It is also a perfectly reasonable question.
Anaerobic digestion plants handle organic materials. Depending on the plant, these may include food waste, manure, slurry, crop residues, food-processing wastes and other biodegradable materials. Some of them can produce extremely unpleasant odours if they are stored or handled badly.
So let's start with an honest answer.
Yes, the materials handled at an anaerobic digestion plant can smell. But that does not mean that a properly designed and operated AD plant should cause an unacceptable smell beyond its site boundary.
Those are two very different things.
Where Does the Smell From an AD Plant Actually Come From?
Interestingly, the sealed anaerobic digester itself should not normally be the main source of odour.
The biological process takes place inside enclosed tanks or vessels because the microorganisms need oxygen-free conditions. The biogas produced is also valuable, so operators have every reason to keep the digestion system gas-tight.
The greater potential for odour generally arises around the handling of materials before and after digestion.
Potential sources can include:
- delivery and unloading of food waste and other odorous feedstocks;
- feedstock reception and storage;
- depackaging and preprocessing equipment;
- open doors and poorly contained reception areas;
- spillages;
- digestate storage and handling;
- solid-liquid separation equipment;
- maintenance operations;
- abnormal operating conditions; and
- the application of digestate to agricultural land.
The precise risks depend heavily on the type of AD plant.
A farm digester processing mainly slurry and crops presents a different odour-control challenge from a commercial food-waste AD plant receiving packaged and potentially decomposing food from many sources.
What Does Digestate Smell Like?
Digestate is the material remaining after anaerobic digestion has converted part of the biodegradable organic matter into biogas.
It can certainly have a noticeable odour, but it is misleading to say that all digestate invariably smells extremely unpleasant.
The character and strength of the odour depend on such factors as the original feedstock, digestion conditions, process stability, storage, subsequent treatment and how the digestate is handled.
Digestate may also be separated into liquid and solid fractions, creating materials with different handling characteristics.
The important point is that digestate remains an organic material containing valuable plant nutrients. It therefore needs to be stored and applied responsibly.
Good AD Plant Design Starts With Containment
One of the fundamental principles of odour control is straightforward:
Don't allow odorous air to escape uncontrolled in the first place.
At facilities handling particularly odorous wastes, reception and processing operations can be carried out within enclosed buildings.
Air can be extracted from these areas so that the building is maintained under suitable containment conditions rather than allowing contaminated air to escape through doors and openings.
The extracted air can then be treated using an appropriate odour-abatement system before discharge.
Depending on the application, such systems may include biofilters and other biological, chemical or physical treatment technologies.
There is no single odour-control technology which is automatically correct for every AD plant. The system needs to be designed for the characteristics and loading of the air requiring treatment.
An Odour-Control System Also Has to Keep Working
Installing an odour-control system is not enough.
Fans fail. Ductwork deteriorates. Filters require maintenance. Doors are left open. Feedstocks change. Operators may receive unexpectedly odorous loads.
That is why good odour management combines engineering with operating discipline.
Operators need to understand where odour could arise, monitor the important parts of the process and have procedures for responding when something goes wrong.
This is particularly important because some of the worst odour incidents at waste-treatment facilities occur during abnormal rather than routine operation.
What Does the Environment Agency Require?
Environmental regulation has developed considerably since I first wrote this article.
In England, the Environment Agency published updated guidance on odour management in December 2025.
For regulated activities, operators are expected to assess their odour risks and use appropriate measures to prevent odour pollution or, where prevention is not practicable, minimise it.
Anaerobic digestion is specifically included among the activities for which an Odour Management Plan (OMP) may be required as part of environmental permitting.
An OMP is not supposed to be a vague statement saying that an operator will try not to create smells.
It should identify the potentially odorous materials and processes on the site and explain how risks will be controlled.
This can include:
- feedstock and waste inventories;
- maximum storage times;
- storage conditions;
- process controls;
- containment;
- air extraction and abatement;
- monitoring;
- performance standards;
- record keeping;
- responses to abnormal conditions; and
- contingency arrangements.
The Environment Agency's current permit requirements are aimed at preventing odour at levels likely to cause pollution outside the permitted site boundary.
If a facility does cause odour pollution, the regulator can require further controls and changes to operating practices. Serious or persistent failures can lead to stronger regulatory action.
What Should Local Residents Look For in a Planning Application?
If you live near the site of a proposed anaerobic digestion plant, I don't think it is helpful simply to be told that "AD plants don't smell".
A better approach is to look at what is actually proposed.
Some useful questions are:
- What feedstocks will the plant accept?
- Will particularly odorous materials be unloaded indoors?
- How will reception buildings be contained and ventilated?
- How will extracted air be treated?
- How long can incoming material remain in storage?
- How will digestate be stored?
- What happens if the odour-abatement equipment fails?
- How close are the nearest homes and other sensitive receptors?
- Is there a credible Odour Management Plan?
Those questions tell you considerably more about the likely risk than simply knowing that the development is called an "anaerobic digestion plant".
Visit an Existing AD Plant
There was one suggestion in the original version of this article that I still think has considerable merit.
If you are concerned about a proposed AD plant, find an established plant of a similar type and visit the surrounding area.
You obviously should not enter private operational premises without permission, but it may be possible to experience conditions from public roads or other legitimate public locations.
Ideally, don't just visit one plant once. Weather conditions, wind direction and plant operations vary.
Nevertheless, seeing an operating AD facility can be much more informative than trying to imagine one from drawings in a planning application.
My Own Experience of AD Plant Odour
In the area where I lived when I originally wrote this article, there were several anaerobic digestion plants relatively close by.
My personal experience was that I did not routinely detect odour from those plants beyond their sites.
On occasions when I did notice agricultural odours in the surrounding countryside, the source was not necessarily the AD plant at all. Normal agricultural operations can themselves generate substantial odour.
That is only my personal observation. It is not evidence that every AD plant is odour-free.
There have unquestionably been AD facilities where poor design, inappropriate feedstock management, equipment problems or inadequate operation have resulted in legitimate complaints from neighbours.
That is precisely why odour needs to be considered seriously during design, permitting and operation.
So, Do Anaerobic Digestion Plants Smell?
The most accurate answer is:
The materials handled at anaerobic digestion plants can be highly odorous, but a well-designed and well-operated AD plant should control those emissions rather than routinely subject its neighbours to them.
It would therefore be wrong to dismiss residents' concerns about odour.
But it is equally wrong to assume that because anaerobic digestion involves decomposing organic material, every AD plant must inevitably make the surrounding area smell.
The questions that matter are about feedstock, design, containment, abatement, management, monitoring and operator competence.
That is where residents, planners and regulators should concentrate their attention.
Steve Last
Chartered Civil & Environmental Engineer (MICE)
Chartered Waste Manager (MCIWM)
Chartered Environmentalist (CEnv)
Updated September 2026. This article was originally published many years ago (on 8 January 2015) and has been substantially revised to reflect current anaerobic digestion practice and Environment Agency odour-management guidance.
Further reference: Environment Agency, Odour management: comply with your environmental permit, published December 2025.










