Biogas Production From Chicken Litter: Advances, Challenges, and Perspectives of Anaerobic Digestion

1. Introduction

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.

2. Bibliometric analysis and Research design

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.

3. Methodological Approach

3.1. Defining the search strategy

Defining the search strategy is a fundamental step in systematic reviews, as it directly determines the scope, accuracy, and quality of the resulting literature portfolio (Page et al. 2021). Accordingly, a search strategy was developed to identify studies related to biogas production from chicken litter, covering aspects such as poultry waste, anaerobic digestion, methane production, and the circular economy.

The literature search was conducted exclusively in the Scopus database (Elsevier), which was selected for its broad multidisciplinary coverage, large number of peer-reviewed journals, and availability of bibliometric indicators compatible with the application of the Methodi Ordinatio, such as number of citations, year of publication, and journal data (Elsevier 2024).

The searches were conducted using the Title, Abstract, and Keywords fields (TITLE-ABS-KEY), which ensures greater thematic relevance of the results and reduces the retrieval of studies with low relevance to the research topic. To develop the search strategies, the Boolean operators AND and OR were used, in accordance with methodological recommendations for systematic reviews, enabling the combination of synonyms and the linking of different concepts (Page et al. 2021).

The following keywords were used: poultry farming, chicken litter, waste, biogas, methane, anaerobic digestion, circular economy, and carbon. These terms were combined in various search queries to cover the multiple dimensions associated with the energy recovery from poultry waste.

The implementation of this strategy resulted in the retrieval of 112 scientific documents indexed in Scopus (search date -15 March 2026). The process flowchart is shown in Figure 1.

3.2. Database selection

The records retrieved from the Scopus database were exported in BibTeX format (112 articles) and organized using JabRef, a software widely used for reference management and the standardization of bibliographic metadata (Team 2025). During the initial data processing stage, a check for duplicates was performed, and no duplicate records were identified. Next, eligibility criteria related to document type were applied, resulting in the exclusion of eight articles published in conference proceedings and three books, since the scope of this review is limited exclusively to scientific articles published in peer-reviewed and indexed journals. After this screening, the bibliographic portfolio consisted of 101 articles deemed eligible for the subsequent stages of applying the Methodi Ordinatio.

3.3. Organization of bibliographic references

The InOrdinatio index is defined by a weighted combination of bibliometric variables and is expressed as the integral of the journal’s impact factor (Fi), number of citations (Ci), year of publication, and the study’s relevance to current research. The formula incorporates three adjustable parameters assigned by the researcher: Δ, λ, and Ω, each ranging from 0 to 10, which represent, respectively, the importance assigned to the impact factor, the recency of the portfolio, and the average number of citations. Additionally, the equation considers the research year, corresponding to the period in which the study was conducted (2026), the year of publication of the articles, and the citation half-life of the journals, based on values from the Journal Citation Reports (JCR), in this case 7.6 (Pagani et al. 2022).

In this study, the intended configuration was to account for the dynamic nature of the field under investigation, where technological innovations are rapidly adopted, making currency and scientific representativeness just as relevant as journal impact and citation volume. To evaluate the parameters, a sensitivity analysis was conducted to verify the consistency of the employed method. The 101 articles were re-evaluated based on three parameter configurations: (i) Δ = λ = Ω = 1; (ii) Δ = λ = Ω = 5; and (iii) Δ = λ = Ω = 10—with the latter being the configuration adopted for the main analysis. The resulting rankings were compared to assess the stability of the top ten studies and determine whether the selection was significantly affected by the parameter values. Factors considered included the number of articles common to the scenarios, the degree of overlap in the *Top 10*, and observed changes in the studies' relative positions. This approach made it possible to assess whether the selection of priority articles remained consistent across different weight assignments for the InOrdinatio 2.0 parameters. To organize the 101 articles comprising the bibliographic portfolio—obtained via a search in the Scopus database—the articles were ranked using the InOrdinatio 2.0 index. This established a priority order for the in-depth analysis of the studies. The ten highest-ranked articles, representing approximately 10% of the portfolio, were selected for detailed qualitative analysis. The method was applied using the RankIn spreadsheet, taking into account the bibliometric variables and the parameters defined in the study (Δ, λ, and Ω).

3.4. Application of Methodi Ordinatio 2.0

This stage involved compiling the bibliographic portfolio using a nine-step protocol based on the Methodi Ordinatio 2.0 methodology. Initially, in steps 1 through 4, studies were searched for and screened exclusively in the Scopus database, accessed through the CAPES (Coordination for the Improvement of Higher Education Personnel, Brazil) portal. The time frame included publications from 2000 onward, with the aim of covering recent developments in the literature on the topic under investigation.

Next, the screening procedures were conducted, during which duplicate articles and studies outside the thematic scope were excluded based on a review of titles, abstracts, and keywords. This was followed by the standardization and extraction of bibliometric metadata using JabRef software (version 5.11), including year of publication, number of citations (Google Scholar), and journal impact factor, reinforcing the multi-criteria nature of the methodology. In the next step, the articles were ranked in the RankIn spreadsheet using the InOrdinatio 2.0 index, calculated according to Equation (1), which incorporates impact factor, number of citations, and year of publication.

In Equation (1), the parameters Δ, λ, and Ω represent, respectively, the weight assigned to the impact factor, the recency of the publications, and the number of citations, ranging from 0 to 10. In this study, Δ = 10, λ = 10, and Ω = 10 were adopted, ensuring equal weighting among the three criteria and promoting a balanced assessment of the scientific relevance of the studies.

After calculating the InOrdinatio index, the articles were organized in descending order of relevance, forming the final bibliographic ranking used in this review. Finally, the resulting portfolio was subjected to descriptive bibliometric analysis, without the use of network visualization tools, since software such as VOSviewer was not employed.

Where:

• Δ (Delta): an adjustable parameter ranging from 0 to 10, defined by the researcher to represent the degree of importance assigned to bibliometric metrics in the study. The choice of the letter Δ is associated with the mathematical concept of variation, symbolizing the relative influence of this component in the evaluation model, which can impact decisions such as publication, scientific visibility, and academic performance evaluation;
• IF (impact factor): or estimated CiteScore if IF is unavailable;
• λ (lambda): an adjustable parameter between 0 and 10 that reflects the importance assigned to the recency of publications. In an analogy with chemistry, the term refers to half-life and is used to weigh the “temporal validity” of articles. This component is normalized to a value of 7.6, corresponding to the median half-life reported for journals indexed in the Scopus database with a JCR indicator;
• ResearchYear: represents the year the study was conducted (for example, 2026), serving as a temporal reference for determining how current the studies are;
• PubYear: corresponds to the year of publication of each article analyzed. Although it generally precedes the year of the study, the model allows for a direct comparison with Research Year to assess the time lag in scientific output;
• Citation half-life: a bibliometric indicator that expresses the median time for which articles from a journal continue to be cited. The reference value used (7.6) is based on the median of JCR journals indexed in Scopus. This indicator is used because it reflects the persistence of scientific relevance over time;
• Ω (Ômega): an adjustable parameter ranging from 0 to 10 that represents the importance assigned to the number of citations an article receives. As it marks the end of the Greek alphabet, it is associated with the final outcome of a publication’s life cycle—that is, its recognition by the scientific community. This recognition is quantified by the number of citations (Ci), obtained from Google Scholar;
• Ci: the total number of citations for each article, used as a measure of impact and scientific visibility within the academic community (Pagani et al. 2022);

After calculating the InOrdinatio index, the articles were ranked in descending order of scientific relevance, forming the bibliographic ranking used in the analysis. Next, a cutoff criterion was applied based on the lowest index value among articles published since 2021, corresponding to 3.754386002, resulting in the exclusion of studies below this threshold. This step ensured that the final portfolio consisted only of studies with the highest scientific relevance, impact, and timeliness.

Finally, the selected articles were reorganized chronologically, from the oldest to the most recent, allowing for an analysis of the temporal evolution of scientific output, as well as the identification of emerging trends related to green synthesis routes and the utilization of agroindustrial waste.

3.5. Presentation and Analysis of Data

The results yielded a final portfolio of 101 scientific articles, all of which were read in full and subjected to critical analysis. The data extracted covered key technologies employed in biogas production from poultry litter, anaerobic digestion operating conditions, methane yield, digestate characteristics, pre-treatment strategies, technical challenges, environmental aspects, and future prospects for the energy valorization of this waste within the context of the circular economy.

Based on the selected articles, in Figure 2, research activity regarding biogas production from poultry litter increased between 2000 and 2026. This trend suggests a gradual maturation of the topic within the field of biogas production. However, the absence of an exponential growth phase suggests challenges in the area.

The data obtained provided the basis for the discussion presented in this paper, making it possible to identify research trends, scientific gaps, and opportunities for technological advances in anaerobic digestion applied to poultry waste.

4. Characteristics of chicken litter and inhibition mechanisms

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.

5. Process optimization strategies

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.

5.1. Poultry manure processing methods for biogas production

Anaerobic digestion of chicken litter produces biogas—used to generate electricity and heat—and digestate, which acts as a biofertilizer, promoting nutrient cycling in the soil (Hemavarshini et al. 2025). This biofertilizer has high concentrations of N, P, and K, making it a potential substitute for chemical fertilizers. Thus, this digestion process offers a promising solution for energy and environmental sustainability in the poultry industry (Montoro et al. 2025), helping to reduce greenhouse gas emissions and prevent eutrophication caused by improper waste management (Cé et al. 2025).

The high nitrogen and lignocellulose content of poultry manure limits anaerobic digestion, whereas strategies such as co-digestion, physical pre-treatments, and micro-aeration enhance biogas production and reduce toxic compounds like hydrogen sulfide (Montoro et al. 2025). Aerobic pre-treatment of chicken litter involves controlled exposure to oxygen, allowing aerobic microorganisms to partially degrade organic matter; this results in a reduction of organic load, moisture content, and ammonia concentration, as well as the inactivation of pathogens. It also improves material homogeneity and increases both compound availability for microorganisms and the operational stability of the digester. This pre-treatment was observed to increase biogas yield by 14% and methane content by 8%, while reducing NH₃ emissions (Rubežius et al. 2020).

Ultrasonic pre-treatment employs high-frequency ultrasonic waves to promote the disintegration of organic matter in the bedding. The cavitation phenomenon relies on the formation and collapse of microbubbles in a liquid medium, generating intense forces capable of disrupting cellular structures and fragmenting solid particles. This process facilitates anaerobic digestion, increases biogas production, and reduces retention time, although its large-scale application remains limited by high energy consumption. The use of ultrasound on the bedding solubilized carbon and sugars, increasing biogas production by 10% and accelerating digestion (Gaspareto et al. 2020).

Thermochemical pre-treatment combined with ammonia stripping involves the use of heat and chemical additives to break down organic matter and remove volatile nitrogen compounds. Increasing the temperature to between 70 and 120 °C raises the pH, solubilizes organic compounds, and converts ammoniacal nitrogen (NH₄⁺) into free ammonia (NH₃). This ammonia is subsequently removed, thereby preventing microbiological inhibition during digestion (Rodriguez-Verde et al. 2018). The most consistent improvements appear in combined or staged systems that lower nitrogen first and then improve digestibility or reactor stability (Adghim et al. 2021). To mitigate ammonia inhibition and optimize the C/N ratio of poultry litter, an anaerobic pre-treatment strategy and thermal ammonia removal (at 80 °C for 24 h) were evaluated. Mono-digestion assays were conducted using raw poultry litter and poultry litter subjected to thermal ammonia removal (at proportions of 5%, 10%, 15%, and 20%). The removal process reduced total ammoniacal nitrogen and volatile fatty acid concentrations by 62–73% and 41–65%, respectively, compared to raw poultry litter. The highest biomethane yield was approximately 215 L CH₄ Kg^-1 COD for 20% TS loads. A direct mathematical conversion to mL CH₄ g^-1 VS was not applicable due to the absence of COD/VS characterization ratios (Markou 2015). The need for careful adjustment of pretreatments—rather than a direct transfer of results—must be emphasized.

Co-digestion involves mixing two or more different organic substrates to be degraded together in the digester. In co-digestion, chicken litter is combined with carbon-rich materials, promoting a better C/N ratio balance, enhancing microbial metabolism, reducing inhibitory compounds, increasing process stability, lowering ammonia levels, improving pH buffering, increasing substrate biodegradability, and boosting biogas production (Mlaik et al. 2024). The use of biochar involves adding a porous carbonaceous material—obtained through biomass pyrolysis—that improves substrate conditions. Biochar possesses a high surface area and adsorption capacity; it aids in compound retention, contributes to pH buffering, promotes microorganism adhesion, and increases the system's metabolic efficiency. Greater process stability, reduced inhibition, and increased biogas production are observed (Cai et al. 2022). The use of co-digestion and biochar can improve the microbiota and increase methane yield (Tawfik et al. 2023).

During anaerobic digestion, a large share of the biodegradable organic matter is converted into biogas, a smaller fraction is converted into sludge, and a residual non-biodegradable fraction remains untreated (Mlaik et al. 2024). Studies focused on waste management, such as Dróżdż et al. (2020) demonstrate that anaerobic digestion, composting, and pyrolysis are complementary technologies in circular systems for energy and nutrient recovery. This is further explored by Tan et al. (2023), who present pyrolysis as a technological pathway capable of converting agricultural and livestock waste into biochar. Another group of studies shows that the value of chicken litter extends beyond biogas production. Goyal, Dhull and Kapoor (2005) and Ogunwande et al. (2008) demonstrate that composting remains an established alternative for organic matter stabilization and nutrient recycling, highlighting the influence of the C/N ratio, microbial activity, and turning frequency on the final compost quality. These studies are relevant for understanding the behavior of organic matter and reinforcing the importance of nutritional balance, a critical aspect for anaerobic digestion performance.

It is worth noting that several studies have utilized alternative agro-industrial raw materials. Although these substrates provide highly relevant mechanistic insights, direct extrapolations to poultry litter should be interpreted with caution, as outcomes may depend on specific conditions and improvements are not uniform. While these alternative raw materials offer valuable indirect evidence to support process optimization, there is a critical need for rigorous, feedstock-specific validation to confirm the technical and economic feasibility of these pretreatments directly on poultry litter matrices (Markou 2015; Rodriguez-Verde et al. 2018). Figure 4, 5 and 6 presents the pre-treatments applied to chicken litter.

5.2. Methane yield comparison

Biogas is a gaseous compound formed, in the absence of molecular oxygen, of 30 to 40% CO₂, 60 to 70% methane, hydrogen, hydrogen sulfide and nitrogen (Golmakani et al. 2022). The calorific value of biogas, depending on the methane concentration, which can reach 60–70%, ranges from 4,800 to 6,900 kcal/m³, and after purification, methane reaches 9,100 kcal/m³, making it more efficient (Jameel et al. 2024).

Anaerobic digestion occurs in four stages: hydrolysis, where hydrolytic bacteria break down complex organic compounds; acidogenesis, where hydrolysis products are converted into hydrogen, CO₂, alcohols, and volatile acids; acetogenesis, where alcohols and volatile acids are converted into acetate and methanol by acetogenic bacteria; and methanogenesis, where methanogenic archaea convert acetate and CO₂ into methane (Amin et al. 2021). Microbial composition varies according to operating conditions, the substrate, and the type of digester, influencing biogas production efficiency (Xu et al. 2021). Co-digestion in multi-stage systems enhances microbial diversity and methane yield, with the control of interactions between bacteria and archaea being essential to optimize anaerobic digestion (Harirchi et al. 2022).

An evaluation of the anaerobic digestion of chicken litter in the studies analyzed reveals that methane yield and process stability are influenced by operating conditions. Rather than showing a linear trend, the literature shows significant variation. This variation can be explained by the interaction of variables such as bedding material, solids content, inoculum-to-substrate ratio, feeding regime, volumetric organic loading (VOL), C/N ratio, accumulation of volatile fatty acids (VFAs), and ammoniacal nitrogen dynamics. When comparing single-digester systems, a stark contrast is observed between high-solids anaerobic digestion and wet/diluted anaerobic digestion. TS and volatile solids (VS) concentrations shape the physical behavior of the system. In high-solids anaerobic digestion, it was demonstrated that a TS threshold of 30% allowed for stable operation, but an increase to 60% TS resulted in severe microbial inhibition. This performance collapse is driven primarily by mass transfer limitations and chemical toxicity. Thus, high density restricts the diffusion of extracellular enzymes during hydrolysis. As a result, intermediate VFA, such as the 5,681 mg/L expressed as CH₃COOH, accumulate faster than methanogenic archaea can consume them, leading to localized acidification, even in the presence of high alkalinity (4,000 mg/L as CaCO₃) (Karumanchi et al. 2026). The increase in solids content in the chicken litter resulted in a decline in efficiency due to a significant increase in the lag phase and a reduction in the hydrolysis rate (Zahan et al. 2018a).

This discrepancy between the 30% TS stability reported in Karumanchi et al. (2026) and the 3% VS restriction in Zahan, Othman and Muster (2018a) highlights how inoculum adaptation and reactor geometry alter the system’s tolerance: high-solids anaerobic digestion systems are often adapted to high-density matrices, whereas sudden increases in organic loading in unadapted wet-batch systems trigger rapid, unbuffered spikes in VFA. This divergence further highlights how water availability alters kinetic constraints. However, when specific chemical conditions—such as chicken manure set at 3% VS containing a high baseline concentration of 4,895 ppm propionate—are established, significant yields of 364.5 mL CH₄ g^-1 VS can still be achieved under mesophilic conditions (Guerrero-Toledo et al. 2019), proving that tolerance to VGA depends heavily on the buffering capacity of the system.

One of the biochemical bottlenecks in the digestion of chicken litter is its narrow C/N ratio. A severe nitrogen imbalance can result in high concentrations of total ammoniacal nitrogen, creating a high risk of free ammonia toxicity, which inhibits methanogenesis. Co-digestion acts as a primary corrective mechanism, although its success depends on the chemical nature of the co-substrate. Co-digestion and adjustment of the inoculum-to-substrate ratio act as vital corrective mechanisms for balancing nutrients, especially since the VS in chicken litter accounts for about 85% of the total solids, which confers high intrinsic biodegradability to the material once the nutrient balance is corrected (Silva et al. 2021). In batch systems, maintaining a high inoculum-to-substrate ratio is crucial for stability Zahan, Othman and Muster (2018a) used an inoculum-to-substrate ratio of 1:2 g g^-1 VS to ensure a sufficient initial methanogenic population to handle the rapid acidogenesis phase. In contrast, Karumanchi et al. (2026) operated batch reactors at 35 ± 2 °C for 30 days, with an inoculum-to-substrate ratio of 1:1, pH 7, and high alkalinity, which prevented a sharp drop in pH during the early stages of digestion. The chemical profile of the raw chicken litter showed a total ammoniacal nitrogen content of 1,500 mg/L and a C/N ratio of 8.9.

Tira and Sutanto (2024) evaluated mesophilic batch digestion (24–30 °C) over a 30-day period, the 50:50 (v/v) achieved an average biogas production rate of 77.22 cm³/day, compared to 14.95 cm³/day for pure chicken litter. The lack of data on the volatile solids content of the substrate and the molar fraction of methane in the biogas prevents the direct conversion of these fluxes into specific methane yield. This limitation highlights the need for standardized and comprehensive analytical approaches to enable quantitative comparisons between different anaerobic co-digestion trials. Adjusting the initial C/N ratio to the optimal range of 25.03 mitigated ammonia inhibition and the acidification of the medium caused by the accumulation of volatile fatty acids. This behavior validates the kinetic optimization predicted by the modified Gompertz model (R² ranging from 0.93 to 0.997) and mesophilic batch tests, in which a multi-substrate mixture—comprising 30–35% bedding, 65–70% yogurt whey, hay, and wheat straw, optimized the C/N ratio to a range of 26 to 27.5, mitigating the risks of ammoniacal nitrogen accumulation and yielding increases of 9% to 85% in specific methane yield compared to mono-digestive trials (Zahan et al. 2018a).

Similarly, Miah et al. (2016) demonstrated that a mixture of 75% bedding and 25% cattle manure balances nutritional synergy, producing 0.469 L g^-1 of specific gas with a methane content of 72.5%. In contrast, the co-digestion of chicken litter with food waste hydrolysate depends on the balance between a biodegradable liquid—with a C/N ratio of 29.1—and the solid matrix of chicken litter. With 30% dry matter, this configuration produced 12.4 L of biogas. However, food waste generally undergoes rapid hydrolysis; therefore, synergy depends on scale and loading rate compared to fiber-rich co-substrates, such as cattle manure or wheat straw, which release volatile acids more gradually (Karumanchi et al. 2026).

The wide variation in methane yields, as shown in Table 2, can be attributed to the operational feeding regime (Continuous/Sequential Feeding vs. Single-Batch Feeding) and batch systems. Single-batch systems expose the entire microbial load to the simultaneous presence of high initial loads of VOCs and ammonia over retention times ranging from 30 to 50 days (Karumanchi et al. 2026; Tira and Sutanto 2024; Zahan et al. 2018b). On the other hand, continuous or semicontinuous systems mitigate this impact through gradual operational strategies. By gradually increasing the volumetric organic loading rate from 0.6 to 3.8 g VS/L/day in a sequential batch anaerobic reactor operating under mesophilic conditions for the co-digestion of chicken litter and wheat straw, Zhan et al. (2022) prevented the toxic accumulation of VFA and enriched the acetoclastic methanogenic genus Methanosaeta. This biological acclimatization allowed for stable operation at a high Volumetric Organic Load of 3.8 g VS/L/day, maintaining the pH between 7 and 9, and achieving a biogas production of 13.06 L/day with an average methane content of 54.38% and robust COD removal ranging from 70.3% to 85.9%. During regular operation, daily methane production fluctuated between 100.41 and 188.65 mL CH₄ g^-1 VS added.

Supplementation with 20 mg L^-1 of magnetite nanoparticles increased the cumulative methane yield to 137.2 mL CH₄ g^-1 VS (a 73.9% increase over the control) by accelerating direct interspecies electron transfer between syntrophic bacteria and archaea, thereby circumventing traditional kinetic bottlenecks. This result highlights the potential of magnetite nanoparticles to enhance methane production under specific operating conditions (Aguilar-Moreno et al. 2020). Applied in situ microaeration combined with sulfur-oxidizing bacteria, which stimulated alternative metabolic pathways, maximizing methane concentration by up to 68% and mitigating biogas impurities by reducing H₂S levels by 80–92% (with production peaks of 270–500 mL between days 14 and 16) (Karumanchi et al. 2026). In addition, enzymatic treatments have proven effective in breaking down lignocellulosic matrices in chicken litter; treating with 1% enzyme under optimized conditions accelerated hydrolysis, producing 460.8 mL CH₄ Kg^-1 VS (Bhatnagar et al. 2020).

5.3. Technical challenges in chicken litter anaerobic digestion

The high nitrogen concentration, low C/N ratio, and lignocellulosic content hinder the anaerobic digestion of chicken litter, potentially causing ammonia toxicity, reducing digestibility, and inhibiting methane production (Bhatnagar et al. 2022). Litter storage and compositional variability can reduce biogas production potential, making it necessary to characterize the substrate and control operational conditions and the microbial community to ensure stable anaerobic digestion and high methane production (Orrico et al. 2025).

In continuous-flow biodigesters, feeding and discharge occur simultaneously without interrupting biogas production, making them suitable for cattle and swine farming as well as other applications. Semi-continuous or continuous systems tend to be more efficient for biomasses with high biodegradability and chemical stability, resulting in greater volatile solids reduction and constant methane production. Batch-type biodigesters are sealed at the start of the cycle and reopened after biomass treatment is complete, making them suitable for chicken litter. They can operate in batch mode—characterized by slower fermentation that limits biogas production—or in sequential mode, which utilizes inoculants containing diverse microbial communities to promote greater stability and methane yield (de Souza Guimarães and da Silva Maia 2023).

Anaerobic digestion occurs within psychrophilic, mesophilic, and thermophilic ranges, with the optimal temperature depending on the feedstock; mesophilic digestion generally offers greater stability in microbial activity and broad applicability, as well as lower energy requirements. Meanwhile, thermophilic digestion often accelerates hydrolysis and can increase biogas/methane production, provided the microbial community is adequately acclimatized and operational conditions are properly controlled; however, this temperature range carries a higher risk of instability and inhibition. Low temperatures generally reduce microbial activity and biogas yield, although psychrophilic digestion can be technically feasible in some cases. In thermophilic digestion, methanogenic activity is affected, showing greater sensitivity to thermal shocks and the accumulation of intermediates. Therefore, the optimal temperature depends on substrate characteristics, the microbial community, reactor configuration, and operational conditions. Furthermore, hydraulic retention time is crucial, as longer periods allow for greater degradation of total solids and increase biogas production (Saadia et al. 2025).

The quantification of total solids (TS) and volatile solids (VS) influences the retention time in the digester. VS represent the biodegradable organic fraction available for conversion into biogas and serve as an indicator of the substrate's methanogenic potential. Therefore, increasing VS or TS levels can enhance production up to an optimal point; however, this varies depending on the substrate and operating conditions, meaning the optima may occur within different ranges (Deepanraj et al. 2021). The reduction of volatile solids during anaerobic digestion indicates greater degradation of organic matter and is associated with increased biogas production (Ahamefule et al. 2024).

Biogas production rates can be estimated based on the physicochemical properties of the material and experimental data. Models based on artificial intelligence—such as artificial neural networks and adaptive fuzzy systems (Sun et al. 2024) or on optimization techniques, such as genetic algorithms, assist in selecting relevant variables and improving the accuracy of predictive models (Beltramo et al. 2019). Approaches combining machine learning with statistical analysis enable accurate production forecasting with reduced uncertainty, thereby facilitating biogas system planning and operation (Sappl et al. 2023). Simple tools based on polynomial regression estimate production under various operating conditions, supporting decision-making regarding system implementation and optimization. Consequently, the combined use of experimental data and computational models provides reliable estimates for planning and energy utilization (Duong and Lim 2023).

Studies on anaerobic digestion are highly complementary—such as the one conducted by Tawfik et al. (2023) which synthesizes biogas production from chicken litter and identifies key process limitations, such as high nitrogen concentration. In line with this Bhatnagar et al. (2022) expands on this discussion by analyzing the potential of anaerobic digestion from a technological and political perspective, highlighting that the successful implementation of this technology depends not only on the efficiency of biodigesters but also on the existence of public policies, economic incentives, and integrated strategies for agricultural waste management. Complementing these reviews, Zahan, Othman and Muster (2018a) demonstrate that co-digestion helps overcome the limitations of chicken litter and that combining it with substrates having a higher C/N ratio promotes greater microbiological stability, improves biodegradability, and increases methane production.

5.4. Digestate management

Chicken litter digestate can function as a nutrient-rich biofertilizer, and anaerobic digestion generally improves hygiene and odor control compared with raw litter, but its agronomic and environmental safety still depends on residual ammonia, salinity, metals, and microbial management (Mortola et al. 2019). The use of digestate must be accompanied by practices that prevent phytotoxicity and promote chemical and biological stability (Toghyani et al. 2025). Composting is recommended following solid-liquid separation, as the liquid fraction undergoes a maturation period that reduces unstable and odor-causing compounds. Digestate can exhibit phytotoxicity if applied prior to complete maturation (Skvortsova and Ablieieva 2024).

Raw digestate is the unprocessed residue from anaerobic digestion, consisting of a mixture of liquid and solid materials. Although rich in nutrients, its high moisture content and variable stability make storage, transport, and soil application difficult. In contrast, separated digestate undergoes physical fractionation—splitting it into liquid and solid fractions—which makes it easier to handle, as the fractions can be managed separately (Fechter et al. 2023). Composted digestate is biologically stabilized through aerobic biodegradation. This product is more stable and mature, making it suitable as a soil conditioner or growing medium; however, it requires an additional aerobic treatment step, meaning its benefits depend on successful composting conditions (Wang et al. 2017). Another type is nitrified digestate, in which ammonium is oxidized to nitrate; this can be used to conserve nitrogen and increase plant-available nitrate, though the associated risks depend on the specific process and conditions (Manu et al. 2021). Finally, dried digestate is characterized by water removal to facilitate handling, transport, and sometimes thermal conversion; however, drying can lead to nitrogen losses unless carefully managed (Ruffino et al. 2022).

It is also recommended to inoculate the biofertilizer with beneficial microorganisms, enriching it with plant growth-promoting bacteria. The application of digestate can increase soil fertility, promote greater plant growth, and improve soil chemical properties (Junior et al. 2025). Regarding the storage of untreated liquid digestate, this can lead to losses of up to 87% of total nitrogen over approximately 100 days, whereas processes such as pre-nitrification reduce these losses to less than 6%, even with agitation (Svehla et al. 2020).

The anaerobic digestion of chicken litter supports the circular economy by converting agricultural waste into biogas and nutrient-rich digestate, which can be used as a biofertilizer to replace synthetic fertilizers and improve agricultural sustainability (Beausang et al. 2020). Furthermore, appropriate digestate management reduces nutrient losses and environmental impacts, while advances in nutrient recovery technologies reinforce its role in sustainable agriculture and climate change mitigation (Chaump et al. 2019).

Chicken litter digestate applied to sorghum crops increased both fresh and dry matter yields and raised soil P and K⁺ levels, with optimal application rates of approximately 20 to 22 m³ ha^-1 (Junior et al. 2025). Digestates and composts derived from chicken litter improve cation exchange capacity and nutrient content; increase levels of calcium, magnesium, phosphorus, and K; and boost microbial activity and crop productivity (Wang et al. 2023). Therefore, the integrated use of these organic residues represents a sustainable strategy for improving soil fertility, increasing agricultural productivity, and reducing dependence on chemical inputs (Tiong et al. 2024).

The application of digestate can increase seed germination rates and improve plant height and biomass, yielding results comparable to or better than those of conventional fertilizers (Tymchuk et al. 2023). Anaerobic digestion also reduces weed seed viability, minimizing the risk of infestation when digestate is applied to the soil (Abbas et al. 2023). Figure 7 demonstrates the benefits of applying stabilized digestate.

Chicken litter digestate should be considered a nutrient-rich fertilizer, but one that carries risks (Table 3 and Figure 8), since the raw material tends to contain more pathogens, ammonia, antibiotic-resistance genes, high concentrations of ammoniacal nitrogen, and a risk of contaminant mobilization. Although treatment can promote stabilization, improve handling, and reduce certain risks, these effects should not be confused with the complete elimination of contaminants. Thus, the transformation of chicken litter into digestate more accurately represents a reduction and redistribution of risks, rather than their elimination, requiring that the quality of the final product be assessed in an integrated manner, simultaneously considering microbiological, chemical, and agronomic aspects (Chaump et al. 2019; Mousavi et al. 2024).

Untreated chicken litter has the highest levels of pathogenic microorganisms, including total and fecal coliforms (Rizzo et al. 2022). Anaerobic digestion is considered suitable for reducing pathogens and antimicrobial-resistant bacteria. Although the process is capable of reducing pathogens and antimicrobial-resistant bacteria, its efficiency depends on operating conditions, retention time, and, most importantly, process temperature (Atanasova et al. 2025). Thus, in biodigesters maintained at room temperature, reduction does not necessarily result in the elimination of genes associated with virulence; on the contrary, the detection frequency of certain genes may increase over time (Agga and Loughrin 2026). Even after digestion, taxonomic signatures associated with pathogens continue to be detected in the digestate and in fertilized soil (Breyer et al. 2026). Therefore, while the treatment reduces microbiological risk, it does not allow us to assume that the digestate is free of resistance agents or markers of public health concern.

The high concentration of ammoniacal nitrogen in chicken litter can compromise the very efficiency of anaerobic digestion, since excess ammonia has an inhibitory effect on methanogenic archaea. Nitrogen recovery through struvite precipitation is a promising strategy; it can recover up to 88% of the ammonia nitrogen from the digestate at pH 11 under alkaline conditions, in addition to reducing the nutrient load of the digestate (Mousavi et al. 2024). It is necessary to consider the fate of this nutrient, the treatment costs, and the possibility of transferring other contaminants to the different fractions generated.

Toxic metals are another factor that limits the use of digestate as a fertilizer. Thermophilic anaerobic digestion can lead to increased release of metals and macronutrients from chicken litter, with concentrations of Zn and Cu potentially exceeding levels considered toxic after treatment (Anjum et al. 2017). On the other hand, mesophilic digestion may produce less pronounced changes in metal concentrations, while heavy metal resistance genes may increase or remain unchanged depending on the animal species of origin (Agga et al. 2020). This variability demonstrates that the outcome depends on both the process conditions and the characteristics of the raw material. Furthermore, the presence of heavy metals at sub-threshold levels—even within regulatory limits—can potentiate antimicrobial resistance in the environment (Błażejewska et al. 2022). The existence of significant correlations between heavy metals and antimicrobial resistance genes reinforces the hypothesis that the presence of these elements may favor the maintenance or selection of resistance in the environment (Peng et al. 2022).

In this regard, strategies for recovering nutrients and contaminants may be important for reducing the transfer of risks to the soil. Struvite precipitation in chicken litter digestate can reduce heavy metals (Zn, Cu, Pb, Ni, Cd, Cr, and Fe) by up to 93% in leachate (Mousavi et al. 2024). Similarly, the addition of 10% biochar during the composting of chicken litter reduced the concentrations of lead, zinc, copper, chromium, arsenic (Ezugworie et al. 2023). However, these technologies should be considered mitigation measures rather than a guarantee of decontamination. The removal of a contaminant from one fraction may result in its concentration or transfer to another, making it essential to assess the mass balance and final fate of the elements throughout the entire treatment process.

The issue of antimicrobial resistance genes further underscores the need for a systemic assessment. Chicken litter exhibits a higher abundance of resistance genes compared to digestate, suggesting a positive effect of anaerobic digestion. However, digestate may exhibit greater diversity of these genes, indicating that a reduction in abundance does not necessarily correspond to the elimination of the problem or a proportional decrease in ecological risk (Atanasova et al. 2025). Thus, the safety assessment of digestate should consider not only the concentration or abundance of these genes but also their diversity, persistence, potential for horizontal transfer, and relationship with microorganisms present in the soil.

The effects of agricultural application also reveal a conflict between agronomic efficiency and environmental sustainability. Furthermore, the direct and repeated application of digestate can alter the soil’s physicochemical properties, although the magnitude and long-term consequences of these changes are not yet fully understood. The possibility of progressive accumulation of heavy metals reinforces the need for periodic assessments of the soil and the applied material, especially in systems involving repeated use (Zapata-Morales and Moreno-Andrade 2025). It is recommended to apply stabilized and mature chicken litter-derived amendments to the soil to reduce negative environmental impacts (Rizzo et al. 2022).

Crude digestate is the untreated or minimally processed material, while treated digestate is stabilized to improve safety, handling, and agronomic performance; the main differences are lower pathogen risk, lower odor/emission potential, and better nutrient management in treated material (Rizzo et al. 2022). For chicken litter digestate, treatment shifts the focus from raw waste with significant biological risk to a managed fertilizer with residual chemical and microbiological restrictions. Thus, thermophilic digestion, composting, acidification, and post-composting can reduce pathogens and some antibiotic resistance genes, but ammoniacal nitrogen, salinity, and the residual presence of metals and antibiotic resistance genes still determine whether the digestate is safe and agronomically useful. antibiotic resistance genes (Mortola et al. 2019).

From a regulatory standpoint, this complexity poses an additional challenge. In the European Union, digestate is covered by a set of legal instruments related to fertilizers, waste, and soil protection, including Regulation (EU) 2019/1009. However, the absence of specific standards for certain concentrations of pharmaceuticals that may be introduced into the soil along with digestate highlights a significant regulatory gap (Czatzkowska et al. 2025). The EU lacks specific criteria for the final disposal of digestate derived from biowaste, which is described as a major barrier to its agronomic use (Di Maria et al. 2018). Digestate must still meet quality standards for pathogens, heavy metals, and PCBs, and its legal status as waste or byproduct is important (Cruz et al. 2021).

Regulation is moving toward more product-specific and nutrient-based criteria (Vaneeckhaute et al. 2016). In the United States, there may be no federal regulations for heavy metals in animal manure, although the U.S. Environmental Protection Agency regulates metals in sewage sludge; the application of chicken litter to soil is managed through best management practices and, in some cases, through liability frameworks such as the Environmental Liability, Compensation, and Remediation Act (Malhotra et al. 2025). Meanwhile, in Brazil, digestate can be compared to Brazilian legal requirements for the reuse of biosolids in agriculture, including CONAMA Law 498/2020 (Leite et al. 2023). An assessment based solely on fertility parameters (N, P, and K) may not account for emerging contaminants or relevant microbiological characteristics. Consequently, the regulation of digestate quality needs to shift from an approach centered on fertilizer value to a perspective based on integrated risk management, incorporating chemical contaminants, antimicrobial resistance, pathogens, and the cumulative effects of agricultural application.

6. Decarbonization and environmental analysis

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.

7. Research gaps and limitations in the anaerobic digestion of chicken litter

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.

8. Conclusions

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.