Pond maintenance and desludging
Well constructed and designed pondage systems require consistent maintenance.
A single storage pond, over 3m in depth, will generally act as an anaerobic pond. A correctly operating anaerobic pond will not be heavily crusted and the rising gases will give the appearance of rain falling on the surface.
Emptying storage ponds
For storage ponds to work effectively, they need to be sufficiently empty at the start of the wetter months, so that all effluent produced over the wetter months or desired storage period, can be contained on the farm.
However it is recommended that ponds are not emptied completely, but that 500 millimetres (0.5m) of effluent or sludge is maintained in the bottom. This will protect the bottom of the pond from cracking, will help prevent outlet pipes or suction lines from blocking and not necessitate the need for reseeding.
Crusting of ponds
Heavy surface crusting should not occur. It usually indicates that the pond has been under-designed for its daily loading. This may mean that the pond needs to be enlarged or cleaned out, so that the microbes are able to break down the manure more effectively.
The ability of microbes to break down manure can also be affected by high salt concentrations within the pond.
Salinity may be a result from:
- poor quality washdown water
- groundwater entering the pond
- incomplete or irregular emptying of the pond.
Salinity in a pond will tend to be greater at greater depths. If the effluent is not emptied from the pond regularly (2 to 3 years) — or if it overflows to another pond — then salt may build up at the bottom of the pond hampering the anaerobic process.
Provided the system is not overloaded, solids may need to be removed from the first or anaerobic pond every 2 to 5 years. This will depend on:
- the quantity of stones and solid material entering the pond
- the quantity of solids taken out in the effluent when it is applied to pasture
- how well the microbes break down the manure.
A solids or debris trap may be recommended to remove a significant percentage of solid material at the dairy before it enters the pond, thus reducing the cleaning frequency and desludging of the ponds.
If sludge is managed correctly in the first pond removal of sludge from second or third ponds should not be required.
Ponds should be de-sludged if:
- the sludge build-up has reduced their storage beyond the required capacity
- if problems occur with effluent conveyance from the pond either via a gravity pipe or pumping application.
It is recommended that ponds are constructed with embankment crest widths of at least 3.5 to 4 metres, to provide easy access for desludging equipment. Long rectangular ponds are also easier to desludge than square ponds.
Sludge can be removed by excavator, pump or vacuum tanker in conjunction with an agitator.
If an excavator is used, costs will be greatly reduced if as much of the water component in the effluent is emptied from the pond first.
Longer reach excavators may cost more to hire per hour, but may be cheaper overall if the sludge does not have to be double handled or if the pond is large or where pond access may be restricted.
Many contractors now provide a pond desludging service using from 7000 to 20 000 litre vacuum tankers and an agitator. The sludge can be spread thinly and evenly over paddocks to gain maximum benefit from the nutrients and organic matter. Usually a 20 to 25mm application is recommended.
Another approach is to remove the liquid, dry out the pond and mechanically dig out the remaining solids. This system may work well if the effluent can be temporarily diverted to an alternate storage area.
The high level of nutrients in sludge makes it a potentially strong pollutant. Therefore care needs to be taken when emptying ponds, stockpiling sludge and applying it to land. If the excavated sludge is not spread out on to pasture immediately, a levee bank may have to be constructed to prevent nutrient runoff from the storage site.
Applying it to poor paddocks, newly lasered paddocks or cropping paddocks may achieve the greatest benefit from the organic matter and nutrients in the sludge.
Cattle must be kept off embankments after initial construction to minimise bank damage. Once compaction has taken place periodic grazing with stock can be useful in controlling grass growth.
Dairy effluent is also a potential health hazard for stock due to its many pathogens.
Weed and tree control
Weed growth across the surface of ponds may reduce the oxygen level in the water and decrease evaporation. Sprays can control weeds and grasses, although killing the grass on embankments should be minimised because of bank erosion.
Any tree suckers should be removed during periodic inspections of banks. Trees may interfere with the stability of the embankments and shade the ponds which will decrease evaporation. Trees may also prevent wind blowing across the effluent surface and thus decrease the gases leaving and entering the pond.
It is also recommended the sun's UV rays penetrate the ponds surface to reduce pathogens and bacteria.
Whole milk is a strong pollutant, and is about 100 times more potent than dairy shed effluent.
A certain amount of diluted milk will enter the dairy effluent pond from washing the milk plant and vat. However if for any reason milk cannot be collected from the farm, the discharge of milk into a dairy effluent pond should be avoided. Milk which enters a pond will give odour problems and the microbes living in the pond which break down the effluent will not be able to work efficiently.
Under no circumstances should milk be discharged to watercourses, due to its significant oxygen depleting characteristics. Milk that cannot be collected should be fed to stock, applied to land or disposed into earthen trenches on the farm.
For more information see Emergency disposal of milk.
Domestic sewage should be excluded from dairy shed effluent ponds and systems, for both human and animal health reasons.