Separation distances for intensive livestock odour

Livestock odour separation distance is a screening tool, not a standard. The Environment Agency and most local planning authorities use the distance between the emission point and the nearest sensitive receptor as a first filter: beyond it, odour is usually accepted as adequately controlled by good design and management, and within it a detailed assessment is required. Clearing the screening distance does not make a unit acceptable, and failing it does not make the unit refusable. It decides how much evidence you have to produce.

On an intensive poultry or pig unit there are usually three separate assessments in play: odour to dwellings, ammonia to designated habitats, and manure or slurry storage as a source in its own right.

Where the thresholds sit

Two regimes bite before planning policy does.

An environmental permit is required for intensive rearing above the thresholds in Schedule 1 to the Environmental Permitting (England and Wales) Regulations 2016, which cover poultry and pig units above stated numbers of places. The permit carries its own odour and ammonia assessment, and the Agency becomes the regulator for emissions while the council remains the planning authority.

Environmental impact assessment is the other filter. Intensive livestock installations above the figures in Schedule 1 to the Town and Country Planning (Environmental Impact Assessment) Regulations 2017 require assessment automatically, and smaller intensive installations fall within Schedule 2 where the new floorspace exceeds the stated threshold, which means screening. Check both sets of numbers against the current Regulations, because a unit can sit above the permitting threshold and below the mandatory assessment one, or the reverse.

Work out which regimes apply before you design the building. The ventilation arrangement that satisfies a permit is not always the one an architect would draw.

What the screening distance does and does not resolve

The Agency's risk assessment approach for intensive farming uses a separation distance to the nearest sensitive receptor as a screening step, with the figure taken from its current guidance. Where the nearest dwelling is further away than the screening distance, a qualitative assessment supported by an odour management plan is usually enough. Where it is closer, detailed assessment follows.

The distance is a proxy for dispersion, and it carries assumptions that may not hold on your site. It assumes a reasonably exposed location rather than a valley where cold air drains towards the receptor. It assumes a conventional ventilation arrangement. It assumes the receptor is a dwelling rather than a school playing field or a caravan site with summer occupation.

So treat a comfortable margin as reassuring and a marginal one as a reason to model. Where the result is marginal, an odour assessment with dispersion modelling is the evidence that resolves it, and councils normally expect a suitably qualified author with professional review before submission.

Topography and prevailing wind are worth addressing explicitly even when the distance is cleared. An officer who can see on a plan that the nearest village lies directly downwind will ask.

Ventilation design and the emission points

Ventilation is the part of the design that most affects odour at the receptor, and it is usually settled for reasons of cost and bird or pig welfare rather than dispersion.

The principle is simple. Discharge high and vertically and the plume rises and dilutes. Discharge at low level through side wall fans and the plume travels across the ground towards whatever is downwind. Ridge-mounted vertical fans therefore perform better for odour than low-level side wall arrangements, at a higher capital and running cost.

Three design decisions follow.

• Emission point height and discharge velocity, which together determine effective stack height.
• The number and location of fans, since a long building with many low-level outlets behaves as an area source rather than a point source.
• Whether abatement is proposed at all, which on poultry units may include acid scrubbers or biofilters on the extract, with their own maintenance and water management requirements.

Set the ventilation arrangement out on the drawings with heights and velocities labelled. An application showing fans as unlabelled symbols invites a request for further information.

Ammonia screening against protected sites

This is the assessment that most often stops an intensive livestock scheme, and it runs on different numbers from odour.

Ammonia emitted from housing and manure stores deposits on vegetation downwind, and sensitive habitats have published critical levels and critical loads. The critical level for ammonia in air is set at 3 micrograms per cubic metre as an annual mean for higher plants, and at 1 microgram per cubic metre where lichens and bryophytes are a feature of the site, which catches many ancient woodlands and upland habitats.

The Agency's air emissions risk assessment sets screening distances within which European sites, Sites of Special Scientific Interest and other designated habitats must be considered, and those distances are larger than most applicants expect. Take the current figures from that guidance rather than from an earlier project.

Where screening identifies a site, the process contribution is modelled and compared with the critical level or load, and a contribution below a stated proportion of the long-term figure is normally treated as too small to matter. Above it, the assessment moves to in-combination effects with other units in the area, which is where applications in poultry-dense parts of the country run into difficulty. Reading the citation for the designated site tells you which features carry the sensitivity, and an Air Quality Assessment is the document that brings the modelling together.

The honest position is that ammonia in-combination assessment has become the binding constraint on new intensive poultry capacity in several catchments, and no amount of odour mitigation addresses it.

Manure and slurry storage as a separate source

Housing is not the only emission point, and a scheme assessed on housing alone will be sent back.

Solid manure stores, slurry lagoons and tanks, dirty water storage and the loading area all emit, and they emit in a different pattern: continuous but weather-dependent, with peaks on agitation and emptying. Covered stores reduce both odour and ammonia considerably, and a cover is one of the more effective mitigation measures available.

Storage also brings its own regulatory requirements. New or substantially altered slurry stores and silage clamps have construction standards and a notification requirement under the silage, slurry and agricultural fuel oil regulations, and spreading is governed by the rules for water. An odour management plan should cover timing of agitation and emptying, weather criteria, and the distance maintained from receptors when spreading.

The parallel with waste composting is close, both in the assessment method and in the role of separation distance, and our notes on bioaerosols and separation distance from composting and on odour from waste transfer and composting facilities cover the same ground from the waste side.

Dispersion modelling when screening fails

Where screening does not clear the scheme, modelling is the route. The inputs are the emission rate per animal place or per unit floor area, the ventilation arrangement, building dimensions for downwash, terrain, surface roughness and several years of hourly meteorological data.

Two points matter in review. Emission factors for livestock housing carry real uncertainty and vary with system, bedding and management, so the choice of factor should be justified and a sensitivity run included. And results should be presented as percentile exceedances of hourly means against the benchmark for the offensiveness category, rather than as annual averages that hide the conditions people complain about.

Conditions to expect

A consent on an intensive unit is normally tightly conditioned, and offering the terms yourself produces a more workable set.

Expect limits on stocking numbers by species and class, the ventilation specification fixed by reference to approved details, an odour and manure management plan approved before the unit is occupied, controls on timing of cleaning out and manure removal, covered storage, restrictions on spreading within stated distances of dwellings, a complaints log, and in some cases a requirement for a periodic review of the management plan.

Stocking numbers are the condition that matters commercially, because they fix the business case. Apply for the number the assessment supports rather than an aspirational figure, since a condition at a lower number is harder to vary than an application is to get right.

This is general guidance rather than legal advice. Check the position with your local planning authority, the Environment Agency or a qualified professional before relying on it.

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