If installing a new pond, consider the distance to neighbours, the prevailing wind direction and topography before deciding on the final site.
Siting a pond in a sensible location is the single most important factor to avoid potential conflict. On existing farms with close neighbours other strategies may be necessary to minimise odours. Many district and regional councils will have minimum distance requirements, but you should also take care to note the direction of the prevailing wind and what will be ‘downwind’ of your pond or facility.
For existing ponds where the siting and design cannot be changed, other strategies are necessary to avoid odour emissions.
Planting trees between neighbours and pond
Although a long term strategy, trees in shelter belts can absorb odorous compounds. They create turbulence that disperses odour upward, particularly under stable night time conditions. Studies indicate that under variable climatic conditions, windbreaks can improve odour dispersion.
More information on trees
The tree species in a windbreak should be selected using the following factors:
- High leaf surface roughness (plants with leaf hairs, leaf veins, and small leaf size)
- Complex leaf shapes
- Large leaf areas
- Medium to rapid plant growth rates
It is usually best to select several different species of trees and shrubs for windbreaks. This helps prevent the loss or destruction of the entire windbreak if insect pests or tree diseases occur on certain species.
Having diversity also offers a better chance for a tree surviving during alternating seasons of drought and/or wet soil conditions. Tree shelter belts will obviously restrict wind run and reduce the amount of dispersion and dilution provided by natural wind run. Tree belts may also reduce perceived odours by providing a visual barrier between the effluent storage pond and neighbouring properties. In effect, this may be a more important consideration than the actual reduction in measurable odours achieved with tree belts.
Regular desludging of ponds
Odour issues can occur when ponds are being desludged as this process mixes the sludge layer at the bottom of the pond. When the sludge is removed, the gases are brought to the surface and are released into the air.
If you leave sludge to build up in the pond for too long, the pond itself can turn odorous very quickly. If the right temperature and pH conditions align, the effluent can turn into a black odorous liquor overnight. It is important to regularly monitor the sludge layer and desludge the pond.
Solid separation prior to effluent storage
With increasing use of feedpads, stand-off pads, covered housing and more supplementary feeds coming onto the farm, effluent has a much higher solid content. If this is stored directly in ponds the sludge can build up very quickly and lead to odour issues. For intensive operations it is strongly recommended to install solid separation and manage the solid and liquid components separately.
Effluent storage ponds should include a stirring or mixing system.
Agitation increases dissolved oxygen, reduces odours and encourages oxygen transfer through algae and direct transfer mechanisms. Mechanically mixing the pond is already a feature of many dairy farms in New Zealand. There are a variety of agitators from tractor PTO driven propellers to electric motor mixers; either pontoon or shore mounted. On farms where there is no existing electrical supply to the effluent storage pond, the installation of an electrical mixer is likely to be an expensive option.
Pond aeration using wind
Increasing the exposure of the pond to wind will help induce oxygen transfer into the pond. The wave action has a limited ability to increase the oxygen transfer to the lower layers of deeper ponds.
Wind aeration systems
A number of systems use wind turbines to compress air, which is then released into the pond through a diffuser. The diffuser can be set at any height in the pond or at any location. These devices have no moving parts in the pond, which allows for easier maintenance. The wind powered compressed air system relies on a constant source of wind power, although some systems can operate at relatively low wind speeds.
There are a number of options to use wind for pond aeration. These systems are frequently used other countries for in municipal/industrial wastewater ponds, amenity ponds and lakes. These systems provide options for different applications within dairy pond systems. Each system needs to be investigated for suitability on a site by site basis. Desktop research did not provide literature evidence of these systems for farm dairy effluent in New Zealand. The Solar Bee mixer has been used in Wanganui since 2007 on a recreational lake.