Don't ignore what is under your nose

Air quality modelling of waste sites is key for effective dust and odour control. Claire Meddings and Joanna Smith offer a step-by-step guide to enable everyone to breathe easy

The planning and design of major waste management facilities has to take many different environmental considerations into account. But there's one that's literally under the noses of everybody living in the vicinity of a proposed new plant, and that's air quality. Air pollution legislation is focused on reducing the adverse human health effects of air pollutants. And with the natural desire to breathe clean air, as well as concern over the number of vulnerable people whose deaths may be brought forward by air pollution, the issue is certain to remain high on the political agenda.

The European Directive 2008/50/EC on ambient air quality and cleaner air for Europe which came into force in June 2008 is the main legislative driving force. As well as merging most of the existing legislation into a single directive, it also imposes on member states new air quality objectives for reducing exposure over time to PM2.5 - fine particles of less than 2.5 microns in diameter which may carry worse health implications than PM10 because of their ability to get further into the lungs.

Although new UK draft regulations are currently at consultation stage, local authorities have already started to monitor PM2.5 levels. The eight main outdoor air quality pollutants for which objectives are currently set - and against which local authorities and the UK government have to report - are benzene, polycyclic aromatic hydrocarbons (PAHs), 1,3-butadiene, carbon monoxide, lead, nitrogen dioxide, PM10 particles and sulphur dioxide.

An air quality assessment is nearly always the starting point as a way of determining whether a proposed development has the potential to impact on local air quality. It is also an essential step in securing planning permission from the local authority, as well as a permit to operate from the Environment Agency.

Assessments can be carried out on all sizes of site - from a small single facility up to large-scale integrated facilities. But air quality modelling is a specialised field, and it requires the use of sophisticated modelling software and detailed input data. The first stage is normally to assess the baseline current situation against any likely change if the development goes ahead. The two scenarios and their probable results are compared against the prescribed air quality objectives at specific selected receptor points. These range from moderately sensitive locations such as residential housing to more highly sensitive ones like hospitals or nursing homes.

A variety of air quality models of different complexity can be used, depending on the development. A screening model is often a good place to start to get an idea of the potential impact and to see if further air dispersion modelling is needed. If so, sophisticated software such as AERMOD or ADMS is used to input greater detail against parameters on receptor locations or over a local grid.

Mitigation measures
Odour can also be modelled for facilities such as anaerobic digestion or in-vessel composting, using data from the technology supplier. Once any probable change in pollutant levels has been assessed, a range of mitigation measures is then recommended to ensure that they stay within acceptable levels. In the case of waste management facilities these can include odour and emission control systems such as biofilters or regenerative thermal oxidisers, increasing stack heights or combining flues to increase buoyancy of emitted gases, and dust filters.

It is not uncommon for existing sites to have dust or odour problems, and to be receiving complaints from local residents, offices or schools. In these cases, a detailed dust or odour assessment needs to be carried out, often including monitoring at different locations to determine the source of the problem. Mitigation measures and management plans can then be put in place - working closely with local environmental health officials to make sure that any concerns are addressed and resolved. Levels will be identified at which additional mitigation is required in particular circumstances.

Their effectiveness can then be checked by monitoring programmes at critical phases of the operation. As far as dust is concerned, the most significant dust-generating operations should be identified for the site. Local meteorological data showing wind speed and direction throughout the year can be used to estimate the number of hours during which winds are likely to blow on dry days from each dusty operation to each receptor. This, together with the distance to each receptor, rates the need for dust control for each situation. Specific mitigation measures can then be designed to target the most significant issues.

These can range from damping to minimise the re-suspension of dust into the air to reducing vehicle drop heights and speeds, covering lorries as they transport waste to and from sites, maintaining road surfaces in good condition, reducing the use of unpaved site roads, maintaining onsite plant and equipment, and using mechanical road sweepers on site and surrounding roads. Keeping stock piles covered or compacted and located as far away as possible from receptors can be very important. Buildings can be fitted with dust control systems for ventilation, particle extraction and minimisation to avoid untreated dusty air being emitting outside.

A wide range of mitigation measures can similarly be put in place for odour control. These can include building ventilation systems, roller shutter doors for automated fast operation when a vehicle approaches, negative pressure systems that continually draw air out using fan extraction, and biofilters that use bugs to eat the contaminants in the air. Regenerative thermal oxidisers can also be used to burn extracted air at high temperatures.

Claire Meddings is an environmental scientist and Joanna Smith is associate director at Wardell Armstrong

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