The intensification of poultry farming in recent decades has led to increased generation of organic waste, such as animal manure and chicken litter. Conversely, growing environmental awareness has driven the search for strategies to mitigate these damages. Such as the search for alternatives to fossil fuels, the global energy crisis, and the demand for biofuels (Tawfik et al. 2023).

In this context, the energy recovery of chicken litter for biogas production serves as an example. This waste contains high concentrations of organic matter and nutrients—particularly nitrogen, phosphorus, and potassium—making it a viable substrate for biogas production via anaerobic digestion. Anaerobic digestion offers additional benefits, such as waste stabilization, nutrient recycling, the reduction of greenhouse gas emissions and odors, and the mitigation of soil and water contamination (Kocetkovs and Zvirbule 2025).

Global chicken meat production has grown from a niche market in the 1960s to the largest meat category, with global production of approximately 107.5 million tons per year (2025/2026). Brazilian chicken meat production reached 15.45 million tons in 2025, making it one of the world’s largest producers and a key driver of future growth (USDA 2026). Poultry meat accounts for about 40% of global meat production and is a rapidly growing category (Nassar 2026). It is important to note that, in terms of impact, on average each bird can emit 3 to 5 Kg of CO₂ during the production cycle, varying according to the system and slaughter age (Henn et al. 2015). In tropical systems, chicken litter production is estimated at approximately 40 Kg per bird per year (Waziri and Kaltungo 2017).

The characteristics of chicken litter can vary among farms, regions, and poultry houses. These variations stem from the amount of material used, its reuse, the composition of the litter, the amount of waste, the drinking water distribution system, and the sanitation and waste disposal practices adopted by the farm. Litter is a dynamic material, heavily dependent on management and infrastructure, which requires monitoring of moisture, nutrients, and microbiota prior to agricultural or energy reuse (Dunlop et al. 2016; Ogundipe et al. 2025).

Despite the energy potential of chicken litter, its use in biogas digesters presents technical challenges. For instance, the high nitrogen content can lead to ammonia formation during anaerobic digestion, which may inhibit the methanogenic microorganisms responsible for methane production, thereby reducing process efficiency (Markou 2015), high concentrations of volatile fatty acids and traces of antibiotics are limiting factors (Tawfik et al. 2023). Another challenge is the low C/N ratio of chicken litter; the ideal ratio is generally between 20:1 and 30:1, yet this waste has a value below this range, which can compromise process stability. Furthermore, the presence of lignocellulosic materials in the manure can hinder the biodegradability of the biomass and reduce biogas production yield (Chaump et al. 2019).

Numerous technologies are used to process chicken litter to produce energy; the most established is anaerobic digestion, which uses microorganisms that break down organic matter in the absence of oxygen, generating biogas in biodigesters. This process allows for the treatment of organic waste while producing biogas, rich in methane (CH₄), carbon dioxide (CO₂), and digestate (stabilized residue), which can be used to generate electricity, heat, biomethane, and biofertilizer (derived from the digestate). Research indicates production potentials ranging from 0.15 to 0.30 L of biomethane per gram of volatile solids, depending on the substrate composition, the biodigester’s operating conditions, and the composition of the litter (Chaump et al. 2019). It is worth mentioning other strategies used to improve biogas production efficiency, such as co-digestion of waste, which involves mixing chicken litter with other substrates to improve the C:N ratio and increase methane production (Zhan et al. 2023a).

Another strategy involves the use of conductive materials or additives, such as biochar or charcoal, which can improve electron transfer between microorganisms, reducing the process adaptation time and increasing methane production in digestion systems with higher solid concentrations (Indren et al. 2020). Regarding microaeration in the biodigester, this involves introducing small amounts of oxygen, thereby reducing the amount of sulfur compounds and improving biogas quality. Another technology is high-solids-concentration digestion, which allows digesters to operate with higher solids content, up to about 30% total solids, thereby increasing energy efficiency. Also worth mentioning is biomass pretreatment, which is used to increase the biodegradability of chicken litter and can be either physical or chemical (Karumanchi et al. 2026).

Anaerobic digestion of chicken litter is a strategic approach for the energy recovery of poultry waste, although its operational efficiency still faces challenges inherent to the high N and lignocellulose content of the feedstock (Bhatnagar et al. 2022; Tawfik et al. 2023). Research is focused on optimizing the C/N ratio and managing ammonia toxicity, using co-digestion strategies and new reactor designs to maximize CH₄ yield (Dhull et al. 2024; DelaVega-Quintero et al. 2025). However, the paradigm of anaerobic digestion has evolved from a waste treatment system to a circular economy unit, where digestate plays a leading role. The application of this byproduct on farms allows for the gradual replacement of synthetic mineral fertilizers, reducing the carbon footprint and mitigating the leaching of nutrients that degrade soils and water resources (Tawfik et al. 2023; Dhull et al. 2024).

The literature on the anaerobic digestion of chicken litter is fragmented, consisting of isolated studies on ammonia inhibition, co-digestion, reactor configurations, and pre-treatments, yet showing little integration among technological advancements, environmental sustainability, and digestate valorization. Previous reviews have adopted conventional narrative approaches; few have utilized bibliometric methodologies capable of objectively mapping emerging trends, collaboration networks, and knowledge gaps (Shapovalov et al. 2020).

The application of the Methodi Ordinatio in studies on the anaerobic digestion of chicken litter allows for the organization and prioritization of the scientific literature based on criteria such as impact factor, number of citations, and year of publication, facilitating the identification of the most relevant and influential studies, as well as scientific gaps in the field. This approach is useful in a multidisciplinary field that encompasses anaerobic digestion, biogas production, waste management, and agricultural fertilization—areas that present challenges such as optimizing the C/N ratio, reducing ammonia toxicity, and increasing methane yield. Methodi Ordinatio allows for the identification of technological advances to improve the biodegradability and energy yield of chicken litter in anaerobic digestion (Bhatnagar et al. 2022; Bist et al. 2024). The systematic review of the literature also highlights significant gaps, such as the need for more efficient operational models and strategies to mitigate the negative effects of pre-storage of the litter. Thus, the Methodi Ordinatio contributes to a robust analysis of the state of the art, guiding future research and technical decisions aimed at the sustainable utilization of chicken litter in energy production. This bibliometric methodology strengthens the integrated understanding of the multiple dimensions involved in the utilization of this agricultural waste (Zarzzeka et al. 2024; Orrico et al. 2025).

This work integrates bibliometric analysis with technological and environmental perspectives, providing a comprehensive overview of anaerobic digestion of chicken litter, including operational optimization, methane production, digestate valorization, greenhouse gas mitigation, and circular economy opportunities. Thus, the objective of this study is to integrate the technological, environmental, and circular economy dimensions of anaerobic digestion of chicken litter through a systematic bibliometric review using the Methodi Ordinatio methodology. By systematically selecting and prioritizing high-impact literature, the aim is to map the evolution of scientific research, the main advances, and existing knowledge gaps. It also evaluates critical operational factors, pretreatments, key technological strategies, and compiles scientific evidence on the contribution of this process to greenhouse gas mitigation and decarbonization. Furthermore, it examines the challenges and opportunities of shifting the use of digestate to high-value-added biofertilizers, ultimately providing an integrated framework to guide future research and technical decisions for sustainable production.

The review goes beyond a simple assessment of methane production, offering an integrated perspective on the anaerobic digestion of chicken litter as a pathway for waste-to-energy conversion, nutrient recovery, the circular bioeconomy, and decarbonization. The discussion covers everything from the characteristics and challenges of chicken litter and technological strategies to optimize anaerobic digestion, to the valorization of biogas and digestate, decarbonization and sustainability, and the identification of priorities for future research.

This study is a systematic literature review aimed at identifying, selecting, and critically analyzing the most relevant scientific evidence on biogas production from chicken litter, with an emphasis on technological advances, operational parameters of anaerobic digestion, digestate utilization, and the integration of this technology into the circular economy. Systematic reviews are recognized for their ability to synthesize scientific knowledge in a structured, transparent, and reproducible manner, reducing the subjectivity inherent in narrative reviews and enabling the identification of trends, gaps, and research opportunities (Page et al. 2021); they are particularly relevant in fields with a large volume and high heterogeneity of publications.

To ensure methodological rigor, we adopted the Methodi Ordinatio 2.0 methodology, proposed by Pagani, Kovaleski and Resende (2015), and refined by Pagani et al. (2022), which is widely applied in fields such as Production Engineering, Chemical Engineering, Sustainability, and the Circular Economy. This multi-criteria approach uses bibliometric indicators, including journal impact factor, number of citations, and year of publication, integrated into the InOrdinatio index, with the aim of reducing subjectivity in literature selection and enabling the identification of studies of the greatest scientific relevance. In its updated version, the methodology incorporates statistical improvements and computational tools that enhance its robustness and automate the ranking process, thereby expanding its applicability in bibliometric and systematic reviews.

In addition, the method combines quantitative ranking with qualitative analysis, since the highest-ranked studies are subsequently analyzed in depth, allowing for critical interpretation beyond numerical indicators. Its application in this study contributes to the construction of a bibliographic portfolio representative of the state of the art, especially in emerging topics such as the anaerobic digestion of agricultural and livestock waste and the production of renewable energy. Finally, the application of the Methodi Ordinatio provides a systematic, reproducible, and scientifically robust process for organizing and analyzing the literature, contributing to the consolidation of knowledge on biogas production from chicken litter and its technological and environmental implications in the context of the circular economy.

As a key component of chicken production systems, chicken litter plays a fundamental role in bird welfare, thermal comfort, and performance. Its physical, chemical, and microbiological quality influences the incidence of lesions, ammonia emissions, pathogen proliferation, and the quality of the rearing environment. Therefore, proper management of moisture, ventilation, stocking density, and litter depth is essential to reduce health issues and improve productive performance (Biesek et al. 2023; Strašifták and Juhás 2023; Kovaleva et al. 2024).

Poultry litter is a heterogeneous mixture of excreta, feathers, wasted feed, and bedding material; its chemical composition varies depending on the diet, bird age, materials used, and management practices (Rogeri et al. 2016; Durmuş et al. 2023). While poultry manure refers mainly to excrement generated by birds without necessarily implying the presence of bedding material, encompassing different production systems, it is distinguished from litter whenever the original study provides sufficient information to determine the substrate composition (Katuwal et al. 2023). The term chicken litter is applied when studies address litter generated by species in general — broilers, laying hens, and turkeys — without restricting the analysis to a single species species; its composition varies, and it is characterized as a mixed material that generally includes manure, bedding, feathers, spilled feed, and moisture (Ball et al. 2024; Tulsi et al. 2025).

Another characteristic of chicken litter is that it is a heterogeneous substrate with physicochemical properties that depend on the birds and their production system. This variability affects both its agronomic potential and its suitability for anaerobic digestion processes aimed at biogas production. This variability affects both its agronomic potential and its suitability for anaerobic digestion processes to produce biogas. High moisture content, the presence of pathogens, antibiotic residues, heavy metals, and high lignin concentrations pose significant challenges for the transport, storage, agricultural application, and energy recovery of the waste (Kyakuwaire et al. 2019; Khodadadi and Masoumi 2025; Haroun et al. 2025). Among the materials that can be used as litter, pine wood shavings are considered the most efficient option due to their high absorption capacity, lower ammonia production, and superior microbiological conditions. In contrast, materials with higher lignin or fiber content can increase compaction, moisture levels, and the incidence of lesions in the birds (Diarra et al. 2021; Lima et al. 2018; Munir et al. 2019; Ahmed et al. 2025).

The chemical characterization of chicken litter is essential for its environmental valorization, as it determines its suitability for use as fertilizer, biochar, a biosorbent, or a substrate for anaerobic digestion. Chicken litter biomass contains high levels of organic matter, carbon, nitrogen, phosphorus, and potassium; however, its composition is highly variable, necessitating preliminary analyses to mitigate environmental risks and optimize its utilization. Studies show that chicken litter, on a dry basis, contains 60–85% dry matter, 28–42% organic C, 2–2.5% total N, 3% total phosphorus, 2.9% potassium, 6–27% ash, and a pH of 7.5–8.5 (Maruthamuthu et al. 2024; Prasetyo et al. 2024; Rogeri et al. 2016). Furthermore, the presence of lignin reduces the material's biodegradability, requiring strategies such as pre-treatments or co-digestion to enhance biogas production efficiency. Collectively, studies demonstrate that the proper management of chicken litter is crucial for poultry productivity, environmental sustainability, and the energy recovery of this waste (Zoghlami and Paës 2019; Bhatnagar et al. 2022).

Projections indicate that global chicken meat production is expected to reach approximately 139 million tonnes (Eremeeva et al. 2025). In 2025, Brazil recorded 5.3 million tonnes in exports, generating total revenue of US$ 9.7 billion, demonstrating the continued expansion of the international market (ABPA 2026). Consequently, Brazilian chicken exports are expected to grow, driven by cost competitiveness and production efficiency resulting from vertical integration and the use of technology (Coronel et al. 2024). Poultry farming generates large volumes of waste throughout the stages of incubation, rearing, and slaughter. It is estimated that 32% to 37% of a slaughtered bird's mass becomes solid waste, with feathers and skin accounting for more than 50% of this total. A bird can produce approximately 1 Kg of fresh manure for every kilogram of feed consumed (Mozhiarasi and Natarajan 2025).

Poultry production systems generate various types of by-products originating from hatcheries (eggshells, infertile eggs, etc.) or farms. Slaughter and processing operations generate blood, viscera, feathers, condemned carcasses, and industrial effluents (Zhang et al. 2023). The quantity of waste generated depends on the number of birds, the rearing system, the type of material, management practices, and the duration of the production cycle (Gonçalves et al. 2019).

Chicken litter can contain contaminants that pose risks to human health and the environment, while improper management contributes to eutrophication and the contamination of water resources (Devnath et al. 2025). The main challenge lies in transforming poultry waste into resources through sustainable technologies and good management practices. Key alternatives include the production of organic fertilizers, biogas, biodiesel, biochar, and animal feed, thereby promoting the circular economy and reducing climate impacts through the valorization of this waste (Alves et al. 2023; Araújo dos Santos et al. 2023). Biogas production is illustrated in Figure 3.

Physical, chemical, and biological treatments for chicken litter aim to reduce ammonia and pathogen levels, control pests, and prepare the waste for agricultural or energy use. Key methods include fermentation, chemical conditioning, composting, incineration, and the use of biodigesters, selected based on the intended application and sanitary or environmental requirements (Vitto et al. 2022; Hemavarshini et al. 2025).

Flat-layer fermentation involves stacking or covering chicken litter to promote anaerobic fermentation; lasting 20 to 30 days, this process reduces pathogens and improves sanitary quality. Chemical conditioners act by lowering pH and reducing ammonia volatilization and microbial load, offering an effective, rapid, and low-cost solution (Gonçalves et al. 2019). The use of ethyl alcohol to control pests in chicken litter—at a dosage of 2.5 mL—resulted in the mortality of the beetle Alphitobius diaperinus in a closed system, while double that amount was required in an open system, demonstrating a viable alternative to conventional insecticides (Fogaça et al. 2017).

Composting reduces the moisture, volume, and weight of chicken litter, while also eliminating some pathogens, weed seeds, and vectors. This treatment can facilitate the reuse of litter within the same poultry house, although it requires daily management and significant labor (Valadão et al. 2011). However, while composting is widely used to convert chicken litter into a more stable organic fertilizer, the process does not automatically reduce greenhouse gas emissions and can cause substantial nitrogen losses through ammonia volatilization. Nitrogen emissions and retention depend on management factors such as pH, moisture, aeration, C/N ratio, composting system, and the use of additives such as biochar, aluminum sulfate, superphosphate, lignite, or acidic additives (Liu et al. 2023).

Incineration can release harmful compounds into the atmosphere, risks involving SO₂, NO_x, CO, HCl, dioxins, heavy metals, CH₄, and toxic ash residues, and generally involves high capital, operational, and pollution control costs. However, its economic and environmental efficiency depends on the technology, emission controls, energy recovery, and the alternative being compared. Even with improved average climate indicators, flue gas purification and waste treatment remain major contributors to the overall environmental impact (Liu et al. 2026).

Biodigestion of chicken litter yields biogas and digestate produced via fermentation which can be separated into solid and liquid fractions. The construction and operation of biodigesters require skilled labor, and the digestate must be treated or evaluated prior to soil application (Carraro et al. 2024). Biodigesters maximize waste utilization by generating biogas from renewable sources (Archana et al. 2024). When chicken litter is diluted with water, it separates into a solid fraction—destined for composting—and a liquid fraction, which is fed into the biodigesters. The liquid fraction yields more biogas per unit of material fed into the biodigester; it is also more cost-effective, as it requires a smaller reactor for the same number of animals and a shorter retention time within the reactor. Compared to the original material, the liquid fraction contains higher levels of soluble nutrients, exhibits greater fluidity, is free of coarse materials, and decomposes rapidly (Bist et al. 2024; Zahan et al. 2018b; Zahan and Othman 2019). Current methods for handling chicken litter are listed in Table 1.

Anaerobic digestion of chicken litter has the potential to reduce the climate impacts associated with conventional management, but this benefit is not intrinsic to the technology and depends heavily on the system configuration, the use of biogas, nutrient recovery, and the control of fugitive emissions. Available evidence indicates that the primary climate benefit does not stem simply from the stabilization of the waste, but from the combination of replacing fossil fuel sources, recovering nutrients, and reducing the emissions that would occur during storage and the direct application of litter to the soil. Life-cycle assessment studies indicate reductions of 27–65% in greenhouse gas emissions compared to direct application, although results vary substantially depending on the energy scenario, the type of co-digestion, and the substitution assumptions adopted (Carneiro et al. 2024). Thus, anaerobic digestion should be viewed as a strategy for the integrated management of carbon, energy, and nutrients, rather than simply as a biogas production technology.

A life-cycle assessment of chicken litter demonstrates that anaerobic digestion can reduce greenhouse gas emissions compared to direct disposal on the ground. The use of liquid digestate as fertilizer resulted in an estimated 34% reduction in greenhouse gas emissions, while the use of autohydrolysis pretreatment prior to digestion resulted in a reduction of approximately 27%. In addition, anaerobic digestion of chicken litter reduced the potential for eutrophication and acidification by 98.2% and 98.4%, respectively, compared to conventional direct application to the soil (Carneiro et al. 2024). However, these percentages should not be interpreted as universal reductions for any anaerobic digestion system. They depend on the system boundaries, the reference scenario, the allocation method, and, most importantly, the credits assigned to recovered energy and nutrients.

The importance of biogas for energy use becomes evident when different utilization pathways are compared. The replacement of fossil fuels is one of the main mechanisms responsible for reducing climate impacts, and biomethane production can offer greater environmental benefits than electricity generation under certain conditions (Mediboyina and Murphy 2024). This means that the same amount of biogas can yield different climate outcomes depending on the energy product it replaces. Consequently, it is not sufficient to quantify CH₄ production; it is necessary to determine which conventional energy source will actually be displaced and what its carbon intensity is.

This dependence on the energy scenario is also observed in larger-scale assessments. In Turkey, it was estimated that co-digestion of all available chicken manure could avoid 12.2 Mt CO₂-eq per year, equivalent to about 2.5% of annual national emissions (Balcioglu et al. 2022). Although this result demonstrates the great mitigation potential associated with the energy recovery of poultry waste, it represents a scenario of widespread technological adoption and, therefore, should not be directly extrapolated to individual systems. Similarly, in the United Kingdom, the gasification of chicken manure was estimated to prevent 1.7 Mt of greenhouse gases per year (Jeswani et al. 2019). Anaerobic digestion of chicken manure has a lower climate impact than fossil fuel-based energy systems in 15 out of 17 environmental categories (Balcioglu et al. 2022). These results reinforce the finding that the greatest climate benefit occurs when the waste replaces a conventional disposal route and, at the same time, provides renewable energy capable of replacing fossil fuels.

At the farm scale, the results also highlight the influence of the system’s configuration. A 22-month study evaluated a 400 m³ biogas digester processing chicken litter and observed CH₄ production of up to 222 L Kg^-1 of volatile solids, in addition to significant nutrient recovery during solids separation. In the life-cycle assessment, the scenario of using biogas in cogeneration—combined with the processing of three times as much biomass—showed a 722% reduction in climate impact and a 510% reduction in eutrophication potential compared to direct application to the field (Hassanein et al. 2022). The magnitude of these results, however, highlights precisely the importance of the model’s assumptions: significant benefits can be achieved when recovered energy and nutrients replace conventional products. Therefore, results from different studies should not be compared solely based on reduction percentages, but also on system boundaries, processing scale, and the products being replaced.

Digestate can replace mineral fertilizers because it contains available nutrients. Fertilizer substitution is an important source of climate credits, especially when digestate is considered a substitute for soil nutrients rather than a waste product, generating credits that offset most of the process’s impacts. When emissions from anaerobic digestion attributed to digestate are added to emissions from field application, the resulting total is lower than the emissions associated with the production and use of mineral fertilizers. The potential for replacing mineral fertilizers is a key factor, as it determines the extent to which the production and application of these fertilizers can be avoided (Kreidenweis et al. 2021). These credits can represent a significant portion of the reduction in the system’s impacts, especially when digestate can be accounted for as an effective substitute for mineral fertilizers. However, this benefit depends on the amount of nutrients actually available to plants and the agronomic equivalence assumed in the model, thereby avoiding an overestimation of the climate benefit.

This point is reinforced by studies that evaluated different scenarios for digestion and digestate application. When emissions associated with anaerobic digestion and digestate application are combined, they may remain lower than emissions related to the production and use of mineral fertilizers (Timonen et al. 2019). However, the management of digestate itself can introduce new sources of environmental impact. In a study involving different co-digestion scenarios for animal manure, the application of digestate showed a higher potential for NH₃ emissions than untreated manure, associated with an increase in pH after digestion, with an estimated increase of 0.14–0.3 Kg NH₃-N per metric ton of biomass. On the other hand, nitrate leaching was reduced across different scenarios, ranging from 0.04 to 0.45 Kg of N-NO₃/t, demonstrating that the environmental impact of digestate is multidimensional and cannot be assessed solely based on fertilizer substitution (Møller et al. 2022).

Anaerobic digestion also does not eliminate emissions after the material leaves the digester. The digestate continues to contain residual organic matter and potentially available nitrogen, so storage, solid-liquid separation, and agricultural application can be sources of CH₄, N₂O, and NH₃. The highest emissions associated with digestate may occur during field application, followed by storage, demonstrating that climate impact assessments must cover the material’s entire life cycle and not just the interior of the bioreactor (Timonen et al. 2019). Thus, the climate benefit attributed to anaerobic digestion may be reduced if the digestate is stored improperly or applied to the soil under conditions that favor N and C losses.

The storage phase of the litter prior to digestion also represents a significant source of emissions. The highest greenhouse gas emissions from digestate occur during soil application, followed by storage. Chicken litter stored uncovered can develop anaerobic regions capable of producing CH₄, as well as aerobic or anoxic regions where N₂O formation may occur. In a study of uncovered stored chicken litter, average emissions of 75 µg CH₄ m^-2 s^-1 and 18 µg N₂O m^-2 s^-1 were measured over 65 days (Wit et al. 2010). These results demonstrate that conventional storage is not an environmentally neutral process and that part of the benefit of anaerobic digestion may stem from replacing this management approach with a controlled energy recovery process.

The bedding cover demonstrates that emissions can be significantly altered by relatively simple management measures. In an experimental study, the cover reduced NH₃ emissions by 92–95%, CH₄ emissions by 25–40%, and N₂O emissions by 82–89%, while total greenhouse gas emissions were reduced by 63–72% during the covering period (Kamp and Feilberg 2024). This has an important implication for the benefits of anaerobic digestion, as part of the avoidable emissions can be controlled even before the chicken litter enters the digester. Thus, the comparison to be analyzed is between complete management systems, including storage, treatment, energy recovery, and final disposal of the digestate.

Residual CH₄ after digestion represents one of the main limitations to climate benefits. Even after biogas production, the digestate may retain biodegradable volatile solids and continue to emit CH₄ during storage. Furthermore, storage conditions can simultaneously favor the formation of CH₄, N₂O, and NH₃. Studies on the storage of digested waste demonstrate that residual methane potential can be significant and that fraction separation can substantially reduce this potential, underscoring the importance of enclosed storage and nutrient separation strategies (Vergote et al. 2020; Kreidenweis et al. 2021). In this study on the decomposition of poultry manure that was either composted or stored in the field, emissions were −165 Kg of CO₂, while composting resulted in emissions of −154 Kg of CO₂ per metric ton during storage (Kreidenweis et al. 2021). Therefore, post-digestion storage may be a controllable residual source, but its specific magnitude for poultry farming needs to be measured or modeled for the actual facility.

Fugitive CH₄ emissions during the system’s operation itself constitute another significant limitation. Methane can escape from the digester, storage structures, gas lines, and cogeneration systems, directly reducing the benefit gained from replacing fossil fuels, making the control of these losses a critical factor in plant operations (Kreidenweis et al. 2021). In a study of biogas plants in Denmark, the average CH₄ loss was estimated at 4.6%, although it varied substantially among facilities; in agricultural plants, the average was approximately 2.4%. It was also estimated that Denmark’s national emissions range from 3,409 to 4,683 metric tons of CH₄ per year, with agricultural plants accounting for 68% to 70% of this total. This overview reinforces the fact that methane losses are a critical factor for the sustainability of biogas; with a loss rate of 5%, fugitive emissions outweigh the logistical and energy benefits, highlighting the need for technical rigor in the management and sealing of systems that operate with biomass rich in solids and nitrogen, such as chicken litter (Scheutz and Fredenslund 2019). These values provide important evidence for analytical comparison when assessing the impact of fugitive emissions on the carbon footprint of biogas systems.

Another aspect that is often underestimated in carbon balancing is the plant’s internal energy consumption. Pumps, agitators, recirculation systems, separators, dryers, and other equipment consume a portion of the energy produced. In chicken litter processing systems, digestate recirculation and solid-liquid separation can account for significant energy demands (Hassanein et al. 2022). Therefore, the climate balance must account not only for the biogas produced but also for the electricity and fuels used for pumping, mixing, drying, treatment, raw material preparation, and ammonia control.

Pre-treatments must be evaluated according to the same principle. Ultrasonic treatment (20 kHz, 2.5 min, 299 kJ/L) of chicken litter, for example, increased biogas production by 10%, from 147.9 mL g^-1 (raw chicken litter) to 163.0 mL g^-1 (pretreated), in addition to promoting the solubilization of 24.7 g Kg^-1 of organic carbon and the removal of 36% of the ammoniacal nitrogen (Gaspareto et al. 2020). However, the ultrasonic equipment requires electricity from a high-power ultrasonic generator, which is not an energy source produced by the biodigester; therefore, the increase in CH₄ production does not automatically result in a greater net reduction in emissions. Hydrothermal pretreatment of chicken litter did not significantly increase emissions when the biogas generated was used as a heat source. Anaerobic digestion of chicken litter—whether combined with hydrothermal treatment or used alone—reduces greenhouse gas emissions by about 30% compared to the baseline scenario when the biogas generated is used as a heat source, but highlights the need to monitor fugitive emissions of CH₄ and N₂O (Carneiro et al. 2024).

The use of biogas to replace fossil fuels is, therefore, one of the main factors determining the final balance. Biogas derived from chicken litter can replace natural gas, fuel oil, or other fossil fuels in boilers, electricity generation, and generator sets (Kelleher et al. 2002). Similarly, biogas from poultry waste can be used for heat and electricity generation, expanding the possibilities for energy substitution (Karuppannan et al. 2021). In a case study in Cameroon, biogas produced from poultry waste primarily replaced LPG for heating and reduced dependence on grid electricity (Baldi et al. 2026). These results reinforce the notion that climate credits can be linked to the conventional energy that is no longer consumed.

Consequently, the carbon balance of an anaerobic digestion plant must include biogas consumed internally, recovered heat, electricity produced or consumed, natural gas used as a backup, biomass fuels, and the chemicals employed. Energy consumption associated with mixing, pumping, drying, ultrasonic treatment, and ammonia control must also be taken into account. Life-cycle assessments show that the results are highly dependent on these choices, with the greatest credits generally related to electricity or natural gas replaced by recovered biogas, in addition to the reduction in impacts associated with waste disposal (Beausang et al. 2020).

The energy demands of the biogas plant for its various operational processes contribute to total emissions and must be accounted for. The results highlight that the calculated emissions are strongly influenced by a limited number of management decisions and by the choice of emission calculation methods. The sensitivity analysis demonstrates the importance of considering parameter uncertainty and management practices when assessing the environmental impacts of broiler waste treatment options. Thus, the actual efficiency of anaerobic digestion depends on the relationship between CH₄ production, internal energy consumption, fugitive emissions, digestate disposal, and the carbon intensity of the energy it replaces. A life-cycle assessment study found the lowest net CO₂ balance, approximately −394 Kg, in the biogas production scenario; although this scenario also had the highest gross positive emissions, the favorable net result was primarily due to credits associated with mitigation and renewable energy generation (Kreidenweis et al. 2021). This result helps illustrate the difference between gross emissions and net climate benefit.

Thus, the anaerobic digestion of chicken litter can offer significant climate benefits, but these benefits are conditional and system-dependent. The greatest gains tend to occur when the technology combines high CH₄ recovery, low fugitive emissions, efficient biogas utilization, the replacement of high-carbon-intensity fossil fuel sources, and the agronomically effective use of digestate. On the other hand, CH₄ leaks, high auxiliary energy consumption, inadequate digestate storage, and overestimation of fertilizer credits can substantially reduce the expected benefit. Thus, it is evident that design and management decisions can determine a large part of the climate outcome. Comparisons between studies must take into account the baseline scenario, system boundaries, allocation methods, the energy mix being replaced, fugitive emissions, and the destination of the digestate.

Anaerobic digestion of chicken litter has advanced, but the process still has limitations. Studies on this topic focus on ammonia inhibition and process optimization, with the main limitations being scale-up, long-term stability, digestate safety, standardization of reports, and comprehensive validation of technical-economic and life-cycle aspects. Furthermore, efforts are focused on reducing the effects of ammoniacal nitrogen through ammonia removal, co-digestion, dilution, biochar, and other additives, indicating that inhibition remains a recurring bottleneck despite the advances made in process optimization (Mansour et al. 2024; Song et al. 2024; Arriagada et al. 2026). Thus, while the ability to mitigate ammonia inhibition is observed, these findings do not yet indicate a universally applicable solution for different operating conditions (Song et al. 2023; Arriagada et al. 2026).

Consequently, these limitations become more significant in digestion systems with high solids content. Thus, the high concentration of solids present in chicken litter simultaneously alters the transport, mixing, and mass transfer properties, making the process behavior dependent not only on the chemical composition but also on the physical characteristics of the substrate. In these systems, the accumulation of volatile fatty acids and ammonia toxicity can occur interdependently, increasing the process’s sensitivity to operational disturbances (Higgins et al. 2021; Collins et al. 2023). Consequently, conditions considered adequate in specific experiments cannot be generalized without considering differences in bed composition, solids content, reactor configuration, and feeding strategy. Uncertainty regarding this transfer of performance between different systems therefore constitutes an engineering gap, not merely a matter of biochemical optimization, and requires further investigation (Collins et al. 2023).

The lack of sufficient evidence from continuous operation and at an expanded scale reinforces this point, as most studies still conduct batch tests using bench-scale reactors or batches. Meanwhile, there are relatively few studies that evaluate performance over extended periods and under conditions similar to those found at the agricultural or commercial scale. This difference is important because issues such as seasonal variation in raw materials, changes in organic load, recirculation, pumping and mixing limitations, foaming, and contamination may not be adequately represented in short-term tests. Thus, the parameters considered optimal at the laboratory scale still need to be evaluated for stability, reliability, resilience, and the maintenance of performance and operating costs when subjected to real-world and prolonged disturbances. The main challenge, therefore, is not merely to increase the reactor volume, but to demonstrate that biological and operational performance remains predictable after scaling up. This difference is important because issues such as seasonal variation in raw materials, changes in organic load, recirculation, pumping and mixing limitations, foaming, and contamination may not be adequately represented in short-term tests. Thus, the parameters considered optimal at the laboratory scale still need to be evaluated for stability, reliability, resilience, and the maintenance of performance and operating costs when subjected to real-world and prolonged disturbances. The main challenge, therefore, is not merely to increase the reactor volume, but to demonstrate that biological and operational performance remains predictable after scaling up (Arriagada et al. 2026; Baldi et al. 2026).

Another critical issue is the quality of the digestate resulting from the anaerobic digestion of chicken litter, as this process does not necessarily eliminate potentially problematic components present in the digestate. On the contrary, some of the nutrients may remain in the digestate or become concentrated and transformed during the process. Therefore, the potential of digestate as a fertilizer cannot be assessed solely by its nutrient concentration. The presence of ammonia, high salinity or electrical conductivity, metals, antibiotic residues, and antimicrobial resistance genes raises additional concerns regarding environmental safety and agricultural use (Chaump et al. 2019; Zheng et al. 2022). From this perspective, characterizing digestate is not a secondary step in the process but a necessary condition for determining its fate and value as a byproduct. Further studies are still needed that integrate chemical and microbiological characterization, the stability of contaminants during digestion, and an assessment of the effects resulting from application to soil (Zheng et al. 2022).

The lack of sufficient evidence from continuous operation and at an expanded scale reinforces this point, as most studies still conduct batch tests using bench-scale reactors or batches. Meanwhile, there are relatively few studies that evaluate performance over extended periods and under conditions similar to those found at the agricultural or commercial scale. This difference is important because issues such as seasonal variation in raw materials, changes in organic load, recirculation, pumping and mixing limitations, foaming, and contamination may not be adequately represented in short-term trials. Thus, the parameters considered optimal at the laboratory scale still need to be evaluated for stability, reliability, resilience, and the maintenance of performance and operating costs when subjected to real-world and prolonged disturbances. The main challenge, therefore, is not merely to increase the reactor volume, but to demonstrate that biological and operational performance remains predictable after the scale-up (Arriagada et al. 2026; Baldi et al. 2026).

Another critical issue is the quality of the digestate resulting from the anaerobic digestion of chicken litter, as this process does not necessarily eliminate the potentially problematic components present in the digestate. On the contrary, some of the nutrients may remain in the digestate or become concentrated and transformed during the process. Therefore, the potential of digestate as a fertilizer cannot be assessed solely based on its nutrient concentration. The presence of ammonia, high salinity or electrical conductivity, metals, antibiotic residues, and antimicrobial resistance genes raises additional concerns regarding environmental safety and agricultural use (Chaump et al. 2019; Zheng et al. 2022). From this perspective, characterizing digestate is not a secondary step in the process but a necessary condition for determining its fate and value as a byproduct. Further studies are still needed that integrate chemical and microbiological characterization, the stability of contaminants during digestion, and an assessment of the effects resulting from application to soil (Zheng et al. 2022).

Pre-treatment strategies and the use of additives share a similar limitation. Different approaches, such as biochar, iron, bentonite, microaeration, acids, enzymes, micronutrients, and inoculum acclimatization, can positively modify digestion performance, while thermal, chemical, ultrasonic, leaching, or separation treatments can improve the availability of organic matter and, under certain conditions, increase methane production (Bi et al. 2019; Mansour et al. 2024; Xiao et al. 2025). However, the heterogeneity of the results indicates that these methods should not be interpreted in isolation as established technological solutions. Their effects depend on substrate composition, solids content, inoculum, and reactor configuration, while the additional costs of energy, reagents, equipment, and management may reduce or even eliminate the benefits obtained in methane yield. The gap, therefore, lies in the absence of an integrated assessment that simultaneously considers the effectiveness, stability, cost, and environmental consequences of these strategies (Indren et al. 2020; Mansour et al. 2024).

Furthermore, our understanding of the rate-limiting steps in the process is still insufficient to allow for the design and control of reactors with a high degree of predictability. Kinetic studies on chicken litter systems indicate that hydrolysis and other conversion steps remain subjects of investigation, particularly in leaching-bed systems (Shen and Zhu 2018). Thus, identifying the rate-limiting steps and incorporating kinetic parameters capable of representing the specific characteristics of chicken litter are necessary to improve modeling and operational control (Alba Reyes et al. 2021; Shen and Zhu 2018). Without this understanding, the extrapolation of results obtained under specific conditions remains limited.

The technical, economic, and environmental assessments of anaerobic digestion of chicken litter do not provide sufficient evidence to support generalizable conclusions regarding the technology’s feasibility. Although anaerobic digestion has potential for energy recovery and waste management, available assessments point to limitations related to the targets adopted, the extrapolation of results, and the reproducibility and robustness of the analyses (Rajendran and Murthy 2019). Furthermore, economic viability should not be inferred solely from the amount of biogas produced, since costs associated with litter collection and transportation, pretreatment, ammonia control, reactor operation, and digestate management can substantially alter the economic balance. Thus, the evaluation of the technology must consider the combined value of energy recovery, waste treatment, and byproducts, rather than using biogas as the sole indicator of benefit (Rajendran and Murthy 2019; Terzich et al. 2000).

Finally, the low level of standardization in experimental procedures and reported parameters regarding the anaerobic digestion of chicken litter. The diversity of feedstocks, inoculum sources, solids content, pretreatments, reactor configurations, and operating regimes makes it difficult to directly compare results (Bercy and Roubík 2024; Song et al. 2023). This heterogeneity also limits the development of cumulative evidence. When key parameters are not reported consistently, it becomes difficult to distinguish whether performance differences stem from litter composition, operational strategy, inoculum, or the experimental setup itself. Consequently, the absence of standardized protocols and metrics slows the identification of truly generalizable conditions and hinders the transition from laboratory results to larger-scale applications (Song et al. 2023).

Artificial intelligence (AI) is being used in the anaerobic digestion of chicken litter primarily to predict and optimize methane/biogas production, adjust operating conditions, and monitor inhibition, rather than to fully automate the facilities (Shen and Zhu 2017; Simeonov and Hubenov 2025). AI is most valuable as a decision-support tool for co-digestion, pretreatment, and the optimization of operating conditions. The most robust models specific to chicken litter are hybrids of artificial neural networks, adaptive neuro-fuzzy systems, fuzzy models, and response surface methodology. These were used to optimize the co-digestion of chicken litter with wheat straw, biochar, duck manure, and other co-substrates (Zhan et al. 2023b; Zhan and Zhu 2024).

In broader approaches to anaerobic digestion, studies suggest that the next step is the use of interpretable virtual sensors for the detection of volatile fatty acids, alkalinity, pH, malfunctions, and real-time control. These methods are relevant to chicken litter systems because they target the same signs of instability (Kazemi et al. 2020; Kazemi et al. 2021; Razak et al. 2025). The development of intelligent anaerobic digestion systems can be considered a key research frontier for the valorization of chicken litter. Future studies could combine experimental data from laboratories, pilot plants, and full-scale digesters with machine learning algorithms to develop reliable predictive models for methane production, ammonia inhibition, microbial stability, and digestate quality. Consequently, future research needs to focus on data standardization, interpretability, transferability, cybersecurity, implementation costs, and validation under real-world farm conditions (Andrade Cruz et al. 2022; Onweh et al. 2025; Razak et al. 2025).

Taken together, this evidence indicates that the main challenge currently facing the anaerobic digestion of chicken litter is determining under what conditions it can operate in a stable, safe, economically viable, sustainable, and beneficial manner in full-scale studies over long periods. Various strategies can improve specific indicators, particularly methane production and the reduction of ammonia inhibition; however, these solutions are still often evaluated in isolation and are not yet fully mature as comprehensive solutions. Advancement in this field therefore depends on the integration of ammonia control, rheological behavior, kinetics, stability in continuous operation, digestate quality, evaluation of pretreatments and additives, technical-economic analysis, life-cycle assessment, and methodological standardization. Another point is the need to explore studies on incremental methane recovered, usable methane effectively utilized, heat and electricity consumed, quantities of chemicals, the volume and composition of wastewater, methane potential in the residual stream, and post-treatment CH₄/N₂O emissions. This shift in focus from yield to demonstrating the robustness and sustainability of the process as an integrated system is essential for determining whether the gains observed on a laboratory scale can be replicated under real operating conditions.

The bibliometric review, conducted using the Methodi Ordinatio 2.0 methodology, revealed an increase in the number of scientific publications over the period (2020–2026), indicating growing recognition of poultry litter as a valuable feedstock for renewable energy generation and nutrient recovery within the circular economy framework. In addition to mapping the evolution of research, the review identified key technological advancements, current limitations, and emerging research priorities in this field. The efficiency of anaerobic digestion is limited by the high nitrogen content of chicken litter, its low carbon-to-nitrogen ratio, and inhibition by ammonia, among other factors. To overcome these challenges, strategies such as co-digestion, physical, chemical, and biological pre-treatments, biochar supplementation, micro-aeration, high-solids anaerobic digestion, and improved operational control have been employed.

In general, the evidence suggests that the anaerobic digestion of chicken litter has the potential to contribute to integrated waste management, renewable energy generation, and nutrient recycling. Its environmental and economic benefits depend on proper process control, such as managing ammonia inhibition and organic load, effectively containing and minimizing fugitive methane emissions, achieving favorable energy balances, ensuring safe and responsible management of digestate, and achieving viable economic performance. Furthermore, studies conducted under laboratory conditions or in small-scale experimental settings may not reflect reality or be directly transferable to commercial chicken production systems.

Therefore, future research must prioritize long-term investigations on a pilot and industrial scale to validate the stability and performance of the process under realistic operating conditions. In addition, technical-economic and life-cycle assessments must be conducted in parallel with experimental studies to determine whether energy and nutrient recovery can provide environmental and economic benefits. Attention must also be paid to digestate quality and safety standards, the monitoring and mitigation of methane emissions, energy consumption, standardized operational reliability, and integration with existing chicken production systems. Multidisciplinary research combining engineering, environmental assessment, agronomy, economics, and chicken production will be essential to determine the conditions under which anaerobic digestion can offer consistent benefits and to support the responsible transfer of the technology to real-world applications and ensure economic and sustainable viability.