Understanding odours around an anaerobic digestion plant

Anaerobic digestion (also called methanisation) is a natural biological process: micro-organisms break down organic matter in the absence of oxygen, in a closed tank called a digester. This process, similar to what happens in a marsh or in the stomach of a ruminant, turns organic matter into two useful products: biogas and digestate.

Incoming materials (feedstock)

An anaerobic digestion unit can be fed with different types of organic matter:

• Livestock manure: slurry, solid manure
• Biowaste: food waste from households, catering and the food industry
• Agricultural residues: crop or silage residues, catch crops grown for energy (CIVE)
• Food-industry co-products: processing by-products (dairies, sugar factories, breweries, etc.)

The resulting products

• Biogas, which can be used to produce electricity and heat, or upgraded into biomethane injected into the natural gas grid
• Digestate, the residue of fermentation, used as an organic fertiliser spread on farmland

How a plant works

The process takes place in closed, airtight digesters (the tanks with a dome on top). In most recent plants, the buildings where materials are received are enclosed and connected to an air treatment system (biofilter or air scrubber) that captures and neutralises odorous compounds before release into the atmosphere. Some products may be stored outdoors (silage clamp) so that the operator can balance what goes into the digester.

Agricultural digestion vs biowaste digestion

Not all anaerobic digestion units have the same odour risk profile. The table below compares the two main families of projects found in Europe. The size of the operation also matters: agricultural digestion plants are often small and linked to existing livestock farms, with regular, local flows. Multi-feedstock plants are often larger and their feedstock comes from more or less nearby sites (farms, food industry, etc.). For these, the odour risk is often higher because of variations in feedstock and in the volume processed.

Sources: ADEME – Anaerobic digestion • INERIS / AIDA – ICPE heading 2781 • ATEE – Club Biogaz • Atmo France

Even a well-managed plant can at times cause noticeable odours. Most of these situations are occasional and identifiable. The table below lists the main odour sources observed around anaerobic digestion units.

Weather factors affecting how odours are perceived

The weather does not create odours, but it strongly changes how they disperse and how residents perceive them:

• Light wind: odorous compounds linger near the source instead of dispersing
• Temperature inversion (often in the morning or evening): cold air stays close to the ground, preventing odours from rising and diluting. This is what you can see in autumn or spring when there is mist early in the morning, for example.
• Heat: it speeds up fermentation and increases the volatility of odorous compounds (at the plant itself as well as in feedstock or waste storage and in associated lagoons)
• Humidity: it helps odour molecules persist in the air

Sources: ADEME / Atmo France – AQAMETHA project • Atmo Hauts-de-France – StoryMap on anaerobic digestion odours • INERIS – studies on odours and anaerobic digestion

Odours linked to anaerobic digestion cannot be reduced to a single olfactory signature. Depending on the source, several types of odour can be perceived, associated with different chemical compounds.

The compounds in a nutshell

• H2S (hydrogen sulphide): gas produced when sulphur-rich matter breaks down; detectable by the human nose at very low concentrations, well below any danger threshold (see INRS)
• NH3 (ammonia): produced when nitrogen-rich matter breaks down, typical of digestate and livestock manure (see INRS)
• VOCs (volatile organic compounds): a broad family of volatile carbon-based molecules, behind many fermentation odours
• Volatile fatty acids (VFAs): intermediate products of anaerobic digestion, responsible for sour or “vinegary” notes

Sources: Atmo Hauts-de-France – StoryMap: anaerobic digestion odours • Atmo France • INERIS – air quality • INERIS - Chemical Substances Portal

In France, anaerobic digestion units are Installations Classified for Environmental Protection (ICPE), governed by heading 2781. The level of oversight depends on the size and nature of the plant:

• Declaration (D): small-capacity plants (less than 30 t/day of material processed), subject to general requirements and periodic inspection (Article 6.2 of the Order of 10/11/09)
• Registration (E): simplified procedure with standardised technical requirements, without a systematic impact assessment. The operator must however carry out a survey of the odours perceived around the site before the plant is commissioned (baseline), (Article 49 of the Order of 12/08/10)
• Authorisation (A): the largest plants, subject to a detailed impact assessment (including modelling), a public inquiry and specific prefectural requirements (Article 29 of the Order of 10/11/09)

The applicable regime depends on how the plant is classified. For plants subject to authorisation, the impact assessment evaluates in particular the expected odour nuisance and the measures planned to limit it. Once in operation, the operator must comply with the requirements set by prefectural order, and the plant is inspected by the DREAL (regional directorate for the environment, planning and housing).

Main obligations relating to odours

• Enclosure of odour sources (reception buildings, unloading areas)
• Treatment of extracted air (biofilter, air scrubber) before release into the atmosphere
• Controlled management of storage (covering, limited duration, dedicated areas)
• Prevention of biogas leaks through regular equipment maintenance
• Maintenance and monitoring of the proper functioning of air treatment systems
• Monitoring of nuisances and implementation of appropriate prevention and management measures

French regulations on odours from classified installations (illustrated by the Order of 12/08/10 for anaerobic digestion) are based on three fundamental principles that shape the approach to prevention and control:

1. Prevention at source

The operator must put in place systematic preventive measures to stop odour nuisance before it occurs. This includes:

• Enclosure of sources: facilities likely to release odours must as far as possible be housed in enclosed premises, ventilated if necessary
• Strategic siting: potential odour sources that are hard to enclose (basins, lagoons) must be positioned taking the prevailing wind direction into account to limit disturbance to neighbours
• Regular maintenance: a preventive maintenance programme must be drawn up to periodically check the airtightness of equipment (every six months for critical systems) and the breather valve
• Prevention of anaerobic conditions: the operator must take all measures to limit odours, in particular by preventing anaerobic conditions from forming in storage basins or open channels
• Treatment of gaseous effluents: ducted odorous gaseous effluents must be collected and sent to a treatment unit (biofilters, gas scrubbers)

2. Obligation of result: traceability and response

The operator is responsible for achieving a quality objective without being bound to specific means. This objective is expressed as a quantified limit:

“The odour concentration attributable to the installation in areas of human occupation within a radius of 3,000 metres of the fenced boundaries of the installation must not exceed the limit of 5 ouE/m³ for more than 175 hours per year (i.e. an exceedance frequency of 2%).”

This obligation of result implies:

• Documented baseline: before commissioning, the operator must establish the existing odour perceptions in the surrounding area in order to assess the impact of the plant
• Complaints register: the operator must keep an up-to-date complaints register including the date, time, location, weather conditions and correlation with critical operations
• Mandatory diagnosis in the event of exceedance: if significant nuisances are reported, the operator must commission a diagnosis and a dispersion study to identify the problem sources and make changes
• Operating log: the operator must document critical operations (start-up, shutdown, emptying) that are more likely to generate odours

3. Freedom of means: a flexible, context-based approach

The regulations do not impose specific technical means, but require the operator to justify the choice of its measures and their suitability for the local context:

Sources: AIDA / INERIS – ICPE heading 2781 • French Ministry for the Ecological Transition • INERIS – ICPE and odour nuisance management

Noticing an odour near an anaerobic digestion site is not abnormal in itself: some occasional emissions (delivery, spreading, opening of a silage clamp) are expected and generally brief. On the other hand, an intense, recurring or unusual odour may reveal a malfunction that is worth reporting.

Situations that are usually normal

• A brief odour during a delivery or unloading
• A silage smell when a clamp is opened
• A spreading odour limited in time, comparable to ordinary manure spreading

Situations to report

• A strong odour lasting several hours or days
• A new, unusual odour, or one very different from what is usually noticed
• An odour associated with repeated nuisance, especially at night or early in the morning

Every report, even for a nuisance that seems modest, helps to establish the facts around a plant and, where necessary, to guide corrective action by the operator or inspections by the authorities. National studies such as AQAMETHA show that dialogue and information-sharing with residents are among the levers for improving air quality and the acceptability of the sector.