 (2.46).

Regarding the B/C ratio, all four projects generate additional returns since their benefits exceed costs. Project 1 yields a B/C of USD1.62; Project 2, USD1.55; Project 3, USD1.73; and Project 4, USD1.65. The estimated time to recover the invested amount reaffirmed the investment viability, with payback periods shorter than the project’s 15-year useful life. The average payback period ranges from 4.56 years (Project 4) to 6.03 years (Project 1).

Based on the economic feasibility evaluation method, all four projects present satisfactory results and are viable for rural producers. Among them, Project 4 is considered the most feasible. This can be explained by the reduction in biogas production costs, which tend to be lower when produced on a larger scale.

This occurs because the costs to build and operate a biogas plant do not increase proportionally with its size. In other words, a larger plant does not incur proportionally higher costs than a smaller one, which contributes to reducing the cost per cubic meter of biogas produced [53]. Thus, the smaller the project is to be installed, the higher the investment cost per unit of capacity [54], which is one of the factors that make the installation of small biogas plants unfeasible. In this case, government subsidies are crucial to encourage investments in smaller-scale projects [55].

The results of the sensitivity analysis are presented in Figure 2. Although variations in five variables were considered, particularly in cooking gas price, biofertilizer price, maintenance cost, diesel price, and initial investment, only two showed significant sensitivity: maintenance cost and cooking gas price. These variables had a substantial influence on the project’s economic viability. The others showed low sensitivity, indicating that possible changes in their values do not significantly impact the results.

The sensitivity analysis showed that the price of liquefied petroleum gas (LPG) is the critical variable for project viability, with elasticity equal to 1.00. This indicates that the economic return of biodigesters directly depends on the savings from replacing LPG, a fuel whose price in Brazil is strongly influenced by public policies and international market volatility. Therefore, in scenarios of higher LPG prices, biodigesters become more competitive, whereas in periods of conventional gas subsidies, their attractiveness may be reduced. This dependence reinforces the importance of bioenergy incentive policies, such as rural credit programs and the valorization of biogas, to ensure long-term stability of the technology.

In contrast, maintenance costs showed a reduced negative elasticity (−0.07 to −0.08), indicating that variations in this factor do not significantly compromise profitability. This suggests that, even with moderately higher operating costs, the system maintains its economic viability. In practice, this operational robustness favors adoption by small farmers, who benefit from greater predictability in maintenance expenses. Taken together, these results highlight that the competitiveness of biodigesters is highly sensitive to fossil energy market dynamics but resilient to internal cost variations, which should be considered in public policies and technology diffusion strategies.

Subsequently, a Monte Carlo simulation was performed (Figure 3), considering a 95% confidence level. The results indicate that the NPV tends to remain within the estimated range for each project, demonstrating robustness even under uncertainty. The lower and upper limits of the NPV were as follows: USD4034.00 to USD6541.00 for Project 1, USD4029.00 to USD6646.00 for Project 2, USD7176.00 to USD10,526.00 for Project 3, and USD7338.00 to USD10,795.00 for Project 4. These results reinforce the economic feasibility of the projects even under risk scenarios, highlighting their suitability for small rural producers.

3.3. Environmental Benefits Assessment

As shown in

Table 6. Biogas production and CO₂eq reduction from mono- and co-digestion of equine manure and vegetable waste using two biodigester sizes.

Description	Results
	Project 1	Project 2	Project 3	Project 4
Biogas production m3/month	184.92	192.85	246.55	257.13
CO2 emissions avoided (tons/month)	2.05	2.14	2.72	2.85
CO2 emissions avoided (tons/year)	24.60	25.68	32.64	34.20

Source: Research data (2025).

, the inclusion of 10% vegetable waste alongside equine manure resulted in higher biogas production, with volumes of 192.85 and 257.13 m³/month for the 15 and 20 m³ biodigesters, respectively, compared to the exclusive digestion of equine manure, which produced 184.92 and 246.55 m³/month. This increased biogas generation contributed to a more significant reduction in CO₂eq emissions into the atmosphere.

Anaerobic co-digestion led to reductions of approximately 2.25 and 3.00 tons of CO₂eq per month, while mono-digestion reduced by 2.16 and 2.88 tons per month in the 15 and 20 m³ biodigesters, respectively. Although co-digestion provided better outcomes, both approaches demonstrated high environmental potential, as any reduction in greenhouse gas emissions represents progress toward greater environmental sustainability.

Co-digestion proved to be a promising strategy for increasing biogas production from organic rural waste, thereby enhancing the mitigation of greenhouse gas (GHG) emissions. The digestion of a single substrate may lack the stability required for anaerobic processes due to the specific characteristics of its composition. Equine manure has a high fiber content, which is linked to the animals’ diet, predominantly based on roughage. Since fiber fermentation in horses occurs in the large intestine, a significant portion of the fibrous material is excreted without complete degradation.

Furthermore, the fiber content in the manure can be even higher when animals are kept on straw bedding, as fecal matter mixes with the plant material. This high fiber content, particularly cellulose and lignin, hinders anaerobic microbial activity, reducing the substrate’s biodegradability and, consequently, biogas production [56].

The addition of vegetable waste to the equine manure mixture improves substrate composition, as these residues are rich in simple sugars, starches, and soluble compounds that ferment quickly. Thus, incorporating vegetables dilutes the fibrous fraction, making the substrate more accessible to microbial degradation and favoring biogas production. As a result, co-digestion promotes greater energy savings through biogas generation and reduces the environmental impacts caused by equine manure and vegetable waste.

The use of biodigesters for recycling equine manure and vegetable waste has proven to be an efficient alternative for renewable energy generation in rural settings. It is a clean and renewable energy source that contributes to GHG emissions reduction, promoting greater sustainability and profitability for family farming. For instance, [11] evaluated the CO₂eq emissions avoided through the anaerobic digestion of residues from olive oil extraction and reported the mitigation of approximately 16.017 tons of CO₂eq per year, based on a daily input of 2 tons of waste. The authors highlighted that co-digestion with other materials could further enhance emission reductions, a result that aligns with the findings of the present study and reinforces the synergistic potential of combining different substrates in anaerobic digestion.

Additionally, the eco-efficiency indicator was calculated (

Table 7. Eco-efficiency indicator of the projects: ratio between NPV and annual CO₂ equivalent reduction.

Project	NPV	CO2eq Avoided (Tons/Year)	Eco-Efficiency (USD/tCO2eq)
Project 1	5283.62	24.60	203.84
Project 2	5331.05	25.68	197.45
Project 3	8848.92	32.64	256.05
Project 4	9063.81	34.20	251.77

Source: Research data (2025).

) to assess both the economic and environmental performance of the projects [57]. This metric was determined based on the ratio between NPV and the annual reduction of CO₂eq emissions, providing an integrated evaluation of project performance across economic and environmental dimensions. The results showed that projects with larger biodigesters (20 m³) had higher eco-efficiency, with Project 3 standing out at USD256.05/tCO₂eq and Project 4 at USD251.77/tCO₂eq. In comparison, Projects 1 and 2, which used 15 m³ biodigesters, showed lower values (USD203.84/tCO₂eq and USD197.45/tCO₂eq, respectively). These findings suggest that, in addition to being economically viable, larger-scale projects deliver greater economic returns per ton of CO₂ avoided, reflecting the benefits of scale and the synergy between productivity and environmental sustainability.

The eco-efficiency indicator (NPV/tCO₂eq avoided) was adopted to integrate economic and environmental performance into a single metric. NPV reflects the project’s financial viability, while avoided CO₂eq emissions represent climate benefits. Their ratio allows for assessing the economic return per ton of emissions mitigated, in line with ISO 14045 [58,59], which defines eco-efficiency as the relationship between economic value and environmental impact. Applied studies in agri-food chains, such as [57,60,61], support the relevance of this integrated approach.

The comparison between the results obtained solely from the NPV and those calculated based on the eco-efficiency indicator reveals important nuances regarding the project’s performance. Although Project 4 presented the highest absolute NPV, indicating the greatest total economic return, it was Project 3 that achieved the highest eco-efficiency, that is, the greatest return per ton of CO₂ avoided. This difference highlights that considering only NPV prioritizes the total profitability of the project, whereas eco-efficiency offers an integrated perspective that combines economic performance with the environmental impact avoided. Therefore, the complementary use of these two criteria allows for a more comprehensive analysis: while Project 4 stands out in terms of scale and absolute return, Project 3 demonstrates a better balance between value generation and environmental sustainability per unit of impact avoided. This finding underscores the importance of adopting composite metrics in feasibility studies involving environmental technologies.

This finding reinforces the importance of adopting multiple evaluation criteria capable of integrating different dimensions into the analysis [61], since approaches that rely solely on traditional financial metrics tend to overlook other aspects of sustainability, such as environmental and social dimensions, as highlighted by Pearce, Atkinson, and Mourato [62].

Such methodological refinement represents an advance over conventional approaches and strengthens the claim of innovation in this study. As noted by [63], methodologies that connect environmental impact, economic value, and social applicability contribute to the development of solutions more closely aligned with the principles of systemic sustainability, including eco-efficiency.

Although Brazil, through the National Program for Strengthening Family Farming (Pronaf), provides specific financing lines for sustainable energy generation projects for rural producers, such as Pronaf Mais Alimentos, which enables the financing of technologies like solar panels and biogas production, the adoption of these solutions remains limited among smallholders.

Studies have shown that access to these credit lines faces challenges such as bureaucracy, lack of information [64], and difficulties in meeting the legal requirements for rural credit processing [65], for instance, the preparation of projects compatible with the criteria required by financial institutions. In this context, the methodology proposed in this research can support producers in developing more structured projects, thereby increasing their chances of securing financing, especially when these projects demonstrate strong environmental and social impacts.

4. Conclusions

This study aimed to evaluate the economic and environmental benefits arising from the production of biogas and biofertilizer through the mono- and co-digestion of horse manure and vegetable residues, using biodigesters of different capacities. The results demonstrated that all four evaluated projects are economically viable, meeting traditional minimum return criteria, with Project 4 standing out by presenting the best economic indicators (NPV of USD 9063.81, IRR of 25.10%, MIRR of 10.95%, and PI of 2.46), as well as the shortest payback period (4.56 years). In contrast, although viable, Project 1 had the longest recovery time, estimated at 6.03 years.

From an environmental perspective, all projects contributed to the mitigation of greenhouse gases, with annual reductions ranging from 25.92 to 36.00 tons of CO₂ equivalent, highlighting the potential of anaerobic digestion as an environmental management tool in small rural properties. Co-digestion with vegetable residues yielded superior results in terms of biogas production and avoided emissions, indicating its relevance as a complementary strategy to mono-digestion.

Additionally, the integrated analysis through the eco-efficiency indicator provided a more comprehensive understanding of the results by linking economic return to environmental benefits. Although Project 4 maintained the highest absolute return, Project 3 exhibited the highest eco-efficiency (USD256.05/tCO₂eq), closely followed by Project 4 (USD251.77/tCO₂eq), demonstrating better relative performance per unit of emissions avoided. Smaller projects, such as the 15 m³ biodigesters, showed lower eco-efficiency, reinforcing the existence of economies of scale.

These results demonstrate that the adoption of biodigesters, even on a small scale, is a viable strategy from both an economic and an environmental perspective, especially when composite metrics such as eco-efficiency are used to guide decision-making. As a limitation of this study, it is important to note the absence of experimental methods to validate the amount of biogas produced. Therefore, future studies are recommended to conduct laboratory experiments to empirically validate this production. We acknowledge that this study was conducted through economic and environmental simulations without direct experimental validation. This methodological choice allowed us to integrate multiple scenarios, evaluate different waste combinations, and comparatively assess project eco-efficiency. However, we also recognize its limitations, particularly regarding real-world operational variability (biodigester efficiency, maintenance costs in different contexts, and market fluctuations). To mitigate this, we relied on well-established parameters from the literature and applied sensitivity analysis to test the robustness of the results Additionally, future research should incorporate alternative financing scenarios and analyze the role of public policies and subsidies in expanding the use of this technology in rural communities.

In addition, this research did not consider the potential costs of collecting and transporting the biomass, which represents a limitation that can affect the applicability of the results in real conditions, especially when it comes to dispersed plant residues. In practice, such costs can significantly impact the economic viability of biodigester systems. To overcome this challenge, future studies and implementations should consider integrated strategies.

It is worth mentioning that, although this research was conducted in the municipality of Dourados (MS, Brazil), which imposes certain limitations on the generalization of the results, the findings still provide relevant insights for reflection and decision-making in similar contexts. This is because other regions may present different climatic, socioeconomic, and structural characteristics. Nevertheless, the results obtained here can serve as a basis for comparative studies and further investigations in different territories.

Finally, although the present analysis concentrated on small-scale biodigesters, it is highlighted that the adoption of larger-scale systems is also presented as a relevant alternative. The relationship between the size of the biodigester and its economic viability can be understood in the light of the theory of economies of scale, according to which units of greater capacity tend to dilute fixed costs, resulting, consequently, in lower unit costs of energy production.

In the evaluated configurations, economies of scale mainly result from the dilution of fixed investment and operation/maintenance costs per unit of biogas generated, as well as from the more efficient use of the workforce already allocated. These gains, however, are constrained by technical factors related to the characteristics of the waste—especially that of equine origin—due to its high fibrous fraction, which can reduce the degradation rate of organic matter and the efficiency of anaerobic digestion, and also by operational conditions, such as the need for proper management of the mixture and moisture to prevent the formation of crusts and the accumulation of solids in the reactor. Thus, although larger units tend to present a higher NPV under the conditions analyzed, the marginal benefit decreases as logistics and transportation costs for collecting and continuously supplying the waste to the biodigester increase.