Transforming Livestock and Aquaculture Waste into Renewable Energy and Materials—A Review

Abstract

In recent years, concerns about sustainability in livestock farming have been raised. The livestock sector is accused of substantial greenhouse gas emissions, environmental pollution (i.e., wastewater with high COD and rich in N and P that can pollute freshwater and cause eutrophication), and resource consumption. The use of fossil resources to produce synthetic fertilizers is the major source of pollution indirectly attributable to livestock farming. However, the polluting load of the livestock sector can be used to produce energy and materials, increasing its sustainability. The scope of this work was to critically review the methods of management and valorization of waste from the livestock sector (slurry, manure, abattoir wastewater, slaughterhouse waste, and aquaculture waste). The various technologies for energy valorization (i.e., bio-H₂ and bio-CH₄) will be represented. The perspectives and challenges for the exploitation of these wastes to produce high-added-value molecules, extraction of bioactive molecules, alternative proteins, biofertilizers, and biopolymers will also be discussed in view of enhancing sustainability. Examples of possible large animal waste-based integrated biorefineries have also been proposed.

1. Introduction

Animal farming is one of the oldest and most deeply rooted industries in the world. Since the second half of the 1980s, demand for animal products has increased significantly, contributing to a shift in the global diet from a predominantly plant-based diet to a more animal-based one [1]. With the global population projected to reach approximately 9.9 billion by 2050 [2], the demand for animal protein is expected to increase further [3]. To meet this growing demand, there has been a strong global expansion, especially in pig and chicken farming, as well as aquaculture [2,3,4,5]. Terrestrial and aquatic animal farming has raised increasing concerns related to environmental impacts, including greenhouse gas emissions, land and water use, nutrient pollution, and animal health and welfare. At the same time, animal farming plays a central role in food systems worldwide [2,3,5,6,7]. It provides essential sources of proteins, contributes to improving nutrition and food security, and supports the livelihoods of millions of farmers [8]. Besides providing food, animal farming is also connected to cultural and social roles and provides raw materials for clothing, furniture, and other sectors. To address concerns about the negative impacts of animal farming, research emphasizes the importance of integrated strategies, including the dietary changes towards more plant-based diets, improved feed efficiency, responsible antibiotic use, enhanced animal welfare standards, and better waste management [9]. In recent years, aquaculture has emerged as a central component of global food production. This rapidly expanding sector now represents approximately 52% of the total aquatic food supply. Projections indicate that by 2032, farmed aquatic animal production will reach 111 million Mg, accounting for about 62% of total fish production [4,6,10,11]. In 2023, global farmed fish production reached 98.5 million tons, in addition to 38 million tons of farmed algae. China is the world’s leading aquaculture producer, generating 55.2 million Mg, which represents about 56% of global aquaculture output; in contrast, Europe’s aquaculture production remains comparatively modest, at approximately 3.4 million Mg (3.5% of the global share), with Norway alone contributing nearly 48% of total European aquaculture production [7]. Globally, the main farmed species include whiteleg shrimp, cupped oysters, carps, Nile tilapia, Atlantic salmon, trout, seabream, and seabass; most of these species are raised in fed aquaculture systems, which require external feed inputs, primarily based on fishmeal and fish oil [7]. Although aquaculture production is generally associated with lower environmental burdens compared to other animal protein production systems, feed use (spanning from feed ingredient production, feed consumption, uneaten feed, and nutrient release) remains the main driver of environmental impacts in fed aquaculture [4,12,13]. In contrast, non-fed aquaculture relies on species that obtain nutrients directly from their surrounding environment. This category includes filter-feeding and extractive organisms such as mussels, oysters, and seaweeds. Non-fed systems are generally considered more environmentally sustainable because they do not require external feed inputs and can even contribute to water quality improvement by removing excess nutrients from the ecosystem. However, global aquaculture production remains dominated by fed species. The share of farmed production coming from fed species increased from approximately 60% to 73% between 2000 and 2022 [8].

As mentioned, one of the impacts associated with aquaculture production relates to the release of nutrients and particulate matter into water bodies. If not properly managed, the discharge of organic substances, including uneaten feed and fecal matter, combined with nutrients like phosphorus and nitrogen, contributes to the deterioration of aquatic environments and sediments, with adverse effects on fish growth and overall ecological balance [10,14]. In particular, such a nutrient-rich substrate can lead to algal blooms (which can cause toxicity of seafood products) and increased metabolic activity of aerobic bacteria that consume available oxygen from the water. The latter can compete with other aerobic organisms characterized by slower growth and vulnerability. Furthermore, in conditions of low oxygenation, sulfate reduction is favored, which in turn inhibits the aerobic nitrification-denitrification process, leading to the death of sediment microfauna [10]. This, in turn, leads to a reduction in the aeration level of the water bodies, which increases the presence of methanogens [10], affecting CH₄ emissions from water bodies. Thus, wastewater treatment is crucial in aquaculture systems for both fish health and environmental protection.

The strong sector intensification and increased production also apply to livestock farming. From 2000 to 2020, the number of farm animals has increased significantly, reaching over 33.1 billion chickens, approximately 2.4 billion sheep and goats, 1.53 billion cattle, and almost one billion pigs [5]. The growth of the sector was largely driven by the increased demand for animal protein, which could no longer be met by small family-based farming systems. As a result, the expansion of intensive farms capable of large-scale production has taken place [1]. However, this transformation has also introduced several challenges, for example, the production of large quantities of manure, slaughterhouse waste, and agricultural residues (waste materials, co- and by-products), with increased excretion of nitrogen, phosphorus, and organic matter (i.e., wastewater with high COD and rich in N and P) that can pollute freshwater and cause eutrophication [1,2,15,16,17,18,19]. Moreover, to maintain animal health and ensure high levels of productivity, the massive use of antibiotics has become common [1,3,4]. However, the antibiotics are excreted and accumulated in animal slurry [3] with the risk of environmental contamination. Thus, even traditional practices that may seem sustainable, such as grazing or manure spreading on agricultural fields, may cause harm to the environment due to excesses of nutrients, unbalanced input of macronutrients, and the presence of microcontaminants such as metals and antibiotics [1,3,20].

As is known, GHG emissions from livestock come mostly from the enteric fermentation of ruminants. Methane is produced by hydrogenotrophic methanogens that live in their rumen. These microorganisms reduce the hydrogen derived from fiber fermentation into methane, counteracting rumen acidification. Unbalanced feed formulations are a key driver of elevated methane (CH₄) production [21]. Thus, natural enteric methanogenic activity can be limited but not suppressed. In addition, livestock manure is a major source of ammonia (NH₃), which can contribute to the formation of fine particulate matter and the generation of nitrous oxide (N₂O) [22]. Agriculture and livestock are also contributors to anthropogenic N₂O emissions [23]. Nitrous oxide is a GHG with a global warming potential 300-fold higher than CO₂ [24], which is released, together with CH₄, in uncontrolled conditions during the degradation of manure. It has been estimated that livestock manure contributes approximately 11%, 29%, and 52% of agriculturally derived CH₄, N₂O, and NH₃, respectively [22].

Overall, the food production sector makes a substantial contribution to greenhouse gas (GHG) emissions, generating 26% of all GHG emissions globally [13]. Of this share, 31% comes from activities directly linked to livestock and seafood production, whereas the remaining 69% refers to emissions related to land-use change (24%), crop production for food and feed (27%), and other activities such as transport, processing, packaging, and retail (18%) [13]. More specifically, approximately 40% are direct emissions, primarily from enteric methane production, and the remaining 60% are indirect emissions, arising from fossil fuel use in feed production, as well as manure and wastewater management, and the disposal of animal waste generated during meat and dairy processing [22]. In Europe, annually, the livestock-related emission accounts for over 247 million Mg CO₂ eq [25].

The reduction in farm-related emissions can be achieved through improved management of farming activities (for example, precision farming or precision aquaculture, or the use of feed with alternative ingredients), as well as through the implementation of technologies that rely on renewable energy sources or lower-emission fuels. On the other hand, one way to reduce indirect emissions is to create a circular economy. In fact, proper management of slurry, manure, and waste generated by livestock-related activities allows for the recovery of energy, high-value molecules, and biofertilizers, increasing the sustainability of the livestock. Consequently, the development of efficient, cost-effective, sustainable, and eco-friendly strategies for managing high volumes of animal waste has emerged as a pressing concern.

Animal-based products (i.e., food, leather, and wool) are integral to the global bioeconomy and continue to play a key role in modern society [5]. As the global population grows, so does the demand for these resources, making sustainable innovation not just desirable but essential. To remain resilient in the face of evolving consumer expectations, rising production costs, and environmental pressures, the animal production sector must embrace a circular economy model [22].

Circular economy models focus on valorizing materials such as by-products and residues that would otherwise be treated as waste. By redesigning production and distribution processes to minimize waste, the circular economy helps reduce environmental impacts and fosters more sustainable resource management. Closing resource loops enables upcycling, which restores or generates new value from used materials. This not only contributes to environmental sustainability but also offers a promising pathway to enhance the efficiency and economic viability of agricultural activities. Political strategies (e.g., the European Green Deal) aim to reduce greenhouse gas emissions by 55% by 2030, to achieve climate neutrality by 2050, and more generally to reduce the environmental footprint of the agri-food sector. The most common paths to achieve these goals are the use of renewable resources and reduced water consumption [10,26]. The reduction in waste through enhanced recycling and re-use using modern and environmentally sustainable technologies and management practices, as well as responsible consumption and production, are other key objectives of several policies and strategies, including the United Nations’ 2030 Agenda for Sustainable Development (e.g., SDG 12).

Like every system in modern society, the agri-food system generates a relevant amount of waste. Globally, the agricultural sector generates approximately 1 billion Mg of waste annually, which is at least 10% of the total waste [18,19]. Recognizing and addressing this dimension is essential for building a truly sustainable and circular economy. By integrating the circular economy and renewable energy into this ecosystem, it is possible to achieve a cascade production system with waste and secondary raw materials as feedstock for an integrated biorefinery. This improves livestock sustainability, resilience, and soil conservation.

The scope of this work was to critically review the methods of management and valorization of waste from the livestock sector (slurry, manure, abattoir wastewater, slaughterhouse waste, and aquaculture waste), which represent a hot topic due to the rise in environmental concerns. The various technologies for energy valorization (i.e., bio-H₂ and bio-CH₄) will be represented. The perspectives and challenges for the exploitation of these wastes to produce high-added-value molecules, extraction of bioactive molecules, alternative proteins, biofertilizers, and biopolymers will also be discussed with a view to enhancing sustainability.

2. Methods

Several articles related to livestock wastewater composition, utilization, and valorization were evaluated for this review. The bibliographic search was carried out in various databases (Google Scholar, Scopus, Science Direct, Web of Science), covering the period from 2014 to 2025 and limited to papers written in English. The search focused on publications related to the following key terms and their combinations: “waste”, “livestock”; “aquaculture”; “slaughterhouse”; “abattoir”; “treatments”; “disposal”. The papers were organized and compiled in Mendeley (v2.137.0), with each paper classified by relevance to the specific topic of this review. A total of 3147 papers were retrieved, of which 180 were selected as the most relevant to the topic after title and abstract evaluation and citation rate by year. In addition to the bibliometric data, information about the experimental methodology and waste characteristics was also evaluated, such as physicochemical properties of the effluent, methods of effluent treatment, and results of new methods proposed. The articles not considered were principally there with no data related to the chemical and physical characterization of the waste. Thus, 180 papers were effectively useful for this review. Possible limitations of this work include the fact that the databases used may not cover 100% of the relevant literature. Furthermore, the section on existing methods of utilizing organic biomass from livestock was based on a narrow selection of the literature, considering only the most recent reviews (i.e., the last 5 years), since these methods are well-known and well-established.

3. Waste Sources, Quantification, and Characterization

Livestock waste mainly falls into two categories: on-farm waste (i.e., manure, slurry, and sludge) and process waste (i.e., abattoir waste, wool, and feathers). Typically, on-farm waste consists of highly diluted animal excreta, collected in large tanks that are difficult to manage due to limitations affecting disposal. Process waste, on the other hand, consists of unusable animal parts. Due to its nature, managing this waste is particularly risky. Aquaculture produces both solid and dissolved wastes. Solid waste sources are primarily uneaten food or undigested components discharged as feces, which can be classified into suspended solids and settled solids. Suspended solids are considered the most difficult to remove and pose significant challenges in all forms of intensive aquaculture. In contrast, settled solids are readily removed through sedimentation as larger particles [4].

If not managed correctly, all the animal waste listed above can cause serious environmental and health problems. Additionally, because they are rich in organic substances and nutrients, they are hard to treat using conventional systems [27].

3.1. Manure, Slurry, and Sludge

In 2022, 9.26 billion Mg of animal manure and slurry [5] were generated globally from land-based livestock farming. Fifteen percent (1.4 billion Mg) of the total manure is produced in the EU [27], although the EU represents only 3% of the total land area. Regarding aquaculture wastewater (AWW), to the best of the authors’ knowledge, there is no precise quantification and updated data. However, considering the global aquaculture production (94.4 million Mg [6]) and the estimation of the sludge produced per kg of farmed fish (about 3 kg sludge kg⁻¹ farmed fish [28]), it can be estimated that 283 million Mg of sludge are generated every year. Therefore, annually, the livestock sector generates over 9.5 billion Mg of manure, slurry, and sludge. Of this, cattle generate approximately 50% of livestock waste; the remaining 50% is generated by other farmed species, in particular sheep, chickens, pigs, and fish [5] (Figure 1).

Figure 1. Annual worldwide livestock manure and slurry generation.

Large livestock such as cattle and pigs generate manure and slurry, which are mixtures of urine and feces. The composition of pig slurry is affected by the type of farming (i.e., closed-loop or fattening only), with only fattening farms generating more concentrated slurry than in the closed-loop farms (characterized by the simultaneous presence of animals at different growth stages fed with specific diets). Similarly, dairy cattle produce more diluted waste than beef cattle, whose diets are richer in concentrates. Nevertheless, livestock wastewater can contain both antibiotics and hormones (i.e., progesterone) commonly used in animal farming. However, regardless of the species and type of farming, for terrestrial animals, manure and slurry can easily be grouped and treated together.

The case is different for chickens and fish. They generate peculiar litter constituted by a heterogeneous mixture of manure, feed, and antibiotics (and feathers in the case of chickens) [14,29]. Given the presence of feed residues, these particular litters are highly hazardous for the environment, both due to the high and easily perishable organic load they abound with and because they are particularly prone to NH₃ emission. Ammonia is formed from the microbial decomposition of nitrogen-containing waste (feces and uric acid) as well as proteins contained in the non-ingested feed that typically constitutes poultry and fish litter. Concerning nutrient release into the water, 14% and 40% of ingested N and P, respectively, are excreted in the feces, while 57% and 34% of N and P are excreted directly in water [30]. Consequently, uneaten or undigested commercial feed, as well as fertilizers used to stimulate primary productivity in fishponds, represents a major source of water pollution in aquaculture due to their high phosphorus and nitrogen content [9,31]. A comparison of the composition of manure, slurry, and sludge is summarized in Table 1.

Table 1. Characterization of manure, slurry, and sludge of different farmed species, according to data reported in the literature.

Type of Waste	Parameters	Range	Reference
Cattle manure and slurry	Total Solids (g kg−1 FW)	72–310	[27,32,33,34]
	Organic Matter (% TS) COD (g kg−1) Total N (% TS) Ammonium N (% TS) Total P (% TS) Total K (% TS) pH	75–89 291–1082 1.77–4.56 0.25–2.37 0.3–1.84 1.06–6.95 6.8–8.6
Poultry manure	Total Solids (g kg−1 FW)	132–849	[27,29,32,35]
	Organic Matter (% TS) COD (g kg−1) Total N (% TS) Ammonium N (% TS) Total P (% TS) Total K (% TS) pH	32–85 35.9–945 2.34–5.45 0.42 1.58–6.13 2.66 6.9–9.0
Pig slurry	Total Solids (g kg−1 FW)	12–124	[2,20,27,32,33,36,37,38,39,40,41,42,43,44]
	Organic Matter (% TS) COD (g L−1) Total N (% TS) Ammonium N (% TS) Total P (% TS) Total K (% TS) pH	66–90 1.3–135 4.96–12.2 2.32–4.6 0.45–3.60 5.1 6.7–7.7
Aquaculture sludge	Total Solids (g L−1)	14.3	[45]
	Organic Matter (% TS) COD (g L−1) Total N (% TS) Ammonium N (% TS) Total P (% TS) Total K (% TS) pH	33 4 1.4 0.08 N.A. N.A. 5.9

FW: Fresh weight; TS: Total solids; COD: Chemical oxygen demand. N.A.: Not Assessed.

3.2. Waste and Wastewater from Slaughterhouses and Seafood Processing

It has been estimated that approximately 220 million Mg of meat is produced annually worldwide. Cattle and buffalo account for 62% of this total production, with each species accounting for 31% of the total production, followed by poultry (11%), pigs (10%), and goats and sheep (10% and 5%, respectively) [46].

Slaughterhouses generate approximately 150 million Mg of solid and liquid waste per year, rich in organic contaminants and nutrients (i.e., proteins, lipids, blood, intestinal contents, manure, and cleaning residues). The waste comes from various stages: preparation, slaughter, recovery, and reprocessing of by-products. This waste presents significant critical issues, such as high biochemical oxygen demand, a tendency to compact, unpleasant odors, and biological risk. The percentage of solid and liquid waste relative to the live animal weight is relevant, corresponding to 66%, 52%, and 80% of the overall live weight of cattle, lambs, and pigs, respectively [47].

For the past 80 years, by-products have been used to produce animal feed. However, regulatory restrictions and growing environmental awareness have made waste management a key challenge for the meat industry. European Union legislation divides animal by-products into three categories, each with specific levels of risk and treatment requirements [48,49], summarized in Table 2.

Table 2. Regulatory classification of animal by-products (EU) [48,49].

Category	Risk Level	Types of Waste	Treatment Requirements
Category 1	Very High	Parts of infected animals, international catering waste.	Prohibited composting or biogas plants.
Category 2	High	Sick animals, intestinal contents, manure	Sterilization at 133 °C, 300 kPa for 20 min.
Category 3	Low	Food waste, meat, pre-cooked foods, by-products from the fish processing	Treatment at 70 °C for 1 h in a closed system.

While manure is categorized as a Category 2 animal by-product (ABP), aquaculture sludge is still not regulated, nor is it merely mentioned in the ABP legislation [49]. Currently, only a small percentage of by-products are valorized to produce feed, fertilizer, or glue. However, landfill is still the primary disposal method, causing environmental contamination, soil degradation, air pollution, and health risks [47]. Nevertheless, by-products are rich in proteins, lipids, collagen, keratin, fibers, and minerals, which can be exploited with an integrated cascade biorefinery approach to produce biofuels, biomaterials, building blocks, and advanced feed. This approach would significantly contribute to reducing soil, air, and water pollution while also reducing health risks and accelerating the green economy transition. In the EU, waste by-product recovery has become important for slaughterhouses with a 50 Mg per day slaughter capacity [50].

The average number of large animals such as cattle, pigs, goats, and sheep slaughtered each year is approximately 2.8 billion heads [50,51]. Half of them are represented by pigs (1.4 billion heads), 22% are sheep, 17% are goats, and 11% are cattle (Figure 2). Please note that for some species, i.e., pigs, the number of animals slaughtered per year is higher than the number of farm animals given in the Introduction because the farming cycle is less than 1 year.

Figure 2. Number of animals slaughtered annually, classified by species.

The main wastes from pig slaughter are intestines, bones, blood, and skin, as well as non-marketable meat and fat. Specifically, 4% of the pig’s weight is waste [46]. Thus, each slaughtered pig generates 2.3–6.8 kg of solid waste (depending on marketing weight), while abattoir wastewater from pigs ranges between 2.2 and 5.5 m³ per day [52]. In the case of ruminants, sheep and goats generate approximately 2.5 kg of solid waste per animal, while cattle generate 275 kg of solid waste per Mg of live weight [46]. The main slaughter waste from ruminants, particularly cattle, is paunch waste. Each slaughtered animal generates an average of 25–60 kg of wet paunch waste [17]. Daily abattoir wastewater generation for ruminants ranges between 0.4 and 3.1 m³ [53,54].

Nearly 74 billion chickens and over 4 billion ducks are slaughtered annually [51]. Global chicken meat consumption, estimated at over 103,046 Mg in 2024, has intensified waste production, reaching 68 billion Mg annually; poultry slaughterhouse effluent is rich in fat, blood, feces, feathers, and wash water from bird slaughter [29,55].

The fish processing industry also generates large amounts of waste. Between 20% and 80% of a fish’s total weight becomes waste, depending on the species and processing methods [56,57,58,59]. Globally, it is estimated that 9.1 million Mg of fish waste are generated annually [60]. This waste is mainly composed of muscle trimmings (15–20%), viscera (12–18%), bones (9–15%), heads (9–12%), scales (5%), and skin and fins (1–3%) [61]. Shrimp/prawn processing can generate up to 60% of waste, which includes carapace and head [62]. In addition, depending on the fish processing (i.e., canning, freezing, skinning, or fileting), 5–15 m³ of wastewater can be produced daily [58]. An overview of the composition of the slaughterhouse and the processing of seafood is shown in Table 3.

Table 3. Characterization of slaughterhouse solid and liquid waste of the different species, according to data reported in the literature.

Type of Waste	Parameters	Range	Reference
Cattle slaughterhouse solid waste	Total Solids (g kg−1 FW)	117–158	[17,46]
	Organic Matter (% TS) COD (g kg−1) Total N (% TS) Ammonium N (% TS) Total P (% TS) Total K (% TS) pH	90–93 111 1.79–10 N.A. 0.6–7.38 N.A. N.A.
Cattle slaughterhouse wastewater	Total Solids (g L−1)	7.5	[17,63]
	Organic Matter (% TS) COD (g L−1) Total N (% TS) Ammonium N (% TS) Total P (% TS) Total K (% TS) pH	75 5–18 2.67 0.21 1.44 N.A. 7.1–7.8
Pig slaughterhouse solid waste	Total Solids (g kg−1 FW)	458	[48]
	Organic Matter (% TS) COD (g kg−1) Total N (% TS) Ammonium N (% TS) Total P (% TS) Total K (% TS) pH	82 N.A. 5.28 0.44 N.A. N.A. N.A.
Pig slaughterhouse blood	Total Solids (g L−1)	179	[48]
	Organic Matter (% TS) COD (g L−1) Total N (% TS) Ammonium N (% TS) Total P (% TS) Total K (% TS) pH	94 N.A. 15.1 0.95 N.A. N.A. N.A.
Poultry slaughterhouse effluent	Total Solids (g L−1)	2.03	[29]
	Organic Matter (% TS) COD (g L−1) Total N (% TS) Ammonium N (% TS) Total P (% TS) Total K (% TS) pH	76 2.4 11.3 N.A. N.A. N.A. 6.9
Seafood processing solid waste	Total Solids (g kg−1 FW)	301–350	[64]
	Organic Matter (% TS) COD (g kg−1) Total N (% TS) Ammonium N (% TS) Total P (% TS) Total K (% TS) pH	82–89 470–530 7.67–7.94 N.A. N.A. N.A. N.A.
Seafood processing wastewater	Total Solids (g L−1)	2–25	[57,58]
	Organic Matter (% TS) COD (g L−1) Total N (% TS) Ammonium N (% TS) Total P (% TS) Total K (% TS) pH	N.A. 1.4–25 2.48 0.8–1.4 N.A. N.A. N.A.

FW: Fresh weight; TS: Total solids; COD: Chemical oxygen demand. N.A.: Not Assessed.

As seen above, slaughterhouse waste is divided into solid and liquid. Although wastewater is generally highly diluted, it has a substantial organic load and cannot be directly discharged into body waters. However, its proper management is challenging given its high-water content, which requires large spaces to handle the daily volumes generated. Adequate valorization of this wastewater can both reduce the pollutant load and generate added value from waste. Still, it can also constitute an important reserve of water from unconventional sources for non-potable use. Interestingly, for seafood processing wastewater, there is a notable difference in terms of COD concentration depending on whether the wastewater is fish processing or shrimp/crab processing (i.e., 3.5 and 16 g L⁻¹, on average, respectively) [57].

3.3. Feathers and Wool

Feathers and wool are two specific wastes generated by chicken and sheep farming, respectively. While feathers accumulate both in poultry litter and, especially, as waste from chicken slaughter, wool is a waste produced both during sheep slaughter and if the sheep are raised for milk production. Obviously, wool is not waste if the purpose of farming is wool production. These two specific livestock wastes share a notable recalcitrance to degradation since they must protect the animal from atmospheric agents, pathogens, and dirt. Therefore, they can accumulate and contaminate the environment even if dumped in a landfill [15,65], and they are also highly resistant to degradation. Although they are classified as category 3 waste, or low-risk waste (see Table 2), in the EU, they cannot be used for feeding domestic animals since animal flour as a protein supplement is strictly prohibited due to the high risk of transmission of animal diseases (i.e., swine fever, bovine spongiform encephalopathy, Creutzfeldt–Jakob disease, etc.) [66].

Approximately 33–37% of the live weight of each chicken is discarded as waste during slaughter [46]. Feathers and skin represent over 57% of this poultry waste [46]. Thus, it has been estimated that approximately 8.5 billion Mg of chicken feather waste is generated every year [67]. Feathers are composed of 90% keratin [68], a protein that is difficult to decompose. Furthermore, pathogens accumulate in feathers. Currently, waste feathers remain largely unused and are disposed of [67]. However, their recalcitrance facilitates their accumulation in the environment even if disposed of in landfills. Moreover, their high protein content leads to the emission of NH₃, which is formed from protein degradation, nitrate leaching into groundwater, and phosphorus runoff into aquatic bodies [68].

Wool, once considered a precious commodity, today finds itself in an ambivalent position: on the one hand, it is an excess by-product, on the other, it represents a potential resource for sustainable agricultural practices. The decline in global demand for wool, caused by the spread of synthetic fibers and high processing costs, has led to inefficient management of the material, with serious environmental consequences. Since only a small number of countries (Australia, New Zealand, China, the United States, Iran, Argentina, Turkey, the United Kingdom, India, Sudan, and South Africa) are the major producers of wool for the textile and fabric industry [15], sheep shearing, although necessary for animal welfare, represents only a cost (about €1.80 per sheep) for the farmer [19].

In Europe, where wool quality is often lower than in the main producing countries, the problem of disposal is acute. In fact, 80% of wool produced in the EU is unused and unmarketable, meaning it is waste [19]. Therefore, 160,000 Mg of wool are discarded annually [69]. In general, wool should be properly disposed of in landfills. However, this represents an unsustainable and environmentally unfriendly practice [15] that adds to the cost of sheep shearing. Thus, illegal practices such as burying or burning raw wool are often resorted to, which not only violate European regulations but also generate soil and water pollution [19]. Due to the widespread nature of these illegal practices, the amount of waste wool produced annually may be significantly underestimated [70]. The presence of lanolin and contaminants such as pesticides, heavy metals, and other substances that can accumulate in wool makes wool a vector of environmental risk if not treated correctly [15].

However, it is precisely this chemical composition that can transform wool from waste to a resource. Rich in carbon, nitrogen, sulfur, and micronutrients, wool can improve soil fertility, regulate pH, and help reduce the use of chemical fertilizers [15]. Furthermore, keratin structure allows for a slow release of nutrients, reducing nitrate leaching and water pollution [71]. Conversely, despite its intrinsic characteristics, wool is currently classified as solid waste, the management and disposal of which pose a growing environmental problem due to the complexity of the operations required for its treatment [19].

In conclusion, the challenge is to rethink the use of the whole animal waste generated. Investing in technologies for the recovery and valorization of this waste can transform a cost for farmers and an environmental problem into an opportunity for sustainable agriculture and the circular economy. In Table 4, a summary of the available amount of waste is given.

Table 4. Summary of the waste quantity (Mg) annually generated, categorized by farm animal.

Farm Animal	Manure/Sludge (Mg)	Processing (Mg)	Total (Mg)
Ruminants	5.8 × 109	8.6 × 107	5.9 × 109
Pigs	4.8 × 108	6.4 × 106	4.8 × 108
Poultry	7.6 × 108	7.7 × 1010	7.8 × 1010
Aquaculture	2.8 × 108	9.1 × 106	2.9 × 108

4. Strategies, Limitations, and Challenges

Over the years, various strategies for managing and disposing of animal waste (i.e., livestock waste, slaughterhouse waste, and dairy wastewater) have been developed, each with its own limitations that research is attempting to overcome. Indeed, with technological advancements and the growing demand for sustainability, new opportunities for utilizing each type of animal waste are emerging.

4.1. Livestock Waste (Manure, Slurry, Sludge, and AWW)

Besides the release of greenhouse gases such as CO₂ and CH₄, the use of livestock waste as fertilizer poses numerous environmental and health risks [3]. For example, it can affect soil and water quality because of the high amounts of N, P, and heavy metals (HMs), soluble substances, and pathogens [1]. In addition, it can affect microbial biodiversity or lead to excessive vegetative growth, irregular fruit ripening, reduced crop quality and yield [1,2,21] and the spread of antibiotic resistance genes (AGRs) [3]. In this regard, it is worth mentioning that livestock waste contains residues of antibiotics and steroid hormones [1,3,4,72], which are difficult to remove with conventional methods and can infiltrate into groundwater and surface water, damaging ecosystems. Furthermore, the concentration of antibiotics in livestock waste is expected to increase as their use is projected to rise by 11.5% in 2030 [3]. Thus, another emerging issue with the current livestock waste disposal system is antibiotic resistance. Indeed, to date, manure management has not been designed to mitigate antimicrobial resistance [3]. The management of livestock waste storage can alter its nutrient content, influencing both emissions generated during treatment and application and nutrient uptake by plants [22,27]. For each Mg of manure directly applied as fertilizer, 43–50 kg CO₂eq are emitted [73]. Currently, livestock waste is treated with physicochemical methods (such as screening, sedimentation, flotation, filtration, adsorption, flocculation, advanced oxidation, and photocatalysis). These methods can remove some contaminants but do not always guarantee the non-toxicity of the degradation products [1,2,14,74]. In addition, they are expensive methods [2,74]. The most consolidated chemical treatment to reduce NH₃ emissions from livestock waste is acidification using sulfuric acid (H₂SO₄). At an acidic pH, ammonia does not volatilize and is transformed into ammonium ion (NH₄⁺). This reacts with H₂SO₄ to form ammonium sulfate, which can be used as a fertilizer. Typically, up to 17 kg H₂SO₄ Mg⁻¹ of slurry may be required [75]. To streamline the process, the NH₃ stripping technique can be integrated [76]. In this case, slurries are heated to promote the volatilization of NH₃, which is conveyed into an air flow that is passed through an H₂SO₄ solution [76,77], which precipitates it as ammonium sulfate. In addition to being heated, the slurry should also be alkalinized: in fact, to maximize NH₃ removal, the best conditions are 70 °C and pH 8.5 [78]. This requires further use of chemical agents. It follows that in any case, the process is impactful, expensive, and corrosive to equipment, and, in any case, leaves residues [77].

Coagulation, which is effective in reducing contaminants, can be made more sustainable by replacing chemical coagulants with natural, biodegradable, and non-toxic alternatives [74].

Although wastewater management is a well-established topic for terrestrial farming, it is an emerging issue for aquaculture. First, the use of direct water is often limited because of its salinity [26]. Then, AWW management is often inefficient [10], mainly due to the large volumes of water typical of these wastewaters. As for livestock waste, conventional treatment methods are expensive, energy-intensive, and release greenhouse gases, without effectively recovering nutrients [10,74]. Furthermore, catalyst dispersion and deactivation represent operational challenges, as well as health risks [74]. Even biological methods, although efficient, raise questions about the management of residual sludge, which may contain persistent pollutants [1]. A robust, economical, and easy-to-apply technology is biofloc [14], which promotes the conversion of NH₃ to nitrite and nitrate by nitrifying and denitrifying bacteria. The nitrogen is then used by heterotrophic bacteria growing in the presence of external sources of C. However, the nitro–denitro system emits N₂O (i.e., 1–20% per N input) [14,79] in addition to water contamination with nitrite or nitrate.

Membrane filtration technology has been implemented for livestock waste treatment remediation. This allows for the reduction in pollutant load and the recovery of resources such as macronutrients. Nanofiltration, ion-exchange membranes, and especially reverse osmosis have been successfully proposed [1,14,33]. Membranes have also been integrated into both aerobic and anaerobic biological treatments, as well as in mixed contexts. In particular, aerobic reactors such as biological filtration sequencing batch reactors (SBRs) or membrane reactors, both aerobic (MBRs) and anaerobic (AnMBRs) [1], combine energy recovery with agricultural resource recovery.

In recent decades, interest in resource recovery from livestock waste has grown. For example, it has been estimated that the production of polyhydroxyalkanoates (PHAs) from European treatment plants could cover 120% of global biopolymer production recorded in 2016 [2]. Considering the availability and composition of livestock waste, it is clear that livestock waste can make a significant contribution to the production of these biopolymers. Furthermore, by exploiting biological treatments, it is also possible to obtain single-cell proteins (SCP) and other molecules.

In any case, the outflows from livestock and aquaculture must be properly managed. Some compounds, such as sulfonamides, are poorly adsorbed by sludge [1]. The degradation of hormones requires a complex process that occurs in several steps: hydroxylation, oxidation, and breaking of side chains [1]. Finally, the removal of pathogens is another parameter to be monitored at the end of the livestock waste management process before disposing of it safely. Mechanisms for reducing pathogen load include biosorption, precipitation, and environmental inactivation [1,3]. The treatment of animal production waste presents several challenges, including high energy consumption [2,10,74]. This aspect is not secondary since it has been estimated that fossil fuels could be exhausted within the next 50 years [2].

4.2. Slaughterhouse Waste

Slaughterhouse waste is commonly incinerated or landfilled [17]. Furthermore, animal processing waste may contain pathogens or hazardous substances, making it difficult to directly reuse in the food and feed industry [16]. Due to rising costs, the industry is continuously investing in the development of technologies capable of maximizing the value obtained from carcasses and minimizing waste [16]. For this reason, alternative solutions that do not compromise safety are needed. Some waste, such as offal and hides, is transformed into useful products (i.e., animal feed, gelatine, pharmaceuticals). This demonstrates how innovation has transformed what was once considered waste into an economic resource.

Conventional techniques (incineration, landfill, spreading on non-agricultural land) are no longer considered best practices [17,46,47,50]. Given the relatively high water content, solid slaughterhouse waste is subjected to dry dumping, which reduces water use by up to 90% compared to wet dumping and limits pollution [17,51]. However, all these techniques either have an impact or do not allow for efficient valorization of this waste. Slaughterhouse waste offers opportunities for energy and nutrient recovery if treated with appropriate technologies. For example, paunch has a high lignocellulose content (over 60%), making it an ideal biomass for renewable energy production [17,50]. Furthermore, slaughterhouse waste can be used to produce biocomposites for use in agriculture or industrial processes [16,47], further reducing the environmental impact.

Recently, a coagulation-flocculation process coupled with electrooxidation has been studied as a method for slaughter wastewater remediation, an alternative to both aerobic and anaerobic biological treatments [63]. The electrooxidation process requires electrical energy and can be direct if it occurs directly on the anode surface or indirect if it occurs through electrogenerated oxidizing agents such as chlorine, ozone, or hydrogen peroxide. Although the method has proven efficient and profitable, it must be taken into account that it requires energy, chemicals, and catalysts that may have an environmental impact [14,63,80]. Moreover, there is no possibility of valorizing the waste from either an energy or resource recovery perspective.

4.3. Strategies to Produce Energy from Waste

The reuse of nutrients from slaughterhouse waste, as well as animal production waste, can increase the efficiency of agricultural systems. However, traditional systems are expensive and often energy intensive [81]. The use of renewable energy has been advocated both to lower costs and reduce the impact [16]. Indeed, livestock and slaughterhouse waste can be used as renewable biomass to produce energy and to recover nutrients useful for agriculture. In this sense, anaerobic digestion (AD) is a flexible and quite conservative technology since it is possible to produce biogas (a mixture of CH₄ and CO₂) and recover macro- and micronutrients that are used but not consumed by the microorganisms in AD. Therefore, this process is useful for both treatment and energy recovery. However, its effectiveness depends on the characteristics of the waste (lipids, proteins, C/N), which can also inhibit the process. The major limitation of livestock and slaughterhouse waste is that it is diluted. To improve profitability, the co-digestion of high-energy waste in centralized plants is proposed [16].

The low C/N ratio implies that these substrates are prone to release NH₃ in AD, a strong inhibitor. The simplest strategy to overcome AcoD is with C-rich feedstock, which allows for nutritional balance in the system, avoiding inhibition phenomena [82,83,84]. Furthermore, the simultaneous digestion of different organic wastes from the same geographical area facilitates management and disposal and reduces their impact because AcoD residues can be recycled within the same territory [83]. Typically, C-rich feedstocks are mostly lignocellulosic by-products or marginal biomass that have to be managed locally if they are not themselves waste.

Although there is thriving research on processes and methodologies for valorizing livestock and slaughterhouse waste, the development of sustainable technologies must be accompanied by favorable policies, economic incentives, and regulations. In particular, the real leap forward for advanced use is recognizing that these biomasses are no longer waste but secondary raw material that can only be used in a closed and safe process such as a large-scale integrated biorefinery.

4.4. Strategies to Reduce Waste in Aquaculture

Aquaculture can be carried out through different systems. The most widely used fish farming system worldwide is pond aquaculture, given the low initial financial investment and technology level. This method is common for species like carp, which is one of the most farmed fish globally [6]. In pond systems, solid waste (i.e., excess and uneaten feed) settles at the bottom. The sediment is decomposed by microbial community processes, which can convert toxic molecules into less harmful compounds [85,86]. Given the semi-natural management of ponds and their dependency on internal processes, problems of water quality and solid waste removal represent a shortcoming [87]. Waste management practices adopted in pond systems mainly rely on pond drainage and sediment removal [85,86].

Cages (or open-net pens) are used primarily in marine environments to farm fish such as salmon, sea bass, and sea bream, but also in freshwater, such as tilapia. These systems are widely used for intensive aquaculture purposes: they allow farming large biomasses in natural water bodies, with relatively little investment. Cages are composed of large floating net enclosures. The water exchange with the natural environment is indeed continuous and passive. The waste types generated in cage aquaculture are similar to those previously listed. However, in cage aquaculture, there is no control of waste discharge, as it is an open aquaculture system. In addition to water quality degradation, cage aquaculture raises concerns because of the accumulation of solid waste on the seabed and escape of farmed fish [88].

Both systems described have a significant impact on the environment due to their extreme proximity to nature. Over the years, less impactful and isolated aquaculture systems have been studied, such as raceways and recirculating aquaculture systems (RASs).

Flow-through/raceway aquaculture systems, commonly used for species like trout, salmon, and tilapia, are composed of narrow and shallow earthen or concrete channels. The water used in raceway systems usually comes from a river nearby, entering at one end of the channel and exiting at the other. This allows for continuous oxygenation and waste removal, but at the same time, the water flow rate (water retention time < 1 h) [86] does not grant sufficient time for organic matter biological decomposition processes, resulting in the discharge of the waste produced into the main stream [88], unless there are sedimentation and biofiltration systems after the outlet of the channel. The main types of waste produced in raceway systems are feces and uneaten feed pellets, sludge (especially if a sedimentation tank is present), dissolved nitrogen compounds, phosphates, and organic matter.

An RAS is a highly controlled and efficient method of fish farming. It is generally used in intensive farming (i.e., salmon, shrimp). An RAS continuously filters and always reuses the same water within the system; only the water lost via evaporation is reintegrated into the system (5–10%/day) [10,14,74]. It also includes settlers for collecting sediment and suspended particles and gas exchange devices to eliminate dissolved CO₂ and add O₂ [88]. Like other closed and semi-closed/open aquaculture systems, the waste generated in an RAS is sludge, feces, uneaten feed, nitrogen and phosphorus compounds, and organic matter.

In raceways, it is difficult to decompose solid waste, and so the effluents have a high organic load that must be treated specifically [4]. On the contrary, in RAS, between 85 and 98% of organic matter can be removed [4], allowing water reuse and reducing pollution [10,14,74]. However, the system presents critical issues such as nitrate accumulation, lowering of pH, and high energy consumption [10,74]. To improve efficiency, biofilters for denitrification are used, which transform nitrogenous compounds into gaseous nitrogen, reducing operating costs, although requiring specific resources and skills [10]. However, not all species are suitable for an RAS. Despite this high efficiency, an RAS still generates wastewater, which annually accounts for between 0.3–3 m³ kg⁻¹ of fish [88] that must be managed.

A further step forward in the sustainability of aquaculture is represented by aquaponics, which combines aquaculture and hydroponic cultivation, allowing plants to absorb nutrients and return purified water, creating a circular and sustainable system [10]. However, this system is not able to absorb all waste flows. That is, it still generates a waste flow that must be managed. In fact, not all the nutrients from AWW are recovered [89] and must be removed to avoid problems for the fish, while hydroponic cultivation obviously has production waste [90]. Moreover, vegetables and waste from hydroponics cannot always be used for fish feed [91,92] and, in any case, in limited quantities [93]. On the other hand, AWW often fails to cover crops’ needs in terms of macro- and micro-nutrients [94,95].

Finally, integrated multitrophic aquaculture (IMTA) (Figure 3), combining different species in a single ecosystem [10], allows for closing nutrient cycles and increasing environmental resilience. Integrated multitrophic aquaculture uses several complementary species to optimize nutrient balance and reduce organic pollution [10], especially by using low-trophic, extractive species whose farming practices displayed low environmental footprints, such as mussels, clams, and oysters [96,97]. In addition, in some instances, the productivity of certain species can even increase in IMTA systems (i.e., shellfish production increased by up to 20% in IMTA [10]). In these systems, organisms from lower trophic levels (such as mollusks and algae) absorb nutrients and organic particles, transforming waste into useful resources. For example, algae can be used as biofilters for AWW and returned to IMTA as feed or to extract commercially valuable substances (i.e., bioactive substances with antioxidant activity such as chlorophyll, carotenoids, vitamins E and C, polyphenols, amino acids, enzymes, and polysaccharides) [10], contributing to a more sustainable and circular management of aquaculture. Although IMTA can improve productivity and profitability, it has limitations regarding the reduction in environmental impact: similarly to RASs, IMTA is not capable of being a zero- or near-zero-waste system. In fact, it only reduces 7.5% of organic deposits compared to finfish monocultures [10]. For instance, IMTA for gilthead bream (Sparus aurata) with Pacific oysters (Crassostrea gigas) still generates 122 Mg of particulate organic matter per year, compared to 132 Mg of finfish monocultures [98].

Figure 3. A schematic representation of integrated multi-trophic aquaculture.

5. Opportunities

Increasing sustainability is imperative for the livestock sector, whose productivity depends on the environment and climate conditions more than any other. Indeed, animal waste is a mine of energy and materials that can also be reused by the industrial sector. Various technologies for the recovery of this waste are available and, potentially, can be integrated with each other to maximize process efficiency. Below are several processes for the recovery of various animal wastes, exploring the possibility of integrating them.

5.1. Biorefining of Feathers and Wool

Feathers and wool are protein-rich biomasses. They offer significant potential for conversion into high-value products (Figure 4), such as protein hydrolysates for high-quality feed and fertilizer [2,15,68] and as a soil improver [71]. However, despite this promising potential, processing facilities for feather and wool waste remain scarce. This is likely due to the high infrastructure costs and low efficiency of current conversion methods [65].

Figure 4. A biorefinery based on feathers and wool.

Composting feathers and wool is a fairly common practice to exploit the nutrients they contain for agronomic purposes [15,68,70]. This is because composting can reduce their recalcitrance. The use of wool waste in agriculture is a practice that has attracted growing interest but presents some critical issues related to its chemical structure. Wool is composed of highly stable macromolecules, held together by cysteine, peptide, and hydrogen bonds, which make it difficult for enzymes present in the soil to degrade [15]. This severely limits its effectiveness as a short-term fertilizer. Moreover, its use can entail some risks, such as nitrate leaching, increased sodium, and reduced soil pH [15,70]. Wool compost has been tested as a method to counteract this particular issue. Mixed with C-rich residues such as olive pomace and straw, composted wool showed good performance as a fertilizer, thanks to the presence of microelements such as iron, copper, and zinc [15]. In addition to stimulating plant growth, wool compost has positive effects on soil fauna and biological pest control. In particular, it was effective in controlling nematodes in tomatoes, significantly reducing the number of eggs and larvae by 77% and 73%, respectively [15]. Another promising approach is wool pelletization, which allows for the reduction in the microbiological load of raw wool, avoiding the costly industrial washing phase and respecting the environmental criteria of the “Do No Significant Harm” (DNSH) certification [15]. Wool pellets have been shown to improve water retention and increase yield by 20–65% in crops such as lettuce, tomato, and poinsettia [15], although with negative effects on some species such as kohlrabi. Furthermore, the combination with microbial inoculants improved nitrate uptake efficiency, addressing a major environmental concern with conventional nitrogen fertilizers [15,98]. An innovative example is the BioWAG geocomposite, which combines a wool biotextile with a superabsorbent polymer and an internal wood structure [99,100]. This system, buried at a depth of 15–30 cm, showed an increase in soil nitrogen content of up to 400%, with a consequent increase in herbaceous plant growth of up to 430% and an increase in root mass between 130% and 200% [15]. These results highlight the role of wool not only as a source of nutrients but also as a structural element for soil water management. In hydroponics, wool panels have been tested as an eco-friendly alternative to rockwool for growing cucumbers, tomatoes, and lettuce. However, the results were controversial: while sheep wool showed a higher porosity than other substrates such as peat, on the other hand, it showed a lower water capacity, with variable yields depending on the crop. For example, cucumber yields increased, while tomato and lettuce yields decreased, likely due to salinity and poor water retention of the wooly substrate [15].

Other non-agronomic uses for unused wool have been explored. For example, in the building sector, thanks to its properties, it is a good thermal and acoustic insulator, as well as being a good regulator of indoor humidity [19], reducing the possibility of condensation forming with consequent formation of mold, which affects human health. It was also tested as a polymer for use in 3D printers, in combination with polycaprolactone (another biopolymer obtained from the fermentation of animal waste and dairy wastewater) [101,102,103].

The ability of wool to absorb water vapor up to 33% of its weight without appearing humid, while simultaneously rejecting liquid water [19], could be used in the textile industry in combination with synthetic technical fabrics to make them waterproof, even partially replacing perfluoroalkyl and polyfluoroalkyl substances (PFAS). Perfluoroalkyl and polyfluoroalkyl substances have been associated with numerous adverse effects on human health [104]. Landfills represent a significant reservoir of these substances, as many products containing PFAS are disposed of in landfills at the end of their life cycle [104]. Therefore, through leachate, they can contaminate both freshwater (and consequently fish) and drinking water, entering the food chain in both ways.

While the use of raw wool has been widely studied, the use of raw feathers is limited to composting with good results [68]. On the other hand, feather hydrolysis is a very flourishing field of research that has also involved wool recently.

Structurally, more than 90% of the TS of feathers and wool consists of keratin [3,46,105]. This high structural stability and rigidity of keratin are due to the specific composition and molecular configuration of its constituent amino acids [4,47]. Thus, research has focused on keratin obtained from both wool and feather hydrolysis. Keratin is a low molecular weight protein that has proven to be extremely versatile and useful in various agricultural fields. For example, it has been successfully used to improve the uptake of minerals such as copper and cadmium (by 850% and 30%, respectively) by tobacco plants, compared to raw wool [15]. Numerous studies have shown that wool hydrolysate can act as a biostimulant, reducing soil pH and increasing the C-to-N ratio by promoting the germination of ryegrass, acting as a source of Zn and Fe for wheat, and doubling the grain yield, which showed 50% more protein than mineral fertilization; moreover, when used to develop biodegradable biocomposites with controlled biodegradation had stimulated the germination of pepper and tomato seeds and increased the biomass yield of turnips by up to 38% in addition to contributing to the reduction in greenhouse gas emissions and to the sequestration of carbon in the soil [15,19].

Products obtained from keratin can also be used in the industrial sector as a replacement for petroleum-based materials. In this sense, a livestock waste-based integrated biorefinery can also contribute to reducing the environmental impact of the industrial sector. One of the emerging uses is in the production of edible packaging. Thanks to its biodegradability and compatibility with materials such as lipids, resins, and hydrocolloids, keratin is an ideal candidate for the development of safe and functional food packaging. In the biomedical field, keratin is being studied for drug delivery, i.e., the controlled release of drugs, and for tissue engineering, where it is used to create scaffolds capable of promoting peripheral nerve regeneration and vascular healing, thanks to its hemostatic properties and biocompatibility [47]. Furthermore, keratin is used in cosmetics, thanks to its UV-barrier properties, while in the industrial field it can be used as an adsorbent; wool and keratin extracted from feathers can also be combined to produce biocomposites that integrate the properties of both materials, making them ideal for tissue engineering, wound dressings, biomedical applications, drug-delivery systems, bioinks, and bioplastics [105].

Research on the energy recovery of feathers and wool, however, is not sufficiently developed. Negligible biogas yields (70 mL biogas g⁻¹ COD) have been reported for feathers [67], without information on Specific Methane Yield (SMY). For textile wool waste, an SMY of 430 m³ Mg⁻¹ VS was reported [66], comparable to those reported for other feedstocks but only if pre-treated. In fact, raw wool produced 20-fold less [66]. However, studies on wool waste from sheep shearing are lacking, which, as previously written, is an impactful waste, especially in Europe.

5.2. Rendering and Feed Production

A practice used to treat various slaughterhouse wastes is rendering them for animal feed, pet food, poultry meal, and animal fats. This practice is widespread, especially for poultry slaughterhouse waste [46,106]. Rendering is a combination of thermal, mechanical, and chemical processes used to manage slaughterhouse waste. Initially, the soft wastes are ground, then autoclaved at temperatures > 133 °C, 3 bar for 20 min. The wastes are further pressed to obtain protein and fat separation [106]. This method allows separating waste into proteins, fats, and water and sterilizing by-products, preventing bacterial growth and facilitating their storage and transportation. However, not all organs can be rendered (i.e., rumen). To avoid oxidation and hydrolysis processes that can deteriorate slaughterhouse waste, rendering must be performed immediately after slaughter [106]. Animal blood rendering is currently used to produce fertilizers or animal feed, or it can be transformed into safe and biodegradable protein-based bioplastics [16], reducing the use of traditional plastics and the risk of contamination in pet food.

Fish processing waste can be used as a supplement to poultry diets. This emerges as an effective strategy: numerous studies demonstrate that the inclusion of these materials, within specific levels, did not compromise production performance, egg quality, hematological parameters, or carcass characteristics [107]. The balanced protein, fat, and mineral content of fish waste make it suitable for poultry feed formulations, satisfying their protein needs while ensuring body weight, weight gain, feed efficiency, and carcass yield comparable to traditional diets In addition, when subjected to fermentation, the nutritional traits of fish waste increased, resulting in higher dry matter digestibility and nitrogen retention (up to 76%) in broilers fed fermented waste compared to the control group [107,108,109].

5.3. Microalgae

Microalgae can play a dual role: they transform organic substances present in wastewater into biomass and can be used as fish feed, also helping to strengthen their immune system [10,14]. Harvesting microalgae is expensive and difficult. However, the use of coagulation/flocculation has demonstrated an efficiency above 90%, making the process economically sustainable [10]. However, microalgae are also effective in the treatment of livestock wastewater, with a nutrient removal (i.e., N and P) of up to 95% [2,110]. In addition to purification, they allow the production of bioenergy (biodiesel, biohydrogen, methane, and bioelectricity) thanks to their ability to accumulate lipids and other energetic compounds [2]. Microalgae have been used for the phycoremediation of dairy and pig wastewater with the aim of producing lipids [2,110]. Biomass yield reached a maximum of 2.04 g L⁻¹, while lipid content ranged from 26 to 55% [2]. The efficiency of this system is limited by the large dilution of the initial wastewater. However, yield depends on the algal species, the composition of the original waste, and the pre-treatment required to break the cell walls; despite the technical challenges (biomass recovery, wastewater variability, environmental conditions), integrating microalgae into treatment systems represents a sustainable solution for waste management and renewable energy production. Indeed, algal biomass can be transformed through dark fermentation, anaerobic digestion, or fuel cells [2,110]. Scenedesmus spp. has been widely used in research: S. obliquus has been tested in different types of wastewater (poultry, swine, cattle, brewery, and dairy). The resulting biomass contained between 31 and 53% protein, 12–36% sugar, and 8–23% lipids, regardless of the wastewater type. The biomass was subsequently fermented with Enterobacter aerogenes, producing between 50 and 390 mL H₂ g⁻¹ VS [2].

In another study, Scenedesmus spp. grown in pig slurry produced a biomass composed of 58% protein, 28% carbohydrates, and 4% lipids. N and P deprivation favored the accumulation of carbohydrates (55%) and lipids (17%), dramatically reducing protein content (24%) [111]. Indeed, N starvation conditions are used in reactors to produce PHA and SCO, exploiting the ability of some microorganisms to accumulate polymers within the cell under unfavorable environmental conditions, to the detriment of biomass yield (i.e., cell division is inhibited).

S. quadricauda, a freshwater microalga, has been used for tetracycline remediation in dairy wastewater. S. quadricauda was able to remove up to 49% of tetracycline, producing a biomass particularly rich in saturated fatty acids (SFAs); in contrast, Tetraselmis suecica, a marine microalga, was less efficient at removing tetracycline (37%), but its biomass was rich in polyunsaturated fatty acids (PUFAs) [112].

Microalgae grown in nitrogen-rich waters produce commercially valuable nitrogen compounds, such as fluorescent proteins (phycobiliproteins), mycosporine-like amino acids (protective against UV rays), and phenolic compounds with antioxidant properties [10]. Therefore, ideally, they should be grown in the liquid fraction of the digestate, which is typically enriched in N and P compared to raw digestate. Recently, microalgae have been successfully grown in a pilot plant in the liquid fraction of the digestate without dilution [113]. In this context, the range of products from an integrated biorefinery increases, as does the added value compared to the mere removal of pollutants. This is the case of Scenedesmus spp. grown on digestate.

Biostimulants for agriculture were obtained from the algal biomass, while spent algal biomass was used in anaerobic digestion (AD) to produce CH₄ and digestate, which then fed the microalgae photoreactor [114]. Furthermore, the biogas can be entirely recirculated in the photoreactor. In this way, it was possible to upgrade biogas to biomethane since only the CO₂ in the biogas is consumed by the algae, resulting in a higher concentration of CH₄ and the simultaneous production of pigmented carbohydrates that can be extracted from the algal biomass; nevertheless, several typical characteristics of the digestate (i.e., high NH₄⁺ content, the presence of heavy metals, turbidity, and pH) are factors that limit the growth of microalgae [115].

5.4. Phytoremediation

Phytoremediation (PR) is an environmentally friendly and cost-effective technology that uses plants and associated microorganisms to remove, degrade, stabilize, or volatilize pollutants from soil and water [1]. It is solar-powered and requires less labor and equipment than traditional methods. The mechanisms of action of PR are phytoextraction, rhizofiltration, phytostabilization, phytodegradation, and phytovolatilization. Phytoremediation can be used to treat a wide range of contaminants: heavy metals, dyes, pesticides, hormones, and antibiotics, as well as almost completely reduce pathogens [1,14,116]. Aquatic plants such as duckweed, water lettuce, water hyacinth, and watermilfoil were widely applied to phytoremediation [1] with good results. Unlike microalgae, the biomass of aquatic plants is easier to harvest after remediation, which is beneficial to the reuse of plant biomass.

Duckweed (Lemna minor spp.) was reported to be an exceptional remover of coliforms and enterococci (>99 and 88%, respectively) [1]. Duckweed has also been tested for PR of AWW in IMTA. It was found to be effective in AWW remediation and to produce a protein-rich biomass, mainly due to the substantial biomass growth in the summer months [11]. On the other hand, the presence of antibiotics is a limiting factor that can reduce the efficiency of phytoremediation; other limitations of PR are the slowness of the process and the variable effectiveness depending on the type of pollutant and surface areas [1]. Moreover, some plants can accumulate dangerous contaminants, which can be released when decomposition occurs. Therefore, it would be useful to integrate PR into centralized livestock waste treatment plants, such as constructed wetlands (CoW), for example, as a replacement for oxidation tanks, and to use plant biomass in an integrated biorefinery, also to recover heavy metals more effectively and cost-effectively than from livestock wastewater. In CoW, several perennial grasses such as giant reed (Arundo donax L.), common reed (Phragmites australis), reed canarygrass (Phalaris arundinacea), napier grass (Cenchrus purpureus), Lemongrass (Cymbopogon spp.), Scirpus (Schoenoplectus lacustris), and Typha latifolia have been successfully tested [117,118,119,120].

These macrophytes allow the concentration of energy from wastewater to plant tissue, which is considerably drier and makes it more profitable to integrate CoW in a livestock waste treatment context. In this sense, giant reed has been used for the remediation of abattoir wastewater and aquaculture wastewater [119,120]. In both cases, the biomass yield per hectare was higher than in controls irrigated with tap water. Notably, the highest biomass yield was recorded for PR of AWW (125 Mg dry biomass ha⁻¹), which outperformed the 30 Mg dry biomass ha⁻¹ that is commonly reported [80,119].

Moreover, CoW is a more sustainable solution for the removal of nitrogen compounds [10]. This makes them particularly suitable for the treatment of livestock waste characterized by substantial levels of N (see Table 1 and Table 3). Constructed wetlands have shown a removal capacity of up to 98–99% of NH₄-N, NO₂-N, and NO₃-N and up to 32% of P [10]. Constructed wetland integration could be useful in aquaculture also in RASs. Although RASs allow for reducing water consumption and environmental pollution by recycling the used water and using only 10% of additional fresh water [10], they present limitations such as nitrate accumulation and high energy costs for water treatment. In fact, as previously written, RASs generate up to 3 m³ kg⁻¹ of fish wastewater per year. For AWW, the limiting factor for CoW integration is the large water content [10]. One solution could be the use of coagulation and then treating a more concentrated wastewater in CoW. Provided that biodegradable coagulants are used instead of traditional coagulants. Tannin-based coagulants, derived from trees, are promising [74] but have limitations related to land use and seasonal availability. Therefore, it is important to explore solutions based on local plant residues.

5.5. Anaerobic Digestion

Anaerobic digestion is a well-established technology, long used for the treatment and simultaneous energy recovery of animal waste. This process represents an effective technology for energy recovery from organic waste. It transforms biomass into biogas. Biogas can be produced from any feedstock that contains substrates such as proteins, fats, carbohydrates, and cellulose. AD consists of 4 phases [121]. During the hydrolysis and acidogenesis phases, macromolecules such as proteins, lipids, and carbohydrates are converted into simpler compounds (i.e., amino acids, fatty acids, and monosaccharides) [46]. During the acetogenesis phase, these intermediate products are further converted into acetic acid, H₂, and CO₂. Finally, in the methanogenic phase, the end products of the previous steps are transformed into CH₄ and CO₂ [121]. Typically, AD is carried out in well-established, completely stirred anaerobic reactors (CSTR) and upflow anaerobic sludge beds (UASB) [1].

The effectiveness of AD in reducing GHG emissions from the agricultural sector is evident: emissions related to the biomethane supply chain range from 9 to 19 Tg CH₄ y⁻¹, while agriculture emits approximately 141 Tg CH₄ y⁻¹ [122], mainly due to livestock farming, particularly due to poor or lack of livestock waste management. Conversely, the AD process “captures” the CH₄ that would otherwise have been emitted. Furthermore, it is interesting to note that emissions from the biogas sector are expected to decrease [122].

Anaerobic digestion is seen as a technology to be implemented with a view to both increasing the amount of renewable energy produced and reducing the impact of livestock waste [121,123]. For example, AD can reduce the volume of sludge output by up to 90%, as in the case of RAS, and destroy most pathogens [14]. Moreover, AD is a very efficient and inexpensive method [81] to reduce environmental contamination of both antibiotic resistance genes (ARGs) and antibiotics. Digestate sampled for one year from a full-scale biogas plant showed an 80% reduction in ARGs and 84%, 92%, and 100% degradation of enrofloxacin, ciprofloxacin, and sulfamethoxazole, respectively [72].

The biomethanation potential (BMP) of various substrates, including animal waste, has been reported in several studies [29,35,36,37,38,39,40,41,42,43,44,45,46,48,64,81,90,124,125,126,127]. An overview of these values is shown in Figure 5a, with particular emphasis on animal waste compared with other common AD feedstocks, particularly maize silage, which has been the feedstock of choice for AD since the beginning.

Figure 5. Biomethanation potential (a) and volumetric methane production (b) from animal waste compared to well-established biomass for anaerobic digestion [29,35,36,37,38,39,40,41,42,43,44,45,46,48,64,81,90,123,124,125,126,127].

For a real comparison it is necessary to correct the BMP for the moisture content and report the value for Mg⁻¹ FW (obtainable from Table 1 and Table 3). The results are displayed in Figure 5b. For example, blood has SMY equal to 490 m³ Mg⁻¹ VS, but the volumetric production (i.e., referred to m³ Mg⁻¹ FW) drops dramatically to 81 m³ Mg⁻¹ FW [48]. This value is what will actually be produced by a plant with the same reactor volume. The same applies to livestock waste and slaughterhouse effluent (Figure 5b). The feedstock must have an adequate energy concentration to maximize biomethane yield from a biogas plant. Energy concentration means high C content with risk of acidification. Thus, anaerobic co-digestion (AcoD) with rich N feedstock is necessary to counteract this eventuality. Animal manures are the complementary feedstocks of choice because they have a high buffering capacity that counteracts the acidification of the medium and rebalances the C/N, avoiding both acidification and excessive ammonia formation. Furthermore, they provide otherwise unusable water necessary to adjust the moisture of the reactor to optimize the process [76,82,127,128], avoiding the use of conventional water.

Some animal wastes can be particularly challenging in AD because they are recalcitrant. For example, it has been reported that only 60% of solid paunch waste was degraded in AD [17]. Fish sludge, however, may not be cost-effective due to the low volumetric CH₄ production caused by the low VS concentration and the high Na concentration [45].

Typically, animal waste is rich in N. In livestock waste, N derives from nitrogenous excretions (i.e., urine and uric acid), whereas in slaughterhouse waste, it derives from the fact that these are mainly composed of tissues consisting mainly of proteins [46]. Feedstock particularly rich in N is prone to generate NH₃ in AD. Excessive levels of NH₃ can inhibit AD, resulting in the accumulation of VFA (i.e., butyric and valeric acid) [29], which, by acidifying the medium, can lead to the blockage of AD. Furthermore, under conditions of excess NH₃, methanogenesis shifts from the acetoclastic to the hydrogenotrophic pathway. It generally occurs when the NH₃ concentration increases because acetoclastics are more sensitive to NH₃ toxicity than hydrogenotrophs; under these high ammonia conditions, syntrophic acetate oxidation can become the dominant process for acetate consumption, counteracting acetoclastic methanogenesis and lowering CH₄ yield [129,130]. In general, in anaerobic digestion reactors, 66% of the CH₄ produced comes from acetoclastic metabolism; the remaining share comes from hydrogenotrophic methanogens [129,130,131,132,133]. Hydrogenotrophic methanogenesis coupled with acetate oxidation further contributes to the lowering of CH₄ yield typical of NH₃-inhibited reactors. In addition, slaughterhouse wastes are rich in lipids [46,48]. During AD, lipids are degraded into long-chain fatty acids (LCFA), which inhibit the activity of methanogenic microorganisms and compromise reactor stability [46,48,134]. Therefore, substrate optimization is essential to ensure the sustainability and efficiency of the AD process.

Although anaerobic digestion can inactivate or significantly decrease the number of microbes, they are still present in wastewater at extremely high quantities [1]. Furthermore, although rare, traces of antibiotics and hormones [135] may be found due to accidental causes that can occur during the regular operation of a biogas plant. Therefore, the management of the digestate also requires caution.

To increase pathogen removal, ethanol fermentation could be adopted before AD having the desirable side effect of improving AD [136]. Indeed, ethanol has an excellent conversion rate to CH4, resulting in faster degradation compared to VFA [137]. For this reason, ethanol fermentation could counteract NH₃ inhibition since the faster the CH₄ generation, the higher the methanogen growth rate. A rich methanogen community can adsorb the NH₃ inhibition. Indeed, some resistance to NH₃ inhibition was detected following ethanol pre-fermentation in a high organic loading rate experiment in AnMBR [138].

A summary of the environmental and economic benefits of the traditional and alternative animal waste treatment methods described above in this review is given in Table 5.

Table 5. Summary of pros, cons, and environmental and economic benefits of the animal waste treatment methods considered in this review [1,2,3,14,16,17,20,22,26,27,33,46,47,50,63,74,75,76,77,78,102,105,110,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155].

Traditional Treatment	Pros	Cons	Total Emission Reduction Efficiency (kg CO2 eq Mg−1 Animal Waste)	Net Economic Benefit ($ Mg−1)	Alternative Treatment	Pros	Cons	Total Emission Reduction Efficiency (kg CO2 eq Mg−1 Animal Waste)	Net Economic Benefit ($ Mg−1)
Direct land application	Low CAPEX and OPEX, low technological level required	Uncontrolled GHG emission, risk of N and P pollution, high salinity, storage cost, high energy demand for transport and application, health risk, antibiotic resistance spread	N.A.	5.8	Anaerobic digestion and digestate	Electricity and biofuel production, reduced GHG emission, N and P recovery, volume effluent reduction, biofertilizer	High CAPEX and OPEX, large reactors, technological equipment, potential presence of heavy metals	94–524	63–321
Composting	Low CAPEX and OPEX, low technological level required, reduction in synthetic fertilizer utilization, pathogen removal	Uncontrolled GHG emission, storage cost	4.1–4.9	4–20	Anaerobic digestion + digestate composting	Electricity and biofuel production, reduced GHG emission, N and P recovery, volume effluent reduction, biofertilizer, pathogen removal, cost-effectiveness and wide applicability, and significant increase in crop yields compared to inorganic fertilizers, reduction of NH3 volatilization, reduced eutrophication and acidification potential	High CAPEX and OPEX, large reactors, technological equipment, potential presence of heavy metals	98–529	76–361
Ammonia stripping	Transforming ammonia into fertilizer	High OPEX cost, corrosion of equipment, chemical and energetic input, lack of P and K recovery	~0	0–174	Physicochemical (flocculation, bioflocculation, advanced oxidation, photocatalysis, coagulation)	Nutrient recovery, volume reduction, high COD removal efficiency, profitable	Energy intensive, high OPEX, 1–20% N2O increased emission, catalyst dispersion, presence of heavy metals and inorganic ash, chemical input, Cl- and O3 emission, potential risk of water quality degradation	N.A.	N.A.
Incineration	Pathogen destruction, waste volume reduced	GHG emissions, presence of heavy metals, formation of toxic compounds, high OPEX	0	N.A.	Membrane filtration	High N and P recovery efficiency, volume reduction, water recovery	Pre-treatment steps, membrane fouling, high OPEX
Landfill	cost- effective	groundwater contamination, GHG emissions, limited space	0	0	Microalgae cultivation	CO2 utilization for biosynthesis, high N and P removal	High OPEX, large photoreactors, dilution needs, high water demand, low efficiency biomass recovery	N.A.	N.A.
Rendering	displacement of fossil fuels, energy intensive, low conversion yield	Nutrient recovery, sanification, pathogen control, biofertilizer and animal feed production, biomolecule recovery	N.A.	N.A.	Phytoremediation	CO2 sequestration, highly effective for heavy metals, cost-effective, high N and P removal, erosion control, preventing dispersion of pollutants into aquatic and atmospheric environments, biomass production	long time required to achieve remediation effects, seasonal dependence, reduced effectiveness at very high contaminant concentrations	N.A.	N.A.

N.A.: Not Assessed; CAPEX: Capital Expenditure; OPEX: Operational Expenditure; GHG: Greenhouse Gas; COD: Chemical Oxygen Demand.

Traditional waste disposal methods are generally less expensive. However, they do not reduce CO₂ eq emissions, but compost does. Indeed, they contribute to GHG emissions both directly and indirectly, as in the case of incineration, which requires the use of fossil fuels in the process, or ammonia stripping, which also requires chemicals. Even the simple application of manure can be impractical and have an environmental impact: the over-application of manure, indeed, altered the composition of P species, with orthophosphate exceeding 95%, while inorganic phosphorus was reduced by ~50%, and phosphorus leached to soil depths of 60–80 cm [149].

Among the alternative methods, those that include AD are the most consolidated. These allow both a good net economic benefit and a significant impact on the reduction in GHG and CO₂ eq emissions (Table 5). Furthermore, producing both an energy carrier (biogas) and recovering nutrients (biofertilizers) contributes to the reduction in indirect emissions related to the livestock sector. In fact, it should be noted that producing 1 kg of nitrogenous synthetic fertilizer emits about 1.6 kg of CO₂ [73]. Moreover, synthetic N fertilization can be responsible for up to 1130 Mg CO₂ eq emissions annually [150]. Although other alternative methods (i.e., membrane filtration, microalgae cultivation, and phytoremediation) are promising, it is worth noting that direct nutrient recovery from manure or slurry does not seem to be a realistic solution, while integrating these methods with AD may be a better solution to improve overall resource recovery [147]. Moreover, it is simple to project that the environmental and economic benefits will increase thanks to the plethora of products that can be obtained.

To effectively reduce emissions from the primary sector, it is necessary to adopt measures that promote highly efficient waste recovery, recycling, and transformation. Among these, the conversion of livestock manure to biogas stands out [156].

6. Integrated Biorefinery Approach to Enhance Circular Economy

The previous sections described how energy and molecules can be recovered from animal waste through various processes. Interestingly, by integrating all the various processes, it is possible to envision a large integrated biorefinery based on livestock waste. Interestingly, livestock waste, in addition to being a feedstock for an integrated biorefinery, also serves as a source of microorganisms that support its operation. In this regard, it was recently reported that starved pig slurry can be a suitable inoculum source for producing biomethane and biohydrogen from various feedstocks, as well as produce chemical building blocks [39,44,136,157,158], given the non-polarized structure of its microbial community [159].

From bacterial fermentation, moreover, it is possible to obtain lactic, caproic, propionic, and hyaluronic acids; vitamins B9 and B12; and polyhydroxyalkanoates (PHAs), which have nutraceutical value or are useful in the food and cosmetics industries, for packaging, or to increase food shelf life [102,160].

An example of a small integrated biorefinery based on dairy and pig wastewater has been previously tested. In that process, the residual liquid of lactic acid bacteria cultivation was digested in AcoD with pig slurry, integrating two livestock wastes from different sectors. Anaerobic co-digestion allowed to increase CH₄ yield by 12% compared to lactic acid monodigestion (i.e., 416 vs. 372 mL CH₄ g⁻¹ VS); in all cases, the LAB cultivation residues (i.e., lactic acid and ethanol) were completely degraded [137]. The most important result was that AcoD allowed the CH₄ production per unit time to increase by 1.5-fold since it reduced the process duration, and the quality of the digestate obtained was found to be suitable for crop fertilization [137], for example, to be used to produce feed for dairy cattle, ensuring circularity.

6.1. Digestate Valorization

The digestate residue from anaerobic digestion is the waste stream of the animal waste-based integrated biorefinery. In general, it can be considered as the ring that closes the circle of the circular economy, being usable as fertilizer to produce feed for animals. Digestate is increasingly considered an effective fertilizer due to its uniformity and the availability of nutrients for plants [21,87]. Its use can improve agricultural productivity by reducing the use of synthetic fertilizers [127], which is the major source of pollution indirectly attributable to livestock farming [19]. Although the direct application of digestate can improve nitrogen availability in the soil, high concentrations of VFA [34] that may remain from the AD process can hinder the conversion of ammonia into nitrate, the form of nitrogen most easily assimilated by plants. Furthermore, during the storage phase, the digestate continues to emit gases such as NH₃ and CH₄, due to the ongoing degradation [34]. Furthermore, the possibility of diffusion of ARGs is not excluded. Thermophilic aerobic composting offers numerous advantages. If well managed, it can reduce the concentration of resistance genes by up to 2.5 log [3]. However, a hybrid solution has shown promising results. In fact, coupling mesophilic AD and composting for 90 days can reduce the presence of tetracycline resistance genes by over 80% [161]. This is due to the fact that the composting process reduces pathogens thanks to the alternation of mesophilic and thermophilic phases. The latter in particular is the one that most significantly affects the reduction in ARGs [3], similarly to what could happen in thermophilic AD, with the exception that, while a thermophilic AD reactor needs to be heated, composting is exothermic. Therefore, the energy balance of mesophilic AD + compost is lower than that of thermophilic AD. In some cases, composting can reduce almost up to 100% of some tetracycline resistance genes [3]. However, it is important to underline that not all genes respond in the same way and that the process can, in some cases, favor the selection of specific ARGs. This aspect highlights the need for further research and careful management of the process. Consequently, correct management of digestate storage is essential for pollution control. Several solutions to counteract these issues have been considered in this review and listed below, effectively making the digestate an additional feedstock to be exploited in the process. Better results can be achieved by separating solids/liquids before composting and by extending the composting times [5].

For slaughterhouse waste, composting is used for solid wastes, while anaerobic digestion is used in wastewater treatment. However, anaerobic digestion has also been demonstrated to treat a variety of solid wastes from meat [21]. This management is severely limited due to the lack of energy exploitation of slaughterhouse solid waste. Indeed, the energy potential is lost in the form of CO₂ during composting. Indeed, even for slaughterhouse waste, it is desirable to adopt a hybrid solution, as for animal production waste. In particular, to counteract the formation of NH₃, the optimal system would be AcoD of slaughterhouse waste with a high C substrate (i.e., a perennial grass with high biomass yield, growing on marginal land) followed by compost in order to also exploit the lignin that remains from the process as a source of slow-release C [82].

6.1.1. Membrane Filtration to Recover Renewable NPK

Both livestock and slaughterhouse waste are particularly rich in N, P, and K. In particular, P can reach up to 50 mg L⁻¹ [18]. Since none of these macronutrients are consumed in AD [82], it is possible to recover them more easily from the digestate, having been concentrated given the consumption of C, H, and O consumed during methanogenesis. After solid/liquid separation, which can be performed through various methods such as belt press, centrifuge, inclined screen, roller press, rotating screen, and screw press [162], the liquid fraction can be filtered. For example, forward osmosis (FO) or reverse osmosis (RO) can be used to concentrate the macronutrients NPK (i.e., recovering 81% N, 99.9% P, and 83% K), but also to recover at least 75% of water from an unconventional, otherwise unusable source [33,138]. This practice can be considered consistent with RENURE, an acronym for REcovered Nitrogen from manure, which refers to fertilizers obtained from transformed manure. Using RENURE fertilizers, their application rate beyond the limits established by the Nitrates Directive will be authorized. Indeed, the membrane separation process applied to the digestate goes beyond RENURE because it also allows for the recovery of P and K, which are renewable P and K [140].

The resulting solid fraction can be further enhanced for the growth of Pleurotus ostreatus [163]. This will provide a liquid phase for fertigation, the production of food or feed, and a sanitized compost as a soil conditioner, increasing the intrinsic value of the production chain of an integrated livestock waste-based biorefinery.

6.1.2. Vermicompost

Vermicomposting is a natural process in which earthworms and microorganisms collaborate to decompose organic materials. Earthworms fragment and ingest organic matter, increasing the surface area available for microbial activity, which completes the decomposition of organic matter [32]. This method accelerates the stabilization of organic matter, modifying its physical and biochemical characteristics, such as increased porosity, aeration, drainage, and water retention; the mineral composition is also well balanced, reducing the mobility of heavy metals [32,140]. Thus, it is potentially an advanced product compared to compost. However, studies on its effectiveness in intensive agriculture and sustainable nutrient management are lacking. Therefore, commercial use is still limited.

6.1.3. Insect Compost

Insect-based technologies rely on the ability of certain insects to transform decomposing organic matter. The most studied composting agent is the larva of the black soldier fly (Hermetia illucens). The larvae of these insects have received increasing attention in animal nutrition because they are rich in protein and fat and capable of utilizing organic waste, including manures, as a source of nutrition [87,139,162]. Black soldier fly larvae have been used as alternative ingredients in fish and chicken feed, as a replacement for other animal meals.

Meal from these insects has been used in up to 60% of the diet of rainbow trout (Oncorhynchus mykiss). In addition to improved growth performance, a lower incidence of distal internal morphological alterations was also observed [164].

Conversely, the use of meal from these insects for poultry feed can have controversial effects. Used up to 12%, it did not negatively affect either the growth of the chickens or the quality of their meat [165]. On the other hand, at 50% of the diet, meat quality was commercially suitable, but growth was negatively affected, while when fed 100% of the diet, chickens grew less and produced lower-quality meat enriched with SFA [166], which was unpopular on the poultry market. However, chickens fed with live black soldier fly larvae were heavier, while egg production and egg characteristics were not affected [167].

Given that black soldier fly larvae grow on organic waste, require little water, and have low substrate requirements [166], it is possible to consider using the solid fraction of the digestate for their growth and to produce compost.

Furthermore, the compost obtained from insect growth could be used to grow Plerotus [168,169], among the most widely marketed edible mushrooms. This mushroom can be used as a source of protein, macronutrients, and vitamins in livestock with ameliorative effects on both terrestrial animals and fish [169,170,171].

If these mushrooms are grown on Hermetia compost, two different types of food/feed ingredients will be obtained, maximizing the use of the digestate fraction.

6.2. Extracellular Polymeric Substances

Other, more advanced methods for exploiting the solid fraction of the digestate involve post-treatment of this biomass to maximize energy and resource recovery. The undigested holocellulose residues still present in the digestate can be enzymatically hydrolyzed to produce H₂ and PHA or lactic acid to produce PLA [80]. This increases the energy output and the production of biodegradable polymers from the process.

Finally, the biofilm and residual microbial biomass can be exploited to recover extracellular polymeric substances (EPS). These biopolymers, produced by microorganisms, are composed of various biomolecules, mainly polysaccharides, proteins, and uronic acids, while nucleic acids and lipids represent minor components [172]. The main function of EPS is to bind to cells, protecting them from adverse environmental conditions and facilitating extracellular electron transfer. Thus, they constitute the structural core of biofilms [173], which is essential in AD. It is possible to recover EPS from the solid fraction of the digestate. Extracellular polymeric substances are highly versatile compounds that can be used in a variety of applications. They can be used to remove persistent pollutants in the soil [173], as bioflocculants, for the adsorption of heavy metals, anticancer drugs, etc. [172,173,174]. Mixed with wool fibers in fireboards, the EPS contributed to a reduction in the heat release rate, which demonstrates that EPS is effective as a flame retardant and also shows self-extinguishing behavior [69].

6.3. Microcellulose and Nanolignin

The digestate’s solid fraction can be further valorized through post-treatments. In particular, the recalcitrant lignocellulosic fraction can be fractionated using deep eutectic solvents (DESs) to separate lignin from cellulose [175].

Deep eutectic solvents represent a promising class of environmentally friendly compounds suitable for biomass pre-treatment applications. Structurally similar to ionic liquids (ILs) [120], DESs offer several advantages: they are non-toxic, cost-effective, biodegradable, and easy to synthesize. These solvents consist of a mixture of hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs), whose interaction leads to a significant depression of the melting point, often reaching the eutectic point at room temperature [175]. Typical DES formulations are derived from naturally occurring substances, including quaternary ammonium salts, amides, amino acids, carboxylic acids, and polyalcohols [121]. Thanks to their unique solvation properties, DESs can effectively fractionate lignocellulosic biomass. Moreover, being biodegradable, they can be energetically valorized in the same area of the biorefinery in which they would be used.

The lignin obtained by fractionation of the solid digestate can be valorized as nanolignin. In recent years, nanolignin has attracted considerable interest as a potential nanofiller for polymer composites due to its remarkable properties, including biodegradability, antioxidant activity, and high specific surface area, as well as UV-blocking properties [176]. However, its widespread use is challenging due to its complex structure and limited chemical compatibility with most polymers.

The cellulose obtained by fractionation of the solid fraction of the digestate can be valorized as microcellulose, characterized by a high specific surface area. It is widely used in the medical and pharmaceutical sectors, where it is used as a modeling agent for drugs and as a disintegrant in tablets; furthermore, microcellulose possesses thixotropic and thickening properties, making it suitable as a thickening and emulsifying agent in water-based coatings [177]. Its versatility allows for simultaneously performing filling, thickening, and emulsifying functions, making it a valuable ingredient in various industrial applications, even in combination with other biodegradable polymers. For example, cellulose recovered from digestate could be integrated with PLA, resulting in a biocomposite with improved mechanical strength [177]. Chitosan coating was enhanced with nanocrystalline cellulose to preserve perishable fruits [178]. Chitosan is a linear polysaccharide obtained through the deacetylation of chitin, a biopolymer naturally present in the exoskeletons of crustaceans, the cuticles of insects, and the cell walls of some fungi [178,179]. Thus, it is possible to easily obtain it from shrimp exoskeleton, which represents up to 60% of shrimp processing waste [62]. Therefore, all the raw materials of this product derive from the recovery of livestock waste and can be easily centralized in a livestock waste-based integrated biorefinery.

Alternatively, the residual cellulose can be enzymatically hydrolyzed to reduce cellobiose to glucose and produce bio-H₂, VFA and PHA, LAB and PLA, or SCP or SCO, increasing the product pool of an integrated biorefinery.

The possibility of producing biopolymers such as PHA and PLA is particularly important considering that both livestock and slaughterhouses generate large quantities of plastic from packaging and protective devices; often contaminated, they end up in landfills because they are difficult to recycle [16]. Compostable alternatives are therefore urgent.

From this perspective, an integrated biorefinery powered by livestock waste addresses both the production of biodegradable plastics and their sustainable end-of-life. Indeed, it is possible to degrade PLA into mesophilic AD; however, the AD duration of PLA is too long [180], incompatible with the AD duration of animal waste.

An integrated biorefinery scheme with several services has been recently proposed [80] and is shown in Figure 6.

Figure 6. Integrated biorefinery scheme with several services [80].

The authors estimated that this system could generate at least 1432 GJ ha⁻¹ annually [80] in addition to technical water. Furthermore, some speculation can be made about the prospective role that KOH may have. By integrating slaughterhouse waste, feathers and wool, and PLA, the common limiting factor can be found in their recalcitrance, which requires pre-treatment. Moreover, considering that giant reed can be used to remediate livestock wastewater in CoWs (see Section 5.4), the number of recalcitrant biomasses included in the scheme increases. Notably, KOH has been successfully used to pre-treat giant reed without the need to remove the resulting liquor [82,181]. Indeed, KOH pre-treated giant reed with pig slurry not only doubled the amount of CH₄ produced per unit volume but also improved the quality of the digestate thanks to the increase in K, which is a plant growth factor [82,182]. Similarly, KOH was used to pre-treat paunch waste, improving CH₄ yield by 60% compared to the untreated control [126]. To close the loop, it is important to note that such a pre-treatment of PLA with KOH allowed for the complete degradation of this biopolymer in 24–30 days, compatible with the duration of animal waste AD; no inhibition was detected up to 31 g L⁻¹ K [182].

In all cases, the digestate can be enriched with K and, in the case of co-digestion with animal slurry and slaughterhouse waste, also with N and P. Furthermore, the abundance of N in animal waste complements the abundance of organic C in perennial grass used for bioremediation in CoW and in biodegradable plastics. In this sense, this AcoD model counteracts both NH₃ raising and medium acidification. Furthermore, an increase in the degradation of the feedstocks used is expected. In fact, a synergistic effect between co-substrates is often observed in AcoD [41,82,83].

The liquid waste from the process just described will be rich in NPK, which can be concentrated with RO technology. This aligns with the circular economy principles, increases food security, and makes livestock more sustainable in general. In fact, crops require abundant NPK. In 2020, the EU listed P and K as critical raw materials (CRM): P and K could generate food insecurity because they are currently extracted from rocks that are unevenly distributed on Earth [20]. It has been estimated that over 80% of P mined annually is consumed by agriculture [46]. Therefore, their increasing scarcity, combined with the geopolitical instability of the areas where these two minerals are most abundant, poses serious risks for food production. Currently, agricultural production depends mainly on chemical fertilizers produced from non-renewable sources (fossil fuels and mineral rocks) [27]. The use of advanced biofertilizers and precision fertigation can reduce the use of P and K and their demand from non-renewable sources. Furthermore, an advanced biorefinery approach can enable the enhanced recovery of P and K from biomass to produce advanced biofertilizers.

7. Conclusions

Sustainability in the livestock sector has become crucial due to climate change and growing food insecurity. Although the livestock sector contributes only 15% of GHG emissions, it has been under scrutiny for its environmental impact. Indeed, in addition to GHG emissions, livestock and related industries and processes (i.e., slaughterhouses, food, and feed) emit significant organic pollutants and environmental contaminants, such as antibiotics, heavy metals, and pathogens that can be dispersed into the environment more or less randomly. This review highlighted how the livestock sector has long been committed to its sustainability. Traditional methods of livestock waste management and the most established and advanced technologies were presented. While in the past, emphasis was placed on pollutant abatement rather than the recovery or valorization of this waste, today the focus is on valorization. Anaerobic digestion has been shown to be a well-established process that generates energy, reduces the volume of sludge to be managed, and eliminates pathogens and contaminants, generally more effectively and efficiently than other systems. It has also been demonstrated how an advanced integrated biorefinery approach can enable the livestock sector to absorb its own emissions and pollutants, valorize them, and produce advanced biofuels (bio-H₂ and bio-CH₄), innovative materials (fiber wool, keratin, microcellulose, and nanolignin), biodegradable polymers (PHAs and PLA), molecules for the chemical industry (VFA and EPS), food/feed (lactic acid, carotenoids, antioxidants, and active peptides), advanced food and feed (single-cell proteins, single-cell oil, and microalgae), and possibly recover metals. This contributes to the reduction in fossil fuels and petroleum-based materials. It has also been shown that livestock waste can also serve as a source of microorganisms for a fully microbial-integrated biorefinery.

In conclusion, the biorefinery approach seems an inevitable solution to enhance sustainability and meet circular bioeconomy requirements. Specifically, the direction to take is for large biorefineries to replace traditional refineries. To achieve this, it is essential to allow for the hybrid treatment of agricultural and municipal waste in a single facility. Considering the benefits presented so far from an integrated biorefinery approach, there is the possibility of obtaining energy, materials, and resources, decontaminating waste, and reducing health risks by reducing pathogens and antibiotic resistance. Indeed, excess antibiotic residues are reduced in AD-integrated biorefineries. Furthermore, even when zero-waste is not achieved, waste generation is near to zero. The major concern in an integrated hybrid agricultural/municipal biorefinery may concern the final output, which can no longer be exploited within the biorefinery and must be returned to the environment in a healthy state.

As depicted in Figure 7, an integrated biorefinery based on animal waste has the ability to absorb waste from the industrial and domestic sectors as well as waste generated by the livestock sector. The high transformation capacity of such a biorefinery allows it to dispose of waste, generating energy and materials that are exchanged from the primary sector with other sectors, actively contributing to reducing their environmental impacts. Moreover, as can be observed, almost all flows are reabsorbed by the system in a zero-waste perspective. In fact, the last outflow (i.e., digestate) returns to the system to feed the crops for animal production. Thus, the livestock sector is a pillar of the green transition, which relies primarily on biomass and its advanced valorization.

Figure 7. The primary sector is a pillar of the green transition. Icons designed by Freepik https://www.freepik.com/.

In conclusion, without livestock and agriculture, a complete green transition is not possible.