Prevention of environmental pollution from agricultural activity: guidance

Code of good practice, giving practical advice to farmers and others on minimising pollution.

Section 7: silos and silage effluent


**1. Comply with the Control of Pollution (Silage, Slurry and Agricultural Fuel Oil) (Scotland) Regulations 2003.

  • notify SEPA at least 28 days before bringing into use any new, substantially enlarged or substantially reconstructed silo and silage effluent storage facility

  • ensure the base of the silo, effluent tank and drains are impermeable. In addition, they, and any silo walls, should be resistant to attack from silage effluent

  • properly maintain all parts of the silo system such that it will meet a 20-year design life

  • ensure the minimum effluent tank capacity requirements are provided. Consult SEPA on the size of tank required

2. Ensure that effluent tanks, channels, silo floors, walls and wall floor joints are inspected annually and any necessary repairs carried out well in advance of the start of silage making.

3. Try, if possible, to wilt the crop that is to be ensiled, so as to reduce effluent production to a minimum.

4. Keep all effluent collection channels and drains clear of blockages.

5. Check effluent tank levels on a frequent basis when effluent is being produced, and regularly throughout the year. Empty as necessary.

6. Regularly check watercourses throughout the year to ensure that they are not polluted. If any signs of pollution become evident, immediate action should be taken to stop any discharge and SEPA should be contacted immediately.

7. Have a contingency plan to deal with structural failures and effluent escapes.

8. Dilute silage effluent with a minimum of one-to-one (1:1) water if spreading on grassland to avoid scorching.

9. Avoid rainwater entering effluent collection systems.

10. Remember that silage effluent can be a valuable feedstuff or fertiliser. Think of it as a potential resource.


**1. Don't site a silo or any part of the effluent collection system within 10m of any inland or coastal waters.

**2. Don't store baled silage within 10m of any inland or coastal waters or remove the wrapping of any bales within 10m of any inland or coastal waters.

**3. Don't allow silage effluent to enter a watercourse as this is an offence under environmental legislation.

**4. Don't allow effluent tanks to overflow, or ever use a by-pass to divert run-off from a silo.

**5. Don't use a soakaway to dispose of silage effluent.

**6. Don't neglect maintenance and inspection of silos, as pollution does not have to occur for a Notice to be served by SEPA.

**7. Don't continue to use silage-making facilities which present a significant pollution risk.

**8. Don't make silage in a free-standing field heap (i.e. without an impermeable base or an effluent containment system).

**9. Don't allow contaminated silo run-off to escape into a watercourse. Collect it - it is very polluting.

10. Don't underestimate the quantity of effluent produced from wet grass silage.

11. Don't add silage effluent to slurry in confined spaces or in buildings, because this will produce lethal gases very quickly.

12. Don't apply silage effluent on sites where there is a danger of direct discharge into field drains (e.g. on cracked soils or recently drained or subsoiled fields).

13. Don't apply silage effluent within 10m of a watercourse and at least 50m of a drinking water supply.

14. Don't apply silage effluent to steeply sloping fields, when the soil is wet or waterlogged, when there is a flooding risk or when heavy rainfall is forecast.

15. Don't apply more than 50m 3/ha (normal rate 25-30 m 3/ha or 2,200 _ 2,700 gallons/acre) of diluted silage effluent to land.


What is silage effluent?

7.1 Silage effluent is produced from any forage crop which is being made, or has been made, into silage. It is also defined as a mixture consisting wholly of or containing such effluent, rain or groundwater emanating from a silo, silage effluent collection system or drain. Silage effluent is the most common cause of agricultural pollution in Scotland as reported by SAPG. Each year, a significant number of serious pollution incidents occur through failure to contain or dispose of effluent satisfactorily. The Control of Pollution (Silage, Slurry and Agricultural Fuel Oil) (Scotland) Regulations 2003 ("the SSAFO Regulations") set minimum standards for installations used for the storage of such substances and their associated effluents.

Why is silage effluent important?

7.2 Silage effluent has a Biochemical Oxygen Demand (BOD) of up to 200 times that of raw sewage. A clamp containing 500 tonnes of unwilted silage has the same polluting potential as the daily untreated sewage from a city of approximately 200,000 people (i.e. about the size of Dundee or Aberdeen). As almost 6 million tonnes of silage are produced per annum in Scotland and there are approximately 10,000 silos, the potential for pollution is high. If silage effluent is allowed to enter a watercourse it rapidly strips oxygen from the water, killing fish, plants and other aquatic life.

7.3 Silage effluent is also highly acidic and attacks steel and concrete surfaces, causing deterioration of cracks and joints in silo floors, collection channels and tanks, making it very difficult to contain and collect all the effluent.

Quantities of effluent

7.4 The volume of effluent produced depends on the moisture content of the crop being ensiled. This in turn depends on factors such as the maturity of the crop, the degree of wilting, the weather conditions, the use of additives and absorbents (e.g. dried sugar beet pulp) and whether the silo is roofed or unroofed. The peak flow of effluent normally occurs within two to three days of ensiling the crop. Up to 50% of the total volume of effluent is produced in the first 10 days. The table below demonstrates that the typical volume of effluent likely to be produced varies significantly with the dry matter of the crop ensiled.

Dry Matter Content of Grass Ensiled and Quantity of Effluent Produced

Dry Matter Content of Grass Ensiled(%)

Effluent Production
(litres/tonne of Grass Ensiled)







7.5 Wilting is very desirable but is highly dependent on weather conditions at the time of silage making. The ensiling of crops with a relatively high dry matter content, such as whole crop cereals, results in less effluent production. Farmers should always be prepared to contain, collect, store and dispose of any effluent and should be aware that the use of some silage additives tends to increase the amount of effluent produced. In a wet year, very high volumes of effluent have to be dealt with and even greater care is required.

7.6 The objective should be to reduce the volume of effluent to a minimum. Rainwater falling directly on the silo cover should be diverted and drained separately from the silage effluent if practical. When the silo is being used, rainwater falling on the floor will become polluted and must be collected. A roof over the silo with an independent rainwater drainage system will minimise the quantities of effluent which require to be handled. Care should be taken by directing rainwater through a sealed system to a suitable outfall to ensure roof drainage does not become contaminated.

7.7 The regular monitoring of collection tank levels should be undertaken at all times but with greater frequency during the first 10 days following ensiling to contend with peak flow rates of effluent, and during periods of wet weather. Many factors influence effluent flow, such as depth of silage, efficiency of drainage within the silo and the use of certain additives. Tank levels must be checked throughout the year especially in the autumn/winter and not just during silage making. A significant number of pollution incidents occur in the autumn from effluent tanks overflowing with dirty water run-off from open clamps. All contaminated water must be collected and by-pass systems must never be used.

Silage making

What legislation must be complied with?

7.8 The Control of Pollution (Silage, Slurry and Agricultural Fuel Oil) (Scotland) Regulations 2003 ("the SSAFO Regulations") are intended to prevent environmental pollution by silage effluent. While the Regulations set minimum standards for the construction and siting of new, substantially enlarged or substantially reconstructed silos, SEPA also has powers to ensure that all existing structures conform if they pose a significant risk of pollution or if they otherwise fail to comply with the requirements of the Regulations. The SSAFO Regulations also apply to facilities used for whole crop forage.

7.9 All silos, effluent tanks and any associated pipes and channels must be designed in such a way that with proper maintenance they will satisfy the requirements of the Regulations for at least 20 years. All parts of the silo must be sited at least 10m from a watercourse, including permeable drains (e.g. field drains) and open ditches, to which any escaping effluent could enter.

7.10 The Regulations state where silage can and can't be made.

7.11 It is prohibited to make silage in freestanding field heaps without an impermeable base and an effluent containment system.

7.12 Silage can be made in any of the following sites:

  • in tower silos

  • by an unbaled, bulk-bagged system which uses an impermeable membrane with sealed ends that effectively prevent any effluent leaking from the bag. The minimum specification to meet the required standard is 3-ply 1000 gauge polyethylene or a material of at least equivalent impermeability and durability. The bags must be kept sealed and stored on a firm level base, a minimum of 10m from the nearest watercourse. It is no longer necessary to pre-notify SEPA of the intention to make silage in this form

  • by baled and bagged or wrapped techniques, providing it is sealed in an impermeable membrane and not stored or opened within 10m of a watercourse

  • in earthbanked silos, providing that the base is impermeable and the walls are lined with an impermeable membrane such as 1000 gauge polyethylene or butyl rubber sheeting. An effluent collection and storage system as described below, but excluding the external channel, must be provided. A kerb is recommended for the edge of the floor adjacent to the earth banks and the lining should be carried down over this to prevent any leakage of effluent into the earth banks. Temporary internal drainage pipes should also be provided to aid the removal of effluent to a storage tank

  • in a conventionally constructed silo


7.13 Silos and effluent tanks must comply with the following:

  • the base and associated effluent tanks, pipes and channels must be impermeable, and along with the walls, should be resistant to attack by silage effluent

  • there should be a slope across silo floors to drains running the length of the silo walls and from back to front to a drain across the front of the silo

  • the base must extend beyond the walls (other than for earth banks) and be provided with channels to collect any effluent which may escape

  • collection channels must lead to an effluent tank, and be similar in profile to a roof gutter. They should be regularly cleaned out

  • retaining walls must be capable of withstanding minimum wall loadings as calculated in accordance with BS 5502 Part 22

  • effluent tanks must have a minimum capacity of not less than 3m 3 for each 150m 3 (or part thereof) of silo capacity for a silo of less than 1500m 3 capacity. For larger silos, the effluent tank requires to have a capacity of at least 30m 3 plus 1m 3 for each 150m 3 (or part thereof) of silo capacity in excess of 1500m 3

7.14 The effluent tank capacities which are stated are a minimum and are unlikely to be sufficient for two days' storage for unwilted silage or silage made in an unroofed silo.

7.15 Farmers should check and empty their tanks as often as is necessary in the light of their own circumstances and experience. Additionally farmers should take as much advantage of storage facilities elsewhere on the farm e.g. slurry towers, for the storage of silage effluent as part of a Manure Management Plan (Farm Waste Management Plan).

7.16 If possible a farmer should try to have an effluent tank which has a capacity in excess of the minimum requirement. An effective alarm system, ideally with audible and visible alarm (i.e. warning lights), will provide an early indication that tanks are almost full, thus avoiding overflow. For unroofed silos, it is important to take account of the volume of surface water run-off and plan accordingly.

7.17 Under exceptional circumstances it may be possible to install an effluent tank which is less than the minimum requirement. This can only be permitted after full consultation and approval with SEPA.

7.18 It is important to remember that effluent can also be produced from other livestock feeds such as draff and whole crop cereals, thus making it essential that such effluent is collected and not allowed to escape and cause pollution.

7.19 If any part of the tank is below ground, it should be either prefabricated in one piece (the preferred approach) or of an alternative construction capable of meeting the requirement of being impermeable and resisting silage effluent for at least 20 years with maintenance. If the farmer intends to opt for this latter alternative, it would be sensible to discuss his proposals with SEPA at the planning stage. In order to protect tanks from attack by silage effluent, it is essential that they be lined with bituminous paint, butyl rubber, glass fibre, proprietary resin or other equivalent material. These linings should be checked at regular intervals and repaired/renewed as necessary.

7.20 SEPA require the farmer to verify that the completed facilities conform to the minimum standards set by the SSAFO Regulations.

7.21 Under the Construction (Design and Management) Regulations (which came into force on 31 March 1995), two aspects of the management of a silage making operation (in addition to health and safety) must be considered:

  • maintenance to retain the minimum performance standards
  • management of a silo at all times but particularly during periods of maximum effluent discharge to confirm that there are no leakages, that the drainage system is working effectively and that the tank is not allowed to overfill

New facilities

What can I do?

7.22 The following general principles of good practice should be adopted when considering and initiating proposals for silage production on the farm.

Planning and siting

7.23 Silos and effluent collection systems should not be sited within 10m of any inland or coastal waters nor in a position where any escape of effluent may reach a watercourse. The risk of flooding from adjacent watercourses must also be avoided. Account should also be taken of public and private water supply sources in the vicinity. Public bylaws may restrict the siting of silage and effluent facilities in such localities.

7.24 Reasonable care must be taken to avoid sites where drains exist. If any drains are present, they must be identified and removed. Any trenches created by such works should be securely backfilled with an impermeable material. Open and permeable drains are not permitted within 10m of any part of the silo, its effluent tank or any associated pipes or channels. Drains should be provided to intercept surface water which may otherwise reach the silo or its drainage system.

7.25 In the case of open silos, the amount of storm water from the cover reaching the silage or floor should be minimised. Where possible, it should be removed at the opposite end to the effluent collection system.

7.26 Storm water from roofed silos should, where practical, be led in the opposite direction to the effluent collection system and removed in downpipes connected to a sealed drainage system. These drains should be backfilled with impermeable material.


7.27 The essential design requirements are to provide facilities which will contain the effluent produced during the making and storage of silage, to collect the effluent by means of a drainage system and lead it to a store of an adequate capacity. With proper maintenance, the whole system must continue to comply with the performance standards for at least 20 years.

7.28 Professionally prepared plans and specifications for all the work are recommended so that the necessary standards are attained on any site. SEPA will require to see these before work commences.


7.29 The ground on which a silo is to be built must be capable of carrying the necessary loads to avoid structural failure due to settlement. This will involve removing all topsoil to a firm base and laying hardcore in well-compacted layers to form the necessary gradients. Avoid made up ground wherever possible, due to the risk of settlement and cracking.

7.30 Adequate steps must be taken to ensure that all work is carried out to a high standard in order to satisfy the SSAFO Regulations. Laying concrete is a skilled operation, and any contractor employed in constructing a silo or effluent tank should be fully proficient in this and associated works.

7.31 Hot rolled asphalt may be used in place of concrete to form the floor slab. It is important to seek specialist guidance on the specification for the floor design and the asphalt mix to be used. Good workmanship including adequate compaction and a suitable 'tack' coat are essential to the success of this material.

7.32 Concrete walls and steel columns which are permanently built-in should be given a surface treatment as protection against corrosion. Such treatments used in accordance with the manufacturer's instructions will minimise maintenance and enable the structures to satisfy the SSAFO Regulations for the statutory period of 20 years.

7.33 Where silo walls are constructed using prefabricated panels the column supports must be secured in concrete bases and care taken to ensure no leakage of effluent at these points. This is particularly important where the columns are set into sockets formed in the base.

7.34 When enlarging a silo, the whole structure must be brought up to standards set by the SSAFO Regulations. It is essential to seek professional guidance on the practicability of the proposal prior to starting any work. Often upgrading is likely to be impractical if lower design criteria were used in the original construction. SEPA should be consulted before any work commences.

7.35 In all cases, SEPA must be notified in writing at least 28 days in advance of bringing a new, substantially enlarged or substantially reconstructed silo or silage effluent tank into use.


7.36 The silo drainage system and effluent tank must be carefully inspected for any signs of surface corrosion, cracking of concrete or fractured pipes.

7.37 The following procedure should be carried out each year immediately after the silo is emptied:

  • empty and, when safe to do so, visually check the effluent tank

Do not enter the tank, as poisonous gas may be present.

If it is necessary to enter for repair purposes, seek advice on safety requirements:

  • clean any areas which cannot be clearly seen

  • inspect walls, floors and wall floor joints for cracking or surface corrosion

  • reseal all sealed joints where the jointing material has been damaged

  • inspect all drains and channels for damage

  • check that all channels and pipework are free flowing

  • check all safety arrangements

  • list all repairs required and prepare a timetable to execute the work

7.38 There are often only a few weeks between silos being emptied and refilled. As many materials require time to cure before being exposed to effluent, plan ahead to ensure the silo is emptied completely and that there is sufficient time before silage making begins to properly complete any works.

Management practice

7.39 Although roofed silos require less management during and after filling, it is essential that frequent checks be made to ensure that the drainage system is free running and that the effluent tank does not overflow.

7.40 The design criteria of BS5502 require that there are internal drains along the bottom of the walls to reduce pressure.

7.41 Where the silage is made as a wedge or grass ramps are used, care must be taken that the ramp does not extend beyond the silo's drainage system, to ensure that the effluent is contained. To deal with this difficulty, many silos have a concrete apron which slopes inwards to the silo floor. The filling of unroofed silos requires planning if the maximum quantity of rainwater is to be diverted from the effluent tank. The aim is to dome or shape the top of the silo so that water falling on the cover is directed away from the working end of the pit. To achieve this in solid walled silos, the covering sheet should extend over and out from the end or one side of the silo. The covering sheet may be run out over the effluent channel to shed stormwater away from it on completion of filling open silos.

7.42 It is important that silo covers are properly secured in place by the use of weights such as rubber tyres, and are regularly inspected to ensure they remain intact.

7.43 While any silage remains in the pit, all effluent and contaminated rainwater (floor, cover and silage) must be collected and disposed of. When empty, the silo may be washed down but on no account should wash waters be discharged to a watercourse.

7.44 All adjacent ditches, burns or watercourses should be checked regularly (preferably daily) for signs of pollution, particularly when silage effluent is being produced just downstream of the point of discharge of the surface water drainage system from the farm. If any pollution is found, immediate action should be taken to remedy the situation. SEPA should also be notified.

Disposal of silage effluent

7.45 Although aeration treatment can significantly reduce the BOD strength of silage effluent, it cannot be treated to a safe level for discharge to a watercourse.

7.46 Silage effluent should be diluted by a minimum of 1:1 with water and spread safely on land. The application rate should be determined by the land suitability and crop cover and should never exceed 50m 3/ha or 4500 gallons/acre (normal rate 25-30m 3/ha or 2,200 - 2,700 gallons/acre) of diluted effluent. In order to avoid scorching, a reduced rate of application will be necessary. Any repeat application should not be made within three weeks. The maximum rate of application should be reduced if the soil has been compacted.

7.47 Silage effluent is also a significant source of nutrients, with a high proportion of these nutrients readily available to the growing crop. These should be accounted for in any farm nutrient budget and in determining crop requirements.

7.48 Soakaways are not an acceptable method of effluent disposal and must not be used.

7.49 Fresh silage effluent can be a valuable feedstuff on which livestock thrive. Increasingly farmers are appreciating this and a growing number are collecting and storing silage effluent to feed back to their livestock.

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