FAQ: Biogas

An agricultural biogas plant uses mainly agricultural feedstocks (slurry, silages, agricultural by-products), while a waste-processing plant processes primarily waste, e.g. from the food industry, municipal biowaste or sludge.

The differences concern not only technology but also law: processing waste requires waste management permits, compliance with BDO requirements, hygienisation and control of the composition of the waste. In Poland there is also a legal definition of agricultural biogas. Agricultural plants usually have simpler logistics, while waste-processing plants have greater variability of feedstock and income from gate fees for accepting waste.

Sources: Ustawa z 13 lipca 2023 r. o ułatwieniach w inwestycjach w zakresie biogazowni rolniczych, Dz.U. 2023 poz. 1597; Dyrektywa 2008/98/WE w sprawie odpadów — EUR-Lex; Ustawa o odnawialnych źródłach energii (tekst jednolity Dz.U. 2024 poz. 1361), art. 2 pkt 2 — definicja biogazu rolniczego — ELI

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Not entirely. The figures on biogas use refer to fiscal year 2004 (JFS, 2008), and the description of B-DASH dates from 2013–2014; they should be treated as a picture of proportions, not the current state. Current statistics are published by MLIT.

Sources: NILIM / MLIT – Technology Introduction Guidelines for the B-DASH Project (Biogas utilization technology); Japan for Sustainability – Japanese Municipalities Targeting Energy Self-Sufficiency at Sewage Treatment Plants (JFS Newsletter No.71, 2008)

Discussed in the article: Biogas from sewage sludge in Japan: how much gas is used and what the B-DASH programme delivered

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Capacity (kW, MW) is the instantaneous ability of a plant to generate energy, and energy (kWh, MWh) is the amount actually generated over a given time.

A biogas plant with an electrical capacity of 1 MW running without a break for a year would theoretically generate 8760 MWh, but in practice capacity utilisation is lower because of inspections, failures, substrate constraints or grid parameters. The capacity utilisation rate is given in equivalent hours of full-load operation per year or as a load factor. When comparing offers, always ask about both the rated capacity and the expected annual yield.

Sources: Basic Information about Anaerobic Digestion — US EPA

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Biomethane is biogas cleaned and upgraded to natural gas quality, with methane usually above 95%, whereas raw biogas has around 50–70% methane.

Upgrading consists in removing CO₂, hydrogen sulphide, moisture and other impurities, e.g. by membrane separation, water scrubbing, pressure swing adsorption or amine scrubbing. Biomethane can be injected into the gas grid, compressed (bioCNG) or liquefied (bioLNG). Biogas is usually burned on site in a cogeneration engine. Detailed quality requirements for injection depend on the grid operator and the standards in force.

Biogas also contains corrosive compounds, mainly hydrogen sulphide.

Sources: Basic Information about Anaerobic Digestion — US EPA; European Biogas Association; European Biogas Association; IEA Bioenergy Task 37 — Energy from Biogas

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A biogas plant can adjust its energy production to demand if it has a larger gas holder, additional cogeneration capacity and a controlled substrate feeding system.

Digestion is a continuous process, so a change in output is achieved primarily through the gas buffer and an additional engine-generator, not by an immediate change of production in the digester. Flexible operation allows energy to be sold in higher-price hours and supports the grid. It requires investment in a tank and a control system, and ensuring that frequent starts and stops do not shorten the engine's life.

Sources: Anaerobic Digester/Biogas System Operator Guidebook — US EPA AgSTAR

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Yes, but it requires separate collection of biowaste, airtight reception halls, odour control and management of the gas and digestate. Examples from Berlin and Oslo show that it is possible with proper organisation.

Sources: BSR — Aus Bioabfall wird Biogas (materiał informacyjny); Berliner Stadtreinigung: Fahrgestelle mit Erdgasmotor — Umweltbundesamt; Oslo — Romerike Biogassanlegg — Cambi

Discussed in the article: Biogas in urban agglomerations: examples from Berlin and Oslo and lessons for Poland

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Yes, kitchen biowaste and food leftovers are a good substrate for biogas plants, but they require segregation, removal of packaging and usually hygienisation.

This waste has a high share of readily degradable matter, so considerable amounts of biogas can be obtained from it, but it can also be a source of inhibitors (ammonia, lipids, salt). Contaminants such as plastic or glass must be removed in the preparation line (depackaging, screens, separators). In the EU, biowaste is subject to a separate collection obligation, and its processing is regulated by the rules on waste and animal by-products.

Sources: Dyrektywa 2008/98/WE w sprawie odpadów — EUR-Lex; Basic Information about Anaerobic Digestion — US EPA

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In a typical agricultural biogas plant, yes — in one digester or in several operating in parallel. Separating hydrolysis from methanogenesis (a two-stage system) is used less often, mainly for difficult substrates or in research.

Sources: Weiland P. (2010), Biogas production: current state and perspectives, Appl. Microbiol. Biotechnol. 85, 849–860; Batstone D.J. et al. (2002), The IWA Anaerobic Digestion Model No. 1 (ADM1), Water Sci. Technol. 45(10), 65–73

Discussed in the article: The four stages of methane fermentation: from hydrolysis to methane

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Yes, many larger wastewater treatment plants stabilise sludge in digesters, obtaining biogas that is used to generate energy and heat for the plant's own needs.

Sludge digestion reduces its volume, improves dewatering and limits odour. Biogas from treatment plants usually contains methane and CO₂ as well as contaminants such as siloxanes, which require cleaning before combustion in an engine. Energy from biogas reduces purchases from the grid and the operating costs of the facility. Sometimes co-digestion with additional substrates is also carried out, if the permits allow it.

Sources: Basic Information about Anaerobic Digestion — US EPA

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A properly operated biogas plant should not cause a significant odour nuisance, but odour may appear during substrate storage, unloading or the spreading of digestate.

The digester itself is closed, and the biogas is collected by a sealed system and directed to a cogeneration engine or to upgrading, so under correct operation it does not escape into the surroundings. Sources of odour can be an open silage clamp, food waste with a high content of readily degradable matter, uncovered tanks or improperly managed digestate. They are limited by closing clamps quickly, sealed tanks, proper planning of deliveries and soil injection of digestate. Good practice is to assess the impact at the siting stage.

Sources: Basic Information about Anaerobic Digestion — US EPA; Anaerobic Digester/Biogas System Operator Guidebook — US EPA AgSTAR

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No. It helps only where there is a real deficiency. At sufficient concentrations it has no effect, and an excess of some metals can inhibit the process and impair digestate quality.

Sources: Chen Y., Cheng J.J., Creamer K.S. (2008), Inhibition of anaerobic digestion process: A review; Weiland P. (2010), Biogas production: current state and perspectives; IEA Bioenergy Task 37

Discussed in the article: Trace elements in a biogas plant: cobalt, nickel, selenium, molybdenum and iron

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No, biogas has a calorific value of around 5–7 kWh/m³, i.e. roughly half that of natural gas, because it contains a lot of non-combustible CO₂.

Pure methane is about 10 kWh/m³ (calorific value under normal conditions), and biogas with 55–60% CH₄ has proportionally less. For this reason biogas fuels engines adapted to low-calorific fuels, and after upgrading to biomethane it reaches natural gas parameters and can be injected into the grid. The calorific value changes with composition, so base energy settlements on measurements.

Sources: Basic Information about Anaerobic Digestion — US EPA

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No. The support is paid per kilowatt-hour of biogas produced from slurry, not per electricity produced.

Sources: Regeringen — Regeringen förlänger stöd för gödselgas (27.06.2023); Riksdagen — Svar på skriftlig fråga: Gödselgasstödet

Discussed in the article: Sweden: the government extends support for biogas from slurry (gödselgasstöd) for Q4 2023

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No. Adapted microorganisms work at concentrations that would be toxic for new biomass. What matters is free NH₃, the rate of change and the symptoms: a rise in VFA, a fall in CH₄ and in gas production.

Sources: Rajagopal R., Massé D.I., Singh G. (2013), A critical review on inhibition of anaerobic digestion process by excess ammonia, Bioresour. Technol. 143, 632–641; Chen Y., Cheng J.J., Creamer K.S. (2008), Inhibition of anaerobic digestion process: A review

Discussed in the article: Ammonia in the digester: inhibition, thresholds and how to respond

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Biogas is usually stored in a double membrane above the digester or in an external foil tank, at a pressure close to atmospheric.

The storage buffers differences between gas production and its consumption by the engine. Its capacity is usually sufficient for several to a dozen or so hours of operation, not for seasonal storage. The membranes require leak checks and protection from wind and damage, and the space between the foils is maintained by blown air. A larger buffer is provided only by compression after upgrading to biomethane, but that is a different technology.

Sources: Anaerobic Digester/Biogas System Operator Guidebook — US EPA AgSTAR

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A well-maintained biogas plant can operate for several decades, but individual components, such as the engine, agitators, pumps or membranes, require regular replacement.

Reinforced concrete digesters are durable, whereas moving parts wear depending on the quality of the substrates and maintenance. Cogeneration units have defined intervals between overhauls (usually counted in thousands of operating hours). Scheduled servicing, oil changes, leak checks and upgrades extend the life of the plant. When assessing profitability, the residual value and the costs of overhauls should be taken into account in the analysis.

Sources: Anaerobic Digester/Biogas System Operator Guidebook — US EPA AgSTAR

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The 2020 SNV brochure gives "over 65,000" biodigesters in five countries; an older SNV project page says "over 50,000", and Hivos 60,000 in phase 2 alone (2014–2019). The differences result from different dates and reporting periods.

Sources: Africa Biogas Partnership Programme (ABPP) — SNV; Africa Biogas Partnership Programme — Hivos; Africa Biogas Partnership Programme — broszura SNV/Hivos (maj 2020); Clemens et al., Africa Biogas Partnership Program: A Review of Clean Cooking Implementation through Market Development in East Africa — Energy for Sustainable Development (2018)

Discussed in the article: Africa Biogas Partnership Programme in Kenya and the region: how many biodigesters were built and on what terms

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The demand for crops depends on the capacity and the share of silage in the recipe: a biogas plant based entirely on maize needs several hundred hectares for every MW of electrical capacity, but many plants use slurry and waste.

With silage maize yields in the order of several tens of tonnes per hectare and electricity production of several hundred kWh per tonne, the calculation can be made independently for a specific capacity. In practice recipes are mixed and also include green forage, beet pulp or waste, so the acreage is smaller. Crop rotation and the use of digestate as fertiliser on the same fields must also be taken into account.

Sources: Basic Information about Anaerobic Digestion — US EPA

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An agricultural biogas plant with a capacity of around 1 MW usually requires several employees, including a process operator, but the number depends on the degree of automation and substrate logistics.

The tasks include dosing substrates, supervising parameters, checking the gas, operating tractors and loaders, maintenance and documentation. Modern controllers with visualisation and alarms reduce the workload but do not replace a competent operator. It is worth remembering cover arrangements and health and safety training, especially concerning explosion hazard zones and hydrogen sulphide.

Sources: Anaerobic Digester/Biogas System Operator Guidebook — US EPA AgSTAR

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A tonne of cattle or pig slurry usually yields only a dozen or so to a few dozen cubic metres of biogas, many times less than maize silage.

The reason is the low content of volatile solids: slurry is mostly water, and part of the readily degradable matter has already been digested in the animals' digestive tract. Despite its low specific yield, slurry is a valuable substrate: it provides an alkaline buffer, microorganisms and nutrients, and its digestion reduces methane emissions from storage. In practice it is used in co-digestion with more energy-rich substrates.

Sources: Basic Information about Anaerobic Digestion — US EPA

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The cost of building a biogas plant depends mainly on its capacity, the scope of technology and the location, so there is no single price; a quotation requires an individual design.

Capital costs are affected by: installed capacity, the number and size of digesters, the substrate and digestate storage system, connection to the electricity grid, cogeneration or upgrading, and environmental requirements. Added to this are the costs of documentation, permits and commissioning. An investor should compare offers with an identical scope of supply and analyse not only the investment cost but also the availability of substrates, the energy sales model and operating costs.

Sources: European Biogas Association

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From one tonne of fresh maize silage you can usually obtain in the order of 350–480 kWh of gross electricity, depending on the dry matter content, the quality of the silage and the efficiency of the engine.

A rough calculation: a tonne of silage typically yields around 95–115 m³ of methane, and 1 m³ of CH₄ contains about 10 kWh of chemical energy. A cogeneration engine with an electrical efficiency of 38–42% converts part of this into electricity and the rest into heat. These are indicative values: the actual yield depends on the variety, harvest date, losses in the clamp, retention time in the digester and the plant's own energy consumption.

Sources: Basic Information about Anaerobic Digestion — US EPA; Towards a standardization of biomethane potential tests — Holliger et al., Water Sci. Technol. (2016)

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A biogas plant is an industrial facility with explosion hazards and toxic hydrogen sulphide, but with proper design and operation the risk to the surroundings is limited.

The main hazards concern workers: a mixture of methane and air is explosive, and H₂S is toxic. Plants are designed on the basis of explosive atmosphere zones (ATEX), and gas detectors, safety valves and an emergency flare are used. Permits, acceptance inspections and staff training are required. The risk depends on the quality of workmanship and supervision, which is why inspections and servicing are crucial for safety.

Sources: Dyrektywa 1999/92/WE — ochrona pracowników w atmosferach wybuchowych — EUR-Lex; Anaerobic Digester/Biogas System Operator Guidebook — US EPA AgSTAR

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Yes, biogas from biomass or biowaste is recognised as a renewable energy source, provided that it meets legal criteria and, in certain situations, sustainability criteria.

Under EU law, biogas produced from biomass falls within the definition of renewable energy (the RED directive), and Polish regulations provide separate rules for it concerning support and registration. Sustainability and greenhouse gas emission reduction criteria apply especially to larger plants and to transport fuels. The specific rules have changed over time, so it is worth checking the current wording of the regulations.

Sources: Dyrektywa (UE) 2023/2413 (RED III) — EUR-Lex; Ustawa z 13 lipca 2023 r. o ułatwieniach w inwestycjach w zakresie biogazowni rolniczych, Dz.U. 2023 poz. 1597

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Not necessarily. In plants running on slurry and with high ammonium nitrogen, a pH of 7.8–8.2 is common. The risk relates mainly to free NH₃, which rises with pH and temperature; assess it together with TAN.

Sources: Chen Y., Cheng J.J., Creamer K.S. (2008), Inhibition of anaerobic digestion process: A review; FNR — Fachagentur Nachwachsende Rohstoffe

Discussed in the article: pH in the digester: which range is correct and what changes mean

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Landfill gas forms spontaneously in the waste body and is collected through degassing wells, while biogas from a digester forms in a controlled tank with a dosed substrate.

The composition of both is similar (methane and CO₂), but landfill gas may be contaminated with air, nitrogen and volatile compounds from mixed waste, and its production is harder to control and changes over time. In a digester it is possible to maintain constant conditions and obtain a stable gas composition. Collecting landfill gas limits emissions and ensures safety, but it is a different technical solution.

Sources: Basic Information about Anaerobic Digestion — US EPA

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Methane fermentation (anaerobic digestion) proceeds in four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis, carried out by different groups of cooperating microorganisms.

In hydrolysis, complex polymers (cellulose, proteins, fats) are broken down into simple sugars, amino acids and fatty acids. Acidogenesis produces volatile fatty acids, alcohols, H₂ and CO₂. Acetogenesis converts these into acetate, hydrogen and CO₂, and methanogenesis, carried out by archaea, yields methane. The slowest stage depends on the substrate: for lignocellulosic biomass it is usually hydrolysis, and for easily digestible substrates, methanogenesis.

Sources: Basic Information about Anaerobic Digestion — US EPA; Anaerobic Digester/Biogas System Operator Guidebook — US EPA AgSTAR

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The profitability of a biogas plant depends above all on the price and availability of substrates, the sale of energy or biomethane, the investment cost and the stability of plant operation.

Substrates are usually the largest operating cost item, so long-term contracts with suppliers and alternative waste feedstocks are very important. On the revenue side, what counts is the support system or market price, the use of heat from cogeneration and the possible sale of digestate. Plant availability (operating hours per year) is also crucial, supported by good servicing and process monitoring. The analysis should be based on current regulatory conditions.

Sources: European Biogas Association; Urząd Regulacji Energetyki

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TS (total solids) is the dry matter of a sample, and VS (volatile solids) is the organic dry matter, i.e. the part of it that burns off at approx. 550°C and can be a source of biogas.

TS is determined by drying the sample at approx. 105°C, and VS by igniting the dry residue at approx. 550°C and measuring the loss of mass. Gas yield is most often given per VS (e.g. NmL CH₄/g VS), because this quantity allows different substrates to be compared regardless of moisture content. In Polish the abbreviations s.m. (dry matter) and s.m.o. (organic dry matter) are also used.

Sources: Anaerobic Digester/Biogas System Operator Guidebook — US EPA AgSTAR

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Mtoe is a million tonnes of oil equivalent. According to the IEA conversion factor, 1 Mtoe is 11.63 TWh, i.e. about 1.21 billion m³ of natural gas.

Sources: IEA — Outlook for biogas and biomethane: Prospects for organic growth

Discussed in the article: IEA on biogas and biomethane: potential to cover approx. 20% of global gas demand

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A digester is a sealed, heated and mixed tank in which microorganisms break down the substrate into biogas and digestate under anaerobic conditions.

Most often it is a reinforced concrete or steel tank with a membrane gas-storage roof, equipped with agitators, heating (usually up to 38–42°C in the mesophilic process), temperature and level sensors, and a substrate feeding system. The substrate is fed in cycles, and the same volume of mixture leaves the tank. Good mixing evens out the temperature and the contact between bacteria and feed, but overly intensive mixing can impair the cooperation of the microorganisms.

Sources: Basic Information about Anaerobic Digestion — US EPA

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Agricultural biogas in Polish law is gas obtained through the methane fermentation of, among others, agricultural products and residues from agriculture, fisheries, aquaculture or forestry (including animal excrement), as well as products, waste and residues from agri-food processing.

The definition is contained in the Act on Renewable Energy Sources (ustawa o OZE) and gives rise to a special legal regime, including entry in the register of agricultural biogas producers kept by KOWR. Biogas from municipal waste, from landfills and (as a rule) from wastewater treatment plants is not agricultural biogas. The precise wording of the definition and the requirements should be checked in the current text of the act, because the provisions have been amended.

Sources: Ustawa z 13 lipca 2023 r. o ułatwieniach w inwestycjach w zakresie biogazowni rolniczych, Dz.U. 2023 poz. 1597; Krajowy Ośrodek Wsparcia Rolnictwa — gov.pl

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Biogas is a mixture of gases, mainly methane (CH₄) and carbon dioxide (CO₂), produced when microorganisms break down organic matter in the absence of oxygen.

The process occurs naturally in marshes, in the rumen of animals and in landfills, and in a biogas plant it is carried out in a closed tank called a digester. The feedstock can be slurry, silage, waste from the food industry, biowaste or sewage sludge. Besides CH₄ and CO₂, biogas contains traces of hydrogen sulphide (H₂S), water vapour and sometimes ammonia. It is methane that carries the energy.

Sources: Basic Information about Anaerobic Digestion — US EPA

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Biogas usually contains approx. 50 to 70% methane and approx. 30 to 50% carbon dioxide, and the remainder is water vapour and trace amounts of H₂S, NH₃, nitrogen and oxygen.

The methane share depends on the type of substrate and the process conditions. Feedstocks rich in fats usually give biogas with a higher CH₄ content, and those rich in carbohydrates, a lower one. The hydrogen sulphide content can range from several dozen to several thousand ppm, which is why plants use desulphurisation. Values for a specific biogas plant should always be taken from current gas analyser measurements, not from general tables.

Sources: Basic Information about Anaerobic Digestion — US EPA

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Co-digestion is the simultaneous digestion of at least two different substrates, e.g. slurry with silage or food waste, in a single digester.

The aim is to achieve a better balance of nutrients, higher biogas production and a more stable process. Slurry provides buffering capacity and trace elements, and energy-rich substrates increase gas production. However, the proportions must be monitored to avoid organic overload, acidification or excess ammonia. The components of the recipe are selected on the basis of analyses and BMP tests, and changes are introduced gradually.

Sources: Basic Information about Anaerobic Digestion — US EPA

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Cogeneration is the simultaneous generation of electricity and heat in a gas engine or turbine fuelled by biogas.

The engine drives a generator, and the heat recovered from engine cooling and exhaust gases is used to heat the digester and, optionally, buildings, drying facilities or a district heating network. The electrical efficiency of biogas engine-generators is typically several tens of per cent, and the total efficiency is markedly higher if the heat has a user. Using the heat significantly improves the economics and environmental balance of the plant, so it is worth looking for external heat users already at the design stage.

Sources: Basic Information about Anaerobic Digestion — US EPA

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The biogas potential in Poland is considered large, mainly thanks to agriculture and organic waste, but actual use depends on regulations, the availability of substrates and the grid.

Estimates of potential differ depending on assumptions (whether they include energy crops or only waste and by-products) and on the time horizon adopted, so it is worth comparing sources and methods. Agricultural plants use slurry, crop residues and food-processing by-products. Current data on the number of plants are published by, among others, KOWR (the National Support Centre for Agriculture) in the register of agricultural biogas producers.

Sources: European Biogas Association; Krajowy Ośrodek Wsparcia Rolnictwa — gov.pl

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Engineering News wrote in 2015 about 4.4 MW, and in July 2016 about 4.6 MW (four engines of 1.15 MW each give 4.6 MW). We found no official explanation of the difference.

Sources: BMW South Africa's Rosslyn plant receives first renewable energy from Bio2Watt biogas plant — BMW Group PressClub; Manure-to-power IPP poised for production after perilous journey — Engineering News; Bio2Watt appoints Anaergia to manage Bronkhorstspruit commercial biogas plant — Engineering News

Discussed in the article: Bio2Watt in Bronkhorstspruit: a feedlot manure biogas plant that powers a BMW factory

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Building a biogas plant usually requires an environmental decision (if required), a building permit, grid connection conditions and entry in the relevant registers, and the steps depend on the capacity and type of the plant.

In Poland there are regulations that facilitate the development of agricultural biogas plants, including the Act of 13 July 2023 on facilitating the preparation and implementation of investments in agricultural biogas plants and their operation (Journal of Laws (Dz.U.) 2023, item 1597). In addition, decisions concerning waste management or animal by-products, integrated permits or water-law permits may be needed. The scope of formalities should be established with a legal adviser and the authorities at an early stage.

Sources: Ustawa z 13 lipca 2023 r. o ułatwieniach w inwestycjach w zakresie biogazowni rolniczych, Dz.U. 2023 poz. 1597

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Methane most often escapes from roof membranes, pressure relief valves, pipe connections, the cogeneration system (unburned methane in the exhaust gas) and from leaking or open digestate tanks.

The losses are small in percentage terms, but methane has a global warming potential many times greater than CO₂, so limiting them makes ecological and economic sense, because a product that is paid for is escaping. Regular detection inspections (gas cameras, portable detectors), prompt repairs and covering of tanks are effective. The EU regulation on methane emissions reduction introduces obligations for measuring and repairing leaks in the energy sector.

Sources: Rozporządzenie (UE) 2024/1787 w sprawie redukcji emisji metanu — EUR-Lex; Anaerobic Digester/Biogas System Operator Guidebook — US EPA AgSTAR

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It depends on the substrate. For fibrous materials (straw, grasses, bedded manure) hydrolysis is limiting; for easily degradable ones (fats, kitchen waste, whey) it is more often methanogenesis, with a risk of acidification.

Sources: Weiland P. (2010), Biogas production: current state and perspectives, Appl. Microbiol. Biotechnol. 85, 849–860; Batstone D.J. et al. (2002), The IWA Anaerobic Digestion Model No. 1 (ADM1), Water Sci. Technol. 45(10), 65–73

Discussed in the article: The four stages of methane fermentation: from hydrolysis to methane

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Organic materials susceptible to anaerobic degradation are suitable for a biogas plant: slurry, manure, plant silages, food production waste, biowaste, slaughterhouse by-products and sewage sludge.

Not all of them are permitted in every plant, however: the type of substrate affects legal requirements (waste, animal by-products), hygienisation and the way digestate is managed. Hard-to-degrade substrates, such as wood, are not suitable without special preparation, and contaminants (plastic, glass, metal) damage equipment. The choice of feedstock is based on analyses and BMP tests, as well as on the stability of supply.

Sources: Anaerobic Digester/Biogas System Operator Guidebook — US EPA AgSTAR

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For an auction to be resolved, at least three bids must have been submitted. When there are fewer interested parties, generators do not receive support in the given basket and must wait for the next auction.

Sources: Największa w historii aukcja dla OZE bez rozstrzygnięcia — Gramwzielone.pl; Urząd Regulacji Energetyki — aukcje OZE

Discussed in the article: RES auction for existing biogas and biomass installations ends without a result

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The breakdown of some intermediate products (e.g. volatile acids) under standard conditions requires energy, so it takes place only when other microorganisms continuously consume hydrogen and acetate. Disruption of this removal favours the accumulation of acids.

Sources: AgSTAR Operator Guidebook — US EPA (listopad 2020); Schink B. (1997), Energetics of syntrophic cooperation in methanogenic degradation, Microbiol. Mol. Biol. Rev. 61(2), 262–280

Discussed in the article: Acidogenesis and acetogenesis: the stages of methane fermentation in which acids and hydrogen are formed

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Prisons have a concentrated source of substrate (excrement and kitchen waste), a constant demand for energy for cooking and controlled operation, which makes it easier to keep the plant running. It replaces firewood and reduces costs.

Sources: Prison waste cuts costs in Rwanda — IOL / Reuters (21.11.2011); Institutional biogas survey report, Rwanda (listopad 2008) — SuSanA; Biogas Source of Energy and Solution to the Environment Problems in Rwanda — Applied Mechanics and Materials

Discussed in the article: Rwanda: biogas in prisons and schools. What evaluations of institutional biodigesters showed

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A flare safely burns off excess biogas when the engine is off or the gas has no user, instead of releasing methane into the atmosphere.

Methane is a potent greenhouse gas, so burning it in a flare is much better for the climate than emitting it. The flare also protects the gas holder from excessive pressure. It should be in working order, regularly inspected and correctly supplied by the control system. Frequent flare start-ups are a sign that something is wrong in the gas system or in the cogeneration unit, and the cause should be checked.

Sources: Anaerobic Digester/Biogas System Operator Guidebook — US EPA AgSTAR

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Hydrogen sulphide (H₂S) is removed from biogas because it is toxic and highly corrosive, and when burned it forms acids that damage the engine, heat exchangers and oil.

The most popular methods are dosing a small amount of air into the gas space of the digester (biological desulphurisation), adding iron salts to the substrate, and beds with activated carbon or iron oxides. The permissible concentration at the engine inlet is specified by the manufacturer. The effectiveness of desulphurisation should be checked by measurements, and excess air should be assessed, because oxygen in biogas affects safety. High H₂S is sometimes related to the composition of the substrates, e.g. a high sulphur content in proteins.

Sources: Anaerobic Digester/Biogas System Operator Guidebook — US EPA AgSTAR; Hydrogen Sulfide — OSHA

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A monosubstrate has low alkalinity and a poor trace element composition, so the process is prone to acidification and deficiencies. Adding slurry provides a buffer, water and some trace elements.

Sources: Weiland P. (2010), Biogas production: current state and perspectives; FNR — Fachagentur Nachwachsende Rohstoffe; IEA Bioenergy Task 37

Discussed in the article: Co-digestion: how to plan a substrate mix without overloading the digester

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Its oxidation to acetate is thermodynamically more difficult and requires a very low hydrogen concentration. When methanogenesis weakens, propionate accumulates and breaks down more slowly, and it can sustain the disturbance on its own.

Sources: Chen Y., Cheng J.J., Creamer K.S. (2008), Inhibition of anaerobic digestion process: A review, Bioresour. Technol. 99, 4044–4064; Weiland P. (2010), Biogas production: current state and perspectives

Discussed in the article: Volatile fatty acids (VFA) in a biogas plant: profile, thresholds and the significance of propionate

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