Innovative System for Animal Waste Utilization Using Closed-Loop Material and Energy Cycles and Bioenergy: A Case Study

Abstract

This study proposes an innovative model for animal waste utilization in the largest Polish meat utilization plant, which assumes an integrated system that processes one part of the meat waste by anaerobic digestion and the second part into meat and bone meal via the hydrothermal method. The solution is based on implementing the concept of industrial symbiosis, using a purposefully directed flow of materials, waste, and energy to create a closed recycling cycle. This study analyzes the key strategic, organizational, and technical circular economy activities that enable the transformation of waste into valuable materials and energy, thanks to the use of closed-loop materials and energy cycles. It estimates the integrated system’s investment costs and economic and environmental outcomes. The presented method allows for biogas production from the bio-fermentation of 160,000 t/y of animal waste; this would more than cover the heat requirements for obtaining 110,000 t/y of meat and bone meal using the hydrothermal method.

1. Introduction

Livestock production and meat consumption are growing worldwide, especially in low- and middle-income countries, due to increased standards of living and population growth [1,2]. The meat supply reached 340 million tons in 2018, 300% more than the amount produced 50 years ago. Pork and poultry meat represent one-third each of global meat consumption, with beef at one-fifth, and the remainder coming from sheep, goats, and other animals [3].

Livestock farming is a significant source of greenhouse gases (GHGs) and contributes around 14.5% of global GHG emissions [4]. It is also a major driver of 10% of anthropogenic freshwater use and biodiversity loss in some areas [5,6]. Proper management of animal waste can minimize the environmental impact of livestock farming [7].

European Union (EU) countries produce 20 million tons of meat waste annually, including 70% low-risk and 30% high-risk material [8]. Caldeira [9] presented a system for calculating the amount of waste produced at the European macro-level. In 2011, the value chains of EU meat and waste producers were (in millions of tons/year) 61.7 tons of consumable meat and 14.2 tons of cumulative meat waste.

Animal waste produced in the EU is mainly processed into meat–bone meal (MBM) [10]. The EU produces 4.5 million tons of MBM annually, which is mainly used as biofuel [11,12] and as a multi-component fertilizer [13,14]. Polish law allows the use of MBM at a maximum dose of 5 tons/hectare, applied every two years. MBM should be applied before sowing and immediately mixed with the soil [13,15,16]. Fertilizing with MBM can decrease GHG emissions by up to 1 ton of CO₂ equivalent (CO₂eq) per ton of MBM [17]. An EC Regulation [18] permits animal protein obtained from MBM based on specific processing methods. The European Commission decided on 17 August 2021 to allow the feeding of farm animals with MBM based on the principle of cross-feeding (for example, pork meal for poultry and vice versa). Feeding the same type of animal protein to the same species is still prohibited [19,20].

In 2021, Poland produced 4.204 million tons of meat [21]. The amount of animal waste, evaluated according to the methodology [9], was estimated to be 967,000 tons per year, processed into MBM, mainly used as biofuel [22,23]. This allows for energy recovery and an overall GHG savings of 600–1000 kg CO₂eq per ton of MBM used, primarily by substituting fossil fuels in the energy sector [17]. Animal waste contains large amounts of organic matter, water, and phosphorus compounds. Their recovery from animal waste is essential for the sustainable utilization of agricultural waste and circular economy (CE) strategies in many countries [24,25].

Meat waste classification and appropriate treatment technologies have been described. Meat waste is defined as whole bodies, animal parts, or animal-derived products not intended for human consumption [18,26,27]. According to [26], meat waste is divided into three categories based on the degree of public health and animal health risk, which determines the various utilization/processing techniques. The first, the high-risk category, includes all animals or parts of animals suspected of being diseased; the second, the medium-risk category, includes animal products recognized as unfit for human consumption. The third, low-risk category, includes carcasses or animal parts unfit for human consumption or due to trading reasons. This category contains heads, feet, feathers, and hides [10]. Long-distance transport of animal by-products raises concerns for biosecurity and the natural environment [28].

Animal waste can also be treated through anaerobic digestion [29]. The yield of anaerobic digestion can be increased, for example, through co-digestion, surfactant addition, pre-treatment, and optimal digester design [30]. Bio-fermentation significantly reduces GHG emissions and produces renewable energy in the form of biogas. In agricultural biogas plants, substrates from crops, raw biodegradable materials, by-products, and meat waste are used [31,32,33]. In 2023, 85,777 tons of Polish slaughterhouse waste were processed through bio-fermentation [34].

This approach promotes the main CE strategies, cleaner production (CP), and sustainable development (SD) [12,35,36]. Implementing industrial symbiosis (IS) at the microeconomic level in production companies leads to the development of strategic CE goals, aiming to maintain the highest value and utility of goods, raw materials, and by-products throughout their life cycles, improving manufacturing systems, reducing the use of natural resources, and providing new reuse and recycling methods for waste [37,38,39]. CP technologies prevent environmental pollution and use renewable materials in all stages of a product’s life cycle. The CP methods include reduction at the source, in-process, on-site, and off-site recycling, and substituting primary resources with waste [35,40,41].

This study proposes an innovative system for managing animal waste at Poland’s largest animal waste utilization plant, assuming the integrated processing of one part of the animal waste through anaerobic digestion and the other part through a hydrothermal method to produce MBM. A model was developed describing the integrated processing system of animal waste into MBM through the hydrothermal method and bio-fermentation into biogas and fertilizer. Key strategic, organizational, and technical CE, SD, and CP activities that consider the conversion of animal waste into valuable materials and energy, developed and implemented according to the new system, were also analyzed. Additionally, variants of the innovative model, estimated investment costs, and the economic and environmental effects of the developed integrated animal waste processing system are presented.

2. Materials and Methods

Proper meat waste utilization is connected with industrial symbiosis (IS), which uses production synergy to develop supply chains, increase economically effective circular loops, and improve resource efficiency through the cooperation of firms in terms of materials, waste, and energy cycles [42,43].

CE model activities involve developing and implementing new systems for product and material flows, using the value chain to manage waste more sustainably and profitably, thereby closing material cycles. High-quality technologies and equipment used for on-site or off-site recycling and reuse of processed waste to obtain bioenergy have facilitated the primary use of materials, reduced energy consumption, and decreased GHG emissions. The CP technologies used include reduction at the source, in-process recycling, waste reduction [44], emission reduction, substitution of natural raw materials with waste, and both on-site and off-site recycling [37,39].

Farmutil HS, the largest Polish meat waste utilization plant, processes 600,000 tons of meat waste annually into MBM using the hydrothermal method (over 60% of Polish meat waste produced). Modernized in recent years, Farmutil’s hydrothermal technology has made its MBM units among the most advanced in European countries. The modernization of the MBM installation, which implemented continuous production lines, reduced production costs by 10% due to lower electricity and steam consumption and more efficient processes. The 14 meat waste collection points are optimally located across Poland, enabling incentivized, high-quality recycling and environmentally safe transport to Farmutil. The OXIDOR system used at the MBM plant can combust all gaseous odors from individual technological operations and air from production rooms (total quantity: 200 million m³/year). This thermal method is the most effective, though expensive and rarely used [22,45]. Its implementation has improved the relationship between Farmutil and the public.

Farmutil’s management system fosters industrial symbiosis (IS). IS traditionally operates through separate entities, engaging in complex resource exchange interactions (materials, water, energy, and by-products). It allows for the full utilization of by-products through networks of independent enterprises exchanging by-products and sharing other common resources, thereby increasing processing efficiency and providing both economic and environmental benefits [35,37]. In the consortium management system, the transition towards CE, design for environment (DFE) [43,46,47], and CP are the main strategies used to promote CE. The basic integrated model of animal waste utilization proposed for Farmutil is presented in Figure 1 [22,48].

Biogas production activities are compatible with a circular economy, mainly consisting of using waste in the anaerobic digestion process. The proposed solutions aim to bio-ferment as much meat waste as possible, especially because meat waste processed by hydrolysis into MBM can be difficult and expensive. The anaerobic digestion process should particularly target waste such as high-risk category meat waste, animal blood, bristles, hatching waste, wasted eggs, manure, screenings, waste fats, and sewage sludge. Another main advantage is replacing as much of the natural gas as possible that is used in MBM production with biogas generated during bio-fermentation.

2.1. Data Analysis

The proposed model assumes the possibility of utilizing meat waste at various scales, with different proportions of waste processed by the hydrothermal method into MBM and the bio-fermentation method. As a result, the amounts of biogas, wastewater, and odors produced may vary when using the integrated model. The equations below allow the calculation of these amounts at various scales of waste processing.

Q_AW = Q_AW1 + Q_AW2

(1)

N_AW2 = N_AW1 · Q_AW1

(2)

B_AW3 = BY · Q_AW2

(3)

where:

• N_AW1—Quantity of natural gas consumed when animal waste is processed with the hydrothermal method into MBM (as CH₄ m³/t)
• N_AW2—Quantity of natural gas consumed when animal waste is processed with the hydrothermal method into MBM (as CH₄ m³)
• Q_AW—Quantity of animal waste processed (t/y)
• Q_AW1—Quantity of animal waste processed with the hydrothermal method into MBM (t/y)
• Q_AW2—Quantity of animal waste processed with the bio-fermentation method (t/y)
• B_AW3—Biogas production when the integrated system is used (as CH₄ m³/y)
• BY—Biogas yield from meat waste (as CH₄ m³/t)

Equation (1) indicates the amount of processed animal waste Q_AW. Equation (2) calculates the quantity of natural gas used for MBM production at different animal waste capacities Q_AW1, and Equation (3) shows the biogas quantities obtained by the anaerobic digestion of animal waste at different animal waste capacities Q_AW2. A value of B_AW3 > N_AW2 means that, in the integrated model, biogas production is higher than natural gas consumption at a determined capacity Q_AW2 of waste processed by the bio-fermentation method and biogas yield BY from processed waste, which are independent variables. Equations (4) and (5) show the quantities of wastewater W_AW3 and odors O_AW3 obtained by the integrated system of waste processing, at the determined capacity Q_AW1 of waste processed into MBM and by anaerobic digestion Q_AW2, respectively. Equation (6) defines the amount of natural gas consumed in the integrated waste processing system.

W_AW3 = W_AW1 · (Q_AW1 − Q_AW2)/Q_AW1

(4)

O_AW3 = O_AW1 · (Q_AW1 − Q_AW2)/Q_AW1

(5)

N_AW3 = N_AW1 ∙ (Q_AW1 − Q_AW2)

(6)

where:

• W_AW1—Quantity of wastewater from MBM (t/y)
• W_AW3—Quantity of wastewater from MBM when the integrated system is used (t/y)
• O_AW1—Quantity of odor emitted from MBM produced with the hydrothermal method (m³/y)
• O_AW3—Quantity of odor emitted from MBM when the integrated system is used (m³/y)
• N_AW3—Quantity of natural gas consumed when the integrated system is used (as CH₄ m³/y)

2.1.1. Estimated Amounts of Biogas Obtainable from Meat Waste

Biochemical methane potential (BMP) tests were used to determine the methane yield of meat waste, indicating its biodegradability and its potential to produce methane via anaerobic digestion. This assesses the substrate quality and predicts the methane production by full-scale biogas plants. Indicators were obtained during laboratory tests of the bio-fermentation process of meat waste [49,50]. In [51], different biogas feedstock analysis methodologies were described, including the following essential analyses: pH, total solids/dry matter, volatile solids/organic dry matter, chemical oxygen demand, total Kjeldahl nitrogen, ammonia nitrogen, and biochemical methane potential. The biogas yield mainly depends on the type of waste. In the case of soft meat waste and bones, it ranged from 229 to 570 m³/t of raw waste. According to [32], the biogas yield was 450 m³ from meat waste and 316 m³ from sewage sludge per 1 t of dry organic mass.

2.1.2. Estimation of the Integrated System of Meat Waste Utilization in Terms of Used Circular Economy Strategic Data

The animal waste utilization method, based on the production of meat–bone meal with the hydrothermal method, and the integrated system combining the production of MBM with hydrothermal technology and parallel bio-fermentation of animal waste, was compared with assessed circular economy (CE) activities, sustainable development (SD) goals, and cleaner production (CP) methods. CE, CP, and SD activities allow for the development of new products or technologies without significant upfront investment, alongside cross-value chain cooperation. The developed integrated system of meat waste utilization wisely selects resources, technologies, and processes. In this model, CE promotes a series of actions, business models, and measures covering the entire lifecycle of the products and materials, from design through manufacturing, consumption, and waste management.

2.2. Utilization of Meat Waste to Produce MBM

One ton of MBM is obtained from approximately 4 tons of animal waste through hydrothermal technology. Due to the highly diversified types of waste, the characteristics of the produced meat meal also vary. MBM is commonly used in low-cost pet food formulations because of its high protein content. However, its nutrient characteristics can vary greatly, and often, additional processing is required [52]. The methods used to produce meat meals are widely discussed [15,45,53]. After pre-treatment (removing metal parts, grinding, and mixing), the feedstock is sterilized (133 °C, 0.3 MPa, for 30 min). Then, the by-products are dried, and the fat is separated by filtration. The solid fraction MBM from the filtration process is then ground and sieved. These process parameters meet EU conditions [45]. Meat waste is categorized into three risk categories [10,18], which are also used to determine the risk categories of the produced MBM. Possible elimination/valorization methods for MBM include incineration or co-incineration (categories 1–3), landfill (categories 2 and 3), biogas production (categories 1–3 after sterilization treatment), and uses as fuels (categories 1–3), organic fertilizers (categories 2 and 3), and animal feed (category 3). According to [15,53,54], meals can be divided in terms of total protein content (in %): meat meal (MM), 55.0; MBM, 40.0; bone meal (MB), 26.5; blood meal (BM) 89.5; skin meal (MS), 45.0; and liquex meal (ML) 70.0. The antioxidant content should be 100–400 mg/kg.

MBM is thermally decomposed up to approximately 550 °C due to the combustion of organic substances in MBM, as evidenced by the high exothermic effect. The mass loss due to the incineration organic MBM phase is around 76%. The average moisture, phosphorus, and calcium contents of MBM samples were 2.43%, 5.8%, and 7.7%. The combustion heat of MBM is 19 MJ/kg [55]. In other studies, MBM samples were calcined in a chamber kiln. The weight loss was 70.0%, and the phosphorus content ranged from 14.5% to 15.0%. And the calcium ranged from 33.8% to 33.6%. Phase composition studies showed that the main crystalline phase of MBM is hydroxyapatite (Ca₅(PO₄)₃OH), with low quantities of impurities such as SiO₂, Ca₃(PO₄)₂, and CaCO₃ [24,56]. The phosphorus content in the ash is comparable to that in phosphorites, with 13.2% to 17.2% P [23].

Water removal is significant for rendering, and the produced aqueous stream represents about 65% of the feedstock mass [57,58]. The meat waste charge [23,45] used for MBM production typically contains ground pork bones with dimensions of 1–3 cm and 35.0–45.0% H₂O, as well as the following components (in percent of dry mass): organic matter 34.0–39.0; fat 14.0–16.0; protein 18.0–23.0; phosphorus 10.0–14.0; and calcium 28.0–30.0. Its bulk density is 0.85 kg/dm³. The contents of cadmium, mercury, arsenic, chromium, lead, and copper were < 0.1 ppm [59]. These confirm the high quality of the obtained hydroxyapatite ashes and the very low amount of heavy metals [23].

2.3. Bio-Fermentation of Agricultural and Animal Waste

Anaerobic digestion utilizes microbiological processes to degrade organic matter, reduce volatile solid content, generate biogas for energy, and remove pathogens, as anaerobic digestion can deactivate them. The process yield depends on the technology parameters, such as temperature, pH, hydraulic retention time, and matter retention time. Salmonella sp. and Escherichia coli elimination increased as the solid retention time increased [60,61,62].

Anaerobic digestion is a composite reaction divided into four biodegradation stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis [61]. During hydrolysis, organic compounds (lipids, proteins, carbohydrates, etc.) are hydrolyzed into simple compounds (amino acids, sugars, fatty acids, etc.) by hydrolytic microorganisms. During acidogenesis, these hydrolyzed substances ferment into volatile fatty acids (VFA), such as butyric, propionic, acetic, and valeric acids, leading to a pH decrease. Acidogens, containing both facultative microbes and obligate anaerobes, manage this fermentation phase [63]. In acetogenesis, fermented acidogens form acetic acid, hydrogen, and carbon dioxide, with acetate being the main product. Methanogenesis is the final stage of anaerobic digestion, where methane is produced from methanol, acetic acid, hydrogen, and carbon dioxide. Two types of microbes are responsible for this phase: acetolactic methanogens, resulting in two-thirds of all methanation, and hydrogenotrophic methanogens, resulting in one-third of all methane production. The optimal pH for anaerobic digestion is 6.5–7.5 [64]. In an anaerobic digester, ammonia content is important for buffering and stabilizing the pH. However, its concentration in the solution should not exceed 1 g/L [65].

The carbon-to-nitrogen ratio (C/N) indicates the proportion of carbon and nitrogen in organic materials. Nitrogen is essential for microorganisms to multiply and create new cells. The optimal C/N ratio for methane production is 20–30, and this is achieved by mixing materials with different C/N ratios. Key parameters that determine the proper course of methane formation include an anaerobic atmosphere, temperature, pH, the chemical composition of the fermented materials, the homogeneity of the fermented pulp, the retention period of materials in the bio-fermentation tank, and the absence of reaction inhibitors (e.g., excess ammonium nitrogen). Methane production by anaerobic digestion occurs in two temperature ranges: 35–40 °C (mesophilic range) and 50–55 °C (thermophilic range) [66]. Thermophilic digestion produces 25–50% more methane than mesophilic digestion. Anaerobic digestion allows for the production of bioenergy and potential by-products, such as soil biofertilizers [29]. The process occurs naturally in the absence of oxygen and by using various types of materials, such as industrial and municipal wastewater, agroindustrial, municipal, food processing, and vegetal waste, to produce biogas [67,68]. Mesophilic units are more common than thermophilic ones, mainly used for wet digestion due to lower energy requirements for heating [65,69]. Methane formation involves anaerobic microorganisms catalyzing the breakdown of organic substances, followed by the generation of methane and carbon dioxide [70].

The energy demand of a biogas plant depends on factors such as the substrate characteristics, transport, storage, pre-treatment, technology used, and the treatment of biogas and residues [71]. Mesophilic fermentation typically has a positive energy balance, whereas thermophilic fermentation lasts 15–20 days, occurs above 40 °C in closed fermentation chambers, and has a negative energy balance. Biological pre-treatment with Bacillus and Lactobacillus bacteria integrated into the digestion process is an attractive strategy to improve the bio-fermentation of the organic fraction of slaughterhouse wastes [32].

Up to 500 L of biogas can be produced from 1 kg of organic material. The main components of biogas are CH₄ (40–80%), CO₂ (20–55%), H₂S (0.1–5.5%), and trace amounts of H₂, CO, N₂, and O₂ [32]. Bio-fermentation requires controlled conditions without access to oxygen and at elevated temperatures [68]. Hermetically sealed tanks ensure that odors from decomposing organic matter are captured and not detectable outside. Fermentation also eliminates pathogenic bacteria, such as Salmonella and E. coli [63]. An additional system often introduced is waste sterilization before processing the feedstock by bio-fermentation, due to the toxic characteristics of some types of meat waste. Bio-fermentation installations are modular, with scaling from 250 kW to 5 MW. A biogas plant requires a surface area of about 2000 m².

2.4. The Developed Strategy for Waste Bio-Fermentation at Farmutil HS

Biogas production activities are compatible with the CE, primarily consisting of using waste in the anaerobic digestion process and the rational use of the digestate [34]. The targeted solutions involve bio-fermenting as much meat waste as possible, especially because meat waste processed through hydrolysis into MBM can be difficult and costly. Replacing as much natural gas as possible for MBM production with biogas generated during anaerobic bio-fermentation supports the development of new methods for anaerobic bio-fermentation of animal blood. Typically, blood is the raw material used to produce meals in animal feeds, such as protein, milk, lysine supplements, and vitamin stabilizers [72]. Blood plasma gel contains 60.0% albumin [73]. Animal blood has a C/N ratio of ~3.5. For typical methane bio-fermentation, the optimal C/N ratio is 20–30. The research aimed to select appropriate raw materials for co-fermentation with blood to maximize biogas yield and the effectiveness of anaerobic digestion. In 2016, the U.S. meat industry discharged 41.8 million metric tons of slaughterhouse waste, of which blood accounted for 7–11% [74,75]. Blood is typically collected and shipped to rendering plants to produce bone meal (BM), an animal feed component. It is estimated that 39.7–41.4 kJ can be obtained from anaerobic digestion of blood waste from each kilogram of slaughtered livestock. Typical blood contains 19–21% solid matter and 13–15% proteins. In [74], a semi-technical scale study showed that the two-stage anaerobic digestion system has a methane yield of 135 m³/t of blood.

3. Results and Discussion

3.1. Expected Volumes of Biogas Generated from Animal Waste

Table 1. The biogas yield from different kinds of meat waste.

Meat Waste Type	Content in Dry Matter		Biogas Yield from Meat Waste (m3/t)
	Mass (%)	Organic Mass (%)	Raw Mass		Dry Mass		Dry Organic Mass
			Biogas	Methane	Biogas	Methane	Biogas	Methane
Mulch	28.27	94.86	124.27	69.69	439.50	246.47	463.33	259.83
			125.01	70.90	442.10	250.75	466.08	264.34
			126.44	71.19	447.17	251.78	471.41	265.43
Average			125.44	70.59	442.92	249.67	466.94	263.20
Animal excrement	14.55	81.94	34.58	22.67	237.68	155.80	290.07	190.14
			34.42	22.45	239.59	156.27	293.92	191.70
			34.64	22.85	238.11	157.06	290.60	191.68
Average			34.55	22.65	238.46	156.38	291.53	191.18
Sewage sludge	14.55	81.94	359.19	254.69	1106.83	784.84	1131.12	802.06
			366.58	258.75	1129.61	797.34	1154.40	814.84
			392.02	227.01	1208.01	853.62	1234.52	872.35
Average			372.59	263.49	1148.15	811.93	1173.35	829.75
Soft waste	32.45	97.85	503.69	350.12	1162.97	808.40	1222.09	849.49
			569.23	398.33	1314.33	919.71	1381.09	966.46
			343.79	236.94	793.78	547.06	834.13	574.87
Average			472.24	328.47	1090.36	758.39	1145.77	796.94
Bones	43.31	95.16	228.99	157.41	399.23	274.42	591.75	406.76
			324.59	226.67	565.89	395.17	838.79	585.73
			398.19	275.80	694.20	480.84	1028.98	712.71
Average			317.26	219.96	553.11	383.48	819.84	568.40
Hooves and horns	57.36	67.47	2.66	1.79	212.51	142.82	305.83	205.54
			2.57	1.68	209.94	137.56	302.13	197.97
			2.68	1.83	211.19	144.54	303.93	208.01
Average			264	1.77	211.21	141.64	303.96	203.84

presents the indicators obtained from laboratory tests on the bio-fermentation process of meat waste. The biogas yield was primarily dependent upon the type of waste (as shown in Table 1). In the case of soft meat waste and bones, the yield ranged from 229 to 570 m³ per ton of raw waste. According to [32], the biogas yield was 450 m³ from meat waste and 316 m³ from sewage sludge per ton of dry organic mass. In the subsequent analysis, a biogas yield of 300 m³ per ton of raw meat waste was considered.

3.2. Obtaining Biogas from Meat Waste and Its Substitution for Natural Gas in MBM Production

The heat consumption at Farmutil is 655,000 GJ/y for the MBM production capacity of 100,000 t/y, and 982,500 GJ/y for 150,000 t/y [55].

Table 2. Biogas yield and production from meat waste for different biogas unit capacities and its consumption to fulfill the heat need for hydrolysis in producing 150,000 t/y of meat bone meal (MBM).

Biogas Unit Capacity (MW)	Meat Waste Charge (t/r)	Biogas Yield from Meat Waste (m3/t)	Biogas Production (Million m3/y) *		Heat Produced (GJ/y)	Heat for an MBM Production Capacity of 150,000 (t/y)
			60% CH4	100% CH4		Heat Need (GJ/y)	(%) of Heat Needed Fulfilled (6/7)
1	2	3	4	5	6	7	8
0.5	20,000	200	4.0	2.4	86,400	1,081,400	8.0
		300	6.0	3.6	129,600		12.0
		400	8.0	4.8	172,800		16.0
1	40,000	200	8.0	4.8	172,800		16.0
		300	12.0	7.2	259,200		24.0
		400	16.0	9.6	345,600		32.0
2	80,000	200	16.0	9.6	345,600		32.0
		300	24.0	14.4	518,400		47.9
		400	32.0	19.2	691,200		63.9
3	120,000	200	24.0	14.4	518,400		47.9
		300	36.0	21.6	777,600		71.9
		400	48.0	28.8	1,036,800		95.9
4	160,000	200	32.0	19.2	691,200		47.9
		300	48.0	28.8	1,036,800		95.9
		400	64.0	38.4	1,382,400		127.8
5	200,000	200	40.0	24.0	864,000		79.9
		300	60.0	36.0	1,296,000		119.8
		400	80.0	48.0	1,728,000		159.7

* 1,000,000 m³ CH₄ = 36,000 GJ.

presents quantities of processed meat waste, with biogas yields ranging from 100 to 400 m³/t of raw meat waste for bio-fermentation unit capacities of 0.5–5 MW. At a biogas yield of 300 m³/t of raw meat waste, it is possible to substitute 72% of the natural gas used at Farmutil to produce 150,000 t/y MBM, with a bio-fermentation of 120,000 t/y of meat waste, and 120% of the natural gas used to produce 150,000 t/y MBM, with a bio-fermentation of 200,000 t/y of meat waste.

Bio-Ferment Management

The bio-ferment from the fermentation process has fertilizing value and can replace about 10% of the currently used artificial fertilizers on Farmutil’s arable lands. Table 3 presents the amounts of bio-ferment obtained at different meat waste-processing rates.

Bio-ferment (digestate) is an alternative to mineral and organic fertilizers, containing valuable plant nutrients. Its low organic dry matter content compared to liquid manure, neutral-to-alkaline pH, and high nitrogen, phosphorus, and potassium contents are some of the parameters that make this by-product an attractive fertilizer. The digestate dry matter content is 3.21–6.62%, and the organic matter is 60.96–76.47%. Its pH is alkaline (8.1–9.4) [76,77]. The chemical composition of the bio-ferment varied [in %], with a total nitrogen, 0.4–0.8; phosphorus, 0.1–0.28; potassium, 0.09–0.12; calcium, 0.22–0.43; magnesium, 0.06–0.17, and (in mg/kg); cadmium, 0.25–0.5; chromium, 1.14–4.55; nickel, 1.07–9.45; lead, 0.5–2.16; zinc, 27.8–105, and copper, 7.9–27.9. Digestate fertilizer from a biogas plant can reduce GHG emissions. Using 30 t of digestate per 1 ha of arable land would decrease GHG emissions by 25.8–44.5 t CO₂eq [78].

Unfortunately, fertilization cannot be conducted year-round in Poland [22] due to limitations in fertilizing ability (9 months per year). Therefore, it should be stored in lagoons, but this is troublesome. Hence, the proposed integrated system provides for the treatment part (about 30%) of the digestate in the existing Farmutil biological treatment plant (processing 1,000,000 m³/y of wastewater from MBM production) when the bio-ferment cannot be used as fertilizer.

Table 3. Material balance of bio-ferment production.

Specification	Units	Amount
Meat waste charge (~86% H2O)	(t/y)	120,000 160,000 200,000
Bio-ferment (70% H2O)	(t/y)	37,333 47,778 62,222
The surface fertilized fields, at a dose of 30 t/ha *	(ha)	834 1112 1390

* The dose used according to [78].

Table 3. Material balance of bio-ferment production.

Specification	Units	Amount
Meat waste charge (~86% H2O)	(t/y)	120,000 160,000 200,000
Bio-ferment (70% H2O)	(t/y)	37,333 47,778 62,222
The surface fertilized fields, at a dose of 30 t/ha *	(ha)	834 1112 1390

* The dose used according to [78].

3.3. The Model of Processing Farmutil’s Meat Waste, with the Integrated System Using Jointly the Hydrothermal Method and Bio-Fermentation

Table 4. Balance of meat waste utilization into meat bone meal (MBM) with the hydrolysis method and anaerobic digestion.

Capacity	Meat Waste Amount (t/y)	Production					Heat Needs for MBM (GJ/y)
		MBM (t/r)	Biogas (Million m3/y)	Methane (Million m3/y)	Bio-Ferment for Fertilization (t/y)	Heat (GJ/y)
					Raw Digestate
MBM	600,000 450,000 440,000	150,000 112,500 110,000					982,500 736,875 720,500
I category MBM	85,000	20,500					139,187
Biogas from I category of meat waste			27.2	16.32	94,250	587,520
Biogas plant power 4 MW	150,000 * 160,000 *		48.0 51.2	28.8 30.72	166,323 177,412	1,036,800 1,240,106

* Biogas yield 300 m³/t of meat waste; 1 m³ of biogas contains 60% methane; 1,000,000 m³ methane = 36,000 GJ.

presents the balance of meat waste processing, with a bio-fermentation of 25% of this waste and 75% processed by hydrolysis into MBM.

The quantities of waste proposed for bio-fermentation are estimated to be 160,000 t/y, and 440,000 t/y proposed for MBM processing. The charge for anaerobic digestion contains 85,000 t/y of I-risk category meat waste; 55,000 t/y of animal blood; and (t/y) bristles, 2883; hatching waste and eggs, 2800; manure, 500; screenings, 6000; waste fats 2400l; and sewage sludge 5617.

Figure 2 presents the balance data of the production of 150,000 t/y MBM by the hydrothermal method, in comparison to the integrated system of producing 110,000 t/y MBM by the hydrothermal technology, with simultaneous processing of 160,000 t/y of waste using bio-fermentation.

Another alternative would be the bio-fermentation of 85,000 t/y of the I-risk category animal waste currently processed into 20,500 t/y of I-category risk MBM. The quantity of biogas generated in bio-fermentation of 150,000 t of waste (25% of all meat waste) would cover ~100% of the heat needed for MBM production from the remaining 450,000 t (75%) of meat waste. This indicates the need to build a bio-fermentation unit with a capacity of 4 MW.

The heat needed to produce 110,000 t of MBM is 720,500 GJ/y. The analysis showed that the biogas obtained by the bio-fermentation of 160,000 t/y of meat waste (1,036,800 GJ/y) would cover 144% of the heat for producing 110,000 t/y of MBM. The developed integrated system would reduce the amount of wastewater produced by Farmutil HS [79] by 321,800 m³/y (40%) and reduce odor emissions by 27%.

3.4. Analysis of an Integrated Meat Waste Utilization System in the Context of Circular Economy Strategy Data

The assessed circular economy (CE) activities, sustainable development (SD) goals, and cleaner production (CP) methods used for the estimation of animal waste management systems are shown in

Table 5. Characteristics of circular economy (CE) activities, sustainable development (SD) goals, and cleaner production (CP) methods used for estimating animal waste management systems.

Animal Waste Management Method	CE, SD, and CP Activities Used for Estimating Animal Waste Management Systems
	CE Activities	SD Goals	CP Methods
Both methods MBM production with hydrothermal method, and the integrated system of production of MBM with hydrothermal method and parallel bio-fermentation of animal waste.	Incentivized high-quality recycling. Transparent and scalable production technology that can be used at other places. Take-back systems and collection, ensuring a continuous flow of materials for remanufacture, reducing damage to the environment. Converting of waste into new materials of higher quality and increased functionality. Rewarding up-cycling of materials. Elimination of odor emission. CE value diversity. Technology innovation.	Sustainable economic growth; diversify, innovate, and upgrade for economic productivity. Build resilient infrastructure, promote inclusive and sustainable industrialization, and foster innovation. Responsible consumption and production. Support domestic technology development and industrial diversification.	Changes in technology, processes, resources, or practices to reduce waste, environmental, and health risks. Minimize environmental damage. Changes in technology, input materials, operating practices. Off-site recycling of all types of animal waste. On-site energy recycling. On-site recycling of wastewater from MBM production.
Only The integrated system of production of MBM with hydrothermal method and parallel bio-fermentation of animal waste	Diversity as key driver of versatility and resilience. Product modularity composing products that can be upgraded with newer features and/or functionalities. Business opportunities, new products. Trade effects. Industrial symbiosis leads to new partnerships, cross-value chain cooperation, cost savings, and changes in the cost structure. Higher energy and material efficiency. Reduced material costs, and net employment. Effects due to technology innovations. Lower energy consumption. Conversion of waste materials into usable bioheat, including combustion, allows decreasing resource dependence and increases system resilience. Net profit impact, higher material productivity at the company level. The high economic value of produced renewable bioenergy. Higher productivity at the company level and reduced energy costs. Reduction of primary material consumption.	Promote sustainable agriculture, improving agricultural productivity Industry, innovation, and infrastructure. Ensure access to affordable, reliable, sustainable, and modern clean energy. Increase the global percentage of renewable energy. Double the improvement in energy efficiency. Improve resource efficiency in consumption and production. Action to combat climate change and its impacts by regulating emissions and promoting developments in renewable energy. Ensure sustainable consumption and production patterns.	Use energy and resources more efficiently. Increase business profitability and increase the efficiency of production processes. Changes in waste use. Waste reduction at the source. Energy recovery from waste and its reuse. Reduction of wastewater amount from MBM production. Production of biofertilizers substituted artificial fertilizers. Substitution of artificial fertilizers with recycled processed waste. Nutrient recovery.

. CE, CP, and SD activities allow for the development of new products or technologies without high upfront investment and cross-value chain cooperation [22,35]. The use of the newly developed integrated system of meat waste utilization wisely selects resources and chooses technologies and processes. In this model, the CE promotes a series of actions, business models, and measures covering the whole lifecycle of products and materials, from design through manufacturing, consumption, and waste management.

Economic growth would be achieved mainly through a combination of increased revenues from emerging circular activities and a lower cost of production due to a more productive utilization of inputs. To meet the CE assumptions, modern and innovative technologies allowing for the recovery of valuable materials were developed. Higher energy productivity and efficiency at the company level increased the economic value of the energy generated by a unit of material consumed. This includes the recovery of materials to serve as secondary raw materials for subsequent production and optimizing resource yields by circulating materials at the highest utility at all times in biological cycles [42].

3.5. The Economic and Environmental Effects of the Implementation of the Integrated System for Meat Waste Utilization

Table 6. Comparison of the economic effects of meat waste utilization in meat bone meal (MBM) production using hydrolysis and an integrated animal waste processing system.

Specification	Hydrolysis Method	Integrated Animal Waste Processing System
Amount of meat waste (t/y)	600,000	600,000
Amount of meat waste processed (t/y)	600,000 1	440,000 1	160,000 2
MBM production capacity (t/y)	150,000	110,000
MBM price (EUR/t)	563	563
MBM production value (million EUR/y)	84.45	61.93
Biogas production capacity (million m3 CH4)	-		28.8 2
Biogas price (EUR/m3)	-		0.78
Biogas production value (EUR/y)	-		22.46
Estimated operating costs (million EUR/y)	67.56	27.08	−22.46
Gross profit 3 (million/y) of a 4 MW plant	16.89	34.85
Investment costs (million EUR)		36.3
Return on investment costs with gross profit EB (y)		1.04
Bio-ferment fertilizer amount (t/y)		124,500
Including nitrogen, phosphorus, and potassium (t/y)		1120
Bio-ferment fertilizer price (EUR/y)		13.5
Bio-ferment production value (million EUR/y)		1.68
Size of the fertilized area (ha)		4150
Gross profit (million EUR/y) with bio-ferment value		36.53
Pay-back with gross profit, with bio-ferment value (y)		0.99

¹ Hydrolysis; ² bio-fermentation; ³ gross profit is the difference between the production value and its operating costs.

shows the calculations for the model presented in Figure 2. Processing 600,000 t/y of meat waste using the integrated system would require building a 4 MW biogas plant. This would be a relatively simple investment and needs no more than 1 year to complete. The estimated investment cost is EUR 36.3 million. The average price of MBM in Poland is EUR 563/t [80]. The recommended dose of bio-ferment fertilizer is 30 t/ha, according to [76]. The amount of nitrogen, phosphorus, and potassium in bio-ferment is estimated at 6, 2, and 1 kg/t, respectively. Its value is EUR 13.5/t. The price of natural gas is EUR 0.78/m³ [81]. For comparison, the profit from a 1 MW biogas plant operating with slaughterhouse waste is EUR 536,651/y [34]. Relatively low investment costs result from the location of the bio-fermentation installation on one of the developed plots, with media connected, inside Farmutil. This reduces the investment cost by half, compared to the investment costs of a new, undeveloped plot. This resulted in a two-times shorter return on investment costs with gross profit. The implementation of the integrated model results in a 26.7% increased capacity of MBM production without investment costs.

The economic benefits are complemented by a positive impact on the natural environment. Managing waste from the agri-food industry under controlled conditions will significantly contribute to energy production and help mitigate climate change.

Table 7. Comparison of the environmental effects of meat waste utilization in meat bone meal (MBM) production using hydrolysis and the integrated system of animal waste processing.

Specification	Hydrolysis Method	Symbiotic Method	Environmental Effects
Meat waste utilization into MBM (t/r)	600,000	440,000
Natural gas consumption (million m3/y)	27.3	0	Substitute 100% of natural gas with biofuel (biogas)
Carbon dioxide emission (t/y)	54,860	-	Elimination of 100% of CO2 emission
Meat waste utilization by anaerobic digestion (t/r)	0	160,000	Zero odor emission and waste release
Biogas production (million m3 CH4)	-	28.8	Production and use of biofuel (biogas) for MBM production
Wastewater treatment (m3/y)	800,000	478,200	40.2% reduction in biologically treated wastewater 40% increased capacity of Farmutil’s biological wastewater treatment plant without additional investment costs
Bio-ferment production (t/y) and use on 4150 ha	-	124,500	Substitute artificial fertilizer with bio-ferment fertilizer on 4150 ha of arable land
Avoided greenhouse gas emission due to bio-ferment use (t/y CO2eq) *	-	147,325	Reduction of GHG emissions due to the use of bio-ferment instead of artificial fertilizer
Sewage sludge (t/y)	7000	-	Elimination of sewage sludge used as feedstock for anaerobic digestion
Odor emission from MBM production (million m3/y)	200 **	146	Reduction of 54 million m3/y of odor emission from MBM production (27%)

* The annual amount of digestate used on a 1 ha area allows for avoiding 25.8–44.5 Mg CO₂eq of greenhouse gas emissions. An average of 30 t/ha of digestate is used [78]; ** [22].

presents the environmental effects of meat waste utilization in MBM production using hydrolysis and the integrated system.

4. Conclusions

The developed model for meat waste management at Farmutil assumes an integrated system processing meat waste through bio-fermentation and into meat bone meal MBM via the hydrothermal method. The material and energy flows of processing 600,000 t/y of meat waste using the hydro-thermal method (for the production of 150,000 t/y of MBM), were compared with the material and energy flow in the integrated model, of processing 440,000 t/y of meat waste into 110,000 t/y of MBM via the hydrothermal method, and simultaneously 160,000 t/y of waste through bio-fermentation. The analysis of the integrated system model showed that biogas production would cover the heat requirements for producing 110,000 t/y of MBM with the hydrothermal method. Implementing the integrated management system would reduce the amount of wastewater produced by Farmutil by 350,000 m³/y (45%) and reduce emitted odors by 25%. Additionally, the spare capacity of Farmutil’s existing sewage treatment plant could be used to treat municipal sewage for approximately 6000 residents. The biogas plant investment is relatively straightforward, and the implementation time does not exceed 1 year. The estimated investment cost for a 4 MW biogas plant is EUR 36.3 million. In this case, the return on investment, with gross profit, would occur in less than 1 year, and the plant should be highly profitable.