An anaerobic digestion plant turns organic material into biogas and digestate. The promise is easy to understand: use livestock manure, crop residues or selected biowaste, produce renewable energy, then return part of the nutrients to soil. Yet the outcome depends on one crucial condition. The gas must remain inside the system until it is used. When methane escapes, energy is lost and the plant’s climate benefit shrinks.

The issue was on the programme at SPACE, the international livestock show in Rennes. On 15 September 2026, an INRAE conference brought specialists together to discuss greenhouse gas emissions from anaerobic digestion. Its agenda highlighted three practical questions: where emissions come from, how methane leaks can be detected, and how much biogas may still be released while digestate is stored. This article is not a report of findings announced at the event. It is nevertheless a timely reminder that the performance of a digester cannot be judged only by the volume of gas it produces.

What is a methane leak at a biogas plant?

Anaerobic digestion uses microorganisms to break down biomass without oxygen. INRAE’s overview says the resulting biogas contains about 55 to 70% methane. Most of the remainder is carbon dioxide, together with smaller amounts of other compounds. The biogas can fuel a boiler or a combined heat and power engine. It can also be upgraded into biomethane for the gas grid or vehicle fuel.

A “fugitive” emission is biogas that leaves the intended circuit without being used or burned. It may escape through a connection, flange, valve, membrane, pipe or upgrading unit. Other releases are not mechanical leaks in the narrow sense. A pressure-relief valve may open, an engine may be unavailable, or digestate that is still biologically active may continue producing gas in storage. From a climate perspective, each route can send methane directly into the atmosphere.

INRAE’s PréDiGem project uses a 100-year warming effect for methane equal to 28 times that of carbon dioxide. This helps explain why a relatively small loss deserves attention. It does not mean that anaerobic digestion is automatically harmful. It means that a sound assessment has to follow the gas from production through upgrading or combustion and on to digestate storage.

What did measurements at French plants find?

The FELeaks research project, funded by ADEME and involving INRAE, AILE, CH4Process and Auvergne-Rhône-Alpes Énergie Environnement, improved a method for estimating methane flows. A measurement campaign then covered 15 sites selected to represent different types of French anaerobic digestion plant.

ADEME’s summary, published online in July 2026, reports a nuanced result. Measured emission factors were generally low, below 0.5% for a large majority of the sites. However, some larger short-lived releases were also observed. That combination matters. A reassuring measurement on one day cannot prove that no release will happen during another operating phase.

The 0.5% figure should not be turned into a universal threshold. The summary does not say that every plant always remains below it, or that this level has no impact. It describes observations from a 15-site campaign. Equipment, feedstock, operating practice and technical events vary between plants. FELeaks therefore produced nine practical advice sheets for preventing fugitive emissions rather than a single conclusion for the entire sector.

Why are methane emissions hard to spot?

Methane does not form a plume that operators can see with the naked eye. A leak may be diffuse, intermittent or located high on a tank. Smell is not a reliable measure of methane flow either. Odours come from other compounds, while a meaningful methane loss may remain unobtrusive.

INRAE describes optical gas imaging, or OGI cameras, as one of the best available technologies for locating these invisible emissions. An operator can scan equipment and visualise certain gas plumes. Portable sensors, measurements around the plant and quantification methods can complement that survey. Finding a leak and measuring its flow are separate jobs: location shows where to intervene, while quantification helps set priorities and assess whether a repair worked.

Timing matters as much as the instrument. Start-ups, shutdowns, maintenance work, pressure changes, new feedstock and unavailable equipment can all alter emissions. A robust plan combines regular surveys with checks triggered by specific events. It also records anomalies, repairs and the measurement made after each intervention.

Where should an inspection look first?

A farm biogas plant contains several zones with different roles. A systematic inventory prevents the inspection from focusing only on the digester vessel.

  • Gas-containing structures: membranes, roofs, seals, hatches and cable penetrations can lose their integrity.
  • Pipes and connections: flanges, valves, drains and joints face vibration, corrosion, temperature changes and human intervention.
  • Safety devices: relief valves protect the plant, but an opening also means gas did not follow the normal route.
  • Upgrading equipment: separating methane from carbon dioxide produces an off-gas stream that has to be included in the overall balance.
  • Energy use: an unavailable engine, boiler or grid-injection unit can cause gas to accumulate and may lead to flaring or release when storage is insufficient.
  • Digestate storage: material leaving the digester may still generate methane. Uncovered storage or insufficiently stabilised digestate can become an important source.

This list is not a remote safety diagnosis. It is a way to structure an inspection using appropriate competence and safety rules. Biogas atmospheres also present physical hazards, so work on plant equipment belongs in the hands of trained people.

Digestate can remain biologically active

Digestate is the material left after digestion. It retains fertilising elements and can return to the soil under regulated conditions. But “after digestion” does not always mean “no further gas production”. Some organic matter may continue to break down during storage. Temperature, retention time, feedstock and storage design all influence that residual potential.

This is the focus of PréDiGem, an action-research project presented by INRAE’s OPAALE unit in May 2026. Researchers sampled digestate from around 20 pioneering plants using different approaches. Options under study include longer retention in the digestion system, a covered buffer tank with gas recovery, and phase separation. Laboratory work is being complemented by monitoring full-scale storage infrastructure over an entire year.

The project does not yet provide one recipe for every plant. Instead, it shows what needs to be measured before a decision is made. Covering storage can capture gas, but its value depends on the remaining methane potential and on whether that gas is recovered. Longer retention may reduce later biological activity, but it changes the capacity and economics of the plant. Phase separation may help with some fertiliser uses while leaving the need to manage nitrogen, water and spreading conditions.

A practical five-step approach

The available results point towards continuous monitoring rather than a one-off certificate. For an operator, the logic can be expressed in five steps, although specialists must adapt the details to each installation.

  1. Establish a baseline. Map equipment, flows and operating situations so that the team knows what has to be checked and under which conditions.
  2. Survey regularly. A periodic campaign should search for invisible emissions along the entire gas pathway, not only around the digester.
  3. Check after events. Maintenance, overpressure, shutdowns or process changes justify a targeted follow-up survey.
  4. Repair according to flow and risk. An accessible minor leak may be corrected quickly; a major or safety-related emission requires a controlled intervention.
  5. Verify the repair. Tightening a fitting or replacing a seal is not the end of the job. A post-repair measurement confirms that the emission has gone and that no new point was created.

This approach also has an economic logic. Lost methane cannot fuel an engine or boiler and cannot be sold into the grid. Reducing emissions can therefore align climate performance, operational safety and energy revenue. FELeaks explicitly identifies all three dimensions.

Is anaerobic digestion always good for the climate?

The serious answer is conditional. Digestion can avoid some emissions from conventional manure storage and displace fossil gas. It can also recover value from material that would otherwise be poorly used. But the outcome depends on feedstock, transport, energy consumption, the final use of biogas, methane losses, storage and the way digestate is spread.

An analysis presented by ADEME in June 2026 concludes that emission reductions require, among other conditions, well-run installations that prevent process leaks. It also stresses that useful energy matters. In the French context, injecting biomethane into the gas grid may have a different value from combined heat and power when the heat is not fully used. This is not an automatic ranking of technologies. It is a reminder that energy produced and energy put to useful work are not the same measure.

Digestate needs the same careful thinking. INRAE notes that it contains nitrogen in a form plants can use more readily, but that nitrogen may also be lost to air or water when spreading is poorly managed. A well-run digester therefore still needs an agronomic plan, controlled storage and suitable spreading equipment.

Questions communities can ask

Emission monitoring is primarily the responsibility of plant operators, designers, technical advisers and regulators. Local authorities and residents can still ask precise questions. Is leak detection planned? Are shutdowns and pressure events recorded? Is digestate storage covered or monitored? How is residual gas recovered? What agronomic controls guide spreading?

Those questions are more useful than reducing the discussion to being “for” or “against” biogas. Two plants using similar technology can perform differently because of their size, feedstock, local setting and operation. Transparent measurements, incident records and verified corrections make it possible to discuss an actual project rather than a general promise.

From biogas to local food networks

A digester does not replace food-waste prevention or the edible use of a harvest. The order matters: eating, donating or processing sound food is generally more valuable than converting it prematurely into energy. Anaerobic digestion is mainly relevant for material that can no longer be eaten, livestock effluent and certain residues, within a plan that fits local soils and needs.

This distinction also matters in short supply chains. A surplus harvest can still feed someone; an inedible residue may enter a suitable organic-treatment route. On Seeed, individuals and producers can give away garden surplus, sell or swap harvests and local products with people nearby. That connection does not fix the performance of a biogas plant, but it helps preserve food value before residual waste has to be managed.

What to remember

Methane leakage is not a side issue in farm anaerobic digestion. It is an indicator of climate, technical and economic performance. FELeaks found generally low measured factors across a large majority of the 15 sites it studied, while also identifying larger episodic releases. Current research on digestate is now working to predict and reduce a source that can be underestimated.

The sensible response is neither to present biogas as perfect nor to reject every plant. It is to require measurements, monitoring over time, verified repairs and serious agronomic management. Renewable gas delivers on its promise when the methane produced is captured, put to useful work and accompanied by controlled nutrient return to soil.

Sources

Image credit: “Biogasanlage-01.jpg”, a biogas plant near Sevelten, Germany, photograph by Cec-clp, via Wikimedia Commons, released under CC0 1.0. Image used without modification.