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6 February 2026

18 Pages

A Comparative Systematic Review of Life-Cycle Assessments of Treatment Strategies for Swine Slurry with a Focus on Anaerobic Co-Digestion

,
and
1
Agriculture School, Polytechnic University of Coimbra, Rua da Misericórdia, Lagar dos Cortiços, S. Martinho do Bispo, 3045-093 Coimbra, Portugal
2
Agriculture School, Polytechnic University of Castelo Branco, 6001-909 Castelo Branco, Portugal
3
Research Center for Natural Resources, Environment and Society, Polytechnic University of Coimbra, Bencanta, 3045-601 Coimbra, Portugal
4
Research Center for Natural Resources, Environment and Society, Polytechnic University of Castelo Branco, Quinta Sra. de Mércules, 6001-909 Castelo Branco, Portugal
Processes2026, 14(3), 573;https://doi.org/10.3390/pr14030573
This article belongs to the Special Issue Advanced Technologies for Bioenergy Recovery from Organic Waste and Wastewater
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Abstract

Intensive swine production contributes significantly to the global protein supply but generates considerable environmental pressure, particularly through greenhouse gas emissions and surplus slurry management. Anaerobic digestion (AD), especially (co-AD), has been widely investigated as a mitigation strategy to enhance renewable energy generation and nutrient recovery. This systematic review synthesizes life cycle assessment (LCA) studies published between 2019 and 2025 that evaluated AD systems treating swine slurry, following the PRISMA 2020 guidelines. Across diverse methodological approaches and regional contexts, the literature consistently shows that AD can reduce global warming potential compared with conventional slurry management, with stronger environmental benefits when biogas is efficiently valorized and when swine slurry is co-digested with complementary organic substrat. Co-AD emerges as a key mitigation option by improving biogas yields, process stability, and overall environmental performance while also enabling better utilization of external organic waste. However, the results remain highly sensitive to operational factors such as methane leakage, digestate management, energy efficiency, and substrate selection. This review highlights the methodological inconsistencies among LCA studies and underscores the need for harmonized assessment frameworks and improved emission data. Overall, co-AD represents a promising pathway for enhancing the environmental sustainability of swine production systems when integrated into optimized, context-specific management strategies.

1. Introduction

The global population is growing, and although some efforts are being made towards more sustainable and environmentally friendly consumption, meat is still largely consumed, which leads to an increase in the number and scale of livestock farms [1,2,3].
Approximately 40% of meat consumption worldwide is from the swine industry [4]; therefore, intensive swine production plays a central role in the global supply of animal protein, but it is also associated with significant environmental impacts. The sector has been steadily increasing in the EU, with almost 21 million tons of meat produced in 2023, according to FAO [5]. It contributes substantially to greenhouse gas (GHG) emissions, such as methane from animal digestion and manure storage, and nitrous oxide from manure management, which is also linked to nutrient surpluses (such as nitrogen and phosphorus) through the direct land application of large volumes of swine slurry/manure, which require careful management to prevent further environmental damage [5,6]. Livestock systems account for approximately 14.5% of anthropogenic GHG emissions, with slurry management representing a major source of methane (CH4) and nitrous oxide (N2O) in swine production systems [7,8,9]. In regions where swine farming is geographically concentrated, such as in Asia, particularly China, several European countries including Spain, Germany, France, Denmark and the Netherlands and in North America, slurry accumulation intensifies problems such as ammonia volatilization, nitrate leaching, soil eutrophication with nitrogen and phosphorus, and dissemination of pathogens and emerging contaminants [5,10,11].
Therefore, effective manure/slurry management is a critical component of sustainable swine production systems. Traditional practices, such as direct land application, remain common but are increasingly constrained by environmental regulations, nutrient limitations, and social concerns.
In this context, anaerobic digestion (AD) has emerged as a promising biotechnology for mitigating environmental impacts and enabling resource recovery [12,13]. By converting organic matter into biogas, AD reduces methane emissions from uncontrolled slurry storage, stabilizes organic matter, and produces a nutrient-rich digestate that can replace synthetic fertilizers [14,15]. Additionally, AD systems often incorporate environmental credits, especially when digestate replaces synthetic fertilizers or when biogas is upgraded or used to displace fossil-based energy [16]. These substitution effects can considerably modify the overall global warming potential and nutrient-related impacts of a system [17,18].
Despite these benefits, the environmental performance of AD systems can vary depending on the design parameters, energy rates, digestate management, and regional characteristics. One way of preventing operational instability of AD systems in different regions and practices is anaerobic co-digestion (co-AD), as mixing different substrates may enhance biogas production by balancing carbon and nitrogen levels, thereby increasing biogas production and methane yield [19,20]. Although this is a relatively emerging topic, studies have already shown promising synergies between different agro-industrial residues and livestock manure in anaerobic co-digestion systems.
To understand the environmental impacts associated with the anaerobic co-digestion of swine manure/slurry with other wastes, several life cycle assessment (LCA) studies have been conducted, showing that co-AD can significantly reduce the overall carbon footprint of pig wastewater treatment systems when compared with conventional management practices, such as storage tanks or aerobic treatments [21,22]. Comparing different LCA of manure treatment technologies is essential to determine the conditions under which co-AD outperforms alternative strategies and to support evidence-based decision-making in swine production systems [23].
Although several reviews have addressed anaerobic digestion or manure management in a general context, a systematic synthesis of quantitative environmental impact ranges directly comparing mono-digestion and co-digestion of swine manure is still limited, particularly with respect to digestate management pathways, which remain insufficiently examined despite their significant contribution to overall environmental performance. LCA is a standardized methodology for evaluating the environmental impacts associated with products, processes, and systems, and has become a central tool for assessing swine production and manure treatment technologies such as AD. ISO 14040:2006 [24] and ISO 14044:2006 [25] remain the primary methodological references guiding LCA practices, providing the principles, framework, and requirements needed to ensure analytical rigor, transparency, and comparability. When applied to the AD of pig slurry, several methodological aspects require particular attention.
A key element is the definition of system boundaries, which typically encompass the digestion process, biogas production and utilization, and handling, storage, and agricultural use of the resulting digestate. The functional unit generally reflects either the treatment of a defined mass or volume of swine manure or the production of a specific amount of biogas-derived energy [21,22]. Fugitive emissions, particularly methane losses from digesters, storage tanks, and digestate spreading, can substantially influence climate-related impact categories and must be quantified as accurately as possible [26].
Therefore, the present study aims to review and synthesize the state of the art regarding the application of LCA to co-AD with a focus on swine manure as one of the substrates, examining their methodological approaches, including system boundaries, functional units, allocation procedures, and impact assessment methods. In addition, this study aims to compare the environmental performance of co-AD with alternative manure treatment strategies and mono-digestion, assessing how technological configurations, digestate management options, and regional energy contexts influence overall outcomes, highlighting methodological gaps or inconsistencies to guide future LCA research in the swine sector.

2. Materials and Methods

Review Methodology

This review followed a systematic narrative approach to examine LCA studies conducted between 2019 and 2025, focusing on the anaerobic co-digestion of pig slurry. A literature search was conducted in the Scopus, Web of Science, ScienceDirect, and Google Scholar databases. The search terms were selected to reflect the scope of the review and included “anaerobic co-digestion”, “swine slurry”, “pig slurry” and “life cycle assessment”. These terms were combined through Boolean operators to build search strings such as “AND” and “OR”. Studies were included if they reported a full or partial LCA of co-AD applied to swine manure or pig slurry. Publications without environmental data or without a defined LCA methodology were excluded, as were studies that applied co-digestion without using slurry/manure as the main substrate. The selection process consisted of title and abstract screening, followed by full-text assessment, in accordance with the PRISMA 2020 guidelines [27]. The analysis of the LCA of PM treatment systems revealed up to 398 studies in the three databases assessed (Science Direct, Web of Science, and Google Scholar), based on the topics “life cycle assessment,” “pig slurry” or “pig manure” and “anaerobic co-digestion” from 2019 to 2025 (Figure 1). First, studies were excluded by duplication using EndNote (n = 26). Most of the studies were found in ScienceDirect, but many of them were off-topic, as they didn’t focus on LCA studies and were promptly discarded (n = 184). From the remaining studies (n = 188), 150 articles were excluded using the AI Rayyan as a screening tool, with all decisions manually verified by the authors [28]. Excluded records did not simultaneously address co-AD, swine manure/slurry and LCA. Ten studies were selected and are summarized in Table 1.
Figure 1. PRISMA 2020 flowchart built from a database of articles. Adapted from the PRISMA 2020 flow diagram [27,29] under the CC BY 4.0 license.: Authors, 2025. * records excluded using Rayyan AI [28].
Table 1. Summary of life cycle assessment (LCA) methodological characteristics of selected studies on pig manure–based anaerobic digestion systems.

3. Results

3.1. Overview of the Selected Studies

Ten peer-reviewed studies on life cycle assessment (LCA) applied to pig manure–based anaerobic co-digestion (co-AD) systems were reviewed. These studies cover a wide range of geographic contexts, including East Asia (China and Taiwan), South Asia (Bangladesh), and Southern and Northern Europe (Portugal, Spain, Ireland, Denmark, Lithuania, and Finland). Most studies adopted a cradle-to-gate system boundary, focusing on manure management and biogas production processes, although two studies extended the analysis to a cradle-to-grave perspective, and one applied a gate-to-gate approach (Table 1). The functional units varied considerably among studies, reflecting differences in system scale and research objectives, and included mass-based units (e.g., tons of manure, kilograms of live weight), energy-based units (e.g., kWh or MJ of biogas energy), and system-scale units (e.g., annual waste or number of animals). The reviewed literature predominantly relied on attributional allocation approaches, typically based on mass, energy, or economic relationships among co-products (e.g., biogas, electricity, heat, and digestate). Only a limited number of studies adopted a consequential approach, explicitly accounting for system expansion.
Table 1 summarizes the key methodological characteristics of recent LCA studies on pig manure–based AD systems, including functional units, system boundaries, impact assessment methods, and software and databases employed. Most studies relied on established LCIA methods, such as CML, ReCiPe, and IMPACT2002+, and commonly used versions of SimaPro from 8.0 to 9.2 or OpenLCA 1.10.3 software in combination with Ecoinvent databases, while some studies incorporated uncertainty analysis through Monte Carlo simulation.
Table 2 presents a comparative overview of life cycle assessment studies examining the anaerobic digestion of pig manure, with particular emphasis on mono- and co-digestion strategies, digestate management options, and their associated environmental performance. The selected studies encompass a range of system configurations, including conventional digestion pathways, co-digestion with various organic co-substrates, and advanced digestate treatment or upcycling technologies, allowing for comprehensive comparisons of alternative management approaches.
Table 2. Summary of life cycle assessment studies on anaerobic digestion of pig manure, focusing on mono- and co-digestion strategies, digestate management, and environmental performance.
Duan et al. [30] evaluated digestate treatment technologies for biogas plants in China, highlighting that while direct land application remains the most economical option, upcycling strategies, such as fractionation followed by composting and microalgae cultivation, can provide environmental benefits, including energy savings and reduced ecological impacts. Co-digestion of swine slurry with microalgae also proved to be energetically favorable, as it led to saving 1.61 × 103 MJ/primary energy per tonne of PM, underscoring the potential of circular approaches for more sustainable digestate management, although further optimization is still required. Although promising results were obtained for the different scenarios, the direct use of digestate remained the most environmentally friendly option [30].
Freitas et al. [20] assessed four different scenarios, showing that co-digestion of swine slurry increases biogas production by up to 50%, however, corn silage generated the highest environmental impacts due to the intensive fossil-energy demand of its production, while elephant grass silage performed better and biochar contributed to additional environmental improvements [20]. These findings are consistent with previous LCA studies by Balcioglu et al. [38] and Mirzaei et al. [39] who also reported high environmental impacts for energy crops such as corn silage.
Girón-Rojas et al. [31] compared pig slurry mono-digestion and co-digestion using pepper waste from nearby vegetable processing plant and showed that co-digestion produces 70% more electricity and offered 5.5 times higher economic benefits compared to mono-digestion. From the environmental categories assessed in the study, co-AD showed two to four times higher benefits per tonne of treated waste and at least 1.5 times the energy required for farm operations compared to mono-AD.
Hossain et al. [32] studied the co-digestion of livestock manure with food waste in Bangladesh, and the scenarios assessed showed benefits by reducing climate change by up to 117%, eutrophication potential by 54.5%, and terrestrial ecotoxicity by 55.7%, respectively. The main reduction was due to minimizing food waste that would have ended up in the landfills. The downside of this co-digestion strategy is the higher amount of hydrogen sulfide and ammonia in the final biogas, which increases environmental burdens. Emerging hotspots include digestate storage and post-treatment of the resulting digestate and biogas.
Jiang et al. [33] compared the traditional management practices in such as direct land application of PM and composting and mono-digestion of FW with the co-digestion of both these substrates. The co-digestion scenario performed better in nine of the 11 environmental impact categories assessed, and the nitrogen availability in the digestate of the co-digestion scenario was 43% higher than that of the organic fertilizer obtained through current practices. There were reductions in GHG emissions, such as methane and ammonia, in co-AD compared to direct land application. Regarding the studied case, a farm production of 16,000 t/year of PM requires at least 2000 t/year of FW to ensure a negative GWP.
Pexas et al. [34] studied pig housing scenarios as levels of barn insulation, in-barn temperature, ventilation efficiency, slurry dilution, frequency of removal, and their interactions with slurry treatment pathways, such as slurry acidification, anaerobic digestion, and co-digestion with organic wastes. Their study concluded that AD significantly reduced environmental impacts for Non-Renewable Resource Use (34.1% compared to baseline), Non-Renewable Energy Use (40.1%), and Global Warming Potential (9.20%). Slurry acidification led to significant reductions in Acidification (28.1%) and Eutrophication Potential (14.2%). Slurry separation significantly reduced only Non-Renewable Energy Use (2.26%). The largest improvement was observed in the acidification potential by increasing the slurry dilution.
Venlauskas et al. [35] conducted a consequential LCA study in Lithuania and showed that using surplus straw for heat generation delivers greater environmental benefits than biogas production, while partial substitution of mineral fertilizers with digestate reduces several impact categories. However, digestate availability was limited, covering only 8.3% of the farm’s cultivated land, which prevented the full replacement of mineral fertilizers.
Zhang et al. [3] applied a LCA to compare conventional land application of PM with anaerobic digestion pathways (mono and co-digestion with grasss silage), followed by digestate land application under Irish conditions. By explicitly accounting for dynamic nutrient profiles, soil nutrient status, and regulatory constraints, the analysis demonstrated that co-AD delivered an overall environmental performance reduction in most impact categories, enhanced energy recovery, and reduced greenhouse gas emissions. The results further showed that nutrient availability in digestate, particularly nitrogen, is influenced by anaerobic processing, whereas phosphorus constraints largely determine land application requirements and transport distance.
Timonen et al. [37] study evaluated climate impacts of anaerobic digestion across the full value chain, explicitly accounting for both energy and digestate by applying different allocation methods. Using finnish conditions and three pig slurry–based co-digestion scenarios, the analysis showed that allocating emissions solely to energy overestimated climate benefits by neglecting digestate-related emissions and credits. While allocation choice strongly affects the distribution of emissions between energy and digestate, all scenarios achieved lower climate emissions than fossil energy and mineral fertilizers. The results highlight that co-feedstock choice, allocation method, and digestate management are critical factors, and that considering digestate use alongside energy production is essential for a balanced and robust sustainability assessment of anaerobic digestion systems.
According to the reviewed studies, co-substrates used in pig-manure co-digestion systems were predominantly sourced locally or regionally, resulting in high geographical and supply chain proximity. Energy crops are often available on farms [20], whereas food waste and agro-industrial residues are typically obtained from nearby facilities or transfer stations [31,32,33], Centralized systems supplied by multiple farms explicitly account for regional transport distances [34,37]. Some studies have reported fully integrated configurations in which co-substrates are produced on-site, such as microalgae cultivated from digestate and internally recycled, resulting in closed-loop systems with minimal transport requirements [30,36]. Grass silage co-substrates were also sourced locally, with reported transport distances of approximately 10 km [3].
Taken together, the reviewed studies considered a wide range of co-digestates, including organic residues and waste, such as food waste, vegetable processing residues, pepper waste, and agricultural by-products, which were found to be environmentally favorable options for co-digestion with swine slurry/manure, while energy crops such as corn and grass silage, although effective in biogas yield, had higher impacts due to agricultural inputs. However, it is important to note that most of the energy crops were positively available on site, whereas organic residues and waste were obtained outside the farms, which impacts were not completely addressed in the studies.

3.1.1. Environmental Impact Categories

Another important aspect is the environmental impact categories (EIC) assessed in each study. As illustrated in Figure 2 and Figure 3, the number and type of impact categories varied widely across the reviewed LCAs, ranging from a single focus on climate change to more comprehensive assessments covering up to 14 midpoint indicators. Climate change is consistently included in nearly all studies, followed by acidification and eutrophication, which are frequently assessed. Impact categories related to human toxicity, ecotoxicity, and photochemical ozone formation have been included in several studies but show greater variability in coverage. In contrast, categories such as water use, land use, ionizing radiation, ecosystem quality, particulate matter formation, and odor emissions are rarely considered in the literature. This uneven distribution highlights a persistent lack of methodological harmonization in manure management LCAs and suggests that, despite adequate coverage of emission-driven impacts, broader environmental dimensions remain underrepresented in the current literature.
Figure 2. Presence–absence heatmap of environmental impact categories assessed in life cycle–based studies on swine manure management and anaerobic digestion. Filled cells indicate the categories included in each study across the reviewed literature [3,20,30,31,32,33,34,35,36,37].
Figure 3. Percentage of reviewed life cycle–based studies assessing each environmental impact category in swine manure management and anaerobic digestion systems. The percentage represents the share of studies (n = 10) that included each impact category, highlighting climate change as the most universally assessed category, followed by acidification, eutrophication and resource use. Ionizing radiation was the least frequently considered impact category in this study.

3.1.2. Economic Assessment of Anaerobic Mono-Digestion Versus Co-Digestion

Across the reviewed studies, economic performance was closely linked to energy recovery, digestate utilization efficiency, and associated logistics costs, with clear quantitative differences between mono-digestion and co-digestion systems. Manure mono-digestion generally provides limited economic returns due to modest biogas yields and constrained fertilizer substitution potential, but limited logistic costs. For example, Freitas et al. [20] reported that pig manure mono-digestion produces approximately 6.4 kWh of electricity per day, which is insufficient to meet on-farm energy demand, whereas co-digestion with elephant grass or corn silage increases electricity production to 55.8–57.5 kWh d−1, generating large surpluses that exceed farm consumption by more than 80% and enabling revenue generation through grid injection. Similarly, Giron Rojas et al. [31] quantified that co-digestion produces approximately 70% more electricity and delivers 5.5-fold higher economic benefits than mono-digestion, with benefits per tonne of waste treated being two to four times higher.
Digestate management strongly influences the economic feasibility of both systems, but co-digestion significantly alters the scale of nutrient recovery. In manure-based systems, digestate availability is often insufficient to substantially offset the use of mineral fertilizers. Venslauskas et al. [35] reports that pig manure and digestate alone allow fertilization of only 91.5 ha of barley at an application rate of 22.9 t ha−1. When pig manure is co-digested with straw and maize, digestate production increases markedly, enabling the fertilization of 457 ha of crops, including 276 ha of barley (22.9 t ha−1) and 181 ha of wheat (34.3 t ha−1), indicating a substantial reduction in fertilizer costs at the farm level. However, several studies emphasize that the low nutrient concentration and high water content of digestate make transport economically limiting. Duan et al. [30] explicitly notes that long-distance digestate transport is uneconomic, contributing to the finding that direct land application of digestate is becoming a costly option for farmers despite its environmental advantages.
The energy efficiency metrics further illustrate the economic contrast between mono- and co-digestion. Duan et al. [30] reports that co-digestion of pig manure with microalgae achieves a net energy saving of 1.61 × 103 MJ of primary energy per tonne of pig manure due to high energy payback, whereas mono-digestion lacks comparable energy recovery potential. Wu et al. showed that co-digestion systems integrated with combined heat and power generate up to 737.2 kWh of electricity and 2242 MJ of heat per 1000 kg live weight of pigs, compared with farm energy demands of 195 kWh of electricity and 239 MJ of heat, indicating substantial potential for operational cost savings. In contrast, mono-digestion systems typically provide only a partial energy offset.
However, co-digestion introduces additional economic trade-offs. Jiang et al. reported higher electricity demand for co-digestion (7.0 kWh m−3) than for manure mono-digestion (5.4 kWh m−3), reflecting increased pre-treatment and handling requirements. Moreover, digestate application costs increase due to higher nutrient loads and regulatory constraints. Zhang et al. [3] demonstrate that co-digestion with grass silage requires approximately 10% more land for digestate spreading than mono-digestion, with required land area increasing by 25–67% as soil phosphorus status rises, leading to transport distances of up to 30 km with higher machinery and logistics costs. Under restrictive nutrient regulations, the demand for chemical nitrogen and potassium can increase by up to 165% and 194%, respectively, substantially eroding economic performance.
Overall, the quantitative evidence indicates that co-digestion consistently outperforms mono-digestion in terms of energy production, fertilizer substitution capacity, and system-level economic value per tonne of manure treated. However, its economic superiority is highly sensitive to digestate management costs, transportation distances, and operational energy demand. Mono-digestion remains economically relevant for small-scale or low-capital systems prioritizing simplicity and minimal logistics, whereas co-digestion achieves higher economic efficiency when local co-substrates are available, digestate can be applied within short transport distances, and surplus energy can be valorized through grid connections or on-farm use.

4. Discussion

4.1. General Findings Discussion

The reviewed studies show that LCA has been widely applied to assess PM management through AD and co-AD, although substantial methodological variability has been observed. Differences in FU, system boundaries, LCIA methods, and allocation approaches reflect diverse research objectives and regional conditions, as Ferreira et al. [40] stated]. Most studies adopted cradle-to-gate system boundaries, while fewer extended the assessment to cradle-to-grave by including digestate LA, highlighting the importance of downstream processes for the overall environmental performance.
The results indicate that co-digestion strategies and innovative digestate management practices generally improve the environmental performance of pig manure–based anaerobic digestion systems compared with mono-digestion and direct land application. Co-digestion with organic residues, such as food waste, grass silage, and agricultural by-products, typically enhances biogas yields, energy recovery, and avoided emissions from alternative waste treatment routes, as reported in a review by Tan et al. [41]. However, these benefits are highly dependent on feedstock characteristics, as differences in biodegradability and C/N ratios directly influence anaerobic digestion performance. In addition, the definition of system boundaries plays a key role in LCA outcomes, as studies adopting broader boundaries provide a more comprehensive assessment, whereas narrowly defined systems may overlook relevant environmental impacts. Allocation choices further influence how environmental burdens and credits are assigned between biogas, digestate, and recovered nutrients, leading to variability in the reported results, as highlighted by Marefat et al. [42]. Advanced digestate treatment pathways, including fractionation, microalgae cultivation, and nutrient recovery, also show potential environmental benefits, although their performance is sensitive to operational conditions and technological maturity. Studies consistently show that co-AD with certain residues achieves lower GWP and resource depletion impacts than mono-digestion. However, using food waste, seaweed, or orange-peel waste can lead to lower environmental impacts than energy crop-based systems, as reported by Negro et al. [43] and Usack et al. [44].
Despite the generally favorable performance of co-digestion relative to mono-digestion, the magnitude and direction of reported environmental benefits vary substantially across studies owing to key methodological and system-configuration differences. Studies adopting broader system boundaries (cradle-to-grave) and energy- or nutrient-based functional units tend to report larger reductions in global warming potential, largely driven by substitution credits for electricity, heat, and mineral fertilizers [3,32]. In contrast, gate-to-gate assessments or mass-based functional units often yield smaller benefits or net impact increases when additional emissions from co-substrate production, transport, and digestate handling are included [20,30]. Differences in process configuration further influence outcomes; studies assuming CHP utilization generally report greater climate benefits than those modelling limited biogas use, while simplified representations of digestate storage and land application tend to underestimate acidification and eutrophication impacts. These contrasts highlight that co-digestion performance is highly context- and assumption-dependent, underscoring the need for transparent reporting and harmonized modelling choices when comparing LCA results.
Biogas use and upgrading can significantly impact the overall results, even though they were not significantly discussed in the studies. Ferreira et al. [38] affirmed that utilizing biogas to produce diesel fuel or vehicular gasoline was preferred over electricity generation, as demonstrated by Balcioglu et al. [40] and Khan et al. [45]. Another important aspect of biogas is yield and consequently biomethane production, two factors that can be highly impacted by the time of substrate storage, as proven by Hollas et al. [46], who reported that a storage time of less than 5 days improved the power generation capacity of the system (up to 4.5 kWh of electricity per m3 of raw manure).
It is also important to highlight that when considering AD or co-AD of swine slurry or manure, their physicochemical compositions vary widely depending on diet, animal age, housing, cleaning products, frequency, and even the time of storage, which can affect biogas yield and system performance [47,48]. The production stage also plays an important role in environmental impact. For example, Santos et al. [49] determined that the fodder production stage had the greatest impact, approximately 60–70% in the categories they assessed.
Digestate storage and LA consistently emerged as key environmental hotspots, particularly for nutrient-related EIC and ammonia emissions, indicating that energy recovery alone does not ensure environmental sustainability. However digestate valorization depends highly on technical, economic, and environmental challenges [50]. Some strategies that can help reduce these emissions and further environmental impacts, such as eutrophication, are to acidify the digestate, as suggested by Beyers et al. [51], or at least consider that incorporation into soil with low moisture can also help prevent ammonia volatilization [11].
Replacing open digestate storage with covered or sealed systems resulted in substantial environmental benefits, reducing human health impacts by 25–165%, climate change impacts by up to 273% (corresponding to a net saving of 41.1 kg CO2-eq FU−1), and ecosystem quality impacts by 20–57%, while also yielding minor improvements (1–3%) in resource damage owing to enhanced nutrient retention and energy recovery [30]. Sensitivity analysis of transportation distances of manure to biogas plants or digestate to field application showed that climate change impacts were highly distance-dependent, with a reduction of up to 118% at 5 km and an increase of up to 629% at 100 km. Human health impacts decreased by 54% and 36% at 5 and 10 km, respectively, indicating that transport distances above 50 km can severely deteriorate environmental performance and compromise the sustainability of anaerobic digestion systems.
AD applied to PM appears widely as a high-impact waste treatment technology for GHG mitigation, where the main climate benefit comes from avoiding conventional manure emissions, in addition to energy production itself [52].
Regarding the EIC of the reviewed LCA studies, co-AD showed highly variable effects relative to mono-digestion. Climate change impacts ranged from substantial reductions (−50 to −149%) when energy and fertilizer substitution credits were included [3,32,33] to net increases or loss of climate benefits where additional feedstock production and methane emissions dominated [20,30,31]. The acidification and eutrophication potentials generally increased under co-AD, typically by 6–108% and 8–81%, respectively, mainly due to elevated NH3 emissions during digestate storage and land application [31,33,34]. Ecosystem quality impacts were reduced by up to 40% [30], whereas fossil resource depletion and water use consistently improved under co-AD owing to higher energy recovery and substitution effects [31,33], highlighting the beneficial effects of co-AD versus mono-AD.
Beyond methodological differences, several engineering and process-related design choices consistently influence the environmental performance of anaerobic digestion systems and warrant explicit discussion.

4.2. Process Design and Operational Drivers of Environmental Performance

The process design and operational parameters before and after AD can strongly shape the results obtained by LCA studies.
Prior to digestion, co-substrate selection emerges as a primary engineering driver, as substrates should be locally available and rich in carbon to improve the C/N ratio, thereby enhancing the methane yields and digestate nutrient quality. Ideal co-digestion based on C/N ratios (20:1–30:1) has indicated suitable inclusion rates for energy crops, such as elephant grass and corn silage [20]. Some studies adopted equal substrate proportions (50% slurry–50% co-substrate) [32], whereas others applied lower co-substrate shares, typically around 20% co-feedstock with 80% slurry [34,37].
During anaerobic digestion, operational parameters such as organic loading rate, hydraulic retention time (HRT), and temperature critically influence process stability and performance. Mesophilic conditions of approximately 38 °C have been reported as optimal, enabling reductions in HRT and corresponding decreases in the required digester volume [3,20,31]. Reported HRT values are commonly fixed at 20, 30, or 50 days [20,33].
Nevertheless, these operational parameters are often insufficiently detailed in the reviewed studies.
After digestion, two main drivers determine the downstream impacts: biogas utilization pathways and digestate management.
Across the reviewed studies, biogas is primarily utilized through on-site combined heat and power (CHP) systems, where it is converted into electricity and heat to meet the internal energy demands of the anaerobic digestion (AD) process and associated farm operations, with surplus energy exported to the grid or nearby users [20,31,33,35,36,37]. Reported CHP efficiencies are typically around 40% for electricity and 45% for heat, assuming a methane energy content of 35.7 MJ m−3 [3].
In simpler system configurations, biogas is directly used as a cooking fuel to substitute natural gas, whereas fewer studies consider biogas upgrading to biomethane for grid injection or indirect electricity substitution, mainly in centralized systems with gas infrastructure [30,34]. Methane leakages from digesters and CHP units, reported at approximately 1–2% of the total CH4 produced, remain a critical factor influencing climate change impacts and overall environmental performance [3,20].
Digestate management, whether direct land application, solid–liquid separation, or further treatment, represents a key interface between AD operation and environmental outcomes, particularly acidification and eutrophication [20]. Ammonia emissions during digestate storage were estimated to be 50% higher than those from raw pig manure, corresponding to an emission factor of 15.8% of the total nitrogen stored. These emissions were also affected by LA methods, with injection, incorporation, and band spreading reducing NH3 losses by up to 80%, 70%, and 55%, respectively, compared to spreading without any incorporation. Seasonal variability further influences emissions, with substantially higher nitrogen losses and lower nitrogen use efficiency in autumn than in spring [3].

4.3. Limitations of Current Studies

This review has several limitations. First, the wide heterogeneity of functional units (mass-based, energy-based, and system-scale) substantially limits cross-study comparability and constrains the potential for quantitative synthesis and meta-analysis. Differences in functional unit choice influence the relative weighting of energy recovery, emission burdens, and nutrient substitution benefits, often leading to divergent conclusions, even for similar systems. Consequently, environmental performance rankings across studies cannot be directly compared without normalization or recalculation.
Second, environmental impact category coverage is uneven; although climate change is assessed in all studies, other relevant categories such as land use, water use, particulate matter formation, ionizing radiation, and odor-related emissions are rarely included, potentially overlooking important environmental trade-offs.
Third, the choice of LCA methods and tools influences the reported results. The commonly applied LCIA methods included CML, ReCiPe, and IMPACT2002+. Although the CML is widely used and robust, it focuses primarily on midpoint indicators and provides limited coverage of emerging or locally relevant impact categories. ReCiPe offers broader midpoint and endpoint coverage but introduces additional uncertainty associated with endpoint modeling and value choices. IMPACT2002+ combines midpoint and endpoint approaches but is less frequently updated and applied, limiting its comparability across studies.
Regarding software platforms, SimaPro (versions 8.0–9.2) and OpenLCA 1.10.3, which are typically coupled with the Ecoinvent database (versions 3.3–3.8), are the most widely used and considered mature and reliable. However, the results were found to be more sensitive to methodological choices, database selection, and background data assumptions than the specific software employed. Consequently, no single LCA tool can be identified as universally optimal for assessing pig manure co-digestion systems, underscoring the importance of methodological transparency and harmonization over tool selection
From a process and engineering perspective, most of the reviewed studies provide limited information on AD design and operational parameters, which are often poorly reported or absent.
Moreover, emerging contaminants such as microplastics, antibiotics, and PFAS, which may be present in pig manure and digestate, are not explicitly considered in the current LCA frameworks.

4.4. Digestate Management and Biogas Upgrading: A Critical Research Gap

Digestate storage, transport, and land application consistently emerged as major environmental and economic hotspots in the reviewed studies. Despite their significance, only a limited number of LCAs have explicitly assessed alternative digestate treatment technologies such as fractionation, microalgae cultivation, and nutrient recovery [30,36].
Similarly, biogas upgrading to biomethane for grid injection or fuel substitution is rarely considered, with most studies limiting biogas use to on-site CHP generation because of technical simplicity and data availability. While these options show strong potential to improve resource efficiency and climate performance, most remain in the early development stages and require further research before large-scale implementation [3].

4.5. Future Research Directions

Future research should prioritize the detailed characterization of digestate management pathways, including alternative treatment options, optimized land application strategies and valorization approaches. Greater attention should be given to process design and operational parameters throughout the AD process, as key variables are rarely reported but strongly influence biogas yield and digestate quality. Accounting for the technical, economic, and environmental feasibility of these strategies is essential to fully capture the sustainability potential of anaerobic digestion and codigestion systems. In this context, particular emphasis should be placed on low-cost and scalable solutions suitable for small-scale swine farms, especially in developing and underdeveloped regions, where simplified reactor designs, locally available co-substrates, direct biogas use (e.g., cooking or on-site heat), and low-input digestate management may represent more feasible pathways. Studies should prioritize greater methodological harmonization, broader inclusion of underrepresented environmental impact categories, and integration of LCA with detailed techno-economic analyses. Advancing methods to account for the fate and potential impacts of emerging contaminants would enable more comprehensive sustainability assessments of anaerobic digestion and co-digestion systems for pig manure management.

5. Conclusions

This review demonstrates that AD and co-AD are effective strategies for reducing the environmental impacts associated with PM management, particularly regarding GHG emissions when biogas is efficiently utilized and avoided emissions are accounted for. In most cases, co-AD outperforms mono-AD owing to higher energy recovery and improved system efficiency; however, the results remain highly sensitive to the FU definition, system boundaries, allocation methods, digestate management, and co-feedstock selection. PM and digestate storage and their land application consistently emerge as environmental hotspots, indicating that energy recovery alone is insufficient to ensure sustainability and that further investigation should be conducted to understand how different storage and land application practices can help mitigate these impacts.
Beyond environmental performance, the review indicates that AD and co-AD are strongly influenced by economic factors, energy self-sufficiency, avoided mineral fertilizer use and digestate handling costs, where co-AD improves economic viability by on one hand increasing biogas yields and consequently more energy production that can be commercialized and, on another hand, enhancing digestate nutrient recycling. The importance of local integration between feedstock supply, cropland availability, and energy utilization pathways are key factors in the sustainability of co-AD.
The analysis further revealed substantial methodological heterogeneity across LCA studies, with climate change being the only impact category assessed universally, while other relevant categories, such as land use, water use, and particulate matter formation, were rarely considered. This limited and uneven coverage constrains cross-study comparability and may obscure important environmental trade offs. Overall, greater harmonization of LCA methodologies, improved emission data, and broader EIC coverage are required to support robust sustainability assessments of AD and co-AD systems for PM management in the future.

Author Contributions

Conceptualization, P.E.; methodology, P.E., A.F. and J.F.; software, A.F. and J.F.; validation, A.F. and J.F.; investigation, P.E.; resources, A.F. and J.F.; writing—original draft preparation, P.E.; writing—review and editing, A.F. and J.F.; supervision, A.F. and J.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

All used datasets are available online and are openly accessible.

Acknowledgments

The authors thank the Portuguese Foundation for Science and Technology (FCT) for the financial support to the Research Centre for Natural Resources, Environment and Society—CERNAS (UIDB/00681/2025) DOI https://doi.org/10.54499/UID/00681/2025. P. Esperanço was supported by an FCT PhD grant (2025.07113.BDANA). During the preparation of this manuscript/study, the author(s) used [Rayyan] for the purposes of [selection/exclusion of articles for further review]. The authors have reviewed and edited the output and take full responsibility for the content of this publication.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
ADAnaerobic digestion
co-ADAnaerobic co-digestion
APAcidification potential
CCClimate change
CSCorn Silage
CH4Methane
CHPCombined heat and power
CMLCentrum voor Milieukunde Leiden impact assessment method
DEA–LCAData Envelopment Analysis–Life Cycle Assessment
EICEnvironmental impact category
EPEutrophication potential
FAOFood and Agriculture Organization of the United Nations
FUFunctional unit
FWFood waste
GHGGreenhouse gas
GSGrass silage
GWPGlobal warming potential
HTHuman toxicity
ILCDInternational Reference Life Cycle Data System
IPCCIntergovernmental Panel on Climate Change
ISOInternational Organization for Standardization
IRIonazing Radiation
LALand application
LCALife cycle assessment
LCCLife cycle costing
LCIALife cycle impact assessment
LWLive weight
MAEMarine aquatic ecotoxicity
MJMegajoule
N2ONitrous oxide
NREUNon-renewable energy use
NRRUNon-renewable resource use
OROdour-Related
ODPOzone depletion potential
PBFPowder Biofertilizer
PMPig manure/slurry
PMFParticulate matter formation
PRISMAPreferred Reporting Items for Systematic Reviews and Meta-Analyses
ReCiPeHarmonised life cycle impact assessment method
SLCASocial life cycle assessment
SSSwine slurry
TETerrestrial ecotoxicity
WUWater use

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