Thermochemical and biochemical conversion technologies for the valorization of coffee-industry residues: a systematic review

1 Introduction

Coffee processing generates substantial volumes of solid and liquid residue streams that often lack standardized management protocols. These streams, comprising coffee husks, pulp, and spent grounds, pose localized environmental liabilities while also offering pathways for circular integration (Tsigkou et al., 2025). Prior investigations have characterized the high valorization capacity of these residues through thermochemical and biological transformation pathways (del Pozo et al., 2021; Lawrence et al., 2025). On the one hand, pyrolysis has been investigated to yield biochar and phenolic fractions (Tsigkou et al., 2025; Bartolucci et al., 2024; Tian et al., 2021). On the other hand, anaerobic digestion represents a viable technology for methane synthesis with energy-recovery and agricultural applications (Tumusiime et al., 2022; Oliva et al., 2025; Hernández-Sarabia et al., 2021; Polańıa Rivera et al., 2024). These outcomes indicate that coffee biomass can be converted into energy vectors, soil conditioners, and platform chemicals of industrial relevance (Ranjbari et al., 2022; Yeoh and Ng, 2022).

At the same time, the extraction of functional compounds and nutrients from coffee derived biochar has been investigated, establishing new paradigms for the formulation of controlled-release organic-mineral fertilizers (Tian et al., 2021; Ranjbari et al., 2022; Yeoh and Ng, 2022). This research trajectory is of critical importance in developing nations facing constrained access to commercial inputs, where amendments synthesized from coffee by-products—such as biochar—enhance soil physicochemical properties and reduce reliance on synthetic agrochemicals (Van Phuong, 2025; Morais et al., 2023; Bartolucci et al., 2024). Furthermore, thermodynamic modeling and Life Cycle Assessment (LCA) studies demonstrate that energy recovery from coffee residues yields a measurable reduction in net CO₂ emissions and overall carbon footprint compared to conventional landfill disposal (Oliva et al., 2025; Van Phuong, 2025; Quynh and Kazuto, 2018; Pazmiño et al., 2024).

However, recent investigations indicate that not all valorization pathways exhibit an optimized trade-off between thermodynamic process energy efficiency (Rivera et al., 2020). Although pathways such as the synthesis of biodiesel from spent coffee grounds are viable from an energy-density perspective, their overall environmental performance is constrained by intensive pre-treatments, operational instabilities, and higher life-cycle impacts compared to conventional thermochemical options or direct application to soil (Tumusiime et al., 2022; Oliva et al., 2025; Zhang et al., 2025). These discrepancies underscore that the thermodynamic potential of a technology does not inherently guarantee its multidimensional sustainability. Despite the technological opportunities, most coffee residues in producing countries are discharged untreated, leading to soil acidification, eutrophication of aquatic ecosystems, and fugitive greenhouse gas emissions (Tumusiime et al., 2022; Oliva et al., 2025; Hernández-Sarabia et al., 2021). This systemic mismanagement results in the loss of carbon-rich fractions that could otherwise support bioenergy grids and agronomic nutrient loops, representing a critical bottleneck for sustainable coffee farming (Hernández-Sarabia et al., 2021). Although technologies such as pyrolysis offer the possibility of producing biochar and compounds of agricultural and industrial interest, their commercial deployment remains constrained by capital cost thresholds, lack of processing infrastructure, and market-entry barriers (del Pozo et al., 2021; Milian-Luperón et al., 2020; Zhang et al., 2025).

On the other hand, comparative LCAs reveal that not every recovery route is environmentally favorable; the synthesis of biodiesel from spent coffee grounds, for example, exhibits higher environmental impacts across multiple categories compared to alternatives such as thermal treatment or direct application to soil (Rivera et al., 2020). In addition, the deployment of these technologies is hindered by the absence of clear market acceptance and adoption guidelines by producers (Ranjbari et al., 2022; Yeoh and Ng, 2022). Finally, the scarcity of regulatory incentives, insufficient technology transfer, and low adoption of cogeneration systems at the industrial level keep most initiatives in experimental phases, limiting their measurable ecological impact on the sustainability of the sector (Campo et al., 2020; Pazmiño et al., 2024).

Prior review papers have addressed the valorization of coffee-processing residues, yet technical and methodological gaps remain. For instance, a review focused on sustainable development analyses primary processing residues but excludes brewing-stage spent grounds, placing these streams outside the industrial boundary (Tsigkou et al., 2025). In contrast, other review efforts focus on spent grounds while omitting agricultural co-products such as pulp, mucilage, or parchment (Yeoh and Ng, 2022). From an evaluation standpoint, bibliometric analyses map scientific production trends at a macro scale but lack synthesis of process-specific operating parameters or mass-energy balances (Ranjbari et al., 2022). This systematic review complements these prior efforts by introducing a standardized evaluation of methodological quality and risk-of-bias across primary investigations.

This work organizes and systematizes technological pathways for resource recovery to provide a consolidated baseline that supports future research and the implementation of circular bioeconomy practices. This systematic review analyses and synthesizes empirical findings on valorization technologies applied to by-product and residue streams generated in the coffee industry. The study focuses on identifying the dominant biological and thermochemical conversion pathways, along with their operational challenges and technical trade-offs, to determine which configurations enable optimized circular resource utilization, mitigate environmental footprints, and enhance process energy efficiency and operational throughput in coffee systems.

The rest of the article is structured as follows: Section 2 presents the methodological approach, focusing on the PIO (Population-Intervention-Outcome) strategy and the PRISMA 2020 checklist. Section 3 synthesizes the dominant valorization technologies, classified by biological and thermochemical pathways, as well as the co-products generated and their environmental, energy, and socioeconomic impacts. In Section 4, the findings are contextualized within the global literature, evaluating technological scalability while delineating current research limitations and implementation bottlenecks. Finally, Section 5 summarizes the study's main contributions, assesses the technical viability of the analyzed routes, and identifies research priorities for the circular valorization of coffee waste.

2 Methodology

This systematic review synthesizes scientific data on recovery technologies for coffee-industry residues published between 2020 and 2025. The study design followed the PICO model and the PRISMA 2020 framework to establish standardized guidelines for formulating research questions and selection protocols. Although this systematic review was not formally registered in a prospective database, the research protocol, eligibility criteria, search strategies, and data extraction framework were established before data collection to ensure methodological transparency and prevent selection bias. Data management used Microsoft Excel to structure and record metadata for the retrieved studies, including title, authors, abstract, Digital Object Identifier (DOI), and keywords. This software enabled the sequential documentation of the screening stages based on title and abstract, full-text retrieval, and the application of inclusion and exclusion criteria. The primary systematic search was restricted to the Scopus and Web of Science (WoS) databases. Google Scholar functioned as a supplementary tool to identify additional records and facilitate access to full-text documents. ResearchGate served as a platform to retrieve full-text documents unavailable through the primary bibliographic sources. Zotero operated as a reference manager to store, organize, and manage the bibliographic records and documents retrieved during this review.

2.1 Application of the PICO model

The research questions were formulated using the PICO model, a structured framework for organizing inquiries by identifying the population (P), intervention (I), comparison (C), and outcomes (O) (Santos et al., 2007). Given the nature of the literature, we excluded component C from the database search strategy to avoid restricting the volume of retrieved process data. Primary investigations in process engineering often focus on the performance of a single technology. Imposing a comparative requirement at the database screening stage would exclude these primary datasets. While individual studies focus on single configurations, this review integrates comparative technological assessments in the results section, contrasting the compiled parameters across the different conversion pathways. Therefore, the study protocol utilizes the PIO framework, which aligns with the exploratory and descriptive scope of the review. In this context, the main research question was: What recovery technologies allow for the sustainable use of coffee industry waste to decrease environmental impacts and increase product yields and process energy efficiency? Based on this question, three specific sub-questions were identified, which are presented in Table 1 along with the search keywords.

Table 1. Subquestions and keywords.

2.2 Search strategy

The literature screening was conducted in Scopus through the Elsevier database interface and WoS Core Collection via the Clarivate Analytics platform to identify papers answering the research questions. Query formulations utilized the keywords from Table 1 combined with the Boolean operators AND and OR. The bibliographic retrieval was limited to peer-reviewed journal articles, English documents, and open-access publications from 2020 to 2025. This 6-year window captures recent technological configurations in coffee-waste recovery. The English-language restriction was established because English represents the international standard for technical reporting, facilitating the uniform comparison of process parameters. This public-access format allowed full retrieval of raw datasets, appendices, and supplementary tables required for systematic quality evaluation. However, these criteria introduce selection and territorial biases, limiting representation from major coffee-producing nations in Latin America and Africa due to the exclusion of regional-language and subscription-restricted literature.

(TITLE-ABS-KEY (“coffee waste” OR “coffee by-products” OR “coffee pulp” OR “coffee husk” OR “coffee mucilage” OR “spent coffee grounds” OR “coffee silverskin”) AND TITLE-ABS-KEY (“anaerobic digestion” OR composting OR vermicomposting OR fermentation OR pyrolysis OR gasification OR “biochar production” OR “waste valorization” OR “circular bioeconomy”) AND TITLE-ABS-KEY (Biogas OR bioethanol OR biodiesel OR biohydrogen OR biofertilizers OR biochar OR “renewable energy” OR sustainability OR “circular economy” OR “environmental impact” OR “life cycle assessment” OR LCA OR “resource efficiency”)) AND (LIMIT-TO (DOCTYPE, “ar”)) AND (LIMIT-TO (LANGUAGE, “English”)) AND (LIMIT-TO (OA, “all”)) AND PUBYEAR > 2019 AND PUBYEAR <2026

Following the same criteria applied in Scopus, the search string for WoS was formulated using the following equation:

(“coffee waste” OR “coffee by-products” OR “coffee pulp” OR “coffee husk” OR “coffee mucilage” OR “spent coffee grounds” OR “coffee silverskin”) (Topic) and (“anaerobic digestion” OR composting OR vermicomposting OR fermentation OR pyrolysis OR gasification OR “biochar production” OR “waste valorization” OR “circular bioeconomy”) (Topic) and (Biogas OR bioethanol OR biodiesel OR biohydrogen OR biofertilizers OR biochar OR “renewable energy” OR sustainability OR “circular economy” OR “environmental impact” OR “life cycle assessment” OR LCA OR “resource efficiency”) (All Fields) and (2025 or 2024 or 2023 or 2022 or 2021 or 2020) (Publication Years) and Article (Document Types) and English (Languages) and All Open Access (Open Access)

The database search was completed on december 31, 2025, to establish a definitive temporal boundary matching the declared study scope from 2020 to 2025. This cutoff date reflects a protocol-driven decision to freeze the dataset, enabling multi-stage screening, technical quality assessment, and engineering process modeling of the selected conversion pathways. Retaining this end-of-year boundary prevents the inclusion of partial 2026 datasets, preserving the temporal coherence of the review. A total of 472 records were retrieved, comprising 215 records from Scopus and 257 from WoS Core Collection.

2.3 Methodological quality and risk of bias assessment

The methodological quality and risk of bias of the 116 primary studies included in this review were assessed using a technical framework adapted from the study validity model proposed by Lorick et al. (2020)) for environmental process engineering reviews. The framework evaluated six criteria: (Q1) feedstock characterization and composition, (Q2) specification of process and operational parameters, (Q3) analytical replication and statistical validation, (Q4) conversion yields and mass and energy balances, where extracted quantitative parameters were systematically standardized into product yield, conversion efficiency, process energy efficiency, net energy gain, adsorption removal efficiency, and benefit-cost ratio, (Q5) definition of system boundaries and process scale, and (Q6) assessment of scalability or techno-economic aspects. Each criterion was scored on a three-point scale: 2 (adequately addressed), 1 (partially addressed), or 0 (inadequately addressed or not reported), with a maximum cumulative score of 12 points.

Based on the cumulative score, methodological quality was classified as High (9–12 points), Moderate (7–8 points), or Low (0–6 points). Quantitative data used for technology performance comparisons and construction of the technology selection decision tree were obtained exclusively from studies classified as High. Studies classified as Moderate were considered only in the qualitative synthesis and contextual analysis. Low-quality studies were not used to derive quantitative comparisons or decision criteria. The study-level assessment is presented in Table A1 in Appendix.

2.4 Selection process

Subsequently, the PRISMA 2020 (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) tool was adopted to structure the process of identifying, evaluating, and selecting articles that contribute to the research (Page et al., 2021). Each stage of the process was accompanied by IC and EC, presented in Table 2, aimed at ensuring systematic and transparent screening of the studies selected in this review.

Table 2. Inclusion and exclusion criteria.

The PRISMA 2020 flowchart, shown in Figure 1, begins with the 472 records identified in the Scopus and WoS databases. In the first stage, 273 duplicate documents were identified and eliminated, leaving 199 reports for the next phase. Through the analysis of titles, keywords, and abstracts, 61 publications were excluded, resulting in 89 records for further analysis. Subsequently, 5 articles were removed due to lack of access to the full text, leaving a total of 138 documents for evaluation. According to the EC, 23 articles were excluded, distributed as follows: 10 according to EC1, 8 according to EC2, 4 according to EC3, and 1 according to EC4. To complement the primary database search, 6 additional records were identified through backward snowballing of reference lists from primary articles and key reviews. These records underwent the same screening and eligibility criteria as the database records. Duplicates were checked and removed in Excel against the Scopus and WoS datasets, and each record was manually verified as a peer-reviewed journal article indexed in either database.

The screening and study selection process was conducted independently by the three authors. Discrepancies arising during study selection were resolved through discussion and consensus. For the six additional studies identified through Google Scholar, duplicate records were checked against the studies retrieved from the primary database searches using Microsoft Excel. A manual verification was also conducted to confirm that all 6 studies were published in journals indexed in Scopus or WoS.

3 Results

This section presents the analysis of the 116 articles selected following the PRISMA 2020 strategy. The findings are organized according to the sub-questions of the PIO framework, describing the thematic consistency of the analyzed works, the types of waste generated in the coffee industry along with their relevant properties, the main valorization technologies reported, and the products obtained with their intended applications in energy, agriculture, or industrial uses. Given the heterogeneous experimental and operational conditions across the analyzed works, numerical values such as temperature ranges, product yields, conversion efficiencies, or process energy efficiencies are interpreted as observed intervals rather than figures that can be directly extrapolated to a single operational scenario.

3.1 Methodological consistency of the studies reviewed

Figure 2 summarizes the degree of alignment between the selected articles and the components of the PIO model. The graph illustrates how each study addresses the problem, the intervention, and the outcomes defined in the review, identifying patterns of complete, partial, or absent coverage of these components. In this regard, the figure serves as an indicator of thematic alignment with the review's analytical framework and not as a measure of the quality or methodological rigor of the included studies. The results show that all the studies considered align with component P, confirming an adequate thematic relationship with waste generated in the coffee industry. Likewise, a high proportion of the studies address components I and O, indicating that most of the selected literature includes valorization technologies and reports products, benefits, or results associated with environmental and productive sustainability. Cases of partial coverage correspond mainly to characterization or exploratory studies; therefore, their inclusion reflects differences in the thematic scope of the studies and should not be interpreted as an assessment of their methodological quality.

Results indicate that all studies address component P, confirming thematic consistency regarding waste generated in the coffee industry. Approximately 90% cover component I, reflecting broad documentation of recovery technologies applied to coffee by-products. Similarly, 90% report on component O, showing that most studies describe products or benefits related to environmental and productive sustainability. This high level of correspondence demonstrates that the selected sample not only adequately identifies the problem, but in most cases includes a clearly defined technological intervention and measurable or discussed outputs. The presence of a small percentage of works with partial coverage of component I or O mainly corresponds to exploratory or characterization works, which do not compromise the overall coherence of the review but highlight the methodological heterogeneity inherent in a consolidating research field.

3.2 Coffee waste and biomass characteristics

The evaluated literature highlights advances in technologies for recovering coffee waste (Tsigkou et al., 2025). The studies focus on residues such as coffee grounds, husks, pulp, and mucilage, generated at different stages of agro-industrial processing (Ranjbari et al., 2022). Within this framework, the system boundary for coffee waste comprises all organic residues and by-products generated along the entire coffee value chain, spanning from agricultural harvesting to final beverage consumption. This classification categorizes these streams into agricultural processing residues, roasting-stage residues, and brewing-stage residues to reflect variations in moisture content, chemical composition, and logistical traits. Their lignocellulosic composition, rich in carbon, hemicellulose, and lignin, offers potential for energy and biochemical conversion (Pérez-Merchán et al., 2025b; Chen et al., 2025; Fernández-Ferreras et al., 2023; Dawerasha et al., 2024). The amount of waste generated depends on the type of processing and the specific stream under consideration; therefore, a single percentage is not valid for the entire coffee value chain. In wet processing, data indicate that solid waste accounts for around 60% of the fresh fruit weight, consisting of pulp representing 67% of the solid fraction, mucilage at 27% and parchment at 6% (Sinclair et al., 2024). This distribution corresponds to around 40.2%, 16.2%, and 3.6% of the fresh fruit weight. Independent data support these parameters, indicating that pulp represents 41% of the wet berry weight (Tolesa et al., 2025). These proportions are valid for wet processing of the fruit and must not be extrapolated to other stages of the production chain, such as roasting or beverage preparation, yielding substantial annual quantities (Tsigkou et al., 2025; Rivera et al., 2020). Improper disposal of these materials leads to leachate contamination, greenhouse gas emissions, and soil nutrient depletion (Silva et al., 2024). Modern conversion technologies enable the recovery of both economic and environmental parameters through biorefineries, pyrolysis, anaerobic digestion, fermentation, composting, and catalytic absorption, transforming this waste into valuable marketable resources with low ecological impact (Sousa et al., 2021; del Pozo et al., 2021; Cancelliere et al., 2025; Polańıa Rivera et al., 2024; Sokolova et al., 2024).

Roasting-stage residues are generated in both producing and importing countries, yielding coffee silverskin, a thin spermoderm tegument that separates from the bean during thermal roasting. This residue represents less than 5% of the bean weight and exhibits low moisture under 10% to facilitate dry transport and stable storage (Page et al., 2021; Vanyan et al., 2024). Brewing-stage residues are generated worldwide at the final point of consumption, yielding spent coffee grounds as the solid powder residue remaining after hot-water beverage extraction. This residue presents a moisture content of 50% to 65%, a carbon-rich lignocellulosic structure, and a dispersed, non-seasonal point source distribution (Kim and Oh, 2021; Ko et al., 2025).

From a physicochemical perspective, coffee residues exhibit characteristics that determine their behavior in recovery processes. Studies report a high organic matter content and a lignocellulosic composition dominated by variable fractions of cellulose, hemicellulose, and lignin, with proportions depending on waste type and processing stage (Tian et al., 2021; Van Phuong, 2025; Zhang et al., 2025). Pulp and mucilage are characterized by high moisture and soluble carbohydrate content, supporting biodegradability and suitability for biological routes (Oliva et al., 2025; Quynh and Kazuto, 2018). In contrast, husks and grounds contain higher carbon and lignin levels and lower moisture content, making them suitable for thermochemical processes (Yeoh and Ng, 2022; Pazmiño et al., 2024; del Pozo et al., 2021). Parameters such as carbon-to-nitrogen (C/N) ratio, ash content, and the presence of phenolic compounds also influence substrate conversion efficiency and product quality, supporting the use of these residues as biomass for energy and agricultural applications (Hernández-Sarabia et al., 2021; Morais et al., 2023). Table 3 summarizes the relationships between residue types, their characteristics, and the most frequent recovery routes.

Table 3. Physicochemical characteristics of coffee residue categories and specific fractions.

*The C/N ratio of 15.63 is associated with washed processing wastewater and should not be confused with pure, undiluted mucilage pulp, which ranges from 14.0 to 54.0.

The C/N ratio varies among coffee waste streams and must be interpreted alongside the physicochemical characteristics of each material. For coffee pulp, values close to 25.6 are reported for the raw biomass, and between 21.7 and 26.7 after anaerobic silage treatments (Getachew et al., 2023). For coffee parchment, elemental analysis shows 41.8% carbon and 1.9% nitrogen on a dry weight basis, yielding a C/N ratio of 22.0 (Heredia Salgado et al., 2021). For coffee husks, elemental analysis shows carbon concentrations of 43.1% –50.1% and nitrogen concentrations of 1.5% to 4.0%, resulting in stoichiometric C/N ratios of 22.0–27.8 (Silveira Junior et al., 2023; Tolesa et al., 2025). This ratio is determined by the stoichiometric balance of total organic carbon and total nitrogen, rather than lignin content. For spent coffee grounds, independent studies report elemental compositions with C/N ratios ranging from 20.1 to 20.3 (Kim and Oh, 2021; Ko et al., 2025). Liquid streams such as mucilage effluent present a ratio of 15.63 in honey water from wet processing (Hernández-Sarabia et al., 2021). This parameter depends on the water volume used during processing and is not representative of pure mucilage.

3.3 Technologies applied to coffee waste

The analysis conducted evaluated various technologies for recovering coffee waste under different operating conditions and associated with by-products from the coffee chain. The most representative technologies include biological and thermochemical processes, evaluated in different operating contexts and with various coffee by-products, showing consistent patterns of performance reported. Table 3 organizes this information and presents the technologies that directly influence the improvement of the recovery process, considering the type of waste applied, the product obtained, and the specific contribution of each technological route (Tsigkou et al., 2025; del Pozo et al., 2021). Such an approach facilitates the visualization of the technical contributions of processes such as anaerobic digestion and composting within the framework of sustainable techniques and circular economy principles (Tumusiime et al., 2022; Hernández-Sarabia et al., 2021). Likewise, it provides a solid basis to understand the potential of each technology within current coffee waste recovery systems (Getachew et al., 2023).

We evaluated the distribution of conversion pathways across the 116 primary studies based on coded technology occurrences. Multi technology evaluations in 25 studies generated 141 total occurrences, indicating that categories are non-exclusive. Thermochemical conversion accounted for 66 occurrences, comprising slow pyrolysis (27.6%), other pyrolysis and activation processes (22.4%), and gasification (6.9%). Slow pyrolysis generated biochar with high thermal stability, low ash content, and absorbent potential (Tsigkou et al., 2025; del Pozo et al., 2021). Researchers carried out processes at 400 °C to 700 °C, with residence times of 1–12 h under inert atmospheres, achieving biochar yields of 25%–40% of the initial dry weight (Zhang et al., 2020; Gonzalez-Piedra et al., 2021; Huyen et al., 2025). These operational ranges reflect typical conditions documented in studies and help explain the recurring application of this technology in agricultural and environmental contexts. Based on the distribution, we classified the technologies by reporting frequency and their relationship to component I. This classification characterizes the most common recovery routes over the last 5 years and their level of consolidation within current technical knowledge. Figure 3 summarizes the data and shows which processes have the most empirical support in coffee waste recovery, following methodologies applied to agro-industrial biomass (Tian et al., 2021; Drewnowski et al., 2025; Oliva et al., 2025; López-Linares et al., 2021). Slow pyrolysis (27.6%) and anaerobic digestion (24.1%) received the highest empirical support, followed by other pyrolysis and activation processes (22.4%), microbial fermentation (21.6%), composting/vermicomposting (10.3%), anaerobic co-digestion (8.6%), and gasification (6.9%). These percentages reflect the relative reporting frequency across the 116 analyzed studies. They should not be interpreted as a direct comparison of efficiency between technologies, but as an indicator of the attention each approach has received in recent applied research.

3.4 Biomass derivatives and recovery

Based on the recovery technologies presented in Table 4, various products derived from coffee waste were identified and classified according to their application type and associated technology. This classification highlights the role of coffee byproducts in the production of bioenergy, absorbents, and renewable fuels through physical, biological, and thermochemical conversion processes (Tian et al., 2021; Huyen et al., 2025; Pérez-Merchán et al., 2025b). The results of the review show that energy applications are the predominant use of products obtained from coffee waste recovery. In particular, the generation of electrical and thermal energy from biogas and bioenergy is supported by 27 studies, making it the most frequently reported application in the analyzed literature. This trend suggests sustained research interest in this alternative for the energy valorization of coffee waste.

Table 4. Technologies and products derived from the recovery of coffee waste.

In addition to energy-related applications, several non-energy recovery pathways derived from coffee waste are extensively reported in the literature. In this context, the use of biochar as an organic soil amendment, fertilizer component, and absorbent material is documented across 48 studies (41.4% of the evaluated literature), making it the most frequently reported non-energy application (Dawerasha et al., 2024; Silva et al., 2024; Araya-Sibaja et al., 2025). This high recurrence reflects sustained research interest in biochar for improving soil structure, enhancing nutrient retention, and mitigating environmental pollutants. At a moderate level of recurrence, the production of organic compost and biofertilizers from coffee waste is reported in 18 studies (15.5%), indicating active adoption of nutrient recycling and organic fertilization practices on coffee farms. Likewise, the production of liquid biofuels, such as bioethanol and biodiesel, is addressed in 14 studies (12.1%), highlighting ongoing technological efforts toward liquid energy vectors despite processing and economic constraints. Finally, 9 studies (7.8%) report the recovery of specialized high-value coproducts, including platform chemicals, battery-anode hard carbons, and bioactive extracts, indicating that these advanced applications represent an emerging frontier within the coffee waste valorization landscape.

Biochar has been shown to adsorb inorganic contaminants and to enhance agricultural soil properties (Morais et al., 2023; Chin-Pampillo et al., 2021; Sokolova et al., 2024; Araya-Sibaja et al., 2025). For example, in alluvial soils contaminated with drimaren dye, the application of biochar derived from coffee husks reduced the mobility of the contaminant by more than 90%, as documented in specific experimental studies, supporting its use as a natural adsorbent and organic fertilizer (Silva et al., 2024; Cancelliere et al., 2025). Co-pyrolysis of coffee grounds with recycled polymers or agricultural plastics represents an alternative pathway to enhance thermochemical energy recovery (Tian et al., 2021; Yeoh and Ng, 2022; Alabdalall et al., 2023). In this context, spent coffee grounds are blended with high-density polyethylene under temperatures from 450 to 500 °C to determine product distributions and synergistic interactions (Ansari et al., 2025). The reported experimental results include a maximum liquid fuel yield of 89.56% under microwave heating at 750 W with a spent coffee grounds to plastic feed ratio of 1:3. This liquid yield decreases to 75.67% and 69.17% when the feed ratio shifts to 1:1 and 3:1, while solid char yields range from 1.20% to 2.53%, while gaseous products span from 7.91% to 29.63%. High-density polyethylene acts as a hydrogen donor and a heat carrier, which limits char formation and promotes volatile deoxygenation, yielding a liquid product with a calorific value of 42.11 MJ/kg (Jeńıček et al., 2025; Alabdalall et al., 2023). These parameters demonstrate that the thermochemical interaction suppresses oxygenated compounds in the liquid fraction, thereby increasing energy recovery and supporting waste integration into circular systems. Furthermore, variants of hydrothermal carbonization are processes that operate in an aqueous medium under autogenous pressure and moderate temperatures from 180 to 250 °C to synthesize carbonaceous materials (Yeoh and Ng, 2022; Nieto et al., 2023; Chen et al., 2025). When used to produce hard carbons for battery anodes, this process offers a sustainable alternative to commercial graphite (Nieto et al., 2023). Within a cradle-to-gate system boundary, this pathway reduces the carbon footprint by avoiding mining and high-temperature purification of mineral graphite (Nieto et al., 2023). The corresponding performance metrics show a specific capacity of 210 to 280 mAh/g and an initial Coulombic efficiency of up to 76% (Nieto et al., 2023). For solid fuel applications, the raw coffee biomass exhibits a higher heating value of around 22 MJ/kg, which increases in the resulting hydrochar due to dehydration and decarboxylation reactions (Zhang et al., 2025; Rosson et al., 2020; Terán-Rivera et al., 2024; Rosson et al., 2020). In comparative terms, gasification of the defatted spent coffee grounds at 900 °C produces syngas with yields from 88% to 95% (Tinoco-Caicedo et al., 2021). Catalytic transesterification of the extracted lipid fraction yields biodiesel with transesterification conversion yields from 82% to 87% (Tinoco-Caicedo et al., 2021; Cárdenas-Piñeros et al., 2025; Fernández-Ferreras et al., 2023). Exergoeconomic analyses in the literature showed increases of up to 91% in overall process energy efficiency and reduced operating costs, pointing to the economic feasibility of integrated thermochemical conversion systems (Pazmiño et al., 2024; del Pozo et al., 2021).

Alcoholic fermentation and anaerobic digestion were the most prominent biotechnological routes. Among the biological pathways identified in the reviewed studies, anaerobic digestion, co-digestion, fermentation, and composting stood out. Because some studies evaluated more than one technology within a single paper, these categories were not necessarily considered mutually exclusive. In other words, their frequency is interpreted as the number of studies in which each technology was evaluated or reported, rather than as a single percentage share of the total number of articles. (Tsigkou et al., 2025; Kim and Oh, 2021; Renaudie et al., 2022). These processes exploit the high organic load and sugar content of coffee pulp and mucilage, as well as the presence of oils and proteins in SCG, to generate bioethanol, biogas, and biohydrogen (Duarte et al., 2021; Kc et al., 2021; Tantayotai et al., 2025; Pardo et al., 2022; Lawrence et al., 2025). Fermentation of pulp and mucilage under controlled anaerobic conditions (30 °C, pH 4.2, 48 h) yielded ethanol conversion yields exceeding 80%, with final concentrations of up to 8.8% v/v (Hernández-Sarabia et al., 2021; Cervera-Mata et al., 2022; Sinclair et al., 2024). In addition, kombucha produced from coffee husks has been reported as a source of functional beverages enriched with antioxidant phenolic compounds, opening opportunities in sustainable nutraceutical markets (Puentes-Escobar et al., 2022; Sales et al., 2023). Dark fermentation without exogenous inoculum has been shown to produce biohydrogen and acetate from raw coffee husks, while also reducing toxic residues generated during roasting (Renaudie et al., 2022). Overall, these biotechnological pathways contribute to wastewater pollution mitigation and support product diversification within the coffee value chain; however, their scalability and economic performance remain dependent on process integration and feedstock availability.

Among the biotechnological routes identified, anaerobic digestion is one of the most widely used technologies for energy recovery from wet waste, such as pulp, mucilage, and coffee grounds mixed with manure (Tsigkou et al., 2025; Sousa et al., 2021). This technology allows the generation of biogas with a high percentage of methane, as well as producing digestate that can be used as organic fertilizer. In various studies included in the review, rural biodigesters and semi-controlled systems have demonstrated increases in energy production and substantial reductions in the organic load of the treated waste (Getachew et al., 2023; Pardo et al., 2022; Polańıa Rivera et al., 2024; Sokolova et al., 2024; Cho et al., 2025). To illustrate this process, Figure 4 shows a representative operational scheme of a biogas plant based on coffee waste.

Figure 4 presents a conceptual diagram of the main stages associated with the anaerobic digestion of coffee waste (Hernández-Sarabia et al., 2021). Its purpose is to provide a general overview of the sequence of feeding, digestion, biogas production, energy recovery, and digestate generation, rather than to describe the detailed design of a specific plant. In real-world systems, the configuration may include additional stages such as substrate conditioning, pH control, biogas storage and purification, removal of moisture and H₂S, as well as subsequent management or treatment of the digestate, depending on the scale, the substrate, and the end use (Hernández-Sarabia et al., 2021). Therefore, this illustration shows the basic operation of an anaerobic digestion system applied to coffee waste, where organic by-products undergo microbial decomposition under anaerobic conditions inside the digester, resulting in biogas that can be converted into electricity and heat by a coupled engine, while the digestate can be used as an agricultural amendment. This diagram reflects the processes described in the studies analyzed and illustrates the potential of biodigesters to enhance overall process energy efficiency on coffee farms, contribute to reduced dependence on Liquefied Petroleum Gas (LPG), and support the transition toward circular bioenergy models. Anaerobic digestion, applied either individually or through co-digestion with manure or honey water, has been identified in the reviewed literature as one of the most stable approaches for the energy recovery of wet waste (Tumusiime et al., 2022). Biogas production results should be interpreted in light of the feedstock and operating conditions of each system. In a study of batch co-digestion of coffee husks and cattle manure, one article reported an observed cumulative potential of 12.37 L of biogas per kg⁻¹ of substrate for a 25:75 mixture of dry coffee husks and manure under field conditions (Paes et al., 2023). Hernández-Sarabia et al. (2021)) evaluated 4 m³ tubular digesters with a nominal hydraulic retention time of 35 days, fed with swine manure, cattle manure, and coffee honey water. These systems exhibited CH₄ contents ranging from 49.1 ± 4.6% to 58.1 ± 2.4% by volume. Although production on these farms was estimated at approximately 19 m³ of biogas per day, this value corresponds to the operational capacity of the evaluated system and should not be used as a comparative indicator without normalization with respect to substrate load or volatile solids. In addition, the application of biological inoculants (Lactobacillus spp.) was associated with increases of up to 70% in methane generation (Oliva et al., 2025; Getachew et al., 2023), while kinetic analyses of mixed digestions involving bovine manure and delignified coffee grounds showed volatile solids conversion efficiency of up to 90% and net energy gains of approximately 25% compared to mono-digestion systems (Sumantri et al., 2022). Finally, the resulting digestate exhibited favorable characteristics for use as a biofertilizer, with C/N ratios ranging from 18 to 22, indicating an adequate balance between carbon and nitrogen for nutrient availability and agricultural application, and enabling the partial replacement of synthetic fertilizers (Zhang et al., 2020).

In the processing of coffee byproducts for the production of a commercial organic fertilizer, reported that the C/N ratio decreased from 12.92 in the raw material to 12.01 after several months of aerobic composting. Subsequently, after incorporating soil as a bulking agent and adding chemicals, including urea and phosphate, the final product reached a C/N ratio of 5.76. Similarly, the concentration of NH4+-N increased from 0.25 g kg⁻¹ after the soil incorporation stage to 8.47 g kg⁻¹ in the final product, equivalent to an approximate 34-fold increase. Therefore, these changes should not be attributed exclusively to composting but also to subsequent stages of fertilizer formulation. Furthermore, a low C/N ratio alone is not a sufficient indicator of compost quality or maturity; therefore, other agronomic and safety parameters must be considered. Several studies report that compost enriched with biochar improves nutrient retention and agronomic nutrient use efficiency, reducing agricultural fertilization costs by up to 40% (Yeoh and Ng, 2022; Dimitrijević et al., 2024). In parallel, the application of modified biochar as an adsorbent facilitated the removal of herbicides (dicamba, sulfonamides) from contaminated water with adsorption removal efficiencies greater than 85%, indicating the potential integration of waste recovery with environmental bioremediation strategies (Zhang et al., 2020; Guimarães et al., 2023; Vanyan et al., 2024). Recent literature also highlights the integration of coffee biorefineries, where multiple processes (pyrolysis, digestion, fermentation, and composting) are combined to maximize by-product recovery (Yeoh and Ng, 2022; Duarte et al., 2021).

Within this context, the circular bioeconomic approach has been described as a framework that transforms waste into various products, such as biogas, bioethanol, biochar, compost, and biofertilizers, thereby generating diversified value chains (Tian et al., 2021). In a life-cycle assessment of instant coffee production, Pazmiño analyzed different energy configurations using a cradle-to-gate approach and a functional unit of 1 kg of instant coffee at the plant exit. The scenario that incorporated a CCHP system, with energy recovery from used coffee grounds, resulted in a global warming potential of 16.90 kg CO₂-eq per unit of instant coffee, compared to 27.08 kg CO₂-eq in the baseline scenario, corresponding to a reduction of approximately 37.6%. This result depends on the energy scenario considered and the system boundaries established in the life cycle analysis (Pazmiño et al., 2024). These hybrid configurations have been proposed as promising options for coffee-producing regions in Latin America, where waste streams are continuously available. In addition, productive synergies have been reported, such as the simultaneous production of bioethanol and biodiesel through fermentation and transesterification in integrated reactor systems (Yeoh and Ng, 2022; Alabdalall et al., 2023; Huang et al., 2024). Moreover, iron-doped biocarbons derived from coffee residues have been explored as catalysts in advanced Fenton-type oxidation processes, broadening their potential industrial applications (Guimarães et al., 2023).

3.5 Environmental, energy, and socioeconomic impacts

The recovery of coffee waste generates environmental, economic, and social benefits, as documented in various studies (Tsigkou et al., 2025; Pazmiño et al., 2024). These benefits are primarily associated with the mitigation of negative environmental externalities and improvements in resource-use efficiency across coffee production systems. In addition, the integration of waste conversion technologies has been reported to contribute to reductions in energy costs, improvements in farm-level profitability, and the progressive adoption of bioeconomy-based solutions in coffee-producing regions (Ansari et al., 2025; Dawerasha et al., 2024). In this context, the reported impacts extend beyond isolated technical effects and encompass complementary dimensions of sustainability, including environmental, economic, and social aspects. Table 5 summarizes these impacts by incorporating environmental, economic, and social dimensions, illustrating how each dimension relates to the application of technologies such as pyrolysis, anaerobic digestion, and composting. This classification facilitates a comparative assessment of the positive impacts reported in field studies and technical evaluations, highlighting waste recovery as a supporting strategy for enhancing sustainability within the coffee sector.

Table 5. Quantitative multi-dimensional sustainability impacts of coffee waste recovery technologies.

Significant reductions in greenhouse gas emissions, primarily CO₂ and CH₄, have been reported, associated with fossil fuel substitution and controlled management of organic residues (Oliva et al., 2025; Kougioumtzis et al., 2024). The application of biochar to agricultural soils enhances physicochemical properties and reduces carbon footprint, as evidenced by life LCAs and comparative analyses (Zhang et al., 2025; Morais et al., 2023; Chin-Pampillo et al., 2021; Cervera-Mata et al., 2022; Matoso et al., 2023). Valorization of coffee residues is also associated with reductions in operational costs through the use of locally generated energy and additional income from value-added products (Yeoh and Ng, 2022; Matula et al., 2023; Huyen et al., 2025). Decentralized pyrolysis systems implemented in Ecuador and Brazil demonstrate reductions in agricultural drying energy costs ranging from 20% to 35%, through partial replacement of fossil fuels with locally produced biochar (Salgado et al., 2024; Heredia Salgado et al., 2021). Similarly, biogas production on coffee farms facilitates partial energy autonomy, reducing dependence on LPG and contributing to increases in rural net income (Hernández-Sarabia et al., 2021). These indicators support the financial and environmental viability of the evaluated technologies, particularly in rural contexts.

Social impacts include technology transfer, rural employment, and community adoption of valorization systems (Salgado et al., 2024; Ansari et al., 2025; Paes et al., 2023). Implementation of appropriate technologies on coffee farms promotes technical training, strengthens local capacities, and enhances acceptance of circular bioeconomy solutions (Tsigkou et al., 2025). Within a circular economy framework, the substitution of commercial adsorbents, synthetic fertilizers, and imported fuels with products derived from coffee residues represents a pathway to more self-sufficient bioindustrial systems, where waste streams are reintegrated into the production cycle (Tsigkou et al., 2025; Yeoh and Ng, 2022). This integrated approach positions coffee waste valorization as a strategy to strengthen environmental, energy, and socio-economic sustainability in coffee-producing regions.

3.6 Research trends and identified gaps

Although this review describes positive impacts associated with the main objective, several structural challenges remain, including heterogeneity in operating conditions, such as variations in reaction time, temperature, and pH, which hinder direct comparison of performance across studies (Tumusiime et al., 2022; Pasquet et al., 2024; Copik et al., 2023). Studies that incorporate LCAs exhibit significant methodological differences that limit their direct comparison. An attribution LCA approach was applied, using the treatment of 1 metric ton of coffee grounds (SCG) as the functional unit and a “cradle-to-grave” system boundary, taking into account substitution credits and avoided loads when comparing different recovery pathways (Rivera et al., 2020). In another study, energy scenarios for instant coffee production were evaluated using a functional unit of 1 kg of instant coffee at the plant exit and a “cradle-to-door” system boundary, assessing eight impact categories using the ReCiPe method (Pazmiño et al., 2024). Similarly, a “cradle-to-grave” framework was used to assess SCG pellets, considering a functional unit of 1 MJ of thermal energy and the allocation of energy impacts in systems with co-products, and establishing comparisons with biodiesel, bio-oil, and direct combustion (Hamedani et al., 2022). These methodological differences regarding functional units, system boundaries, treatment of byproducts, impact assessment, and reference scenarios limit the direct comparison of absolute environmental results across different studies.

Persistent gaps in technology transfer to rural communities have been identified (Tsigkou et al., 2025; Getachew et al., 2023; De Freitas et al., 2023). A quantitative capacity distribution across the 116 primary studies shows that 58.6% (68 studies) operate at laboratory scale below 5 kg h⁻¹, 27.6% (32 studies) correspond to pilot systems between 5 and 50 kg h⁻¹, and 13.8% (16 studies) reach demonstration or industrial capacity above 50 kg h⁻¹. This concentration at small scales limits the assessment of process performance under industrial conditions and constrains robust techno-economic evaluations, reflecting an ongoing research trend that remains distant from commercial implementation and validation under real operational conditions (Sousa et al., 2021; Drewnowski et al., 2025). Furthermore, public policy analyses in specific coffee-producing countries show limited integration of waste valorization within national bioeconomy and energy transition frameworks (De Freitas et al., 2023; Nurmalita et al., 2022; Kim and Oh, 2021).

Coffee valorization technologies face structural constraints that limit their adoption and scaling, particularly in producing regions with limited infrastructure and access to specialized equipment (Drewnowski et al., 2025). Installed capacity for operating thermochemical and biological systems is often insufficient, as these technologies require continuous process control, stable energy supply, and specialized equipment such as pyrolysis reactors, gas cleaning units, or instrumented biodigesters. Several studies consistently report that high implementation and operating costs remain major barriers to the widespread adoption of processes such as pyrolysis and anaerobic digestion (Tsigkou et al., 2025; Pazmiño et al., 2024; Tinoco-Caicedo et al., 2021). These costs are associated not only with initial investment but also with feedstock preparation, energy consumption, and specialized technical maintenance, affecting all technologies summarized in Table 6. The regulatory constraints identified are not uniform across producing countries and depend on both the jurisdiction and the type of product obtained. In Colombia, for example, the agricultural use of organic and digested products is supported by regulations such as NTC 5167 and Decree 1287 of 2014, which provide criteria for certain agricultural applications (Hernández-Sarabia et al., 2021). In Ecuador, however, the literature identifies a more specific gap regarding small-scale thermochemical recovery; existing provisions on the energy conversion of agricultural waste are primarily geared toward larger-capacity facilities and do not sufficiently cover pyrolysis systems that can be implemented by rural cooperatives. In this context, the need has been highlighted to develop specific criteria for small- and medium-scale facilities—those under 3 MWth—as well as for pyrolysis-derived products, particularly biochar (Heredia Salgado et al., 2021). Therefore, regulatory barriers should be understood as heterogeneous and specific to each country, technological scale, and product category, rather than as a generalized absence of regulation. In parallel, limited technical training in rural areas and low levels of technology transfer continue to hinder the development of pilot projects and industrial partnerships (Carneiro et al., 2025; Salgado et al., 2024). These structural, economic, and regulatory constraints explain the concentration of evidence at laboratory and pilot scales and the slow transition toward demonstrative and industrial applications, particularly in the Latin American context. Table 6 summarizes these limitations and organizes the main obstacles identified in recent literature.

Table 6. Main gaps or limitations in the recovery of coffee waste.

Source: Prepared by the authors based on gaps identified in the reviewed literature (2020–2025).

The valorization of coffee waste through interconnected thermochemical and biological processes enables the generation of high-value energy, agricultural, and industrial products (Tian et al., 2021), such as biochar and fuels with elevated calorific value and adsorption potential (del Pozo et al., 2021). The main recovery pathways comprise slow pyrolysis, typically operated between 400 and 600 °C; mesophilic anaerobic digestion at 35–37 °C; and controlled alcoholic fermentation at approximately 30 °C and pH 4–6, applied to coffee mucilage and pulp for bioethanol production or sustainable fermented beverages (Kc et al., 2021; Terán-Rivera et al., 2024; Jeńıček et al., 2025; Islam et al., 2024; Rosson et al., 2020). Integration of these technologies within circular biorefinery frameworks allows simultaneous production of biochar, biogas, compost, and adsorbents, contributing to reductions in greenhouse gas emissions and leachate generation (Drewnowski et al., 2025; Pérez-Merchán et al., 2025a,b). Coffee pit-derived biochar exhibits high adsorption capacity for contaminants and can partially replace commercial activated carbon (Zhang et al., 2020; Araya-Sibaja et al., 2025; Moreno et al., 2026), while its combination with plant growth-promoting bacteria enhances soil fertility and supports nutrient cycling (Dimitrijević et al., 2024). Integrated digestion–pyrolysis systems further facilitate renewable energy recovery in the form of biogas and bioethanol (Duarte et al., 2021; Torboli et al., 2024). Such synergies strengthen local income generation and energy autonomy for small producers by reducing dependence on fossil fuels and synthetic fertilizers (Hernández-Sarabia et al., 2021). Figure 5 illustrates the main transformation routes and their integration within a circular economy model. The reviewed studies indicate that coffee waste can be utilized as a secondary raw material for the production of energy, agricultural products, and materials; however, the extent of these benefits depends on the technology applied, the characteristics of the waste, and the implementation conditions.. Future challenges involve scaling these technologies through cooperative networks and regional valorization policies, considering the structural limitations summarized in Table 6, including infrastructure, regulatory frameworks, and mechanisms for technology transfer that enable implementation under real operational conditions (Santos et al., 2007; Tinoco-Caicedo et al., 2021).

The overall analysis confirms that coffee waste valorization constitutes a viable and sustainable technological pathway for mitigating environmental impacts and reinforcing circular economy principles within the coffee sector. An analysis of the included studies reveals a recurring use of both biological and thermochemical pathways for the utilization of coffee waste. Among the most studied technologies are anaerobic digestion, pyrolysis, fermentation, co-digestion, composting, and gasification. Since some articles evaluate more than one technology in a single study, these pathways should not necessarily be interpreted as mutually exclusive categories. Consequently, their frequency should be considered an indicator of the presence of each technology in the reviewed literature, rather than a single percentage distribution of the total set of studies. Regarding the products obtained, these alternatives enable the production of biogas, biochar, bioethanol, compost, and other value-added materials, although their environmental and energy performance depends on the waste used, the operating conditions, and the methodological limitations established in each study. The integration of different pathways into biorefinery schemes offers the potential to improve energy and material utilization, but their benefits must be evaluated using comparable indicators and clearly defined reference scenarios. Among the configurations evaluated using life cycle analysis, the integration of a CCHP system that recovers energy from used coffee grounds reduced global warming potential by approximately 37.6% compared to the baseline scenario, using 1 kg of instant coffee at the plant outlet as the functional unit (Pazmiño et al., 2024). Additionally, applying a circular bioeconomy approach allows the valorization of a single residue into multiple value-added products. Positive impacts were observed across three dimensions: greenhouse gas mitigation, rural employment generation, and substitution of fossil-based inputs. Nevertheless, substantial barriers persist, including high implementation costs, limited infrastructure, and insufficient technological maturity, which continue to restrict industrial-scale deployment in Latin America. Therefore, strengthening local technical capacities, fostering institutional coordination, and establishing enabling frameworks remain essential conditions to translate laboratory- and pilot-scale achievements into sustainable production-scale applications.

4 Discussions

The technical performance of individual recovery pathways depends on the physicochemical composition of each coffee residue fraction, process parameters, and operational scale. Biological routes such as anaerobic digestion and fermentation achieve high volatile solids conversion efficiency when treating high-moisture, carbohydrate-rich substrates like coffee pulp and mucilage, where moisture levels above 60% and low recalcitrance facilitate rapid enzymatic hydrolysis (Tumusiime et al., 2022; Oliva et al., 2025; Kc et al., 2021). However, monodigestion of coffee pulp or spent grounds induces operational instability due to methanogenic inhibition caused by polyphenols, caffeine, and unbalanced nitrogen stoichiometry (Getachew et al., 2023; Drewnowski et al., 2025). Co-digestion with livestock manure or inoculation with specialized microbial consortia resolves these kinetic bottlenecks by stabilizing system pH and adjusting the carbon-to-nitrogen ratio to the optimal 20 to 30 range, increasing biomethane yields up to 3.6-fold compared to unadjusted monodigestion (Pasquet et al., 2024; Hamedani et al., 2022; Nieto et al., 2023; Sinclair et al., 2024).

In contrast, thermochemical conversion via slow pyrolysis and gasification is optimal for dry, recalcitrant lignocellulosic fractions such as coffee husks, parchment, and dried spent grounds (Tsigkou et al., 2025; del Pozo et al., 2021; Kim and Oh, 2021). High fixed carbon and lignin contents promote stable biochar mass yields between 25% and 40% with elevated calorific values (Zhang et al., 2020; Gonzalez-Piedra et al., 2021; Huyen et al., 2025). Processing wet spent grounds or raw pulp through pyrolysis introduces a severe net energy loss due to the enthalpy required for water evaporation prior to thermochemical reactions (Hamedani et al., 2022; Ko et al., 2025). This energetic penalty explains why direct pyrolysis or solvent extraction of wet spent grounds yields lower net process energy efficiency unless integrated within multi-product biorefinery configurations (Yeoh and Ng, 2022; Rivera et al., 2020).

Discrepancies in reported performance metrics across published literature originate from variations in process parameters, including peak temperature, residence time, heating rate, and feedstock pre-treatments (Bartolucci et al., 2024; Copik et al., 2023). For instance, elevating pyrolysis temperatures from 350 to 600 °C reduces solid char mass yield while increasing fixed carbon concentration, specific surface area, and pore volume (Lataf et al., 2022; Ko et al., 2025).

Figure 6 structures these technological selection criteria into a decision tree, differentiating wet from lignocellulosic fractions and linking feedstock properties to specific recovery pathways. The diagram establishes quantitative operational thresholds, such as moisture limits below 10% for gasification and above 60% for anaerobic biodigesters, providing a systematic basis for process selection (Hernández-Sarabia et al., 2021; Salgado et al., 2024; Heredia Salgado et al., 2021).

Regional disparities in technology adoption reflect local economic constraints, infrastructure availability, and investment capacity (Salgado et al., 2024; Heredia Salgado et al., 2021). High-capital technologies like hydrothermal carbonization and catalytic activation dominate research in European and Asian institutions equipped with advanced facilities (Bartolucci et al., 2024; Tian et al., 2021; Kim and Oh, 2021). In contrast, Latin American coffee systems emphasize low-complexity configurations, including farm-scale biodigesters and decentralized pyrolysis units that reduce thermal drying costs by 20% to 35% through fossil fuel substitution (Hernández-Sarabia et al., 2021; Salgado et al., 2024; Heredia Salgado et al., 2021). Expanding commercial adoption requires overcoming technical barriers through standardized quality metrics, biofertilizer certification frameworks, and comprehensive life-cycle assessments (Pazmiño et al., 2024; Rivera et al., 2020; Rosson et al., 2021).

5 Conclusions

This systematic review shows that the valorization of coffee waste is a promising alternative for recovering energy, materials, and products of agricultural interest from various waste streams. The technologies analyzed—including anaerobic digestion, pyrolysis, fermentation, and composting—have demonstrated the potential to generate products such as biogas, biochar, bioethanol, compost, and other value-added materials. However, the available evidence does not allow us to conclude that these alternatives are universally viable or sustainable under all conditions. Their performance depends on the type of waste, the scale of the process, operating conditions, available infrastructure, implementation costs, and the environmental and regulatory context. The coffee supply chain generates significant amounts of waste at various stages of processing and consumption, including pulp, husk, mucilage, silver skin, and used coffee grounds. Because these waste streams have different generation rates, moisture content, and compositional characteristics, it is not appropriate to express their production as a single percentage applicable to the entire supply chain. Their recovery for use as energy, fertilizers, and functional materials represents an opportunity to reintegrate resources into more circular production systems. The technologies analyzed—including anaerobic digestion, pyrolysis, fermentation, and composting—demonstrate the technical feasibility of producing biogas, biochar, biofertilizers, and biofuels while maintaining favorable environmental performance. The reviewed studies report environmental, energy, and agronomic benefits associated with different recovery alternatives; however, the magnitude of these benefits varies depending on the waste used, the technology applied, the experimental conditions, the scale of the process, and the baseline scenario. Therefore, the quantitative values obtained in individual studies should be interpreted within their specific conditions and should not be extrapolated as general results for the entire coffee waste recovery sector.

These findings suggest that coffee waste valorization not only offers environmental benefits but also provides practical solutions for local agriculture and energy generation. Future opportunities emerge along two complementary dimensions: (i) the consolidation of public policies, economic incentives, and specific regulations to support the scaling up of recovery technologies, and (ii) the advancement of research focused on the design of modular biorefineries, comprehensive life cycle analyses, and the development of value-added products from coffee waste. Addressing these areas is critical to establishing sustainable, replicable, and economically viable recovery strategies, positioning coffee waste valorization as a strategic element for enhancing efficiency and promoting circular bioeconomy principles in the coffee sector.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/frsus.2026.1947220/full#supplementary-material

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Footnotes

Edited by: Nabajyoti Saikia, Kaziranga University, India

Reviewed by: Sourav Poddar, Haldia Institute of Technology, India

Jairo Smith Triviño Pineda, Colegio Cooperativo La Presentacion, Colombia