Environmental Authorisations (Scotland) Regulations 2018: ​Proposed Standard Conditions for Registration level activities

Closes 24 Nov 2024

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3.2.9. Anaerobic digestion of non-waste materials

Anaerobic digestion of waste materials already requires an authorisation from SEPA. To date, anaerobic digestion of non-waste materials has not required an authorisation, but the potential risks to the environment are similar. To address this and create a level playing field, the Scottish Government has proposed the addition of this new activity to EASR.

This activity often takes place on farms where purpose grown energy crops, or slurry produced by animals is used as feedstock for the process. This non-waste feedstock is digested and produces methane/biogas, which is often combusted to produce energy for heating or power on site or sold to the grid. The resulting digestate is used as a fertiliser.

Applications for this registration level activity will include engineering plans for the liquid storage and management system, which will be assessed by SEPA.

Activity description

  • Anaerobic digestion of less than 100 tonnes per day of non-waste materials and associated feedstock and digestate handling and storage; and
     
  • The burning of the resultant biogas in a gas engine and/or Combined Heat and Power plant with a rated thermal input of greater than or equal to 1MW up to and including 50MW; and
     
  • The storage of effluent associated with the anaerobic digestion activity.

Standard conditions

Below is the list of core and activity-specific standard conditions for anaerobic digestion of less than 100 tonnes per day of non-waste materials.

  1. All potentially odorous materials must be stored within suitable closed containers or covered to prevent odour.
     
  2. All storage and processing of feedstock, digestate and effluent must take place on an area with a sealed drainage system.
     
  3. All liquid feedstock, digestate and effluent must be kept within a liquid storage and management system.
     
  4. The liquid storage and management system must be designed, constructed, maintained, managed and of sufficient capacity to prevent any emissions.
     
  5. The liquid storage and management system must be constructed as detailed in the liquid storage and management system engineering plans.
     
  6. Liquid digestate storage lagoons with walls made of earth must not be filled to a level that results in less than 750 millimetres of freeboard.
     
  7. Liquid digestate storage tank(s) must not be filled to a level which results in less than 300 millimetres of freeboard.
     
  8. Non-liquid feedstock must not be stored above the height of the vessel it is stored in.
     
  9. Liquid feedstock, digestate and effluent storage lagoons with walls made of earth must:

    a. be lined with an impermeable sheet material; and

    b. have suitable leak detection.
     
  10. The impermeable liner at the mixing and filling zones of any earth-banked liquid feedstock, digestate or effluent storage lagoons must be protected with a layer of concrete.
     
  11. Wherever any flare is in use to combust biogas it must operate at a minimum temperature of 1,000ºC.
     
  12. All biogas condensate must be:

    a. collected and contained; or

    b. recirculated back to the digester.
     
  13. The pressure in the biogas system must be maintained below 3.2 millibar at any time.
     
  14. All pressure relieve valves (PRVs) in the biogas system must be linked to an alarm system(s) which must be connected to telemetry 24 hours a day to monitor and record, as a minimum, temperature and pressure within the PRV system.
     
  15. Emissions of unburned biogas and the operation of the auxiliary flare must be minimised.
     
  16. Any activation of:

    a. the flare; or

    b. the PRV

    must be recorded as an environmental event.
     
  17. Point source emissions to air from the authorised activities must only be made from the gas engine and/or the combined heat and power (CHP) plant stack(s).
     
  18. Any gas engine and/or combined heat and power (CHP) plant stack height associated with the authorised activities must be:

    a. greater than or equal to 3 metres above the roof ridge height of the building on which it is located; or

    b. greater than or equal to 3 metres above the ground if it is located separate to any building; and

    c. greater than or equal to the height of any part of a building which is located within a distance of 5 times the uncorrected stack height.
     
  19. Emissions of substance(s) to the air from the gas engine and/or CHP plant stack(s) must not exceed the specified emission limit value specified in Table 1.
     
  20. Air must not be added to dilute emissions in order to achieve emission limit values specified in Table 1.
     
  21. The emission of any substance, not specified in Table 1, from the authorised activities must not cause environmental harm.
Table 1: Emission limit values from gas engine and/or CHP
Substance Emission Limit Value
Dust 50 mg/m³
Oxides of nitrogen (NOx)  190mg/m³
Sulphur dioxide 15 mg/m³
Total volatile organic compounds  20 mg/m³
Carbon monoxide  1,000mg/m³
  1. Monitoring of emissions of substances to air must be undertaken at the gas engine and/or combined CHP plant stack(s) at the sample port, frequency and using the monitoring standard specified in Table 2.
     
  2. Sample points must be installed, maintained and appropriately identified so that representative samples may be safely obtained.
     
  3. Monitoring must be undertaken:

    a. during normal operation;

    b. under stable conditions; and

    c. at a representative, even load.
     
  4. The first monitoring of emissions must be undertaken within four months of the start of operations.
Table 2: Monitoring requirements for gas engine and/or CHP
Substance / Parameter Emission Point Reference Monitoring frequency Monitoring standard (1)
Dust Sample port determined in accordance with BS EN 15259 Manufacturer’s guarantee

OR

Annually 		 BS EN 13284-1
Oxides of Nitrogen (NOx)  Sample port determined in accordance with BS EN 15259 Annually  BS EN 14792
Sulphur Dioxide Sample port determined in accordance with BS EN 15259 On commissioning of new plant OR a new feedstock is introduced to the process BS EN 14791
Total Volatile Organic Compounds  Sample port determined in accordance with BS EN 15259 Annually  BS EN 12619
Carbon Monoxide  Sample port determined in accordance with BS EN 15259 Annually  BS EN 15058
Biogas flare temperature (ºC) Sample port determined in accordance with BS EN 15259 Continuous BS EN 16911-2
  1. All releases to the air from the authorised activities during normal operation, other than condensed water vapour, must be free from visible emissions.
     
  2. Measures must be taken to prevent, or where that is not practicable, minimise:

    a. dust;

    b. odour;

    c. the presence of vermin;

    d. biogas emissions; and

    e. heat emissions

    from the authorised activities.
     
  3. Dust from the authorised activities, which has a significant impact on the environment, people or property, must not be emitted beyond the boundary of the Authorised Place.
     
  4. Offensive odours from the authorised activities as perceived by a SEPA officer must not be emitted beyond the boundary of the Authorised Place.
     
  5. SEPA must be notified via its pollution hotline contact telephone number as soon as reasonably practicable, and in any case within 24 hours of identification of an event, of any of the following:

    a. an event that has caused or could cause adverse impact to the environment or harm to human health;

    b. an event that results, or could result, in an emission to the environment that is not authorised; and

    c. an event that has caused a breach of a condition of this authorisation.

    In this condition, the meaning of ‘event’ is as defined in the Interpretation of Terms of this authorisation.
     
  6. All measures that are reasonably practicable must be taken to stop an event and to minimise its effect on the environment.
     
  7. Within 14 days of an event a report must be submitted to SEPA detailing:

    a. the reason(s) for the event;

    b. the action(s) taken to stop the event and minimise the impacts; and

    c. the action(s) taken to prevent the event from reoccurring.
     
  8. All information recorded, kept or submitted to SEPA in accordance with a condition of this authorisation must be:

    a. true and accurate; and

    b. kept for a minimum of six years; and

    c. provided to SEPA upon request.
     
  9. Records must be kept of all monitoring results and verification of compliance with the emission limit values specified in Table 1.
     
  10. The results of the monitoring of emissions, as described in condition 22, must be submitted to SEPA within eight weeks of the date the monitoring took place via email to registry@sepa.org.uk.

Interpretation of terms

Interpretation of terms
Term Definition
Application The application submitted for this authorisation
Digestate Material remaining after anaerobic digestion of biodegradable feedstock including whole digestate (mixture of liquor and fibre), liquor, and separated fibre. 
Dust  Suspended solid particles and liquid droplets suspended in air which may be deposited on surfaces and may cause air pollution and/or nuisance. 
Effluent a mixture consisting wholly of or containing liquid produced from the storage of any forage crop or other feedstock, the storage of digestate, and rainwater or groundwater emanating from an effluent collection system or drain.
Emission The direct or indirect release of substances or heat from individual or diffuse sources in the authorised activities into air, water or land.
Emission Limit Value

The mass, expressed in terms of specific parameters, concentration or level of an emission, which may not be exceeded during one or more periods of time.

All emission limit values are defined at:

  1. temperature of 273.1K;
     
  2. a pressure of 101.3kPa;

without correction for water vapour content.
Environmental harm
  1. Harm to the health of human beings or living organisms,
     
  2. Harm to the quality of the environment, including:
     
    1. harm to the quality of the environment taken as a whole,
       
    2. harm to the quality of air, water or land, and
       
    3. other impairment of, or interference with, ecosystems,
  3. Offence to the senses of human beings,
     
  4. Damage to property, or
     
  5. Impairment of, or any interference with, amenities or other legitimate uses of the environment.
Event
  • Any accident which has caused or could cause environmental harm; or
     
  • Any malfunction, breakdown or failure of plant, infrastructure or techniques which has caused or could cause environmental harm; or
     
  • Force majeure or action taken to save human life or limb.
Feedstock The non-waste materials to be treated in anaerobic digestion plant.
Forage crop Any crop grown as food for livestock or for use in energy production.
Freeboard The vertical distance between the crest of a tank, container or lagoon and the digestate surface.
Gas engine An internal combustion engine which operates according to the Otto cycle and uses spark ignition to burn fuel.
Impermeable A surface constructed of impermeable material to a standard sufficient to prevent the transmission of liquids beyond the surface. Should be read in conjunction with the term ‘sealed drainage system’
Impermeable sheet material
  1. synthetic rubbers, EPDM (ethylene propylene diene monomer rubber) and butyl,
     
  2. plastics, including polyvinyl chloride, low density polyethylene and high density polyethylene, and
     
  3. reinforced geomembranes.
Leak-proof To be constructed of impermeable material of sufficient strength and structural integrity such that it is unlikely to burst or leak in its ordinary use.
Liquid Any aqueous material but excluding sludge.
Liquid storage and management system

An impermeable system designed to contain liquid, comprising the base(s) and walls of liquid feedstock, digestate or effluent storage tank(s), lagoon(s), pit(s), vessel(s), tower(s) plus any connected channels, pipes and valves which:

  1. has sufficient capacity to accommodate contaminated run-off (including any rainfall) that directly or indirectly enters the liquid storage and management system and the volume of digestate produced during periods when application is not authorised under activity 18 of column 1 of Schedule 9 of the Regulations

    or would not comply with the requirements of the Action Programme for Nitrate Vulnerable Zones (Scotland) Regulations 2008;
     
  2. has an effluent tank which is at least:
     
  3. for a feedstock storage tank, lagoon, pit, vessel or tower with a capacity of less than 1500m³, 20 litres for every 1m³ of silo capacity; or
     
    1. for a feedstock storage tank, lagoon, pit, vessel or tower with a capacity of 1500m³ or greater, 30,000 litres plus 6.7 litres for every 1m³ of silo capacity over 1500m³;
       
    2. is protected against corrosion in accordance with paragraph 7.2 of the Code of Practice on Buildings and Structures for Agriculture published by the British Standards Institution and numbered BS 5502-50:1993+A2:2010;
  4. is capable of withstanding characteristic loads calculated on the assumptions and in the manner as set out in paragraph 5 of the Code of Practice on Buildings and Structures for Agriculture published by the British Standards Institution and numbered BS 5502-50:1993+A2:2010;
     
  5. where the system incorporates a system of pumps and sumps, it must be fitted with an automatic overfill prevention device with a dedicated electrical supply and an alarm;
     
  6. If new, reconstructed or enlarged on or after 1 January 2022, has a life expectancy of at least 20 years, with proper maintenance, from its construction, reconstruction or enlargement;
     
  7. where the liquid digestate storage tank is fitted with a drainage pipe—
     
    1. there must be two valves in series on the pipe and each valve must be capable of stopping the flow of liquid digestate through the pipe and must be kept shut and locked in that position when not in use;
       
    2. sub-paragraph (g)(i) does not apply in relation to a liquid digestate storage tank which drains through the pipe into another liquid digestate storage tank of equal or greater capacity or where the tops of the tanks are at the same level.
Liquid storage and management system engineering plans The liquid storage and management system engineering plans submitted as part of the application.
Normal operation Operation of authorised activities excluding start-up and shut-down periods.
Oxides of nitrogen Nitric oxide expressed as nitrogen dioxide (NO2) and nitrogen dioxide, expressed as nitrogen dioxide (NO2).
Point source emission  Single, identifiable source of emission. 
Sealed drainage system

A drainage system with impermeable components and surfaces which does not leak and which will ensure that:

  1. no liquid will run off the surfaces otherwise than via the system; and
     
  2. except where they may be lawfully discharged, all liquids entering the system are collected in a sealed sump.
SEPA officer Any person who is authorised in writing by SEPA under section 108 of the Environment Act 1995.
Sludge A by-product of the anaerobic digestion process with a suspension having a high solids content.
Storage tank Includes a lagoon, pit (other than a reception pit), vessel or tower used for the storage of feedstock, digestate and effluent. 
Sump

In relation to a sealed drainage system, the container in which all liquid collects which must:

  1. hold at least:
     
    1. for a single container, 110% of its capacity;
       
    2. or for two or more containers, the greater of:
       
      1. 110% of the capacity of the largest container; or
         
      2. 25% of the capacity of all containers together.
  2. catch all spills from the container(s) and related parts;
     
  3. be leak-proof;
     
  4. be located, and/or protected, to prevent damage as far as reasonably practicable; and
     
  5. have any spills and/or rainwater removed as soon as reasonably practicable.
Uncorrected stack height The stack height before any required increases in height are made to account for any nearby buildings. 
Water environment All surface water, groundwater and wetlands.

Rationale

The standard conditions are new and consistent with the conditions for the anaerobic digestion of waste material registration. There are controls related to odour, dust, biogas and heat emissions; feedstock, digestate and effluent management; biogas system management; and monitoring of emissions from the gas engine or combined heat and power plant is required.

 

90. Do you agree with the list of standard conditions for the anaerobic digestion of <10 tonnes per day of non-waste materials?