Prevention of environmental pollution from agricultural activity: guidance

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

Section 13: prevention and control of emissions to air


**1. Seek a permit from SEPA if you have more than 40,000 places for poultry, or 2,000 places for production pigs or 750 places for sows at an installation.

**2. Submit an application between 1 November 2006 and 31 January 2007 if your farm exceeds the above thresholds.

**3. Apply for a permit from SEPA if you intend to construct a new installation for rearing pigs or poultry and where the number of places for animals or birds will exceed the thresholds specified in the Pollution Prevention and Control (Scotland) Regulations 2000, as amended ("the PPC Regulations").

**4. Consult SEPA about any proposals you have to substantially change an existing pig or poultry installation in advance of 1 November 2006, as this may require you to seek a permit to operate such an altered installation under the PPC Regulations.

**5. Consult Scottish Natural Heritage (SNH) regarding any areas designated for their nature conservation value within 2km of any new or substantially changed installation that exceeds the thresholds in the PPC Regulations.

**6. Comply with the 'Standard Farming Installation Rules' developed by SEPA for pig and poultry installations operated under the PPC Regulations.

*7. Follow the latest edition of the Muirburn Code. [GAEC measure 6]

8. Consult SEPA if in any doubt about the requirements of the PPC Regulations.

9. Spread slurries and manures when the wind direction is away from public/residential areas and areas designated for their nature conservation value.

10. Seek professional advice about how to prevent and control emissions to air if in doubt.

11. Use low-emission techniques for slurry spreading e.g. trailing-shoe, shallow (open slot) injector, deep (closed slot) injector or band spreader. When this is not possible, use a broadcast slurry spreader that gives a low and downward trajectory and large droplets.

12. Minimise odours from livestock housing by collecting and transferring all slurry every day to a suitable store and cleaning buildings regularly.

13. Cover slurry stores where practicable to reduce emissions of ammonia. This will also reduce levels of waste production by excluding rainfall.

14. Incorporate applications of slurry and solid manure to uncropped land as soon as practical, preferably within 6 hours for slurry and 24 hours for solid manure.

(Italics indicates mandatory if specified in a permit issued by SEPA.)


**1. Don't apply for a permit if your pigs or poultry are reared outdoors, as this type of production is not covered by the PPC Regulations.

2. Don't spread slurries or manures in a manner that may cause pollution of air or result in odour nuisance.

3. Don't be a 'bad neighbour' by spreading livestock slurry and manures:

  • close to domestic or public buildings

  • at weekends or public holidays

  • when the wind direction is towards public/residential areas

  • in areas designated for their conservation value

  • during the hours of darkness, unless unavoidable

4. Don't burn plastics, rubber, tyres or other materials which will produce dark smoke.

5. Don't light fires near a public road.


13.1 Increasingly, the impact of agricultural activities on air quality is being recognised. It is known, for example, that agriculture is the dominant source of ammonia emissions in the UK, mainly arising from the storage and application of livestock manures and slurries.

13.2 Agricultural activities can also give off various "greenhouse" gases, such as carbon dioxide, methane and nitrous oxide, which contribute to climate change. Carbon dioxide is emitted when soil is disturbed (e.g. by ploughing) and when peat is drained. Methane is formed from the decomposition of organic matter and is also produced by ruminant animals. Nitrous oxide, a powerful greenhouse gas, can be formed in wet soils from using nitrogen fertiliser and from the treatment and application of animal manures and slurries. Using energy in buildings and transport also leads to emissions of greenhouse gases.

13.3 Methyl Bromide, a chemical pesticide/fumigant, is an ozone-depleting substance. Production and supply of methyl bromide for general use ceased on 31 December 2004. Where suitable alternatives are not available, an application for a critical use exemption must be made.

13.4 Complaints about agricultural odours arise mainly from slurry or manure spreading, farm buildings and slurry or manure stores.

13.5 A permit is required from SEPA for the operation of certain large intensive pig and poultry installations to control the overall impact on the environment, including emissions to air.

What legislation must be complied with?

13.6 The Pollution Prevention and Control (Scotland) Regulations 2000, as amended ("the PPC Regulations"), which are enforced by SEPA, apply to a wide range of industries. In terms of agriculture, however, only large installations used for the intensive rearing of pigs and poultry that exceed the following thresholds will become subject to control:

  • 40,000 places for poultry

  • 2,000 places for production pigs (over 30kg)

  • 750 places for sows

13.7 The Regulations are being phased in, with complete implementation required by 2007. However, new intensive pig and poultry installations, and those that become substantially changed, will be required to comply immediately by applying for a permit from SEPA. Note that installations where pig and poultry are reared outdoors are not covered by the PPC Regulations.

13.8 The overall environmental impacts of intensive pig and poultry installations are controlled under the Regulations. As well as focusing on issues such as waste minimisation and noise generation, PPC involves looking at raw material inputs, the transfer, collection and storage of manures and slurries, odour control, and efficiency of water usage. The storage and spreading of manures is a critical issue, given that the pig and poultry sectors contribute to 23% of the UK's ammonia emissions.

13.9 Guidance on meeting the requirements of the PPC Regulations for pig and poultry rearing installations has been incorporated into the Standard Farming Installation Rules for Pig and Poultry PPC Installations developed by SEPA in conjunction with the industry and other UK organisations. Contact your local SEPA office, or access the SEPA website, for further guidance on the requirements of PPC. Scottish Natural Heritage (SNH) must also be consulted regarding any areas designated for their nature conservation value within 2km of any new or substantially changed installation that exceeds the thresholds in the PPC Regulations.

13.10 Consult SEPA if you are planning to treat slurry and FYM on your farm, including the processes highlighted in Appendix 1, as this may require to be controlled under waste management legislation.

Farm odour

13.11 The statutory nuisance provisions under Part III of the Environmental Protection Act 1990 give local authorities power to act on complaints arising from agricultural activities. Using these powers, an abatement notice may be served requiring a decrease of the nuisance or prohibiting or restricting its reoccurrence.

13.12 The use of appropriate practices allied to good planning and management of both new and existing installations/structures can help minimise the risk of causing a nuisance. The aim is to contain the smell within the farm buildings or surrounding agricultural land by reducing emissions at source and allowing the diluting effect of air movement and distance to complete its dispersion.

Biological treatments

13.13 Biological treatment of slurry and manures by anaerobic or aerobic techniques has proved successful in significantly reducing odour emissions in certain circumstances and it may be worth considering the use of such treatments to tackle specific or severe problems. Further advice on this can be found in Appendix 1 to this section. 

Minimising odour from livestock buildings

13.14 When designing new buildings, consider their siting in relation to residential accommodation, and avoid sites within 400m of such developments. Where possible, sites downwind of residential areas should be chosen. Ensure buildings are properly ventilated to control temperature, humidity and the concentration of gases, and to provide a good distribution of clean air under a wide variety of external weather conditions. Ventilators should be thoroughly cleaned between batches of stock and maintained to ensure operation at the correct airflow for the stock requirements. The removal of dust will help to reduce the level of odour. Humid conditions caused by poor ventilation result in smells and the build up of high levels of ammonia.

13.15 Professional advice should be sought on the positioning of ventilator outlets. The higher the outlet, the greater the dilution factor from air movement. Ventilation outlets positioned along the sides of buildings, below slatted floors and immediately over slurry collection channels can result in poorer dispersion of odours. The use of bioscrubbers, biofilters and tall chimneys as exhaust ventilators for odour removal can be effective technologies, but they are expensive and it is generally better to control odours at source.

13.16 To control odour, it is essential to maintain a high standard of hygiene and cleanliness:

  • plan the collection and storage of slurry and manure (see Section 4 for further details)

  • minimise open concrete areas used for livestock. Drainage arising from this area must drain to the slurry storage system. It should not be allowed to run off on to roads where evaporation will increase smells, in addition to posing a road safety risk

  • maintain or replace drinking systems to avoid overflow and spillage which leads to wet, smelly areas, increased levels of waste production, slurry dilution or wet poultry litter or bedding

  • clean livestock buildings regularly and thoroughly using the correct type and quantity of disinfectant and volumes of wash water

  • ensure that all cleaning material/drainage water is collected. Never discharge wash waters to watercourses as these can give rise to conditions, giving rise to odour. Contact SEPA for advice on suitable disposal arrangements

  • comply with the stocking density recommendations set out in the appropriate Codes of Practice for the Welfare of Livestock. Further advice on this can be obtained from Scottish Executive Animal Health Offices, the Farm Animal Welfare Council and SAC

Minimising odour from slurry and manure stores

13.17 New slurry store sites should not be located within 400m of a residential development. In solid manure stores, natural composting should be encouraged by allowing air penetration to stimulate the breakdown of the manure.

13.18 Offensive smells from slurry stores are produced by:

  • long periods of storage under warm condition

  • the addition of waste foods, milk products or silage effluent

13.19 Where safe and practicable, odour emissions can be reduced by the use of a floating or fixed cover. Take care as poisonous and explosive gases may collect under the cover.

Minimising odour from land application

13.20 The whole-farm production and management system should be considered in relation to the overall control of emissions. The release of odours during the spreading of slurries, manures and other organic fertilisers can be detected at large distances from the field. The level of odour is affected by:

  • the type of manure or slurry, especially slurries containing waste foods, milk or silage effluent

  • the age of the manure or slurry

  • the time of year, length and method of storage

  • the weather conditions during and following spreading

  • the rate of applicatio

  • the method of spreading/equipment used

13.21 The following guidance should be adhered to:

  • obtain a weather forecast and select suitable conditions for spreading. The best spreading conditions are bright, sunny and windy days, followed by cloudy, windy evenings. Avoid spreading when the forecast indicates very damp, humid weather with very light winds and clear, still nights with light winds, as these conditions prevent odour dilution and dispersion

  • use machinery with low emission techniques for slurry spreading e.g. trailing shoe, shallow (open slot) injector, deep (closed slot) injector or band spreader. When this is not possible, use a broadcast slurry spreader that gives a low and downward trajectory at low pressure. High trajectory applicators, such as those fitted with a splash plate or rain guns, should only be used on sites remote from residential housing or areas to which the public have no access

  • inject or incorporate applications of slurry and solid manures to uncropped land as soon as practical, preferably within 6 hours for slurry and 24 hours for solid manures

  • seek to immediately plough or work slurry into the soil on arable land, as this can reduce emissions by up to 90%. Rates of loss are highest during the first few hours after spreading

  • adopt a 'good neighbour' policy and, where possible, avoid slurry, sewage sludge and poultry litter spreading at weekends, public holidays and in the evenings. Do not spread during the hours of darkness, unless unavoidable, as it is difficult to supervise and control such activities or see any impacts on local watercourses etc. Where possible, slurry spreading should not be undertaken less than 1km upwind of residential development and a wide margin should be given to public footpaths and private gardens

  • avoid spreading in areas designated for their conservation value including unimproved grassland and rough grazings

  • consider exporting the slurry to another farm where land application does not cause environmental problems, if the above precautions are inadequate to control nuisance at an acceptable level. The additional slurry must be included in that farm's Manure Management Plan (Farm Waste Management Plan) / Risk Assessment for Manures and Slurries. Please note that, in some circumstances, the movement of manures and slurries from one farm to another will be considered movement of waste. In these cases, the exporting farmer will have to produce a waste transfer note within seven days if SEPA asks. Further details are available from SEERAD and from SEPA.

Ammonia emissions

13.22 Ammonia emissions from agriculture not only represent the loss of a valuable nutrient but can also cause the following detrimental effects to the environment:

  • acidification of the soil through deposition of ammonia and transformation to ammonium and then nitrate

  • addition of nitrogen, by deposition to areas of high nature conservation value, which may result in vegetation changes or increased nitrate leaching because many ecosystems are adapted to low nutrient conditions. This may give rise to increased nutrient enrichment of watercourses

13.23 Ammonia losses are considerably higher from manures and slurries containing a high ammonium nitrogen content e.g. cattle and pig slurry and poultry manure. While the PPC Regulations only apply to intensive pig and poultry installations above certain size thresholds, smaller livestock units (including dairy and beef) can give rise to potentially damaging emissions if sited or managed inappropriately.

Minimising ammonia losses from buildings and slurry and manure stores

13.24 Several techniques can be used to reduce ammonia emissions from buildings and slurry and manures stores. Slurry stores should be covered if this is safe and practical, which will also help to reduce levels of waste production by excluding rainfall. In addition, additives can be added to stored manure to manipulate its pH value and techniques can be employed to encourage fast air drying of manure or litter in poultry houses.

Minimising ammonia losses from land application of slurry

13.25 Of the total ammonia lost through slurry spreading, over 50% can be accounted for within the first 24 hours of application. The amount lost varies according to the following factors:

  • soil condition

  • slurry type and composition

  • crop

  • weather conditions (e.g. rainfall, wind-speed, temperature)

Techniques used for odour reduction are also effective in reducing ammonia emissions resulting from slurry applications.

General on-farm burning

13.26 Burning plastics, rubber, tyres or other waste produces dark, polluting smoke and could constitute a statutory nuisance under Part III of the Environmental Protection Act 1990. It is not recommended that these materials be burned.


13.27 Muirburn is a means of managing the vegetation of upland grazings to maintain them in a productive condition. In the planning and control of muirburn, farmers and landowners should follow "The Muirburn Code" and its supplement "Guide to Best Practice" published by SEERAD.

Straw and stubble burning

13.28 Straw and stubble burning presents several risks, including:

  • the farm - buildings, crops and machinery may catch fire

  • the countryside - hedges, trees, wildlife and game can all be affected

  • general - serious risks can be caused to nearby businesses and homes and, if smoke is allowed to drift onto roads, this can result in unsafe driving conditions

  • loss of valuable organic material for soils

  • emissions of greenhouse gas

13.29 As straw is of value to livestock farmers, baling and carting is the preferred alternative to burning where this can be achieved within cost and time constraints. Alternatively, straw can be chopped and incorporated into the soil prior to establishing the next crop.

13.30 If stubble burning is to be carried out, the following precautions should be taken:

Before burning

  • make a fire break at least 10m wide

  • warn neighbours in order to prevent unnecessary alarm or inconvenience when burning starts

  • consult the local authority Environmental Health Department, if burning near a residential area

  • check the up-to-date weather forecast before you burn. Avoid burning in still conditions or if wind is forecast to exceed Force 3 (4-6mph)

When burning

  • each separate fire must be continually supervised by a sufficient number of competent staff

  • limit the area of burning to controllable blocks of not more than 10 hectares, with not less than 150m between any two blocks being burned simultaneously

  • burn against the wind

After burning

  • check carefully that no straw remains alight or smouldering

  • incorporate ash as soon as possible

Never burn

  • when the fire may get out of control e.g. in windy or exceptionally dry conditions

  • within 15m of the trunk of a tree or a hedgerow

  • within 50m of any housing or farm buildings

  • within 100m of any road

  • after dusk

  • on weekends or public holidays

Appendix 1
Biological treatment of manures and slurries

Biological treatment occurs either in the presence of air/oxygen (aerobic systems) or in its absence (anaerobic systems). Most livestock slurries are present in highly concentrated forms so their biological treatment has been directed towards the reduction of odour and soluble Biochemical Oxygen Demand (BOD) to reduce environmental pollution and the production of energy in the form of biogas or heat.

Biological treatment of slurry and FYM by anaerobic or aerobic (aeration and composting) techniques has proved successful in significantly reducing the amount of odour from waste products experimentally and has potential to deal with specific or severe problems. The cost of providing and operating treatment systems can be high. Expert advice should therefore be sought to ensure the treatment is appropriate and cost-effective.

a. Anaerobic digestion

Controlled anaerobic digestion takes place in an insulated gas-tight tank. Slurry is fed in on a regular, usually daily, basis. The contents of the tank are mixed regularly and heated to 35 or 55°C. The treatment period is usually 12 to 15 days for pig slurry and 20 days for cattle and poultry slurries. The main benefits of anaerobic digestion are:

  • a very substantial reduction of slurry offensive odour

  • reduction of dry matter and BOD, thus minimising the risk of creating anaerobic soil conditions and minimising pollution of drainage water after field application of digested slurry. However, neither the digested slurry nor the liquid decanted from the digested slurry can be discharged to a watercourse

  • some destruction of harmful organisms (e.g. pathogens) and weed seeds

  • improved fertilising quality of digested slurry

  • energy generation in a form of biogas

b. Aerobic treatment

Whole or separated slurry can be aerated either in specially built tanks, slurry storage tanks or lagoons using compressed air or mechanical aerators. Correctly designed aeration systems can treat slurry in a relatively short period of time, between 3 to 10 days, to achieve the following objectives:

  • offensive odour removal

  • reduction of BOD

In addition, aerobic treatment can manipulate the nitrogen content by either increasing the proportion of ammonia nitrogen in slurry (resulting in higher nitrogen availability to crops compared to untreated slurry) or by removing up to 70% of total nitrogen in the form of nitrogen gas. This may be useful in certain circumstances for some farms in Nitrate Vulnerable Zones.

The heat produced during aeration of slurry increases the temperature within insulated tanks or lagoons to over 30°C. This accelerates the treatment and where temperatures of 50°C are achieved this will increase the kill of harmful organisms and weed seeds in the slurry. Heat from the treatment can be extracted and used for heating water for on- farm use or direct heating of farmhouses.

Note: While aerobic and anaerobic treatments are effective in reducing slurry odour, they can increase ammonia loss from land by increasing the pH value of the slurry. A modification of aerobic treatment of manures and slurries can convert ammonia into nitrogen gas. This minimises ammonia and odour emissions while reducing the nitrogen input to soil.

c. Use of slurry additives and deodorants

Additives to reduce odour emissions can be used as a short-term treatment for a batch of slurry or a store which is causing a nuisance. Masking agents may be used to control site smells where it is not possible to deal effectively with the source. As most additives and deodorants have not proved very effective, seek professional advice before making a purchase.

d. Composting

Composting is an aerobic process, which stabilises organic matter such as solid livestock and vegetable wastes, straw, grass and hedge cuttings, leaves and other biological wastes. It prevents a further degradation of wastes which, normally stored under anaerobic conditions, generate leachates, gases and obnoxious smells. High temperature, which occurs during composting, substantially decreases the viability of weed seeds and significantly reduces harmful organisms (pathogens). Compost, made by this process, can be used as a fertiliser or soil conditioner depending on its manurial value. Compost is a potentially marketable product but the farmer should seek professional guidance on technical feasibility and economic viability of composting before commencing such an enterprise.

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