Statutory guidance

SR2021 No 10: generic risk assessment for anaerobic digestion of non-hazardous sludge at a waste water treatment works, including the use of the resultant biogas

Updated 17 May 2022

Applies to England

The Part A installation must have the capacity to treat more than 100 tonnes of waste, or a combination of waste and non-waste in any one day.

The Environment Agency produces the generic risk assessments for all standard rules permits. These list the potential risks and how to manage them.

Check this generic risk assessment to understand:

• the potential risks of a Part A installation operating an anaerobic digestion facility which includes using the resultant biogas
• if your proposed activity has the same risks and you can apply for the standard rules permit
• how to manage the risks effectively

Each risk comprises:

• information about the source, pathway and receptor – and the potential harm to that receptor
• a judgement of the level of risk and justification of that judgement
• actions for managing the risk (through permitting) and a residual risk rating after managing it

Risk management involves breaking or limiting the source-pathway-receptor linkage to reduce the risk. If we set minimum distances, we explain the basis of the distance, for example by modelling.

We will control the residual risk (after risk management) when we assess compliance.

If you need to check the meaning of any terms we have used (in the context of this risk assessment), see the explanation of terms.

This generic risk assessment is based on the following parameters.

Parameter 1

Permitted activities s5.4 A(1)(b)(i) are:

• the storage and recovery of waste (R13, R1, R3)
• incineration on land (D10)

Parameter 2

The permitted wastes are biodegradable wastes suitable for digestion – sewage sludge and septic tank sludge only).

Parameter 3

The maximum quantity of waste received on site each day including solids and liquids shall not exceed the designed storage capacity of the site.

Parameter 4

Except for the auxiliary flare, the aggregate rated thermal input of all appliances used to burn biogas must be less than 5 megawatts.

Parameter 5

These rules do not allow any point source emission into land, surface waters or groundwater under control of the permit

Parameters 6 to 14

The site must be a minimum distance from certain types of sensitive receptors. It cannot be within:

• 200 metres of the nearest receptor as measured from any combustion stack or stacks, unless the stacks are at least 7 metres high and the effective stack height of each stack is greater than 3 metres
• 500 metres of a European site (within the meaning of Regulation 8 of the Conservation of Habitats and Species Regulations 2017) or a Site of Special Scientific Interest, including candidate or proposed sites
• a groundwater source protection zone 1 or 2, or if a source protection zone has not been defined then within 50 metres of any well, spring or borehole used for the supply of water for human consumption (including private water supplies)
• 250 metres of the presence of great crested newts, where it is linked to the breeding ponds of the newts by good habitat
• 10 metres of any watercourse
• 50 metres of a Local Nature Reserve, Local Wildlife Site, Ancient Woodland or Scheduled Monument
• 50 metres of a site that has species or habitats of principle importance (as listed in Section 41 of the Natural Environment and Rural Communities Act 2006) that the Environment Agency considers at risk to this activity, these are also often referred to as priority habitats and species
• a specified Air Quality Management Area

1. Risk to local human population

1.1 Releases of emissions NOx, SOx, CO and other gases

NOx, SOx, CO and other gases travel through the air and can be inhaled.

We have assessed the potential harm to human health as follows:

• respiratory irritation
• illness

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as medium
• overall severity of potential consequences as medium
• overall risk rating as medium

The reasons for giving the activity this rating is because there is potential for exposure to:

• anyone living close to the site
• members of the public at locations to which they could be regularly exposed

Managing the risk

The following manages the risk:

• set back distances apply for effective stack height and open processing of waste
• monitoring of emission levels from biogas driven combined heat and power plants and specified generators as required by the permit and in accordance with guidance ‘Monitoring stack emissions: environmental permits’ (formerly M2) and ‘Monitoring Stack emissions: low risk MCPs and specified generators’ (formerly M5)
• activities shall be managed and operated in accordance with a management system (including the inspection and maintenance of equipment and engines) and the point source emissions to air with emission limits requirement
• other monitoring shall be reported as permit requirements
• the activities shall not be carried out within a specified Air Quality Management Area
• leak detection and repair programmes will be in place to mitigate and prevent methane emissions
• gas pressure will be monitored to minimise the release of biogas and all pressure relief systems will be inspected and calibrated as required by the permit

Taking these actions will control the risk and rate it as ‘low’.

1.2 Release of micro-organisms (bioaerosols)

Bioaerosols travel through the air and can be inhaled.

We have assessed the potential harm to human health as follows:

• respiratory irritation
• illness

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as medium
• overall severity of potential consequences as high
• overall risk rating as high

The reasons for giving the activity this rating is because there is potential for release when waste is received and treated.

Managing the risk

The following manages the risk:

• the permit does not allow anaerobic digestion activities to be carried out within 200 metres of the nearest sensitive receptor
• operations have to take place within a closed system with appropriate filters or a scrubbing system
• the required set back distances are sufficiently precautionary so that bioaerosols are at background levels
• bio-filters are monitored for efficiency

Taking these actions will control the risk and rate it as ‘low’.

1.3 Odour

Odour travels through the air and can be inhaled.

We have assessed the potential risk to the local population as:

• nuisance
• loss of amenity

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as medium
• overall severity of potential consequences as medium
• overall risk rating as medium

The reasons for giving the activity this rating is because local residents are sensitive to odour. Odour can result from:

• a range of sewage related wastes, particularly when receiving them
• the release of biogas
• digestate and dewatering liquors

Managing the risk

The following manages the risk:

• all waste reception areas and digestate storage shall be managed to prevent and mitigate odours, as required by an odour management plan
• all channelled emissions will be treated before release
• operators will take additional measures to ensure ammonia release is controlled
• all appropriate measures and Best Available Techniques shall be implemented
• non-point source emissions of biogas shall be minimised
• all abatement systems are designed, monitored and maintained to treat specified emissions and off-gases
• no fugitive emissions are permitted
• storage tanks are required to be covered

Taking these actions will control the risk and rate it as ‘low’.

1.4 Noise and vibration

Noise travels through the air and vibration through the ground.

We have assessed the potential risk to the local population as:

• nuisance
• loss of amenity
• loss of sleep

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as medium
• overall severity of potential consequences as low
• overall risk rating as medium

The reasons for giving the activity this rating is because local residents can be sensitive to noise and vibration. However, there is low potential for exposure.

Managing the risk

The following manages the risk:

• noise and vibration shall be minimised and not cause nuisance
• fans and condensate traps will be checked for water
• fans and extraction systems will be checked
• flares must be kept to a minimum to reduce noise impact and a record of use kept and reported
• a noise and vibration management plan may be required

Taking these actions will control the risk and rate it as ‘low’.

2. Risk to local human population, livestock and wildlife

2.1 Gaining unauthorised access to the site

There is a risk of direct physical contact with all on-site hazards such as wastes, machinery and vehicles.

There is a risk of causing injury to humans or livestock.

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as low
• overall severity of potential consequences as low
• overall risk rating as low

The reasons for giving the activity this rating is because it is carried out within enclosed digesters so physical contact is minimised.

Managing the risk

The following manages the risk:

• activities shall be managed and operated in accordance with a management system which includes site security measures to prevent unauthorised access
• all sites will have a DSEAR assessment which covers unauthorised access to site
• maintenance workers or contractors are not permitted on site without a suitable qualification and they must have permission to do the work

Taking this action will control the risk and rate it as ‘low’.

3. Risk to local human population and local environment

3.1 Arson and vandalism

Arson and vandalism risk causing the release of polluting materials to the air (smoke or fumes), water or land.

Arson and vandalism can cause:

• smoke to travel through the air
• spillages and contaminated firewater to run-off from the site and through surface water drains and ditches

We have assessed the potential harm as:

• respiratory irritation, illness and nuisance to local population
• injury to staff, fire fighters, arsonists or vandals
• pollution of water or land

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as medium
• overall severity of potential consequences as medium
• overall risk rating as medium

The reasons for giving the activity this rating is because:

• biogas is flammable
• the consequences of an incident may be serious, including risk to the safety and wellbeing of people and loss of containment causing releases to the environment

Managing the risk

As risk 2.1 plus the following manages the risk:

• an accident management plan is required as part of the management system (covers fire and spillages)
• a Hazard & Operability Study (HAZOP) or similar risk identification technique is done to produce a schedule of planned maintenance (as identified by the HAZOP or risk assessment or suppliers), and is documented  
• visitors shall be accompanied
• fire control processes and procedures as set out in the DSEAR plan and they shall be communicated to the local fire service
• leak detection schemes are in place to prevent and minimise fugitive biogas release
• the site shall be secure
• any security breaches shall be reported
• contingency measures must be in place in the event of loss of plant

Taking these actions will control the risk and rate it as ‘low’.

3.2 Accidental explosion of gas

An explosion of biogas risks causing:

• fire and smoke to travel through the air
• spillages and digestate to run-off the site through surface water drains and ditches

We have assessed the potential harm as:

• respiratory irritation, illness and nuisance to the local population
• injury to staff or fire fighters
• air, water or land pollution

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as low
• overall severity of potential consequences as medium
• overall risk rating as medium

The reasons for giving the activity this rating is because the risk is reduced by an effective management system being in place, as required by the permit.

However, biogas is flammable, and the consequences are likely to be serious, including risk to:

• safety and wellbeing of those working or in close proximity to the site
• loss of containment may be detrimental to the environment

Managing the risk

This risk is managed in the same way as risks 2.1 and 3.1 and these additional measures:

• the management system is required to include the risk management measures specified in the HAZOP and DSEAR plans and cover planned maintenance
• the measures and procedures for any potential accidents are fully documented in the accident prevention and management plan, as part of the management system
• training and regular ‘tool-box’ talks are given to site operators and all staff understand their role in an emergency
• the management system must include measures for bunding of tanks
• leak detection and repair is required

Taking these actions will control the risk and rate it as ‘low’.

3.3 Accidental fire

Fire can cause polluting materials to travel through:

• the air (smoke or fumes)
• run-off from the site through surface water drains and ditches (spillages and digestated from loss of containment)

We have assessed the potential harm as:

• respiratory irritation, illness and nuisance to local population
• injury to staff or fire fighters
• air, water or land pollution

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as low
• overall severity of potential consequences as medium
• overall risk rating as medium

The reasons for giving the activity this rating is because biogas is flammable and the consequences are likely to be serious and include risk to:

• safety and wellbeing
• loss of containment

Managing the risk

This risk is managed in the same way as risks 2.1, 3.1 and 3.2 along with:

• assessing the need for lightning conduction equipment which must be put in place where necessary or required for insurance purposes
• safe storage of activated charcoal and other combustible materials is required

Taking these actions will control the risk and rate it as ‘low’.

4. Risk to surface waters close to and downstream of site

4.1 Spillage of digestate tank failure liquids, including oil – acute effects

We have assessed the acute effects as:

• water supply pollution
• fish kills

We have assessed the harm as:

• direct run-off from the site across the ground and through surface water drains and ditches

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as medium
• overall severity of potential consequences as medium
• overall risk rating as medium

The reasons for giving the activity this rating is because of the potential for spillage from digestion tanks and storage vessels.

Managing the risk

The following manages the risk:

• digestion tanks require appropriate design and validation
• an impermeable surface is required for storage of all wastes
• no point source emissions are allowed to water
• run-off is restricted to clean surface water using appropriate measures
• the site drainage plan is documented
• all staff are trained in the event of an emergency or accident
• tanks are bunded in line with CIRIA 736 and industry standards
• run-off is restricted by the ‘emissions of substances not controlled by emission limits’ rule
• all biogas condensate shall be discharged into a sealed drainage system
• condensate taps are checked daily

Taking these actions will control the risk and rate it as ‘low’.

4.2 Spillage of digestate tank failure liquids, including oil – chronic effects

We have assessed the chronic effects as deterioration of water quality.

We have assessed the harm as:

• direct run-off from the site across the ground surface and through surface water drains and ditches
• spillages and digestate through direct run-off from site and through surface water drains and ditches
• indirect run-off through the soil layer or from loss of containment

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as medium
• overall severity of potential consequences as medium
• overall risk rating as medium

The reasons for giving the activity this rating is because of the potential for spillage from digestion tanks and storage vessels.

Managing the risk

The following manages the risk:

• preservation of critical infrastructure requirements
• all storage areas shall be constructed to an approved standard
• secondary containment is in place and validated by a chartered engineer to CIRIA 736 standard
• all tanks are inspected to ensure appropriate freeboard is maintained
• all transfer of digestate and material is supervised and the flow rate controlled

Taking these actions will control the risk and rate it as ‘low’.

5. Risk to water abstracted from a watercourse

These risks relate to watercourses downstream of a facility and to water for agricultural or potable use.

This receptor is at risk from spillage of digestate liquids due to tank failure.

There is a risk of contaminants, including oil, travelling through direct run-off from the site across ground surface, through surface water drains and ditches and finally through abstraction. This could have acute effects and cause the closure of abstraction intakes.

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as medium
• overall severity of potential consequences as medium
• overall risk rating as medium

The reasons for giving the activity this rating is because of the potential for spillage from digestion tanks and storage vessels.

Managing the risk

Risk management is as set out in risks 4.1 and 4.2.

Taking these actions will control the risk and rate it as ‘low’.

6. Risk to groundwater

Groundwater is at risk from spillage from digestate tanks and storage vessels.

There is a risk of contaminants travelling through soil to groundwater which can then be abstracted from a borehole. This could have a chronic effect resulting in the groundwater requiring treatment or causing closure of a borehole.

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as medium
• overall severity of potential consequences as medium
• overall risk rating as medium

The reasons for giving the activity this rating is because of the potential for spillage from digestion tanks and storage vessels.

Managing the risk

Managing the risk is the same as risk 5 as well as:

• the activities shall not be carried out within a groundwater SPZ 1 and 2, or if a SPZ has not been defined then within 50 metres of any well, spring or borehole used for the supply of water for human consumption (including private water supplies)
• an impermeable surface is required for the anaerobic plant
• all sites must be risk assessed in line with CIRIA 736 and where necessary have validated secondary containment
• all primary tanks must have a delegated inspection regime
• process parameters are monitored and understood by site operatives

Taking these actions will control the risk and rate it as ‘low’.

7. Risk of diffuse emissions from polluting and greenhouse gases such as methane and ammonia

There is a risk of fugitive releases of volatile organic compounds such as methane from storage of gas bags, lagoons, tanks, vents and pipe work.

We have assessed the harm as:

• acute effects and long term effects on air quality
• longer term effects of volatile organic compound releases and adding to global climate change

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as medium
• overall severity of potential consequences as medium
• overall risk rating as medium

The reasons for giving the activity this rating is because:

• biogas contains high levels of methane and is a known contributor to climate change
• digestate and digestate storage releases ammonia which can severely impact air quality
• burning biogas and biomethane can produce harmful pollutants

Managing the risk

The following manages the risk:

• tanks and lagoons are covered and where necessary emissions are abated
• venting is minimised by correctly fitting under and over pressure relief valves and is recorded as abnormal events
• leak detection and repair is employed

Taking these actions will control the risk and rate it as ‘low’.

8. Risk to protected sites

Protected sites include:

• National Parks and Areas of Outstanding Natural Beauty
• Marine Conservation Zones
• Sites of Special Scientific Interest
• Special Areas of Conservation
• Special Protection Areas
• Ramsar wetland sites

Protected sites can be at risk from any source and by any pathway. However, the main risk is from ammonia and NOx.

The risk of harm to protected sites include (but are not limited to) the following:

• toxic contamination
• nutrient enrichment
• leachate
• contaminated surface water run-off
• smothering
• disturbance
• predation from pests and vermin

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as medium
• overall severity of potential consequences as medium
• overall risk rating as medium

The reasons for giving the activity this rating is because physical disturbance and emissions to air may cause harm to and deterioration of nature conservation sites.

Managing the risk

The following manages the risk:

• the parameters listed at the beginning of this document do not allow operations in close proximity of conservation sites to use these standard rules
• emissions of substances not controlled by emission limits (excluding odour and noise) shall not cause pollution
• the operator shall not be taken to have breached this rule if BAT and appropriate measures, including, but not limited to, those specified in any approved emissions management plan, have been taken to prevent or where that is not practicable, to minimise, those emissions
• emission limits are in the permit
• storage of high ammonia bearing material will be covered at all times and an ammonia reduction plan will be implemented where necessary
• emission limits for stack gases are specified

Taking this action will control the risk and rate it as ‘low’.