Dry Anaerobic Digestion of Selectively Collected Biowaste: Technological Advances, Process Optimization and Energy Recovery Perspectives

Abstract

Given the increasing volume of selectively collected bio-waste and the requirement to increase waste treatment system energy efficiency, dry anaerobic digestion (DAD) represents a more sustainable choice for the treatment of municipal organic fraction instead of conventional technologies. The current paper provides an overview of the existing knowledge on DAD of green waste or kitchen waste collected selectively. Key substrates characteristics (chemical composition, methane potential), novel reactor design and process conditions relevant to effective digestion at elevated dry matter content are considered. Of special interest is the process intensification techniques, impact of contamination and co-fermentation opportunity with other biodegradable wastes. This article also discusses energy and economic performance of DAD plants and puts their environmental burden in perspective versus other bio-waste treatment processes. The current legislation and DAD’s role in the circular economy are also considered. Selectively collected biowaste has significant energy potential and dry anaerobic digestion is an effective technology, especially in areas with limited water availability, offering both waste volume reduction and minimized energy losses. The aim of this work is to introduce the potential of this technology as a sustainable option within the context of renewable energy and modern waste management.

1. Introduction

Energy recovery from waste (EfW) is a crucial component of modern waste management systems, particularly in line with the principles of the circular economy. Waste-to-Energy (WtE) technologies convert waste into electricity, heat or fuels. Their goals are twofold: to reduce waste volumes and to recover energy and materials. The main WtE technologies are combustion, gasification, pyrolysis, plasma treatment, anaerobic digestion and landfill-gas recovery [1,2].

Anaerobic digestion deserves particular attention. It is efficient and consistent with the Sustainable Development Goals [3,4,5,6,7]. The process converts the organic fraction of waste into biogas. The biogas can generate electricity and heat or be upgraded to biomethane. The residual digestate can be used as fertilizer, which strengthens its relevance for a circular economy [8,9,10]. Anaerobic digestion is a mature technology and is already widely applied, for example, to sewage sludge and municipal biowaste. Thermal technologies are often deployed alongside it within EfW systems.

The most common thermal route is waste incineration with energy recovery. Incineration cuts waste volume and produces energy. It also causes irreversible loss of material resources and can meet public resistance [11,12,13]. Gasification is an alternative. It transforms waste into synthesis gas (syngas). Plasma gasification is its most advanced form. The process exposes waste to plasma, an ionized gas at several thousand degrees. It almost completely decomposes organic and inorganic matter, suppresses dioxin formation, and yields a high-energy, nearly contaminant-free syngas. The gas can be burned or upgraded to hydrogen or synthetic fuels [14,15]. Pyrolysis is another thermochemical route. It proceeds without oxygen and splits waste into gas, oil, and a solid fraction [16]. The method is used mainly for plastic and tire waste. Operational constraints and the need to refine pyrolysis products still limit its wider uptake. Landfill-gas recovery captures methane produced during anaerobic decay of organic waste in landfills [17,18]. Although the energy yield is modest, the approach offers supplemental power and cuts greenhouse-gas emissions.

Recent studies show that thermochemical technologies-pyrolysis, gasification, and incineration—are increasingly configured for co-production of several outputs, such as bio-oil and biochar. This design improves economic performance and lowers energy losses along the entire processing chain [2]. There is also a growing trend toward integrating EfW processes with CCS and CCU technologies. Carbon Capture and Storage (CCS) and Carbon Capture and Utilization (CCU) aim not only to reduce CO₂ emissions but also to use the captured CO₂ for the production of synthetic fuels or chemicals. Technology selection must consider the specific properties of the waste stream. Key factors include moisture content, presence of inorganic compounds and susceptibility to thermal degradation. The goal of Waste-to-Energy (WtE) is not limited to waste disposal. It also aims to convert the energy potential of waste into useful forms such as heat and electricity (Figure 1).

For biodegradable waste, anaerobic digestion is a particularly effective solution. It enables energy recovery in the form of biogas and produces digestate, which can be used for agricultural purposes. But in response to the diverse nature of biowaste, new approaches to anaerobic digestion are being developed. One such method is dry anaerobic digestion, applied where conventional wet systems are inadequate or economically unfeasible. This technology aligns well with the principles of sustainable waste management. It enables energy recovery, reduces greenhouse gas emissions and allows for the use of digestate as fertilizer [19,20,21,22].

This article aims to present the current state of knowledge on dry anaerobic digestion of selectively collected organic waste, with a focus on technological, energetic, economic and environmental aspects.

2. Characterization and Pretreatment of Organic Waste for Dry Anaerobic Digestion

2.1. Quantitative Aspects of Selective Biowaste Collection in Municipal Waste Management

In the European Union, biowaste is primarily composed of food and garden waste. It constitutes a significant portion of municipal solid waste, accounting for approximately 36%, as shown in Figure 2.

Annually, the European countries generate between 118 and 138 million tons of municipal biowaste. However, only about 40% of this amount, roughly 47 million tons per annum (tpa), is separately collected and processed into high-quality compost and digestate [24]. Of this, 38 million tons were generated in the EU27, with 70% sent for composting and 30% sent for anaerobic digestion [25]. Differences in biowaste management across EU Member States are notable. Countries such as Denmark, the Baltic states, and France exhibit limited separate collection of food waste, focusing instead on garden waste. In contrast, Belgium, the Netherlands, Austria, and Germany, which are leaders in biowaste recycling, employ systems for the commingled collection of food and garden waste. However, within these systems, the Netherlands and Belgium often exclude meat and fish. Germany and Austria, by contrast, target all food waste and provide kitchen caddies for household use. Commingled collection systems without compostable liners tend to capture only 10–30 kg of food waste per capita annually. More advanced models, widespread in parts of Norway, Italy, the UK, and Spain, focus exclusively on food waste, using small containers with compostable liners and vented caddies to enhance usability and reduce moisture. These systems achieve significantly higher capture rates, typically between 60–100 kg per capita per year, by avoiding garden waste collection and promoting home composting [26]. The separate collection rates vary across Europe, from 80% or more in Austria and Slovenia to less than 10% in Bosnia-Herzegovina, Cyprus, Portugal, Spain, or Turkey [23]. For instance, Germany leads with over 14 million tons of bio-waste collected annually, followed by the United Kingdom with 8.9 million tons, Italy with 6.5 million tons, and France with 4.6 million tons. Conversely, some countries have yet to implement comprehensive separate collection systems, resulting in lower capture rates. In Poland, biowaste, including both kitchen and green waste, constitutes the largest fraction of the municipal waste stream. In 2023, its share reached 37.4%, exceeding the EU average of 34% [27]. Experts estimate that biowaste may account for up to two-fifths (40%) of all collected municipal waste, amounting to over 4 million tons. Potentially, each Polish citizen may generate 247 kg of biowaste annually, including 112 kg of food waste and 135 kg of green waste [28]. This dominant position of biowaste in the municipal waste stream in Poland highlights the crucial role of its effective management in achieving national and EU recycling targets. Poland anticipated the EU mandate on the implementation of separate biowaste collection. According to the National Waste Management Plan 2022 (KPGO 2022) and the Regulation of 29 December 2016, all municipalities in Poland were required to implement a unified system for the separate collection of biowaste by 2021 at the latest [29]. In accordance with this regulation, a uniform waste segregation system was introduced, establishing a five-container separation scheme, including a dedicated brown container intended for biowaste. In 2023, the average Polish citizen generated 54 kg of biodegradable waste. By comparison, in 2020, this amount was 42 kg. In total, 2042.6 thousand tons of biowaste were separately collected in 2023, representing the highest recorded volume up to date. The key indicators characterizing biodegradable waste management in Poland are summarized in

Table 1. Trends in the separate collection of biodegradable waste in Poland [6,8].

Year	Total Mass of Collected Municipal Waste (Thousand Tons)	Amount of Municipal Waste Generated per Capita (kg/a)	The Mass of Separately Collected Biodegradable Waste (Thousand Tons)	Relative Share of Separately Collected Biodegradable Waste to the Total Municipal Waste Stream (%)	The Amount of Biodegradable Waste Collected per Capita (kg/a)	Proportion of Separately Collected Biowaste in Relation to the Total Biodegradable Waste Generated (%)
2015	10,863.5	283	657.0	-	-	25.9
2020	13,116.9	344	1610.0	12.3	42	32.4
2021	13,673.6	360	1843.0	13.5	49	33.9
2022	13,420.3	355	1913.6	14.3	51	35.7
2023	13,447.9	357	2042.6	15.2	54	37.4

. These include the total mass of collected municipal waste (in thousand tons), as well as the amount of municipal waste generated per capita. The table also presents the mass of separately collected biodegradable waste (in thousand tons), along with its share relative to the total municipal waste stream (expressed as a percentage). Furthermore, the data illustrate the per capita amount of biodegradable waste collected (in kg/year), as well as the proportion of separately collected biowaste in relation to the total biodegradable waste generated. These indicators provide a valuable insight into the progress and effectiveness of biowaste source separation and collection practices. However, when compared to the potential for biowaste generation, a significant gap still remains. This indicates that the current collection systems and public engagement are still ineffective, leading to the loss of valuable organic material, which most likely ends up in mixed waste.

In 2024, the Institute of Environmental Protection—National Research Institute (IOŚ-PIB) conducted a comprehensive nationwide study on the quantity of separately collected and recycled municipal biowaste. The results provide key data for fulfilling Poland’s obligations towards the European Union in the area of waste management [30]. The study was carried out following the methodology set out in Annex II to Commission Implementing Decision (EU) 2019/1004 and covered three research groups: collaboration between IOŚ-PIB and municipalities, research conducted by an external entity and research carried out by private individuals between January and December 2024. The findings indicate that the average annual amount of source-separated kitchen biowaste recycled by Polish residents using home composters amounted to 97 kg per person per year. Additionally, an average of 1.92 kg of green biowaste per 1 m² of green area belonging to a household was also composted in home composters [31]

2.2. Biochemical Characteristics of Organic Waste Streams and Their Potential for Anaerobic Digestion

Separately collected biowaste constitutes a key substrate for biogas production through anaerobic digestion. Its physicochemical properties, such as dry matter content, carbon-to-nitrogen (C/N) ratio and biochemical composition determine the efficiency of the process and the final methane potential [32]. Kitchen waste shows the highest biogas production efficiency among all tested substrates. Cooked rice achieved a record yield of 2821 ± 31.03 mL of biogas per gram of volatile solids (VS) over 28 days, with a methane content of 62.8% [33]. Cooked beans yielded 983 ± 10.81 mL/g vs. in 11 days, while cabbage reached 800 ± 8.8 mL/g vs. in 10 days [33]. A key factor in the success of kitchen waste is the optimal carbon-to-nitrogen (C/N) ratio. Cooked rice had a C/N ratio of 30.9 and a pH of 7.2, which falls within the optimal range for the methanogenesis process. In contrast, cabbage, with a low C/N ratio (13.9) and a pH of 6.2, showed a tendency for rapid acidification, which limited its biogas potential [33]. Green waste, represented primarily by grass, demonstrates stable performance in the dry fermentation process. Fresh grass achieved a biogas yield of 628.9 mL/g VS, while dried grass yielded 565.9 mL/g VS, and ensiled grass 573.4 mL/g VS. The differences in yield result from preservation methods, drying led to an 11.5% reduction in yield, while ensiling caused only a 10.4% decrease [34]. This results, higher than expected by authors, can be explained by considering that the feedstock was loaded in the reactors briefly after being harvested with the highest availability of organic matter. In general, green waste, rich in lignocellulose exhibits lower biodegradability (approximately 52%) and a methane potential. A significant advantage of green waste is its stable lignocellulosic structure, which provides suitable physical support for the fermentation process and relatively consistent availability throughout the year. The methane content in biogas from grass is approximately 60%, which is satisfactory for energy applications [34,35,36]. Food industry waste is characterized by varying biogas potential depending on its origin and composition. In Polish studies, apple pomace showed the highest biogas yield per ton of fresh matter among all tested industrial waste [37]. Waste potatoes and residue from fruit filtration also achieved significant results, with the retentate exhibiting high biogas output despite its low dry matter content [37]. Mixed fruit and vegetable waste, when co-digested with dairy industry waste, achieved a yield of 720 mL of biogas per g VS, with a methane content of 436 mL/g VS. The process maintained a stable pH and a balanced C/N ratio, which ensured efficient conversion of organic matter [38]. Mixtures of wastes that optimize the C/N ratio demonstrate high biogas potential [39,40,41,42,43]. In addition supplementation with trace elements reduces inhibition caused by volatile fatty acids and indicates the enhance of operation stability during anaeraobic digestion [44]. The dry anaerobic digestion process, designed for substrates with a high dry matter content (>15–20%), enables efficient biogas production while minimizing the generation of liquid waste [45]. It should be noted, however, that lignocellulose poses a challenge in the dry fermentation process due to its recalcitrant structure. Corn straw, after ethanol production, achieved a biogas yield of 330 mL/g vs. with a methane content of around 250 mL/g over 8 days of fermentation. The process required pH adjustment to 7.5 and a temperature of 37 °C. Lignocellulosic biomass without pretreatment shows yields ranging from 200 to 600 m³/kg of volatile solids, which is significantly lower compared to other types of waste [46].

Table 2. A comparison of selected biowaste types in terms of their biogas potential in the dry fermentation process.

Waste Type	Biogas Yield (mL/g VS)	Methane Yield (mL/g VS)	Fermentation Time (days)	C/N Ratio	pH	Experimental Parameters	Source
Kitchen waste
Cooked rice	2821	~1770	28	30.9	7.2	Lab-scale batch BMP (1 L serum bottles), mesophilic 37 °C, TS ≈ 8%, S/I 1:1 (w/w)
Cooked beans	983	~590	11	-	-		[33]
Cabbage	800	~480	10	13.9	6.2
Green waste
Fresh grass	639.7	~384	-	-	-	Lab BMP, mesophilic 37 °C; freshly harvested; TS ≈ 20%
Dried grass	565.9	~339	-	-	-	Lab BMP, mesophilic 37 °C; sun-dried; TS ≈ 90%	[34]
Ensiled grass	573.4	~344	-	-	-	Lab BMP, mesophilic 37 °C; ensiled 60 d; TS ≈ 30%
Agrofood industry
Fruit-vegetable mix	720	~436	-	-	-	Batch co-digestion test with dairy sludge; mesophilic 35 °C; TS ≈ 12%	[38]
Corn straw	330	~250	8	-	7.5	Batch BMP, mesophilic 37 °C; alkaline-adjusted; TS ≈ 25%	[46]

presents a comparison of selected biowaste types in terms of their biogas potential, while

Table 3. Biowaste and its suitability for the dry fermentation process.

Waste Type	Dry Matter Content (%)	Optimal Temperature (°C)	Retention Time (Days)	Advantages	Challenges	Experimental Parameters	Source
Kitchen waste	8–33	35–40	15–30	High biodegradability, easy availability	Low carbon content, possible acidification	Mesophilic 35–37 °C; TS adjusted to ≥15%	[33]
Green waste	20–40	35–40	20–35	Stable structure, good biodegradability	Seasonal availability	mesophilic 37 °C; batch mode	[34]
Food industry	10–30	35–40	15–25	Diverse organic composition	Variable chemical composition	mesophilic 35 °C; codigestion	[37]
Lignocellulose	85–95	35–55	25–45	Low water content, easy storage	High biodegradation resistance	mesophilic 37 °C; pH adjusted 7.5; TS 25–30%	[46]

presents types of biowaste in terms of their suitability for the dry fermentation process.

Such a comparison enables an informed selection of substrates for dry fermentation, optimizing the process in terms of biogas production efficiency and operational stability. In practice, the best results are achieved through co-digestion of kitchen biowaste with green or industrial waste, which allows for obtaining an optimal C/N ratio and improving the physicochemical properties of the substrate [16]. Kitchen waste, green waste and food industry waste represent promising sources of biomass, whereas lignocellulose requires the application of pretreatment techniques.

2.3. Pre-Treatment Methods for Organic Waste Prior to Anaerobic Digestion

Pre-treatment of substrates used in anaerobic digestion represents a critical step for enhancing the overall efficiency of the process. The application of appropriately selected physical, chemical or biological techniques facilitates the intensification of biochemical transformations, reduction of hydraulic retention time (HRT) and an increase in methane yield [47,48,49,50,51]. The main objectives of pre-treatment include improving substrate accessibility for microorganisms through disruption of cellular structures, enhancing the solubilization of organic compounds (soluble chemical oxygen demand, SCOD), accelerating the fermentation process, increasing methane production efficiency, and improving process stability while reducing the presence of pathogens and inhibitory substances.

Biomass pretreatment strategies vary substantially with the anaerobic digestion mode: dry anaerobic digestion (DAD, 20–40% total solids) or wet anaerobic digestion (WAD, 2–10% total solids) [52]. In dry anaerobic digestion (DAD), conducted at a high total solids content, a key factor is enhancing the availability of organic compounds to microorganisms by breaking down the lignocellulosic structure and increasing the surface area of contact between the substrate and the microbial biomass [53,54,55]. Of particular significance are substrates with high lignin content, such as straw, grasses, garden waste, paper, and the organic fraction of municipal solid waste (OFMSW), which, in their raw form, exhibit low biodegradability. Therefore, their pretreatment is a prerequisite for the efficient course of dry anaerobic digestion (DAD). Among the most effective pretreatment methods applied in DAD are primarily mechanical and alkaline treatments. Mechanical size reduction (milling, micronization, ball milling) significantly increases the reaction surface area and improves the accessibility of cellulose and hemicellulose. Chemical treatment with alkaline solutions (e.g., NaOH, Ca(OH)₂, NH₃) leads to partial lignin solubilization and improves the C/N ratio, thereby enhancing methanogenic activity. These methods are particularly effective under low-water conditions typical of DAD. Thermal and thermo-alkaline methods can further improve efficiency, but they require control of inhibitor formation. Under DAD conditions, acid pretreatment should be avoided due to the risk of methanogenic inhibition. Biological methods, while environmentally friendly, are generally too slow for technical applications [53,54]. In wet anaerobic digestion (WAD), the presence of a liquid phase and the ease of mixing place greater emphasis on methods that enhance the solubility of organic matter—such as hydrothermal, acid, or enzymatic treatments. Miscible, liquid substrates (e.g., slurry, wastewater) do not require aggressive size reduction, and the process environment is more tolerant to pH fluctuations and the presence of inhibitors. In contrast, dry anaerobic digestion (DAD) requires a more advanced and selective pretreatment strategy than WAD, due to the limited availability of water and the more challenging conditions for biomass degradation.

The efficiency of dry anaerobic digestion (DAD) is highly dependent on the method of substrate pretreatment. A well-designed pretreatment strategy—taking into account the physicochemical properties of the feedstock and the process constraints specific to DAD—is essential to ensuring stable plant operation and achieving high methane yields.

Table 4. Overview of the most commonly applied pre-treatment methods for selectively collected waste prior to dry anaerobic digestion.

Method	Mechanism	Description	Source
Mechanical shredding	Increased specific surface area, rupture of cell walls, shorter diffusion path for enzymes, substrate homogenization, removal of contaminants (glass, metals)	Shredding to <10 mm, often using hammer or knife mills.	[47,56,57,58,59]
Alkaline treatment	Deacetylation and loosening of lignin fibers, increase in soluble sugars.	Biomass treated with alkaline solution (e.g., NaOH, Ca(OH)2).	[60,61,62]
Hydrothermal hydrolysis	Decomposition of hemicellulose into simple sugars, partial degradation or redeposition of lignin.	Biomass decomposition in hot pressurized water without chemical additives.	[63,64,65]

provides an overview of the most commonly applied pre-treatment methods for selectively collected biowaste.

The selection of pre-treatment methods is based on their suitability for both the characteristics of the biowaste fraction and the operational conditions of dry anaerobic digestion. The selectively collected biowaste fraction exhibits considerable morphological variability, comprising both soft organic residues (e.g., vegetable and fruit scraps) and fibrous components (e.g., leaves, stems, peels, grass). This heterogeneity necessitates the application of complementary pre-processing strategies to effectively enhance biodegradability and process efficiency.

Among the pre-treatment methods for biowaste, mechanical techniques are of particular importance. These include ball mill grinding, flail milling, abrasion in flow conditions and screw extrusion. They operate on the principles of impact, shearing or compression and are especially effective in dry anaerobic digestion, where substrates are denser and less susceptible to natural degradation. Mechanical extrusion disrupts the structural integrity of materials through high pressure and shear force, enhancing methane yield without the use of chemical additives. Mechanical pre-treatment plays a key role in improving anaerobic digestion performance. By reducing particle size, it increases the surface area available for microorganisms, facilitates lignocellulosic breakdown and accelerates the hydrolysis phase. Heller et al. [59] compared several mechanical techniques for plant-based substrates, including ball milling, flail mowing and cross-flow grinding. The highest methane yield (279 L CH₄/kgVS) and the shortest lag phase were achieved with the ball mill, but at the highest energy cost. In contrast, the flail mower offered a positive energy balance, despite slightly lower efficiency. Vigueras-Carmona et al. [56] demonstrated that excessive micronization (below 500 µm) may lead to excessive volatile fatty acid (VFA) accumulation, causing acidification and inhibition of methane production. The authors identified an optimal particle size range of 500–1000 µm as a compromise between increased microbial accessibility and process stability. The study emphasized that while size reduction improves contact area and hydrolysis rate, too fine particles can result in VFA build-up and methanogenesis inhibition. Mozhiarasi et al. [58] showed that screw extrusion of vegetable, fruit and floral waste can increase methane yield by up to 40% by improving substrate availability. However, they also highlighted the high energy and capital costs of the technology which may limit its large-scale application. In the study by Vlierbergh et al. [60], alkaline treatment with CaO (60 g/kg TS) was applied to pre-treat oat and rye biomass for anaerobic digestion. While this approach effectively preserved biomass without reducing methane potential, it led to significant organic matter losses (18.1% for oats, 9.0% for rye) and butyric acid accumulation due to early-stage fermentation and pH drop. The method did not enhance fiber solubility or methane yield, which was attributed to insufficient exposure time to alkaline conditions. Khor et al. [61] assessed low-temperature (10 °C) alkaline pre-treatment using Ca(OH)₂ for lignocellulosic biomass (grass, corn straw, and sprout stems). The highest methane increase was observed for grass, with 7.5% Ca(OH)₂ treatment for 20 h resulting in a 37.3% yield improvement. Additional screw extrusion prior to chemical pre-treatment further enhanced methane production by 15.2% for grass, 11.2% for corn straw and 8.2% for sprout stems. Soluble COD increased and organic matter conversion efficiency improved. The process energy balance was favorable, the energy recovered from additional methane was about six times greater than the energy required for pre-treatment. Gueri et al. [62] investigated co-digestion of food waste and grass pre-treated with 3% NaOH for 12, 24, and 48 h. BMP tests conducted over 31 days showed the best results with 12-h treatment, lignin content decreased by 40.95%, and the C/N ratio reached the optimal value of 25. Methane yield increased by 5% and substrate conversion rate by 25%. The treatment improved biomass degradation, process stability and overall digestion efficiency, confirming its value as a supporting method for lignocellulosic biomass processing. In the study by Sato et al. [63], hydrothermal pre-treatment of rice straw was performed at 140 °C for 1 h with NaOH additions of 0%, 3%, and 5% (w/w). The treatment resulted in significant hemicellulose solubilization and partial lignin degradation, increasing cellulose content in the residual solid fraction. Biogas yield increased from 23.9 L/kg (raw straw) to 57.1 L/kg (hydrothermal only), 95.8 L/kg (with 3% NaOH) and 108.8 L/kg (with 5% NaOH). NaOH addition proved more effective than temperature alone; however, higher alkali doses led to elevated concentrations of fermentation-inhibiting by-products (e.g., furfural). The highest biogas production was achieved with 5% NaOH, although better process stability was observed at 3%.

Dry anaerobic digestion requires the application of pre-treatment techniques, particularly for substrates with low inherent biodegradability such as lignocellulosic materials. Among available technologies, mechanical methods are the most widely used due to their simplicity, absence of chemical additives and ease of integration with fermentation systems. Physical and chemical methods, while highly effective, are often associated with high energy demands or operational costs. In practice, integrated approaches combining multiple mechanisms tend to deliver the best outcomes by maximizing substrate accessibility for methanogenic microorganisms. Comparative analyses of pre-treatment effectiveness show that performance depends on substrate morphology, solids content and technical capabilities of the digestion facility.

3. Key Technological Aspects of Dry Anaerobic Digestion

Dry anaerobic digestion (DAD) is a relatively new technology that allows the conversion of biomass with a high dry matter content under anaerobic conditions. This type of fermentation is particularly effective with smaller digester volumes. The process is also characterized by ease of maintenance and no or limited mixing, which reduces the costs of conversion [66]. In addition, DAD installations can operate at high organic loading rates (OLRs) [67]. However, this technology still faces some challenges. There include difficulties with homogenization resulting from the high solids content, which is usually in the range of 15–40% of the waste volume. Another challenge is often the lower methane content in biogas, which has a significant impact on the efficiency of the process. Limited contact between feedstocks and microorganisms hinders biochemical transformations [47]. Current challenges, but also benefits from biomass conversion, provide an incentive to optimize the dry fermentation process and thoroughly understand its basics.

3.1. Overview of Used Digesters and Feeding/Unloading Techniques

The key element of the biogas production installation is the digester. Due to the type of reactor used, we can distinguish batch reactors—where the substrate is loaded once and reactors for continuous and semi-continuous fermentation, where the feedstock is dosed regularly

Batch reactors represent the simplest technological configuration for DAD. In this setup, feedstocks are introduced into the reactor once, and the fermentation process typically lasts from several to dozen days. Although these systems generally lack mechanical mixing, percolate recirculation is frequently employed to enhance the availability of nutrients for microbial activity [68]. The batch mode is undoubtedly characterized by its ease of use compared to continuous and semi-continuous fermentation systems. In batch reactor systems, the total solids (TS) content typically ranges from 20% to 40%, which corresponds to the upper limit of solids concentration in AD. Under these conditions, methane production generally falls within the range of 0.2 to 0.5 m³ per kilogram of volatile solids (VS) [55]. Many studies are based on fermentation in batch reactors. However, the key aspects are the selection of appropriate volume ratios of substrates, especially important in co-digestion, and the adjustment of optimal parameters, including retention time. In their research, Sganzerla et al. [69] used a 6.8-L batch reactor for dry fermentation of brewery grains for a period of 40 days. The batch filled 60% of the volume of the substrate mixture consisting of waste, inoculum and water. The remaining 40% was the space intended for biogas production. Another study using batch reactor was conducted by Zhen et al. [70] to optimize methane production from maize straw and cattle manure. The authors analyzed values such as dry matter content and substrate to inoculum ratio (S/I). The developed kinetic models allowed to state that moderate increase of TS and S/I enhanced methane yield. However, too high S/I ratio causes destabilization of the process. Moreover, this parameter also affects the diversity of microorganisms.

In the case of continuous and semi-continuous reactors, substrate is fed to the digester regularly. The process allows for better management of the feedstock, which can be added to the reactor. Moreover, it is considered that the stability of digestion is higher compared to the use of batch reactors. Among digesters operating in continuous mode, the most common are mixed reactors—continuous stirred-tank reactors (CSTRs) and plug-flow reactors (PFR). Due to their design, plug-flow reactors allow for an even flow of material without the need for intensive mixing, which reduces energy consumption and the risk of dead zones. The substrate is dosed and moved in the reactor most often by means and screws or hydraulic pistons, which allows for continuous operation and maximization of the use of the working volume [71]. The main technical components include the feed and discharge systems, mixing equipment, percolate recirculation system, heating and insulation units, unloading mechanisms, process monitoring devices, and facilities for biogas collection and storage. For instance, Rossi et al. [67] used a 37-L semi-continuously reactor in a pilot study of the fermentation of the organic fraction of municipal waste. The reactor was equipped with, among others, a stirring system, an external water jacket to maintain thermophilic conditions, and probes to measure basic parameters. Such technological solutions facilitate in-depth analyses and provide a foundation for scaling up the full-scale operations.

Due to the specificity of dry fermentation, it is necessary to implement appropriate loading and unloading systems. Mechanical material transport systems are most commonly used, which are adapted to the viscosity and low water content of the substrates. Batch reactors typically use front end loaders and conveyors to feed substrate into the reactor, while digestate removal is performed mechanically [72]. Continuous and semi-continuous dry fermentation systems, on the other hand, employ screw feeders and hydraulic pistons [73].

3.2. Comparison of Dry and Wet Fermentation

Differences in the liquid content of substrates form the basis for distinguishing between dry and wet fermentation. Although both processes result in biogas production, the choice of the appropriate fermentation type and its implementation should be justified by analyzing the key differences (

Table 5. Comparison of selected parameters of dry and wet fermentation [72,75,76].

Parameter	Dry Anaerobic Digestion (DAD)	Wet Anaerobic Digestion (WAD)
Dry matter content (TS)	High solids content: 20–40% TS	Low solids content: <10–20% TS
Water and energy demand	Low—pre-treatment of feedstocks does not require mechanical particle size reduction or dilution	High—preparation of substrates requires dilutions, energy demand during mixing
Methane yield	Typically higher methane yield	Typically lower methane yield
Foaming problem	Not applicable	Occurs
Space requirements	Reactors with smaller working volumes	Reactors are usually larger than in DAD due to the dilute feedstocks
Process stability	Often lower process stability resulting from, among others, the risk of acidification	More stable process
Percolate recycling rate	High	Low or lack
Investment and operating costs	Possible higher costs of mechanical processes, lower energy costs—no mixing equipment required	Higher water and energy costs

). Notably, microorganisms involved in biomass decomposition in both fermentation types do not show significant differences at higher taxonomic ranks. Dry fermentation, however, is dominated by specific methanogens, for example archaea related to Methanocelleus bourgensis MS2^T, which are well adapted to dry conditions [74].

3.3. Process Parameters Overview

An important aspect in dry fermentation is the selection of optimal process parameters. The most critical parameters include temperature, the dry matter content of substrates, hydraulic retention time (HRT), the C/N ratio, pH, and mixing (Figure 3).

Temperature directly influences the metabolic activity of microorganisms. In dry anaerobic digestion, the typical range is 35–40 °C for mesophilic conditions and approximately 55 °C for thermophilic processes [68]. Although higher temperatures generally accelerate the decomposition of complex organic matter and can increase methane yields, they also raise the risk of process destabilization due to elevated ammonia concentrations and pH fluctuations. For this reason, the mesophilic range is more commonly applied, as it provides greater process stability despite requiring a longer retention time. In the case of municipal waste used in the study by Hossain et al. [77], the best results were achieved at a temperature 37 °C and a mixed inoculum in the ratio of 1:2 = anaerobic sludge/cow manure. Another equally important parameter is dry matter content. A higher TS content reduces water demand and minimizes the amount of leachate generated. However, it can also create challenges with mixing and substrate flow, which can be mitigated by using effective recirculation systems. Typical high-solids feedstocks suitable for DAD include agricultural residues such as wheat straw [78] and corn stover [79], garden waste like leaves [80] and grass [81], as well as bio-waste from selective collection, including kitchen waste [82]. Hydraulic retention time (HRT) is defined as the average residence time of the feedstock in the digester which it remains in contact with methanogenic microorganisms. Determining the optimal HRT is crucial for maximizing the decomposition of organic compounds. A retention time that is too short results in incomplete waste degradation, whereas excessively long retention times are neither technologically justified nor economically feasible [68]. It is generally accepted that a longer retention time results in a higher degree of lignocellulosic conversion. Bokhary et al. [83] also found that, in the case of corn stover, the application of pre-treatment plays a significant role, especially at lower HRT values, by increasing the solubility of hemicellulose and lignin. The stability of the process is also ensured by a balanced carbon to nitrogen (C/N) ratio, which requires the appropriate selection of substrate proportions. Tang et al. [84] achieved the optimal C/N ratio for ammoniated straw and swine manure biogas residues at a substrate mixing ratio of 3:3. Different co-substrates require specific mixing ratios to prepare the feedstock blend, so analyzing the characteristics of each feedstock separately is essential. The C/N ratio is a key parameter because the calorific value of biogas largely depends on carbon, which mainly originates from plant-based substrates and is also a fundamental building block of microbial cells. In turn, the amount of nitrogen, which is a component of amino acids and proteins, influences microbial metabolic processes [85]. Another important parameter indicating the proper course of dry anaerobic digestion is pH. Excessively low pH poses a risk of acidification and process inhibition, while an increase in pH may lead to the release of ammonia, which exerts a toxic effect on methanogenic microorganisms. The recommended optimal pH range for DAD is approximately 6.8–7.5. As indicated by Song et al. [75], maintaining pH above 6.5 has a positive effect on the stability of food and kitchen waste conversion.

Due to the specific nature of the anaerobic digestion of dry substrates, mixing is carried out differently than in the case of wet fermentation. Mechanical mixing is widely used, and its duration should be adjusted depending on the characteristics of the process. Mixing reduces sedimentation and increases the contact between the feedstock and microorganisms. What is more, mixing can be performed continuously, throughout the process or intermittently, which helps reduce energy consumption. In addition to mixing time, the mixing speed should also be controlled, as excessively high revolutions per minute can damage bacterial cells, whereas mixing too slowly can contribute to acidification and the formation of dead zones [86].

3.4. Analysis of Factors Limiting Process Performance

For stable fermentation, it is necessary not only to properly adjust and control the anaerobic digestion parameters but also to identify and mitigate the effects of inhibitors that may arise during the process. Figure 4 presents a classification of the main factors limiting the efficiency of dry anaerobic digestion.

The main inhibitors of the dry fermentation process are ammonia and volatile fatty acids (VFAs). Ammonia affects the disruption of key enzymes in the process. Free ammonia is toxic at concentrations of 300–800 mg/L [67] and may also cause corrosion and environmental pollution [87]. Both high concentration of ammonia nitrogen and VFAs cause a decrease in biogas production. However, Xiao et al. [88] found that the addition of biochar can stimulate the process, increasing methane yield and accelerating the DAD of chicken manure by over 120% even in the presence of these inhibitors. Other solutions reported include membrane distillation and the supplementation of trace elements. Another serious threat to DAD performance is the presence of excessive amounts of heavy metals and antibiotics in the feedstocks. The negative effects of these factors include, among others, changes in the abundance and activity of microorganisms, inhibition of methane production, and disruption of mass transfer. For example, barium at concentration of up to 2000 mg/L can inhibit cellulose degradation. In the case of antibiotics, their presence can suppress key stages of fermentation, including hydrolysis and methanogenesis. For instance, tetracycline at a concentration of 8 mL/L significantly slowed down these processes [89]. It is difficult to eliminate the presence of antibiotics in feedstocks due to their widespread use in animal husbandry for both treatment and disease prevention. Moreover, gene transfer increases bacterial resistance to antibiotics, which in turn drives the development of new medical solution, the use of which also contributes to environmental contamination. Anaerobic digestion alone does not ensure the effective removal of antibiotics, due to their diverse sources, varying concentration, and differences in fermentation technologies [90].

Sulphates present in organic substrates can also have inhibiting effect on the anaerobic digestion process. These compounds are reduced by sulphate-reducing bacteria (SRB), leading to the formation of hydrogen sulphide. These bacteria, as well as methanogens, utilize organic acids or hydrogen sulphide, which causes competition for substrate [91]. Even at low concentrations (approximately 50 mg/L), H₂S is toxic and inhibits methane production by negatively affecting methanogenic archaea. Removal of hydrogen sulphide from the system typically involves processes such as precipitation, oxidation, or adsorption [92].

Plastics, especially microplastics, defined as particles smaller than 5 mm, pose a significant threat to anaerobic digestion. Although comprehensive studies on the impact of microplastics on dry anaerobic digestion are still lacking, available research indicates that kitchen waste may be contaminated with considerable amounts of polypropylene microplastics (PP MP). Zhao et al. [93] demonstrated that the presence of PP MP with a particle size of 50 μm delays the hydrolysis stage under mesophilic conditions and reduces methane yields. What is more, that their negative impact intensifies with increasing concentration. Microplastics, depending on their type, properties and concentration, can also have negative effects by demanding microbial cells, generating reactive oxygen species (ROS) and inhibiting enzyme activity. For instance, polyethylene has been shown to reduce the activity of acetate kinase and protease [94].

Another challenge of AD is the presence of pathogens such as bacteria, viruses and fungi in substrates. However, this mainly concerns bacteria found in animal manure, including toxin-producing bacteria of the genera Bacillus, Clostridium and Escherichia. Their inactivation during fermentation requires higher temperatures and often high concentrations of VFAs, which is difficult to meet in the operating conditions of biogas plants [95]. Pathogens not only negatively affect the waste conversion process, but also prevent the use of digestate for fertilization purposes [96]. It is also established that diversity and composition of the anaerobic digestion microbiome considerably affect biogas yield. The individual stages of fermentation are dominated by different bacteria species [97]. However, process conditions and substrate type can stimulate or suppress the abundance and activity of these microbial communities. Such fluctuations may lead to process instability, which in turn can reduce the overall efficiency of anaerobic digestion.

An improperly selected or contaminated inoculum may also inhibit the anaerobic digestion process. The inoculum is the main source of fermentation microorganisms, which is why its quality is crucial. Problems with the inoculum can lead to an extended reactor start-up period, reduced methane yields and the formation of process inhibitors. Acidification and destabilization of the process may occur if the inoculum is not properly selected for the type of feedstock and technological conditions. Moreover, in addition to its origin, the waste-to-inoculum ratio is also an important factor influencing process efficiency [98].

3.5. Selected Intensification Techniques

Co-fermentation is a common practice to intensify both dry and wet fermentation. Processing different substrates in one reactor allows for optimizing the C/N ratio, which determines the stability of the conversion process. Co-fermentation also improves the efficiency of processing time, which shortens the HRT. For example, research conducted by Abdelsalam et al. [66] confirms the validity of co-fermentation of waste such as manure and plant waste, including potato peelings, lettuce leaves and pea peelings. The authors, using mixtures in a weight ratio of 2:1 (manure/plant waste), noted an increase in methane production by almost 41% after co-fermentation of lettuce and manure compared to the control sample, i.e., mono-fermentation of manure [66]. Musluoğlu et al. [99] analysed the efficiency of the fermentation and co-fermentation process of organic fraction of municipal solid waste (OFMSW), chicken manure and sewage treatment plant sludge. They used a dry fermentation installation operating in a continuous system. The authors paid special attention to the need to adjust key parameters such as C/N, OLR and optimize the composition of substrates in order to obtain satisfactory effects in the form of efficient biogas yield and stability of the co-digestion process. Moreover, they also recommend conducting experimental studies before the stage of scaling up biogas production from waste containing high nitrogen content.

The problem that occurs during dry fermentation of kitchen waste is low biogas efficiency and a drop in pH. In order to prevent undesirable changes, neutralizing additives can be used, including calcium oxide, which in the right dose can improve biogas production by up to 8 times. CaO causes the leaching of organic matter, increases the rate of degradation of volatile fatty acids and increases microbiological diversity, which directly affects the metabolism of proteins and sugars [100]. In addition to chemical compounds, improved microbiological starters are also used to intensify the process. Studies of the microbiological composition of the inoculum provide knowledge on the type and role of key microorganisms serving as adapters for AD. The analysis can be carried out using classical methods, including isolation and culture, as well as modern methods, such as 16S rDNA metabarcoding. Bioinformatic analyses are becoming increasingly important, as they allow for assigning specific sequences to known taxa. This provides the basis for developing new, improved microbial starters and implementing them in dry anaerobic digestion technology [101]. Other intensification techniques include pre-treatment technologies such as mechanical grinding and disintegration, which improve biogas production efficiency [102].

4. Energy Efficiency and Economic Aspects of Dry Anaerobic Digestion

4.1. Biogas Plant Energy Balance

The energy balance of a biogas plant includes both the production of electricity and heat in cogeneration systems, as well as energy consumption for internal needs, such as the drive of mixers, pumps, fermenter heating systems, or automatic control systems. According to literature data and experimental analyses, the electrical efficiency of gas engines used in biogas plants is around 35%, while the thermal efficiency averages 50%, giving a total system efficiency of approximately 85% [103,104]. Compared to other renewable energy sources, such as wind power (efficiency of ~35%) or photovoltaic farms (efficiency of PV systems depending on the quality and type of panels is approximately ~25%), biogas plants are much more efficient due to their specific operation. The scheme of energy balance of a biogas plant is shown in Figure 5.

For a 0.5 MW (biogas) installation, the following parameters are assumed: chemical energy in biogas: approx. 1.5 MW, electricity production: 0.5 MW, heat production: 0.9 MW, and own consumption: 0.05 MW. For a 1 MW biogas plant, chemical energy in biogas is approximately 3.0 MW, electricity production is 1.0 MW, heat production is 1.8 MW, and own consumption is 0.1 MW [105,106,107]. It is important to note that the actual electricity consumption for internal use in a well-optimized biogas plant does not exceed 8–9% of total electricity production. The largest consumer of energy is usually the agitator system, which can account for more than 50% of internal consumption.

4.2. Cost and Revenue Analysis

The economics of biogas investments depend on a number of factors, including capital costs (CAPEX), operating costs (OPEX), revenues from the sale of electricity, heat, and digestate, as well as available support mechanisms such as investment subsidies or auction systems. The estimated costs and revenues for the two types of installations are given in

Table 6. Estimated costs and revenues [108].

Parameter	0.5 MW	1.0 MW
Capital expenditure (CAPEX)	PLN 6.5–7 million	PLN 13–14 million
Annual operating costs (OPEX)	PLN ~2 million	PLN ~4 million
Revenue from the sale of electricity	PLN ~1.8 million/year	PLN ~3.6 million/year
Revenue from the sale of heat	PLN ~1.2 million/year	PLN ~2.4 million/year
Revenue from digestate	PLN ~0.2 million/year	PLN ~0.4 million/year
Available subsidies	45–65% of CAPEX	up to 65% of investment costs

.

The profitability analysis indicates a return on investment (ROI) period of 6–10 years, assuming stable energy prices and maximum use of available support mechanisms, such as the “Energy for Rural Areas” program or RES auctions [109]. The data indicate that 1 MW biogas plants have a significantly shorter return on investment period, assuming similar conditions for the sale of electricity and heat. This is due to a more favorable energy balance, economies of scale in operating costs and greater opportunities to utilize surplus energy, according to an analysis of reports by the Institute for Renewable Energy and the European Biogas Association from 2022–2023.

The following factors are key to optimizing the energy and economic balance of both 0.5 MW and 1 MW biogas plants: substrate quality and fermentation efficiency, modern high-efficiency cogeneration systems, infrastructure enabling the use of waste heat, and the availability of a local market for electricity and heat.

4.3. The Importance of Biogas in the Energy Transformation

Biogas obtained from the anaerobic digestion of organic waste is not only a source of renewable energy, but also a tool for the efficient management of bio-waste and the reduction of greenhouse gas emissions. According to reports by the IEA and the European Commission, the development of the biogas sector is in line with the objectives of the European Green Deal and the methane reduction strategy [110].

Biogas (and biomethane—its purified version) represents an important, though still small, share in the energy mix. According to data from the European Commission and industry reports, total biogas and biomethane generation in the EU in 2021 was approximately 18.4 billion cubic meters (bcm) (around 196 terawatt-hours (TWh). Germany is among the leaders in terms of the number of installations, with more than 10,000 biogas plants operating there, generating ~87 TWh per year. The European biomethane sector is showing dynamic growth, with total biogas production (biogas + biomethane) in Europe reaching 22 bcm (~234 TWh) in 2023, according to data reported by the European Biogas Association (EBA), representing 7% of the EU’s annual natural gas demand. At the same time, the number of biomethane installations continues to grow, with a generation capacity of about 6.4 bcm/year (installations worth €27 bn by 2030) reached in 2023, according to the EBA [111]. Experts indicate that the potential for biogas production in the EU could be more than doubled by 2030, but this requires appropriate financial, legislative and technological measures. From a climate policy perspective, biogas plants play a crucial role in the circular economy, enabling the management of organic waste, reducing methane emissions from agriculture and generating renewable energy. The country’s technical balance sheet predicts a real potential of 2186 million m³ of biogas per year, which can produce 4627 GWh of electricity and 18,694 GWh of heat (2023 statistics) with full cogeneration [110]. Thus, the Polish biogas sector has a high potential for development, but with the proper design, construction and management of cogeneration infrastructure. EU and international climate and energy policy outlines ambitious plans for the biogas sector. As part of the REPowerEU package, the European Commission has set a target of achieving 35 bcm of biomethane per year (~342 TWh) by 2030. Analyzing the data in

Table 7. Example biogas production and number of installations in selected countries (source data from IEA/industry)—own elaboration based on [112].

Country	Estimated Number of Biogas Installations	Biogas Production (TWh/Year)
Germany	>10,000	87
France	~1600	25
Italy	-	22
Denmark	-	7

, there is still a long way to go to achieve the set targets without further rapid, but considered, development of biogas plants in the EU. The experience of recent years shows that growth must continue to accelerate; the current rate of sector development (several percent annual production growth) is insufficient to meet this target. Various industry analyses (e.g., Gas for Climate) estimate that ~35–40 bcm could be realised by 2030, and by 2050 even approx. 95 bcm of biomethane per year (or ~923 TWh) in the EU, making full use of available feedstock [111,112].

High capital expenditure and operating costs constitute the primary barrier to large-scale deployment of biogas technologies. Additional systemic risks arise when energy monocultures replace diversified agriculture: reduced biodiversity and soil degradation can drive up the price of agricultural commodities. Germany’s recent debate over dedicated energy crops illustrates the tension between biogas expansion and food-production security.

Regulatory challenges further impede growth. In many EU member states, including Poland, the environmental and construction permitting process for biogas facilities remains unpredictable, and national development strategies often lack coherence with broader European roadmaps. Technical factors can also suppress performance: inadequate process control or sub-standard feedstocks markedly reduce energy yields and slow sectoral uptake. Despite these constraints, biogas aligns closely with the objectives of the European Green Deal. When managed effectively, it provides dispatchable electricity and heat while simultaneously curbing methane emissions, positioning the technology as a valuable complement to other renewable energy sources.

5. Environmental Impact and Legal Regulations

5.1. Comparison of the Carbon Footprint of Dry Anaerobic Digestion and Other Organic Waste Treatment Methods (Composting, Landfilling, Wet Anaerobic Digestion)

Waste management is currently at a pivotal juncture where not only economic and technological efficiency but also climate impact is becoming central criteria for evaluating treatment technologies. The carbon footprint, understood as the total greenhouse gas (GHG) emissions generated throughout the life cycle of a given process, product, or system, is emerging as a key indicator of environmental sustainability and technological performance [113]. This is particularly relevant for the management of biowaste, which, under uncontrolled decomposition, can generate significant quantities of methane (CH₄) and nitrous oxide (N₂O), gases with high global warming potential (GWP) [114]. The aim of this analysis is to compare the carbon footprints of four commonly used methods of organic waste treatment, dry anaerobic digestion, wet anaerobic digestion, composting, and landfilling, from both environmental and energy perspectives.

Dry anaerobic digestion (dry AD), a form of anaerobic decomposition of organic biomass under limited moisture conditions (typically 20–35%), is considered one of the most environmentally sustainable approaches to biowaste treatment [55]. The absence of the need to add large volumes of water simplifies the technological logistics and reduces energy consumption, particularly for heating and mixing in the reactor. Furthermore, dry AD generates significantly less leachate and condensate than wet AD, reducing the need for energy-intensive wastewater treatment [55,68,115,116]. The main output is biogas containing 50–60% methane (CH₄), which is suitable for combined heat and power (CHP) units, compressed biomethane production, or upgrading to pipeline-quality gas [103,117]. The digestate, depending on the input material quality, can be used as an organic fertilizer, supporting the closing of the nutrient cycle [118]. From a GHG emission standpoint, dry AD demonstrates a favorable environmental profile. Conducted in a closed system, it minimizes CH₄ and N₂O emissions and enables precise gas flow monitoring [55,119]. Life Cycle Assessment (LCA) studies show that, with high energy recovery efficiency and effective digestate utilization, dry AD can achieve negative net emissions ranging from −60 to −100 kg CO₂-eq per Mg of processed waste [119].

In comparison, wet anaerobic digestion, prevalent in many agricultural and municipal biogas plants, relies on processing biomass with high water content (typically above 85–90%) and operates under different process conditions than dry AD. This method requires the dilution of feedstock and intensive mixing to maintain homogeneity, resulting in higher mechanical and thermal energy consumption, particularly under mesophilic or thermophilic conditions. These factors lead to increased overall energy demand and consequently, a higher carbon footprint [55,68,120]. The biogas produced typically contains 50–60% CH₄ and can be used in CHP units or upgraded to biomethane for transport or gas networks [103].

From an environmental efficiency perspective, the total carbon footprint of wet AD is generally higher than that of dry AD, mainly due to greater energy use and emissions associated with leachate treatment and suboptimal process conditions. Literature data indicate average net emissions around −55 kg CO₂-eq/Mg, though this value is highly dependent on local technological parameters [121]. Feedstock quality plays a critical role: substrates rich in easily biodegradable compounds improve biogas yield and emissions balance, while nitrogen-rich or low-quality inputs may result in increased nitrous oxide emissions and higher energy demand for their mitigation [122,123].

The environmental performance of wet AD is also scale-dependent; larger facilities tend to achieve better energy and economic efficiency due to improved heat recovery optimization. Digestate management is equally important; its use as fertilizer or for further processing (e.g., liquid-solid separation) significantly affects the final carbon footprint. Improper digestate storage or long-distance transportation can negate climate benefits [124,125,126].

Although composting is technologically simple and relatively low-cost, it does not offer climate benefits comparable to anaerobic digestion. As an aerobic process, composting relies on microbial decomposition of organic matter in the presence of oxygen, inherently resulting in carbon dioxide (CO₂) emissions as a byproduct of aerobic respiration [114,127,128]. Under suboptimal conditions, such as excessive compaction, high moisture, insufficient aeration, or imbalanced C:N ratios, nitrous oxide (N₂O) may form, a GHG with a 100-year GWP over 270 times that of CO₂ [129,130,131]. Composting lacks energy recovery, meaning that GHG emissions are not offset by energy generation. Consequently, the total carbon footprint of composting can range from approximately +30 to +140 kg CO₂-eq/Mg, depending on process technology, conditions, and feedstock type [132]. Open windrow systems tend to have higher emissions than closed or in-vessel systems, highlighting the variability of environmental outcomes [114,133].

Another limitation of composting lies in the quality of the final product. If the input contains physical contaminants (e.g., glass, plastics, metals), heavy metals, or pathogens, its use in agriculture may be legally restricted or completely prohibited, especially under EU Regulation 2019/1009 on organic-based fertilizers. Low-quality compost is often relegated to land reclamation or municipal landscaping, reducing its utility and contribution to nutrient recycling. Additionally, the high volume of compost relative to input waste mass creates logistical burdens and transportation/storage costs, particularly where local demand is lacking [128,134,135].

From a climate perspective, landfilling is the least favorable method of organic waste treatment. Conducted under anaerobic conditions, it results in the generation of methane, a GHG with a GWP 28–36 times greater than CO₂ over a 100-year horizon [130]. While some methane can be captured and utilized for energy, landfill gas recovery systems rarely exceed 50–60% efficiency over the landfill’s operational life [136]. The remaining methane is emitted uncontrolled, often in the post-operational phase, making its true climate impact difficult to quantify and frequently underestimated [137,138]. The total carbon footprint of landfilling organic waste can range from +230 to +710 kg CO₂-eq/Mg, making it the most emissions-intensive method among those analyzed [132]. Additional issues include emissions of nitrous oxide (N₂O) and hydrogen sulfide (H₂S), which arise under anaerobic conditions in the presence of nitrogen and sulfur compounds. Beyond climate impact, landfilling also poses risks to groundwater, air quality, and public health due to odor and toxic gas emissions such as ammonia [139,140,141].

Based on the above analysis, dry anaerobic digestion, assuming high energy efficiency and effective digestate utilization, emerges as the most carbon-efficient technology. When integrated with local energy systems and agriculture, it aligns with circular economy principles and climate neutrality targets. To provide a more systematic assessment of its comparative advantages and limitations, a SWOT analysis (Strengths, Weaknesses, Opportunities, Threats) can be applied (

Table 8. SWOT analysis of organic waste treatment methods (dry AD, wet AD, composting, landfilling).

Strengths	Weaknesses	Opportunities	Threats
High energy efficiency and potential for negative carbon footprint	Higher initial investment compared to composting	Alignment with circular economy and climate neutrality targets	Policy or market instability affecting renewable energy support
Reduced leachate and condensate compared to wet AD	Limited applicability for very high-moisture feedstocks	Development of biomethane markets and energy system integration	Competition from other waste treatment technologies (e.g., incineration, composting)
Production of digestate usable as organic fertilizer	Technical complexity requiring skilled operation	Access to EU and global green funding instruments	Public acceptance issues related to odor or local siting
Closed system minimizes fugitive GHG emissions	Lower experience base compared to wet AD in some regions	Potential role in energy security and regional bioeconomy strategies	Risks related to feedstock availability and contamination

). This method complements quantitative indicators such as LCA results and carbon footprint by highlighting not only the environmental but also the economic, technological, and policy-related dimensions of each technology.

This dual assessment-quantitative (carbon footprint) and qualitative (SWOT)-offers a comprehensive framework for decision-makers, allowing them to evaluate not only environmental impacts but also socio-economic and regulatory conditions that shape the future of dry anaerobic digestion.

5.2. Legal Framework for Waste Management in the European Union and Globally

The assessment of organic waste treatment technologies must consider the applicable legal framework. The European Union’s environmental and waste policies, along with international regulations, constitute a complex system of legal norms that define operator responsibilities and guide the development of low-emission technologies by shaping environmental and economic frameworks. Directive 2008/98/EC on waste, known as the Waste Framework Directive, is of particular importance. It establishes the waste hierarchy and promotes material and energy recovery over landfilling [142]. Amended by Directive (EU) 2018/851, the revised Waste Framework Directive forms the cornerstone of the EU’s waste policy and circular economy strategy. It introduced ambitious, binding targets that require systemic transformation of waste management across Member States. These include achieving 55% municipal waste recycling by 2025, 60% by 2030, and 65% by 2035, alongside limiting landfilling to no more than 10% of the total waste generated [143]. These targets not only reduce environmental impact but also stimulate the development of innovative, low-carbon waste technologies and strengthen the secondary raw materials market.

At the international level, three fundamental conventions underpin global waste governance and environmental protection: the 1989 Basel Convention on the Control of Transboundary Movements of Hazardous Wastes and Their Disposal, which aims to prevent the uncontrolled export of waste to developing countries; the 2001 Stockholm Convention on Persistent Organic Pollutants (POPs), targeting the elimination or restriction of highly hazardous chemicals due to their persistence, toxicity, and bioaccumulation potential; and the 1998 Rotterdam Convention, which introduced the Prior Informed Consent (PIC) procedure for trade in certain hazardous chemicals and pesticides. Together, these conventions form a legal and institutional framework for global responsibility over hazardous substances, reinforcing the principles of precaution, environmental accountability, and transboundary equity. They also influence regional and national legal systems, establishing uniform environmental safety standards worldwide [144,145].

Beyond Europe, several countries have introduced their own ambitious frameworks for waste and resource management that shape the global landscape. In Japan, the Basic Act on Establishing a Sound Material-Cycle Society (2000) and subsequent legislation set clear priorities for waste prevention, recycling, and energy recovery. Japan has been a global pioneer in promoting waste-to-energy facilities, with more than 70% of municipal solid waste being incinerated under strict emission standards, often coupled with energy recovery. In South Korea, the Waste Control Act and the Framework Act on Resource Circulation (2018) introduced extended producer responsibility schemes and mandatory food waste recycling, supported by digital tracking systems that ensure transparency and citizen compliance. In China, the Circular Economy Promotion Law (2009) and the 14th Five-Year Plan (2021–2025) prioritize large-scale development of anaerobic digestion and composting for urban biowaste, alongside strict restrictions on landfilling and open dumping. China has also introduced mandatory municipal waste sorting in megacities such as Shanghai since 2019, significantly accelerating the uptake of biowaste treatment technologies.

In North America, the United States applies a fragmented but influential framework, with the Resource Conservation and Recovery Act (RCRA, 1976) forming the basis of federal waste regulation [12]. Many states, such as California, go beyond federal requirements—SB 1383 mandates a 75% reduction of organic waste disposal by 2025 and large-scale deployment of composting and anaerobic digestion facilities. Canada, under its Zero Plastic Waste Strategy and Canadian Environmental Protection Act, has launched parallel initiatives that increasingly integrate biowaste valorization into climate and resource policies. At the same time, Latin American countries are advancing progressive waste strategies: Colombia introduced a National Circular Economy Strategy (2019), emphasizing organic waste valorization, while Brazil’s National Solid Waste Policy (2010) promotes landfill diversion and encourages municipal investment in anaerobic digestion [146,147].

An important complement to these regulatory frameworks is the standards issued by the International Organization for Standardization (ISO), particularly ISO 14001 (environmental management systems), ISO 14040 and ISO 14044 (life cycle assessment), and the newly released ISO 24187:2023, which focuses on environmental aspects of waste management [148,149,150,151]. These standards support the implementation of effective and transparent environmental strategies in both public and private sectors. Their significance is increasing in light of emerging climate reporting regulations, such as the EU’s Corporate Sustainability Reporting Directive (CSRD), and in response to financial markets’ and investors’ growing preference for projects aligned with sustainable investment criteria under Regulation (EU) 2020/852 [134,146,148,149,150,151].

Within this context, bio-waste treatment technologies, particularly dry anaerobic digestion, are gaining recognition as key tools in decarbonization policies. Thanks to its low carbon footprint, high energy efficiency, and alignment with circular economy principles, dry AD can play a crucial role in meeting EU and global climate targets. However, realizing its full potential requires not only regulatory support but also the strategic application of financial, planning, and investment instruments. The European Green Deal, the EU’s 2020 Circular Economy Action Plan, and the Just Transition Fund (JTF) are especially relevant in this regard, as they aim to reduce emissions while ensuring equitable integration of environmental technologies into the economic transition of carbon-intensive regions [147,152,153,154]. At the same time, comparable initiatives outside the EU—such as Japan’s Green Growth Strategy through Achieving Carbon Neutrality by 2050, California’s Cap-and-Trade Program, or China’s National Carbon Emissions Trading Scheme (ETS)—demonstrate that integrated regulatory and financial frameworks are increasingly converging worldwide to support circular and low-carbon bio-waste technologies.

Modern waste management increasingly demands an integrated governance model that balances environmental, legal, economic, and societal considerations. The development of dry anaerobic digestion as a low-emission technology aligned with legal and environmental standards represents a promising direction for countries and regions striving toward climate-neutral waste systems.

5.3. The Role of Dry Anaerobic Digestion in the Circular Economy: Soil Carbon Sequestration and Greenhouse Gas Emission Reduction

The circular economy (CE) represents one of the fundamental pillars of the European Green Deal. In this context, anaerobic digestion (AD) and particularly dry anaerobic digestion (DAD), enables the efficient treatment of the biodegradable fraction of municipal organic waste. This process not only facilitates energy recovery in the form of biogas, a renewable energy carrier, but also generates digestate, which can be used as an organic fertilizer contributing to soil carbon sequestration [155].

DAD technology allows for the processing of high-solids-content substrates (e.g., manure, biowaste fractions, green waste) without the need for large volumes of process water or recirculation, thereby reducing both water consumption and the overall energy demand of the installation [156].

The application of digestate to soil increases the level of soil organic carbon (SOC), which enhances water retention, soil fertility, and structure [157,158]. Experimental studies indicate that digestate derived from DAD may increase the stable fraction of soil organic matter by several percent, depending on soil type and application frequency [159].

Moreover, the use of digestate slows down carbon mineralization and promotes its long-term stabilization in biologically recalcitrant forms. This mechanism aligns with the principles of carbon farming and soil-based carbon sequestration strategies at the farm level [160]. Dry anaerobic digestion contributes to greenhouse gas (GHG) emission mitigation on several levels:

• Methane emissions reduction from organic waste: Diverting biowaste from landfilling to DAD prevents uncontrolled anaerobic decomposition, which would otherwise generate significant amounts of methane (CH₄) [117].
• Replacement of synthetic fertilizers: The use of digestate reduces reliance on mineral fertilizers, the production of which is associated with high carbon dioxide (CO₂) emissions [161].
• Production of green energy: Biogas generated through DAD can be converted into electricity, heat, or upgraded to biomethane, thereby reducing dependence on fossil fuels.

According to life cycle assessment (LCA) studies, DAD systems demonstrate a GHG mitigation potential of 50–85% when compared to conventional organic waste treatment methods [162].

Integrated DAD systems align with the principles of closed-loop material and energy flows. In the municipal sector, the selective collection of biowaste and its processing in urban DAD facilities supports climate neutrality targets and reduces the volume of landfilled waste [162].

From the circular economy perspective, DAD enables the recovery of essential nutrients (N, P, K), promotes carbon storage in soils, and reduces GHG emissions, thereby addressing environmental, climate, and energy goals simultaneously [163].

6. Priority Research Directions and Policy and Strategic Planning Recommendations for Municipal Integration of DAD Systems

6.1. Overview of Mature DAD Technologies and Emerging Hybrid Optimization Methods

Under commercial conditions, the process of anaerobic digestion may encounter technical difficulties, i.e., delayed degradation of intricate substrates, especially plants biomass, inhibitory compound accumulation, and poor biogas yield during treatment of heterogeneous substrates. Thus, a significant factor in the development of DAD technology is significant intensification of process effectiveness with the introduction of hybrid strategies. The development of DAD technologies has led to the commercialization of several advanced systems tailored for high-solids organic waste treatment. The table below (

Table 9. Comparative analysis of mature and technologically advanced DAD systems in commercial practice.

Technology	Reactor Type	Operating Temp.	Mixing Strategy	Advantages	Limitations	Reference
Dranco	vertical plug-flow	thermophilic (~50 °C)	none (gravity)	simple design, low OPEX, good for high TS	no mixing may cause channeling; limited flexibility	[164]
Valorga	vertical with gas injection	mesophilic/thermophilic	gas injection	handles heterogeneous waste, stable performance	mixing energy costs; risk of sedimentation	[165]
Kompogas	horizontal plug-flow	thermophilic (~55 °C)	slow internal paddles	effective mixing, high reliability	maintenance of mechanical parts	[68]
Bekon	batch (garage-type)	mesophilic (~37 °C)	none	low-tech, suitable for decentralized use	lower throughput; risk of local inhibition	[166]
Aikan	integrated dry digestion + composting	thermophilic + aerobic stage	combined stages	maximizes recovery, integrates composting	high CAPEX; system complexity	[68]
Linde BRV	horizontal plug-flow with percolate recirculation	thermophilic	percolate recirculation	good efficiency, moderate energy needs	clogging risk in recirculation	[68]
BioPercolat	leach-bed	mesophilic	percolate irrigation	low water demand, modular design	uneven percolate flow; inert buildup	[167]
Iska	two-stage (dry + wet)	mesophilic + wet methanogenesis	separate dry/wet	improved hydrolysis and methane yield	more space and complexity needed	[168]

) summarizes key technical parameters, advantages, and limitations of selected DAD technologies currently implemented at full scale [68].

Recent research has established that combining DAD with novel intensification equipment can enhance biomethane production while lowering solid retention time (SRT) concurrently. One of the proposed techniques is electro-anaerobic digestion (EAD), with the application of a low-voltage electric field (<1.0 V) inside the digester, increasing microbial activity and direct electrons transfer. Application of this process has increased CH4 yields in different studies by 10–37% [169]. Further work is needed to clarify optimal electrode materials, electron flux densities and stability over the long term at high total solid (>20%) DAD conditions. The most promising new hybrid pretreatment method is oxidative pre-treatment (e.g., wet oxidation with water and supplementation with oxygen, air or chemical reagents such as H₂O₂) can enhance methane potential of lignocellulosic kitchen and green waste by 49%, or 4–5 folds [68].

6.2. Holistic Process Optimization in DAD: Addressing Limitations Through Synergistic Engineering

The current knowledge gaps are still how to best integrate hybrid approaches, delivering synergistic effects and blend oxidative, alkaline, fungal and thermal pre-treatments, including energy and cost balances when pre-treatment is integrated with high solids feeding systems. The other important knowledge gap is also real-time rheological monitoring to minimize viscosity peaks that hamper mixing in dry digesters. Another area of unavoidable research advancement is next generation bioreactor engineering and modelling technology. Next-generation bioreactors combine spatial compartmentalisation with selective biomass retention. Most DAD plants utilize plug-flow or leach-bed systems, as mechanical mixing is energy inefficient at total solids (TS) concentrations over 20%. A one-dimensional convection diffusion reaction model has recently been developed and validated against laboratory scale plug-flow reactor (PFR) operation with cattle manure at 23–28% TS [170].

Although there are significant developments in reactor design and modeling packages, dry anaerobic digestion remains the most difficult mode in mechanical and biological complexity in the total anaerobic digestion technologies. Mass transfer limitations in dry anaerobic digestion become increasingly problematic as increasing total solids concentration leads to scaling non-linearly with substrate dryness. As TS rises from 10% to 30%, the diffusive capacity of volatile fatty acids (VFAs) and dissolved CO₂ decreases by one to two orders of magnitude, usually leading to local acidification. In addition, inhibited mass transfer between gas and liquid phases leads to H₂ and CO₂ supersaturation, shifting the carbonate equilibrium and enhancing pH heterogeneity. Also, in leach-bed reactors, process leachate recirculation may cause settling of heavy particles such as sand, grit, or bone particles, which, with time, clog liquid distributors and deform the actual residence time distribution in the system [68]. In DAD, there is a critical issue with thermal management since there is no free water available. This absence leads to one-dimensional conductive heat transfer to override the convective pathways, which itself tends to generate radial temperature gradients exceeding 3 °C in massive plug-flow reactors with, say, ~500 m³ volume and operating at ~30% total solids (TS). Such gradients generate uneven microbial activity and localized performance issues. In addition, latent heat of vaporization is crucial at higher solids content: removal of only 1% water would lead to energy loss of about 25 kWh per ton of substrate, increasing the formation of self-drying zones near gas or air inlets and the development of hot spots, both of which lead to process instability. These problems are comprehensively documented in recent high-solids anaerobic digestion reviews, pointing out that heat management is a major handicap to the scaling of DAD systems [171].

Despite its growing importance in the treatment of organic wastes, DAD still faces significant scientific and technological limitations to its optimization and scale-up. Low water activity inherent in high-total-solids (TS) systems significantly aggravates inhibition phenomena, particularly ammonia toxicity, where the threshold concentration for microbial inhibition decreases at ≥25% TS [166]. Additionally, trace element availability is limited because essential metals tend to precipitate on dry surfaces, reducing bioavailability and rendering conventional supplementation strategies ineffective. Monitoring and process control are further complicated by the opaque, viscous character of high-solids substrates, which rapidly foul optical sensors and attenuate spectroscopic signals, making online pH and VFA measurements unreliable and forcing recourse to time-consuming offline techniques [47]. At scale-up, mix dynamics and inoculum distribution are distorted, with effects such as wall-slip at lab scale over-estimating full scale efficiency, while unaided inoculum injection can leave over 30% of reactor volume biologically inactive for extended periods [171]. From a modelling point of view, existing digital twins based on ADM1 do not capture hydrolysis kinetics in dry systems well, which are governed by surface-based processes rather than bulk-liquid diffusion [170]. Furthermore, there are material and energy integration issues stemming from the need for reactors to be capable of withstanding enhanced internal pressures and having narrow thermal margins, which restricts the scope for the integration of solar or renewable heat sources. End-of-life treatment of DAD reactors is also a challenge because the dried digestate can create hard, cement-like blocks that have to be mechanically stripped, and corrosion underneath deposits can reduce the lifespan of construction materials by up to 50% compared to wet AD systems [168]. Cumulatively, these limitations define critical research needs in process kinetics, sensor technology, reactor design, and long-term operational durability, highlighting the need for interdisciplinary innovation to fully exploit the potential of DAD technologies.

To address the technical limitations of DAD discussed in this section (i.e., thermal instability and insufficient process control), a Technical Optimization Framework is proposed (

Table 10. Technical Optimization Framework-integrated roadmap for closed loop system.

Step No.	Technical Optimization Framework
1	Pretreatment strategy
	Goal: improve feedstock properties before entering the reactor
	Freezing-thawing—disrupts cellular structure, improves hydrolysis efficiency and reduces viscosity. Alkaline/hydrothermal methods—solubilize organic matter, making it more accessible for microbes. Mechanical shredding—increases surface area, enhances microbial contact.
Effect: improves substrate rheology, reduces energy needs for mixing and heating.
2	Reactor design optimization
	Goal: Adapt reactor to handle high-solids content and improve heat/mass transfer.
	Enhanced mixing systems—spiral, paddle, or vertical-axis mixers. Insulation & heat retention—thermal jackets, phase-change materials. Batch vs. continuous mode—match to substrate type and loading rate.
Effect: ensures homogeneity, stabilizes temperature, reduces dead zones.
3	Smart monitoring & control
	Goal: enable real-time process optimization and fault detection.
	Sensor networks—monitor temperature, pH, VFA, biogas yield. SCADA systems with AI algorithms—predict failure, optimize feeding schedules. Remote diagnostics—early detection of process instabilities.
Effect: Increases reliability, reduces downtime, supports automation.
4	Integration & feedback loop
	All three areas are interconnected:
	Pretreatment alters substrate—reactor operates more efficiently. Optimized reactor supports better microbial performance—easier to monitor. Smart monitoring feeds back to adjust pretreatment or reactor settings in real time.
Effect:
Enhanced process synergy: integration ensures that modifications in one area (e.g., pretreatment) immediately benefit the others (e.g., reactor efficiency, monitoring accuracy). Real-time adaptability: smart monitoring provides continuous data, enabling dynamic adjustments to pretreatment intensity or reactor settings in response to operational changes. Improved system stability: stable temperature and consistent mixing achieved through adaptive feedback reduce microbial stress and increase biogas yield. Resource efficiency: optimized energy use across pretreatment and reactor operations; reduced water input due to better rheology; overall lowered environmental footprint. Learning-based optimization: AI-driven control systems learn from historical performance, leading to predictive maintenance and optimal scheduling. Closed-loop circularity: continuous refinement of parameters leads to higher substrate conversion, lower residue generation, and improved recovery of energy and nutrients.

). This integrated roadmap links a three-pillar strategy aligning: pretreatment methods (e.g., freezing-thawing to improve feedstock rheology), reactor design adaptations (e.g., improved mixing systems, heat retention structures), and smart monitoring technologies (e.g., real-time temperature and biogas yield sensors). By connecting these elements, the system can enhance substrate accessibility, energy efficiency, and operational stability under high solids content. Moreover, the synergy among these domains creates a closed-loop system enhancing digestion performance, reducing operational risk, and increasing process scalability.

In this roadmap, the very critical component is the Integration & feedback loop, which connects pretreatment strategies, reactor design, and smart monitoring into a dynamic, adaptive system. This integration ensures continuous process optimization, enhances operational efficiency, and supports circular resource recovery.

6.3. From Waste to Value: Economic and Governance Pathways for Circular DAD Integration

DAD is a promising technological solution for sustainable waste management within circular economy systems, particularly in areas where water is scarce. Unlike wet digestion or composting, DAD is a low water input process and is therefore highly suitable for areas where freshwater resources are sparse or where reception capacity of treatment facility is absent. Recent LCA analyses have indicated that, with system boundaries incorporating avoided synthetic fertiliser production and displaced fossil-derived thermal energy, DAD has a 20–45% lower GWP compared to composting and 30–60% lower GWP compared to incineration with energy recovery. These benefits render DAD a high-performance solution for urban areas attempting to decarbonize their waste schemes and reduce overall environmental footprints. To ensure the alignment with circular economy objectives, municipalities are required to incorporate quantitative sustainability metrics into public tendering and concession contracts [172].

Under DAD, traditional flat-rate gate-fee systems tend to ignore fluctuations in substrate quality, biogas yield, or digestate value. This discourages process improvement and innovation. Improved systems are based on flexible, performance-based gate-fee mechanisms, variable operator remuneration in response to measurable outputs like methane yield, nutrient capture, or contamination rate. Such models align the interests of stakeholders with circular economy goals. The dynamic pricing assists in improving economic as much as environmental outcomes. Implementation, however, requires proper monitoring, open data procedures, and flexible contracts. Current scholarship further indicates the role that these models have in internalizing environmental externalities. Overall, evidence-based gate fees are a strategic tool to maximizing the efficiency and sustainability of DAD systems [173]. Regional co-digestion centers for DAD make it possible to optimize logistics by bringing together a number of organic fractions, such as municipal biowaste, residues of agro-industry, and food waste, which makes the process more stable and efficient in terms of methane production. Clustering substrates of different characteristics makes it feasible to optimize the C/N ratio and concentration of total solids, which in turn results in higher and more stable biogas production [174]. To support this, the inter-municipal partnership (PPP)-based or joint municipal utility governance models enable sharing of risks and transparence of revenue. Moreover, digital intake monitoring systems (inline intake tracking) have to be implemented to ensure efficient hub operation, as they allow for feedstock quality control and stabilization of co-digestion processes. Regulated and coordinated along with stakeholder engagement, social acceptance and operational stability improve. As a whole, regional co-digestion centers constitute a scalable and resilient infrastructure framework for DAD, enabling optimized logistics, open governance, and efficient management of organic resources [175].

7. Conclusions

DAD of selectively collected biowaste holds significant scientific and practical potential. The topic is highly relevant and addresses the current challenges in modern waste management, where priorities include energy recovery, greenhouse gas (GHG) emission reduction, and the sustainable use of resources. DAD technologies utilizing source-separated green and kitchen waste can make a tangible contribution to the development of green energy and the advancement of the circular economy. The technological design of DAD facilities significantly influences both capital and operational expenditures. The choice between plug-flow reactors and leach-bed configurations should be guided by local spatial constraints, logistical considerations, and substrate characteristics. Moreover, economies of scale play a critical role in DAD system feasibility, as larger facilities benefit from lower unit processing costs, thereby supporting the establishment of regional co-digestion hubs. For municipalities, identifying the optimal facility capacity and matching it with an appropriate business model, whether public ownership, public-private partnership (PPP), or private investment, is essential to ensure financial sustainability. Importantly, spatial planning and integration of DAD systems with local heat and fertilizer users (e.g., agriculture or industry) can improve return on investment while reducing environmental impacts. DAD systems operate as interconnected entities; thus, cross-sectoral collaboration involving municipalities, industry, agriculture, and academic institutions is vital for their successful design and scale-up. Local circular economy strategies should treat DAD as a core element for energy recovery, nutrient cycling, and greenhouse gas (GHG) mitigation. Currently, DAD systems often struggle to maintain economic self-sufficiency, and their payback periods tend to be longer compared to composting, thereby requiring a stable and predictable regulatory environment. However, policy instruments such as capital subsidies, feed-in tariffs for biomethane, or tax incentives can significantly improve DAD profitability, particularly during early implementation stages. Municipalities should therefore explore diversified financing models, including national funding, EU grants, and private investment, to reduce project-related economic risk. In light of sustainability and circular economy strategies, many countries have adopted or are developing regulations that mandate the selective collection of biowaste, which directly enhances substrate quality and consequently improves fermentation performance and economic outcomes. Nevertheless, the actual quality of selectively collected waste often deviates from technological expectations. To address this, a variable gate-fee structure indexed to substrate quality or energy output should be implemented as an incentive for optimized collection and co-digestion practices. Public policy must also support education of both residents and plant operators to improve source separation and plant performance.

Despite substantial development, DAD technologies still lag behind wet digestion systems in terms of efficiency. Several technological and scientific gaps remain that limit the broader deployment of DAD. In particular, there is a pressing need to modernize or develop new monitoring and reporting systems to track key performance indicators (e.g., energy return on investment, nitrogen and phosphorus recovery, mass reduction). Knowledge gaps persist in integrating hybrid pre-treatment strategies that synergistically enhance biomass conditioning for DAD. Another critical research frontier involves real-time rheological monitoring to ensure efficient mixing. Although reactor design and modeling have advanced, dry anaerobic digestion remains the most mechanically and biologically challenging configuration within the anaerobic digestion family. One of the major operational challenges in DAD is thermal management, given the absence of freely available water. Additionally, material and energy integration is hindered by the need for reactors to withstand elevated internal pressures and operate within narrow thermal tolerances. Taken together, these constraints highlight essential research needs in process kinetics, sensor technologies, reactor engineering, and long-term operational resilience, emphasizing the importance of interdisciplinary innovation to fully harness the potential of DAD. In conclusion, DAD projects will only be competitive if technology, public policy, and financial models are cohesively integrated and tailored to local conditions.