Evaluating the Performance of Biogas Plants Operating on Organic Waste: A Practical Approach

Abstract

Biogas plants utilizing organic waste play an important role in the transition toward a circular economy and renewable energy systems. However, evaluating their actual performance is challenging due to the diversity of anaerobic digestion technologies, the wide range of feedstocks, and the use of various pretreatment methods. Consequently, assessing operational efficiency requires a comprehensive approach that goes beyond installed capacity alone. This paper synthesizes and systematizes existing approaches for evaluating the efficiency of biogas plants based on key operational indicators reported in the literature. The analysis considers a broad spectrum of feedstocks, highlighting the variability of input materials and their influence on plant performance. Particular attention is given to the internal energy consumption of electricity and heat, which directly affects net energy output and overall efficiency. The relationship between annual energy production (MWh) and installed capacity (MW) is analyzed as a core performance indicator enabling comparison between plants using different technologies, substrates, and scales. The proposed framework supports transparent performance assessment, operational optimization, and evidence-based decision-making in the development and management of waste-based biogas systems.

1. Introduction

In recent decades, as societies have become more affluent, the amount of bio-waste produced has increased significantly. Until recently, and in some countries still today, organic waste has been disposed of using the simplest and often illegal practices such as land spreading or discharge into water bodies [1,2]. In the case of some organic waste, such as slaughterhouse waste or sewage sludge, this could lead to environmental contamination and, on the other hand, increased epidemiological risk [3]. However, dedicated treatment technologies are available for organic waste management. These include primarily anaerobic digestion, where bio-waste is utilized as a feedstock in biogas plants to produce biogas and digestate for use as fertilizer [4], as well as composting, which produces high-quality organic fertilizer. Composting technologies are generally cheaper to implement on an industrial scale; however, they do not enable the recovery of energy (heat) or capture of carbon dioxide generated during decomposition [5]. While the anaerobic digestion (AD) process is more expensive at the investment stage, a functioning biogas plant can deliver significant energy benefits (stable production of “green” electricity and heat, or biomethane and bioCO₂), economic benefits (reduced disposal and waste costs, and additional revenues), environmental benefits (reduction in greenhouse gases (GHG) and ammonia emissions), and social benefits (odor reduction associated with biowaste treatment) [6,7].

According to International Renewable Energy Agency (IRENA) [8], the electric capacity of biogas plants reached 21,283 MW by the end of 2024. Europe is by far the leading region, with 13,845 MW installed.

Table 1. The global maximum net generating capacity of electricity from biogas [MW].

Region	2015	2016	2017	2018	2019	2020	2021	2022	2023	2024
Europe	11,314	11,795	12,222	12,946	13,348	13,860	13,465	13,577	13,674	13,845
South America	301	367	397	421	512	590	633	672	700	712
North America	2622	2626	2730	2686	2649	2549	2521	2463	2340	2319
Asia	1047	1212	1447	1701	1949	2105	2445	2631	2851	2885
Africa	26	35	38	44	55	55	100	100	110	110
Australia	282	241	240	240	240	247	247	247	247	216
Eurasia	266	311	390	452	561	754	1006	1145	1200	1196
Σ:	15,858	16,587	17,464	18,490	19,314	20,160	20,417	20,835	21,122	21,283

presents the evolution of this capacity over the period 2015–2024. According to IRENA, Eurasia includes Russia, Turkey, and Azerbaijan; however, it should be emphasized that the vast majority of installed capacity is located in Turkey (1177 MW at the end of 2024). North America also includes Central America and the Caribbean, while Australia is grouped with Oceania.

Nevertheless, it should be noted that China is the global leader in biogas production; however, its use for electricity generation (installed capacity of 1520 MW in 2024) is less significant than its utilization for biomethane production. China is a country where there are no subsidies for either electricity or biomethane production (unlike, for example, most EU countries, where both electricity production from biogas plants and biomethane is subsidized). Therefore, only a small share of biogas plants generate electricity, because the price of electricity is very low; therefore, it is more economically viable to produce biomethane or use biogas in local off-grid energy systems.

Consequently, biogas production has reached approximately 300 PJ, most of which is used in residential applications, particularly in rural areas. In 2022, China’s biomethane production capacity was approximately 200 million cubic meters per year [9].

As shown in Figure 1, the installed capacity of biogas plants for electricity generation in Europe is declining. This trend is associated with the growing importance of biomethane production in such installations, particularly in France, Germany, Italy, and Denmark in recent years. It is expected that this trend will also emerge in other countries in the coming years, due to the growing importance of biomethane in European economies.

Figure 2 presents electricity generation from biogas plants in Europe in 2023 [8]. The leading role of Germany in biogas-based electricity generation is clearly evident. Economies such as France, as mentioned earlier, place greater emphasis on biomethane production than on electricity generation from biogas. The potential for electricity generation, for example, in Poland is significantly higher than the current output, and a substantial increase has been observed in recent years [10].

As mentioned, there are many ways to utilize biogas produced in the anaerobic digestion process (Figure 3). For example, in Poland, as of 2026, due to government policies aimed at establishing a biomethane market to partially replace natural gas consumption, and owing to some of the highest support tariffs in the European Union (EU) for biomethane injected into the gas grid (PLN 545/MWh, or approximately EUR 127/MWh), biomethane production is currently much more profitable than producing electricity and heat via cogeneration [11]. However, the applications of biomethane extend far beyond its use as a renewable substitute for natural gas. Faced with the need to reduce emissions from urban transport, increasing efforts are being made to deploy buses powered by compressed natural gas (CNG), subsequently utilizing compressed biomethane as a fuel [12,13]. This solution is significantly cheaper than hydrogen or electric buses, and most large urban agglomerations can produce biomethane within their own wastewater treatment plants [14]. Trucks equipped with liquefied natural gas (LNG) systems, which use liquefied biomethane, are also becoming increasingly common [12]. Therefore, a substantial expansion of the biogas sector can be anticipated, especially in Poland, where the resource potential is comparable to that of Germany (the European leader in biogas), yet the sector remains significantly underdeveloped relative to Western European countries.

This paper synthesizes and systematizes existing approaches for evaluating the efficiency of biogas plants based on key operational indicators reported in the literature. In accordance with the general-to-specific principle, the discussion begins with a broad overview and concludes with the presentation of a case study. A novel aspect of this study is the integration of literature data with operational parameters from full-scale biogas plants operating in Poland, Europe and China. It should be emphasized that micro-scale biogas plants (below 50 kW) were excluded from this analysis due to their limited operational efficiency in harsh climates. During the winter of 2025/26 in Poland, many such installations ceased production due to low ambient temperatures or required supplementary heating from coal-fired systems.

2. Types and Characteristics of Feedstock Used in Biogas Plants

The performance and efficiency of biogas plants are strongly dependent on the type and characteristics of the substrates used as feedstock [15]. Biogas plants use a broad spectrum of organic materials, which differ significantly in terms of chemical composition, dry matter (DM) content, biodegradability, and energy potential. The choice of feedstock directly influences biogas yield, methane concentration, process stability, plant scale and overall plant economics [16,17]. Feedstocks used in biogas production can generally be classified into high-energy substrates, characterized by high dry matter content [18,19], and low-energy substrates, which typically contain a high proportion of water and lower organic matter concentrations [20,21]. Modern biogas plants increasingly apply co-digestion, in which substrates with low methane potential are combined with highly energy-rich materials to improve the overall efficiency of anaerobic digestion. This strategy allows operators to balance key process parameters such as the carbon-to-nitrogen ratio, nutrient availability, and organic loading rate. As a result, co-digestion can enhance microbial activity, increase methane yields, and reduce the risk of process instability or inhibition. Furthermore, the use of multiple substrates enables more efficient utilization of locally available biomass and organic waste streams [22,23].

In Europe, particularly in countries such as Germany, Denmark, Italy, and France, biogas plants have historically relied on maize silage, livestock manure and slurry. These substrates provide a reliable source of organic matter and nutrients and have been widely used due to their high availability and predictable composition. However, reliance on a single substrate, such as maize silage, can lead to, for example, seasonal fluctuations in feedstock supply [24,25]. Agricultural biogas plants have served not only as energy production facilities but also as systems for the deodorization of pig slurry. In contrast, Asian biogas systems, especially in China, India, and Southeast Asia have traditionally focused on animal manures, household organic waste, and agricultural residues. These systems often integrate with rural sanitation and smallholder farming, supported by government programs such as China’s household digester initiatives, which emphasize decentralized, low-cost production over high-yield feedstocks. Although large-scale industrial plants utilizing energy-rich substrates are becoming increasingly common [26,27].

Figure 4 and Figure 5 depict scenarios for the agri-food sector with and without the integration of a biogas plant. They illustrate the benefits associated with biogas plant operation and the challenges encountered when a processing facility lacks an effective organic waste management strategy. The biogas plant scenario highlights the potential for utilizing both organic waste and animal feces as substrates in the anaerobic digestion process. This enables the production of biogas, which can be used in a combined heat and power (CHP) unit to generate electricity and heat, or, after upgrading, converted into biomethane. These energy carriers can be used internally by the agri-food sector or sold externally.

Additionally, the anaerobic digestion process produces digestate, which can partially or fully replace mineral fertilizers due to its content of nutrients such as N, P, and K. In contrast, without a biogas plant, animal feces stored as manure lead to the emission of greenhouse gases such as CH₄ and N₂O into the atmosphere [6].

2.1. High-Energy Feedstocks: Characteristics and Role in Biogas Yield

High-energy substrates are defined by their high dry matter content (typically above 20–30%) and a predominance of readily biodegradable organic compounds, such as carbohydrates, lipids, and proteins [28]. Unlike dilute substrates (e.g., slurry), they generate higher specific methane yields. As a result, they contribute significantly to maximizing energy production in commercial biogas plants while reducing transportation costs [29]. In Europe, energy crops have been one of the most important groups of high-energy feedstocks [30]. Maize silage, grass silage, and cereal silage are widely used due to their predictable composition, high methane potential, and year-round availability [31]. The most common feedstock in Europe is maize silage because it is characterized, by among other factors, high biogas yield, a stable composition that support anaerobic digestion, a favorable carbon-to-nitrogen ratio (C:N), the possibility of long-term storage with relatively low energy losses and very high biomass production per unit area [32]. Maize silage is generally not classified as organic waste; however, it is the most commonly used co-substrate in substrate mixtures, which justifies its inclusion in this study. Additionally, sorghum is increasingly utilized for biogas production. It is characterized by high biomass production, a high biogas yield, and strong tolerance to drought and elevated temperatures, allowing it to be grown in poorer soils with lower water and fertilizer consumption than maize. In the context of decreasing rainfall during the growing season in Central Europe as a result of climate change, sorghum cultivation may achieve higher average yields than maize, which has substantially higher water requirements. Furthermore, the plant’s short growing season and good ensiling properties enable its effective use as a substrate in biogas plants, especially in regions experiencing increasing water scarcity [33].

However, due to EU regulations, in recent years, increasing attention has been paid to the competition between energy production and food supply. According to the Renewable Energy Directive III (RED III), the share of biofuels and biomass fuels produced from food and feed crops is limited in order to reduce pressure on agricultural land and avoid competition with food production. Therefore, greater emphasis is currently placed on the use of waste, residues, and non-food biomass as feedstocks for bioenergy production [34,35].

Therefore, biogas plant operators who use technologies capable of utilizing other high-energy feedstock are increasingly seeking alternatives to the aforementioned maize silage. Consequently, for example, food waste and waste oil are increasingly used as substrates in biogas plants. Food waste exhibits high methane yields due to its high fat and carbohydrate content. However, waste oil (e.g., used cooking oils) may require pretreatment (in some older type of installations) due to the risk of inhibition by fatty acids [24,36]. In addition to energy crops, agro-industrial by-products are becoming an increasingly important category of high-energy substrates in biogas plants [37]. These include food processing wastes such as sugar beet pulp, molasses, brewer’s grains, which are characterized by high biodegradability and alignment with circular economy principles [38]. These substrates are commonly characterized by a high organic matter content which translates into methane yields often exceeding those from energy crops [39]. However, seasonal substrates from fruit and vegetable processing can also be characterized by high biogas yields, as is the case with wine production residue—grape pomace [40].

In Asia, high-energy substrates in biogas plants are closely linked to the rapidly developing agro-industrial sectors. In China and Southeast Asia (e.g., Indonesia, Malaysia, Thailand), waste from rice processing, palm oil mills (POME—palm oil mill effluent and solid residues), cassava, and the sugar industry predominate [41,42]. POME, although a liquid waste, is classified as a high-energy feedstock due to its high organic matter content and high chemical oxygen demand (COD). A high COD means that POME contains a large amount of organic matter that microorganisms can break down under anaerobic conditions, releasing large amounts of methane [43]. In Asia, maize straw and poultry manure are increasingly used as substrates in agricultural biogas plants, but their potential is still not fully exploited [44,45]. Maize straw is a cheap and widely available lignocellulosic raw material that, after appropriate pretreatment, provides fermenting microorganisms with structural sugars, increasing overall biogas yield [46]. A high-energy by-product of sugarcane processing is bagasse, which is also commonly used as a substrate in biogas plants in South America [47].

However, dry fermentation (i.e., anaerobic digestion at high total solids (TS) content) is technically more demanding, and wet and semi-dry fermentation remain the dominant operational mode, with feedstock total solids typically below 15% [48]. Consequently, co-digestion of high-energy substrates with low-energy once’s, mixing with diluting materials or digestion circulation is commonly applied to ensure process stability and appropriate hydraulic and organic loading rates [19].

Table 2. Methane yield of selected high-energy feedstocks.

Substrate	Methane Yield	Advantages	Disadvantages
Maize silage	294.5–376.2 m3 CH4/t vs. (Volatile Solids) [49]	High starch content (up to 35%) ensures rapid fermentation, has an optimal C:N ratio [50]	Low C:N ratio (may require nitrogen supplementation) and risk of soil degradation with intensive cultivation [51]
Grass silage	341 m3 CH4/t vs. [52]	High availability and low production costs (marginal agricultural soils) [51]	Lower methane yield than maize (more lignin), greater structural requirements of the fermenter [51]
Cereal silage	300.8 m3 CH4/t vs. for winter wheat, 277 m3 CH4/t vs. for oat [49]	Balanced composition (protein, carbohydrates); quantitatively available raw biomass; contains sugars and starch (e.g., wheat, triticale) that facilitate fermentation [53]	Conflict between food production and feedstock production for biogas plants [54]
Sorghum	287.8 m3 CH4/t vs. [49], 277.25–301.86 m3 CH4/t vs. [33]	Drought-resistant, an alternative to maize in Europe, tolerates higher temperatures [33,55]	Lignin present in sorghum hinders microbial decomposition of the cell wall, which may negatively impact biogas production [56]
Oil	up to 1014 m3 CH4/t vs. [57] with 70% methane content in biogas	Huge energy potential—very high biochemical methane potential (BMP) [57]	Risk of inhibition by LCFAs (long-chain fatty acids) [58]
Whey	395.6–565.8 m3 CH4/t vs. [59]	Very high bioavailability (high lactose content) [59]	Low alkalinity levels, tendency for rapid acidification [60]
Sugar beet pulp	open silos 337–420 m3 CH4/t VS, closed silos 411 to 451 m3 CH4/t vs. [61]	Sugar beet pulp contains 60–70% carbohydrates in dry matter, of which almost 90% is composed of cellulose, hemicellulose and pectin [62]	Seasonal availability from sugar refineries [63]
Raw brewer’s spent grains	301 m3 CH4/t vs. [64], 387 m3 CH4/t vs. [65]	Rich in protein and readily available carbohydrates; a waste product [66]	Often requires pretreatment, partially difficult to decompose [67]
Food waste	467–529 m3 CH4/t vs. [68] *	Very high biodegradability (high content of sugars, fats, proteins), gives a quick and high level of biogas [69]	High protein and nitrogen content, therefore, nitrogen-containing organic compounds, such as proteins, amino acids, and nucleic acids, can be hydrolyzed to ammonium nitrogen [70]
Fruit and vegetable waste	420 m3 CH4/t vs. grape pomace, 231 m3 CH4/t vs. beet leaves [71]	Very easily biodegradable (lots of sugars, pectins), digests quickly [72]	Often high humidity, variable composition, rapid acidification, leading to decreased pH within the bioreactor [19,73]

* depending on the composition (high fat content increases the value).

presents selected high-energy feedstocks.

2.2. Medium and Low-Energy Feedstocks: Slurry and Liquid Substrates

Low-energy substrates are characterized by low dry matter content and lower specific methane yields, but they play a crucial role in ensuring process stability and sustainable biogas production. Livestock manure and slurry are among the most widely used medium- and low-energy feedstocks worldwide, primarily due to their continuous availability and environmental significance [74,75]. Cattle slurry and pig slurry typically contain high water content and relatively low concentrations of volatile solids. While their methane yields per unit mass are lower than those of energy crops, these substrates provide essential buffering capacity, micronutrients, and a diverse microbial community that supports stable digestion processes [76]. In Europe, manure-based biogas plants are often promoted as a tool for reducing greenhouse gas emissions from agriculture and improving nutrient management [77]. Co-digestion of manure with high-energy substrates is a common practice, allowing operators to combine the stability of manure with the high biogas yield of energy-rich materials [78]. Another commonly used substrate is distillery stillage, a highly hydrated and readily biodegradable material with a high content of microelements (e.g., potassium), generated during alcohol production. In 2024, it was the most commonly used substrate in Polish agriculture biogas plants, over 1,500,000 tons of this substrate were used [10]. Another important substrate is whey, a by-product of the dairy industry. Its use in biogas plants supports waste management and reduces environmental burdens [79]. Depending on the degree of dehydration, whey may exhibit medium or low dry matter content.

In Asia, small-scale biogas systems have historically relied almost exclusively on animal manure and human excreta, particularly in rural areas [80]. Although these systems produce relatively low amounts of biogas, they provide significant social and environmental benefits, including improved sanitation, reduced indoor air pollution, and decentralized energy access [81]. Moreover, poultry production in Asian countries, especially in China, is very developed, which has led to an increasing use of poultry manure for biogas production [74]. Poultry manure, rich in nitrogen and readily biodegradable organic matter, is often used as a co-substrate, improving nutrient balance (C:N) and the stability of the fermentation process [82]. In South America, for example, in Brazil, waste from ethanol production, such as vinasse is also a frequently used substrate for biogas plants [83].

For a biogas plant operator, the energy value of the substrate, expressed in terms of substrate mass, is crucial. This is typically optimized by combining raw materials with high, medium, and low dry matter content (Figure 6). It is important because some substrates require transportation to the biogas plant site, necessitating an assessment of the economic viability of transporting a given substrate [84]. Furthermore, transportation can impose additional burdens on local communities.

The comparison of substrate energy quality in relation to dry matter content presented in Figure 6 should be interpreted with caution, as the dry matter content of most substrates can vary considerably. Therefore, under typical operating conditions, a biogas plant operator should request a specification sheet from the supplier when a substrate is delivered for the first time, including at least basic information (DM and ODM content and, preferably, results of biogas yield tests). If such documentation is not available, these parameters should be determined independently in cooperation with a specialized laboratory collaborating with the biogas plant.

The summary of the energy quality of substrates in relation to their dry matter, shown in Figure 6 is quite fluid due to the fact that the DM content range can vary significantly for most of these substrates. Therefore, under typical operating conditions, the biogas plant operator should request a safety data sheet from the supplier of the substrate being used for the first time, containing at least basic information (DM content, ODM, desired biogas efficiency tests), or if it is unavailable, perform it independently in a specialized laboratory cooperating with the biogas plant.

2.3. Co-Digestion Strategies and Optimization of Feedstock Mixtures

One of the key developments in modern biogas technologies is the widespread adoption of co-digestion, defined as the simultaneous anaerobic digestion of two or more substrates with complementary properties [85]. Co-digestion allows operators to balance the limitations of individual feedstocks, improve process stability, and enhance overall biogas and methane yields [86]. High-energy substrates, while attractive due to their high methane potential, often lack sufficient buffering capacity and micronutrients [87]. Conversely, low-energy substrates such as manure and slurry provide alkalinity, trace elements, and a stable microbial environment but generate limited amounts of biogas per unit mass. Combining these substrate types enables synergistic effects that improve digestion kinetics and reduce the risk of process inhibition [88].

However, it is important to emphasize that monofermentation is a process that often requires macro- and micronutrient supplementation in most cases [89]. Generally, a single substrate does not contain all the required micronutrients, which are essential for the proper metabolism of the bacteria involved in the anaerobic digestion process [90]. Liebig’s law of the minimum is particularly relevant in this context, as it states that the dynamics of a given process depend on the nutrient most deficient (Figure 7). Macronutrient deficiencies can also occur, for example, in the case of straw monofermentation [91], where a significant deficiency of nitrogen limits the maintenance of an optimal C:N ratio in the fermenting biomass.

For this reason, co-digestion of substrate mixtures represents a more effective approach. This can significantly reduce the need for mineral supplementation, or even eliminate it altogether [91]. However, regular monitoring of the fermenting pulp, including its macro- and micronutrient composition, should be conducted during biogas plant operation. Operational experience indicates that the best results are achieved by co-digesting substrates with complementary physicochemical characteristics. For example, substrates low in nitrogen and micronutrients, such as cereal or maize straw, are effectively combined with substrates high in nitrogen and micronutrients, such as animal manures (e.g., liquid manure or poultry manure) [46,92]. It is worth emphasizing that anaerobic digestion of liquid manure alone, especially pig slurry, is not very efficient or economically viable due to its low dry matter and organic matter content [93]. Monofermentation of chicken manure, on the other hand, poses significant technological challenges due to the possibility of ammonium inhibition caused by its very high nitrogen content [94]. According to Angelidaki and Ellegaard [95] an economic analysis of existing biogas plants shows that a biogas installation can become economically viable when the average biogas yield exceeds about 30 m³ of biogas per m³ of biomass (which corresponds to roughly 15–20 m³ of CH₄ per m³ of biomass).

To properly plan the substrate mix for an effective fermentation process, several key criteria should be considered, such as:

• Achieving the appropriate C:N ratio in the substrate mix (ideally in the range of 20–30:1) [96].
• Ammonium nitrogen concentration should remain within an optimal range. According to Procházka et al. [97] optimal concentrations for methanogens were 2.1, 2.6 and 3.1 g/L of ammonia nitrogen in dependence on inoculum origin. Higher ammonium nitrogen content may increase the risk of ammonium inhibition, while too low NH₄⁺-N concentrations can cause nitrogen starvation [98]. However, variations in operating temperature, reactor design, and the level of ammonia nitrogen inhibition tolerated by microbial communities across different systems preclude the definition of universal threshold values [99].
• Certain substrates, such as various types of straw or beet pulp, are significantly deficient in essential micronutrients and trace elements essential for the metabolism of microorganisms involved in the anaerobic digestion process. Therefore, substrates should be supplemented with those rich in these elements, such as animal manure [100].
• The average dry matter content of the substrate mix fed to fermenters in wet fermentation should be approximately 12%. A lower value will result in a low organic loading rate (OLR), and as a result, the anaerobic digestion potential of the fermenters will be poorly utilized [101].
• Depending on the substrate mixing technology used and its pumpability, a specific dry matter content (TS) level of the substrate mixture should not be exceeded. Some high-solids technologies allow AD at dry matter contents often exceeding 15% [102].

In general, the primary substrates used in biogas plants should be locally available biomass or biowaste generated by the respective farm or enterprise [103]. However, in some cases, these substrates, such as the previously mentioned straw or beet pulp, can cause technological difficulties and limit process performance due to a lack of certain trace elements [104]. In such cases, it is beneficial to purchase substrates with significantly different physicochemical properties that will compensate for the deficiencies. Otherwise, to maintain the proper anaerobic digestion process and adequate efficiency, targeted micronutrient supplementation may be required [105,106].

2.4. Influence of Feedstock Properties on Biogas Plant Performance

The selection of substrate mixes to create the optimal feedstock has a direct impact on the efficiency of a biogas plant. In the case of wet fermentation, the most widely used method worldwide, the substrate mix should ensure:

• Proper organic loading rate which is the amount of organic matter added to the fermenter daily. It is a key parameter controlling the efficiency and stability of a biogas plant. An OLR that is too high can lead to the accumulation of organic acids and acidification (and consequently to process failure), while an OLR that is too low can result in underutilization of reactor capacity. An appropriate OLR in the digester may vary depending on the substrate or substrate mixture used, temperature of the process and technology used [107,108]. For agricultural biogas plants, it typically should not exceed approximately 4–5 kg of VS/m³/d. Seick et al. [109] in laboratory conditions reached 5.8 g VS/L/day using maize silage and changing the regime from mesophilic to thermophilic conditions; Lindorfer et al. [110] in real-scale biogas plant conditions increased OLR from 2.11 to 4.25 kg of VS/m³/d in an agricultural biogas plant using pig manure, solid energy crops such as silages from maize and rye, ground grains from maize and wheat, and residues from vegetable processing as feedstock. Tamborrino et al. [111] described an installation in Italy fermenting olive waste (pulp and pitted olive pomace) and pulp pitted olive pomace and wheat bran shorts in the amount of 10% of the total powered biomass operated at an OLR of approximately 5.3–5.5 kg VS/m³/d. In turn, in the study by Jiang et al. [112] digested organic fraction of municipal solid waste (OFMSW) in a semi-continuous mode showed that the maximum achievable OLR was 9.00 g VS/L/d but the recommended operating conditions for the anaerobic digestion of OFMSW is an OLR of 7.50 g VS/L/d and thermophilic condition.
• Efficient mixing and substrate homogenization are essential for stable anaerobic digestion. Mechanical pretreatment such as grinding or chopping of biomass improves substrate accessibility for microorganisms and enhances the hydrolysis stage of the digestion process [113]. Particle size reduction increases the specific surface area of the substrate, facilitating microbial attachment and enzymatic degradation of organic matter. As reported in experimental studies, smaller particle sizes significantly improve methane production due to enhanced mass transfer and substrate availability for anaerobic microorganisms [114,115]. Mechanical pretreatment also improves reactor hydrodynamics and mixing efficiency. In industrial biogas plants, substrates are often milled or shredded before feeding to produce a pumpable slurry that can be effectively mixed in continuously stirred tank reactors. Adequate mixing ensures uniform distribution of microorganisms, nutrients and heat, which is essential for maintaining stable digestion performance [33,116].
• The absence of inhibitory compounds (e.g., antibiotics, toxic chemicals, excessive ammonium nitrogen). Antibiotics are among the most frequently reported inhibitors of anaerobic digestion because they directly affect microbial metabolism. These substances can disturb the activity of microorganisms responsible for the degradation of organic matter and the final stage of methane formation. According to Koniuszewska et al. [117] antibiotics in wastewater and cattle slurry used as substrates in the AD process can pose serious problems for anaerobic microorganisms, because they may reduce the activity of methanogenic microorganisms or alter the microbial community structure responsible for methane production [24]. Experimental studies confirm that many antibiotics can significantly reduce biogas production. For example, the addition of antimicrobial compounds to cattle manure during mesophilic anaerobic digestion caused substantial decreases in methane generation; in some cases, amoxicillin caused a 75% decrease in CH₄ production in comparison with the control samples. Enrofloxacin, tetracycline, oxytetracycline, and chlortetracycline reduced the amount of biogas produced by 36, 39, 45 and 53%, respectively [117]. Therefore, maintaining substrates free from inhibitory compounds is essential for stable biogas plant operation. In practice, this requires careful control of feedstock sources, avoiding wastes contaminated with pharmaceuticals, disinfectants, or toxic industrial chemicals. In cases where inhibitors are present, strategies such as dilution, co-digestion with clean substrates, or adsorption using materials like biochar may help mitigate their negative effects on the anaerobic digestion process [73,118]. Another inhibitor may be ammonium nitrogen, which is found in large quantities in poultry manure, which can affect the operation of biogas plant [119]. In this context, cooperation between the farmer supplying animal manure to the biogas plant and the biogas plant operator is crucial [120]. If a herd becomes ill and a veterinarian administers a strong dose of antibiotics to the animals, a significant portion of the antibiotic will be excreted by the animals and found in the slurry or manure. There are numerous cases where digested pulp containing antimicrobials and antibiotics reduces or even stops the fermentation process [121,122,123].
• Removal of mineral and other non-biodegradable contaminants from the feedstock is a critical step in the operation of biogas plants. Substrates used for anaerobic digestion, such as manure, agricultural residues, food waste or municipal organic waste, often contain inert materials including sand, stones, soil particles, glass, metals or plastics [124,125]. These materials are not degradable in the anaerobic digestion process and may lead to both operational and mechanical problems and can cause damage to mixers, pumps, or valves. In turn, Hoffmann et al. [126] observed a positive effect of sand on methane yield—sewage sludge degradation to CH₄ was significantly enhanced in the presence of sand.
• The pH of the input substrate is generally of secondary importance, as a well-managed biogas plant maintains a stable pH in the individual fermentation reactors (maintained spontaneously by cooperating groups of bacteria). Unless substrate is fed too intensively, which would overload the fermenter and cause acidification. The appropriate pH level, is difficult to determine, it depends on the technology used and on whether the digestion process phases are separated. Ward et al. [127] assumed that the pH level suitable for methanogenesis ranges from 6.8 to 7.2; on the other hand Khalid et al. [128] assumed that the pH levels suitable for hydrolysis and acidogenesis are 5.5 and 6.5, respectively. Some experts (biogas plant operators) even recommend values for methanogenesis at pH level of 7.4–7.7.

Figure 8 schematically presents the key feedstock characteristics influencing the anaerobic digestion process.

The combination of feedstock properties and operational strategies presented in Figure 8 can be integrated within a biogas plant to generate synergistic effects that enhance overall process efficiency. For example, some chemical inhibitors present in the feedstock (e.g., antibiotics) can be decomposed during fermentation in a biogas plant with a separated acid hydrolysis stage, which reduces the risk of inhibition [129]. Moreover, this combination (substrates and low-pH hydrolysis) can also significantly shorten the hydraulic retention time (HRT) of the feedstock used. Shortening the HRT is also possible through additional activities, such as mechanical and/or thermal pretreatment, which can ultimately lead to increased biogas productivity.

3. Internal Consumption of Electricity and Heat in the Biogas Plant

The energy efficiency of a biogas plant depends not only on its ability to produce biogas efficiently but also on the amount of electricity consumed for its own operation. Reducing the internal energy demand is crucial to the profitability of a biogas investment. The greatest potential for optimization lies in mixing systems (electricity) and tank design (thermal energy). Typical biogas plants use approximately 15–25% of the generated electricity to power equipment such as mixers, pumps, grinders, and substrate conveyors [130,131]. In small-scale biogas plants, auxiliary equipment may even affect electrical power quality [132]. Mechanical grinders are among the devices with the highest power requirements, reaching up to 100 kW in a 1 MW biogas plant. However, grinders operate only intermittently and do not consume electricity continuously. The equipment that consumes the most electricity in biogas plants is mixing system. Lemmer et al. [133] stated that mixing consumes up to 51% of total electric energy consumption in the biogas production process at the research biogas plant of the University of Hohenheim. A typical 1 MW biogas plant usually has two or three fermentation chambers with diameters ranging from 26 to 38 m. Each fermenter is equipped with two to four lateral mixers, with power ratings typically between 8 and 15 kW. These mixers are designed to prevent the formation of a crust and sediment, and they operate either continuously or in an intermittently optimized mode [134]. Consequently, when the electricity consumption of all the mixers in the fermentation tanks and periodically in the digestate tank is combined, mixing represents the most energy-intensive process in the operation of a biogas plant [133]. In the study by Björn et al. [135], the rheology-driven power requirements for mixing varied from 0.0000001 to 0.002 kW/m³ of digester volume (depending on sludge viscosity), which was lower than the typical range of 0.005 to 0.008 kW/m³ reported by Appels et al. [136].

It is worth emphasizing that there are solutions that can significantly reduce electricity consumption during mixing. One of the most effective approaches is vertical mixing. Two main vertical mixing methods are commonly used: a vertical agitator with a vertical axis, centrally positioned in the reactor and either mounted at the bottom or ending slightly above it, and a central jet agitator (as shown in Figure 9) [137,138]. Using a single centrally located vertical agitator, the pulp can be homogenized with just one motor, typically rated between 15 and 25 kW. This setup ensures thorough and symmetrical mixing without dead zones, while achieving the same level of homogenization with lower electricity consumption compared to conventional fermentation tanks. The shape of the digester also influences mixing efficiency because if the diameter of the tank is several times higher than its height (typical shape of old-type fermenters)—then mixing becomes difficult and ineffective [138].

Another method that can reduce electricity consumption is hydraulic mixing. In this approach, substrate slurry is recirculated using a hydraulic pump located outside the fermenter and reintroduced into the fermenter at high velocity at a different location. This method is often combined with heating the pulp in a heat exchanger to enhance process efficiency. However, hydraulic mixing can be problematic when processing poorly ground substrates containing lignocellulosic biomass, as there is a high risk of scum formation on the surface of the fermenting pulp [139].

The most energy-efficient mixing method is pneumatic mixing, which can be implemented in two principal configurations. The first approach involves drawing biogas from the top of the fermenter and injecting it through a small pump into perforated tubes located at the bottom of the fermentation tank. A particularly effective variant consists of sequentially activating individual tubes, creating a wall of upward-flowing biogas bubbles that gradually circulate throughout the entire tank [140]. This method minimizes electricity consumption, as a pump with a power rating of less than 1 kW is sufficient to achieve effective mixing. However, the most energy-efficient mixing method is pneumatic mixing, in which the movement of the pulp is caused periodically by compressed biogas produced by the bacteria.

Biogas plants using modern mixing methods, such as central, hydraulic, or pneumatic mixers, instead of traditional systems can reduce their electricity consumption for internal needs. Another crucial element in operating a biogas plant is synchronizing the operation of devices so that they are activated sequentially rather than simultaneously [19,137].

The heat consumption of a biogas plant’s own needs is closely related to the type of installation and the technology used. Biogas plants operating in thermophilic mode, meaning most often at temperatures between 50 and 55 °C [103,141], will require higher heat consumption than those operating in mesophilic mode (35–42 °C) [142]. Similarly, biogas plants using substrates requiring thermal hygienization (e.g., slaughterhouse waste) require a larger amount of heat for thermal pretreatment [143]. A similar situation occurs when the biogas plant is equipped with a thermal digestate dehydration system. The evaporators used for digestate dehydration can even consume all of the heat produced by the cogeneration engine [144].

Thus, traditional biogas plants, with large-diameter reinforced concrete fermentation tanks up to 36 m in diameter and covered with flexible single- or double-layer membranes, consume on average 25–35% of the heat generated for internal purposes. Therefore, other energy sources are also sought to ensure the proper operation of biogas plants [145,146]. Steel tanks constructed in other proportions, where the tank height is equal to or greater than its diameter, require significantly less heat to maintain stable fermentation or triple or even quadruple membranes, consuming about 15–20% of the generated heat [147,148].

The problem of heat management in biogas plants is significant because the amount of heat generated during electricity generation is slightly greater than the amount of electricity produced. For this reason, biogas plant owners are trying to implement various methods of heat utilization [149], the most popular of which include:

• Building local heating pipelines and heating supply to nearby buildings [150];
• Building drying facilities for wood, agricultural products or digestate drying [151];
• Heating greenhouses [152];
• Combining biogas production with ethyl alcohol production [153];
• Producing cold from heat using trigeneration technology and utilizing the cold in industrial installations (dairies, cold stores, etc.) [154].

There are also other, much more advanced ways of utilizing heat, but they are not widely used. For example, near Poznań, Poland, there is a unique in Europe biogas installation with an electrical capacity of 1 MW, which uses the generated heat to produce deep cooling (−8 °C) for the needs of a nearby vegetable cold store, chilled water (4 °C) for cooling sow farms in spring and summer, and electricity in an Organic Rankine Cycle (ORC) system used for the biogas plant’s own needs, with the remaining heat used to heat the farm buildings (52°04′35.9″ N 17°10′27.7″ E).

4. Energy Efficiency in Biogas Plant Operation

The operational electric efficiency of a biogas plant is as the annual electricity production (MWh) per 1 MW of installed electrical capacity (MWh/MW). This indicator can therefore also be applied to biogas plants with CHP of any capacity. It is commonly referred to as the electrical efficiency of a biogas plant. Since a year consists of 8760 h, 100% efficiency would mean producing 8760 MWh from 1 MW of installed power [155]. In practice, however, this level is unattainable, as the cogeneration engine must be serviced every 700–1200 operating hours (changing oil, spark plugs, filters, etc.), which requires the cogeneration plant to be idle for several, sometimes even several dozen, hours. Furthermore, other cogeneration unit failures also occur, which also reduce the electrical efficiency of biogas plants [156,157].

As a result, the electrical efficiency of biogas plants in Germany is estimated at over 70%, with a production of approximately 7300 MWh per 1 MW installed but this is variable due to biogas plants operating in demand-driven mode. In Poland, the latest data covering the entire biogas market were published in 2019 by the Ministry of Energy. According to these data, biogas plants classified as renewable energy installations produced on average 5250 MWh per 1 MW of installed capacity, corresponding to an efficiency below 60% [158]. However, since some installations in Poland generate even over 8500 MWh/1 MW (over 95% efficiency), it can be assumed that there are also biogas plants operating with efficiency below 50% [6]. Low electrical efficiency is primarily associated with suboptimal plant operation, including insufficient substrate feeding, inefficient fermentation processes, and operational errors [159,160]. It should also be noted that the inefficient anaerobic digestion process, which results in low electrical efficiency of the biogas plant, also results in poorly decomposed digestate and, as a result, may contain large amounts of dry matter [77,161]. Such digestate often requires mechanical separation to remove excess undegraded solids. Only then can the liquid fraction be used as a diluent for the substrate fed to the biogas plant [162]. Conversely, an effectively conducted AD process produces highly digestible digestate, with a dry matter content below 3%. This result is typically observed in biogas plants operating at efficiency levels exceeding 90% (Figure 9). Therefore, analyzing the dry matter content of the digestate is also one of the best indicators of the fermentation process’s effectiveness [163].

5. Discussion and Future Outlook

Biogas production via anaerobic digestion is primarily suited for the treatment of so-called “wet” biowaste, while biowaste with a higher dry matter content (above 50%) particularly that rich in lignocellulosic compounds can also be processed in thermal processes such as pyrolysis or gasification [164]. Dozens of different biogas technologies are currently in operation worldwide, ranging from primitive installations without heating and mixing systems constructed of plastic or bricks, to technically and technologically advanced biogas plants controlled by automation and remote control systems, also capable of processing over 1000 tons of substrate per day [27,165]. Technological diversity is particularly evident in the global leader in biogas markets—China, where industrial biogas plants operate using a wide variety of technologies. This diversity also applies to substrates, especially biowaste, which have very different compositions and yields (Table 2, Figure 6), which influences the efficiency of the fermentation process. Furthermore, biogas production can be more efficient if appropriate operational mechanisms are used for specific feedstock types in the biogas plant (Figure 8).

In Europe, the situation is less diverse; of the 20,000 biogas plants, over 90% use technologies developed in the 1980s [166,167]. Poland, with fewer than 500 biogas plants (including only 204 agricultural plants), is currently one of the smaller biogas markets in the EU [168]. However, it currently has the greatest investment potential in Europe, estimated to be equal to or slightly greater than Germany, which has nearly 10,000 biogas plants. Currently, the Polish market is becoming increasingly diverse, attracting interest from companies from virtually all over Europe and China [167,169]. Therefore, it seems that investors planning to build biogas plants should verify the technologies offered in terms of AD efficiency, the range of substrates used, electrical efficiency, and the quality of the digestate obtained. This will impact the Polish energy market in the near future, where coal must be phased out by 2040, and biogas is the only stable and fully controllable renewable energy source available on a large scale.

6. Conclusions

This study demonstrates that installed capacity alone is an insufficient metric for evaluating biogas plant performance; instead, full-load equivalent indicators (MWh/MW) provide a robust, technology-neutral benchmark enabling cross-comparison of systems operating under diverse conditions. Feedstock quality emerges as a primary driver of process efficiency, with substrate heterogeneity (in terms of dry matter, biodegradability, and nutrient composition) critically shaping methane yield, process resilience, and economic outcomes. Co-digestion is identified as one of the most important elements of optimization strategies, enabling synergistic enhancement of digestion kinetics, improved nutrient balance, and mitigation of inhibition phenomena. Internal energy demand, particularly for mixing (electric energy) and maintaining process temperature (thermal energy), represents a major efficiency bottleneck, highlighting the need for advanced reactor design and energy optimization strategies to maximize net energy output. The presence of inhibitory compounds, including antibiotics and ammonia, constitutes a critical operational risk; however, technological configurations such as separated acid hydrolysis stages can partially mitigate these effects and enhance process robustness.