A further way of minimising odour conflict is to modify the concentration or type of odours released so they are not detectable at nearby residences or are considered to be inoffensive.
Various means of encouraging biological or chemical interactions or increasing dispersion have the potential to mask or dilute the concentration of odorous gases released. However, a very large reduction in odour concentration (up to 90%) seems to be necessary for effective abatement of odour problems (McGahan et al. 2002).
The options currently available that are able to significantly reduce odour tend to be poorly suited for commercial scale meat chicken sheds. However, a number of options warrant further research, as noted in the following discussion of existing and emerging technologies and in Table 2.
Odour neutralising or inhibiting agents.
Various commercially available products are used as additives to feed, drinking water or litter to help inhibit the anaerobic degradation of shed litter or as agents that react with odour compounds to neutralise their offensiveness. Most are relatively easy to adopt and involve low capital costs but there is limited scientific assessment on their effectiveness in controlling odour.
Many neutralising agents use chemical interactions to remove or reduce ammonia released inside the shed but this does not necessarily reduce odour emission rates. This has been further supported in a 1997 study on two commercially available products, clinoptilolite zeolite and De-odorase®, reported by McGahan et al. (2002). Other compounds such as zeolite may be effective in removing odours but because they also attract moisture they are impractical in poultry sheds.
Adapted from: McPherson (2000) and McGahan et al. (2002)
Jiang & Sands (2000) suggested that adding lime to fresh litter may inhibit odour generation by restricting anaerobic breakdown. Further research is required to quantify the effects on odour emission rates, possible impacts on bird health and the cost-effectiveness of this option.
Tree planting (vegetative screens or windbreaks)
Appropriately designed and located windbreaks can help redirect air movement around sheds, and reduce dust movement. Mature windbreaks may also help mask and disperse odours over short distances (EPA 2000), but their main benefit is to reduce the visibility of sheds.
Odour complaints are often a result of heightened sensitivities and the visual impact of poultry amenities. By reducing the visibility of sheds it is possible to influence perceptions of how appropriate it is to have the enterprise within a locality. Screening sheds also encourage the identification of other possible odour sources.
The effectiveness of vegetative windbreaks depends on good design, site preparation, species selection (preferably local to the site) and effective maintenance including regular weed control and watering during the first two years. Windbreak design also needs to avoid adversely reducing airflow within the sheds or creating unwanted shading.
Constructing a fixed 3 m high barrier a few metres from exhaust fans can help reduce the concentration of odours at nearby dwellings or public areas by directing expelled air upwards, which increases turbulence and the dispersion of odours. Windbreak walls also encourage odour-carrying dust particles to drop out of the airflow.
Preliminary studies by Bottcher et al. (2000) identified that tarpaulin windbreak walls could reduce odour concentration in sensitive locations by 30% to 90%. However, the dilution effect is limited to relatively short distances and is significantly reduced during calm conditions. In strong wind conditions the dilution effect is also reduced due to the existing high levels of airflow and turbulence. Windbreak walls are relatively easy to install on new or existing sheds at relatively low cost. Possible materials include concrete panels, sheet iron, hay bales, brushwood or tarpaulins stretched over timber or metal frames – but the visibility of the structure must be considered.
Short stacks attached to exhaust fans direct odorous air upwards, increasing the chance for the plume to disperse before reaching the ground. A study by Pollock & Friebel (2000) on the effectiveness of 5 m tall stacks in diffusing night-time odours found that the distance from the poultry farm at which odour levels of 5 Odour Units were detected (99.9 percentile for an averaging period of 3 mins) was reduced from 300 to 170 m (McGahan et al. 2002).
The additional turbulence from short stacks and corresponding reduction in odour concentration at ground level is most effective over relatively short distances (200–500 m) from the sheds. Strong winds or unstable conditions will minimise this effect.
Recent research (reported in Mc Gahan et al. 2002) identified short stacks as being more effective than taller stacks because of the additional turbulence that air flows around the sheds create in the stack discharge zone. (Such effects would be less pronounced during calm conditions.)
Air scrubbers and washing walls.
Air scrubbers cause exhaust gases to absorb into a liquid stream and are an effective means of removing airborne contaminants and odours from industrial exhausts. The removal of odorous air from fish processing and rendering plants by high pressure Venturi scrubbers is known to be greater than 99%. However, such technology is unlikely to be viable for meat chicken sheds due to the complexity and cost of such systems (McGahan et al. 2002).
A simpler pressure scrubber system tested by McPherson (2000) generally removed around 10% of dust particles and some ammonia but was ineffective at reducing odour from meat chicken sheds. The large airflow at relatively low odour concentration from poultry sheds would also require substantial infrastructure and water flows which are not economically or environmentally justified.
An alternative is to create a washing wall by placing a vertical pad of continually wetted material 1.5 m upstream of the exhaust fans (McPherson 2000). While capable of removing significant dust and ammonia at medium ventilation rates, washing walls are not that effective in extremes of ventilation conditions that are common in poultry meat sheds.
An even simpler method bubbles odorous air through a tank of water (McGahan et al. 2002). While a significant proportion of ammonia may be removed, the water needs to be continually bled off and replaced to remain effective which creates a large volume of effluent and increases the cost.
Fixed-bed packed scrubbers minimise the use of water by directing odorous airflow through towers packed with plastic or ceramic material over which a thin film of water flows. The volume of air that can be treated is limited by the height of the tower and water pressure. To remain effective both the water and materials also need regular replacement. Hence this option is not well suited to poultry sheds (McGahan et al. 2002).
Oxidisation (ozone and oxygen treatments)
Ozone is used in various industries to kill airborne bacteria, deodorise odours and remove particles. Its strong oxidising properties are claimed to neutralise a broad range of odorous compounds, including those known to occur in poultry sheds. However, quantifiable information on the odour-removing effectiveness of ozone is limited and it may not work in humid conditions.
McGahan et al. (2002) reports that trials using low levels of ozone (0.1 ppm) to deodorise and reduce airborne bacteria in sheds stocked with birds are promising. Further research is required on the cost-effectiveness of ozone in reducing odour and dust, any impacts on production or bird health and its safety. Ozone systems also involve considerable capital and operating costs that are not currently justified for the meat chicken industry.
Active oxygen is a recent technology which passes oxygen over charged electrical sources to increase the capacity to oxidise odorous compounds. This system relies on mixing energised oxygen with odorous compounds in a stack or exhaust chimney but the set-up and running costs are too expensive for use in meat chicken sheds.
In biofiltration a steady flow of exhaust air is passed through a bed of moist organic material inhabited by specialised bacteria that break down and oxidise odorous compounds. Constant airflow, temperature and moisture levels are required to maintain the micro-organisms. Excessive moisture increases air?ow resistance and may encourage anaerobic conditions. Insufficient moisture deactivates the microbes and allows channels to form in the substrate. Ongoing control of rodents and excessive vegetative growth is also necessary to ensure effective operation.
Correctly designed and maintained biofilters have proven effective for pig and calf sheds overseas but are less suitable for commercial chicken meat sheds due to:
• high ventilation rates involved, and the capital cost of suitable systems
• high maintenance requirements to ensure the substrate and organisms are effective
• effluent produced from maintaining and cleaning the filter material.
A simpler variation is the biomass filter, which removes dust and odours by forcing exhaust air through a vertical wall of organic material (e.g. chopped corn stalks) held in place by a timber and wire frame. Such options are still being trialled but the cost may be relatively similar to biofilters.