Dairy effluent pond construction
Updated: October, 2002
Note Number: AG0425
Barrie Bradshaw, Ellinbank
Good site investigation and construction are essential for the long-term success of dairy shed effluent pond systems. Refer to the Agriculture Note AG0424; Dairy effluent: Pond site selection before constructing ponds.
Local government requirements
Check whether your local government requires a permit for the construction of a pond. Whether a permit is required will vary between areas and can depend on whether the natural drainage or active flow path in the area is affected.
Ponds should be designed to hold the maximum quantity of effluent produced over the wetter months to avoid waterlogging and pugging of pastures. Effluent may need to be stored between 4–6 months depending on the farms location. Refer to the Agriculture Note, AG0441:Dairy effluent: Storage pond sizing for more information. It is important to ensure that the contractor follows the plans, which have been designed, and that the minimum size specifications are adhered to.
Types of ponds
The type of pond or ponds that you select will depend on the topography of the farm and the type of effluent system implemented to return nutrients back to pasture. Excavation costs will depend on the storage/excavation ratio of the pond. The storage ratio is the number of cubic metres of water stored in the pond for each cubic metre of earth moved to build the pond.
Hillside ponds generally have a 3-sided or curved bank. These ponds have relatively poor storage ratios but often have the advantage of allowing gravity flow to and from the system. A hillside slope of 8% gives the best storage ratio.
Excavated ponds or below ground ponds will supply excavated material which can be stockpiled for other use. Whilst effluent can usually be gravitated to an excavated pond, usually only the overflow from the pond can be gravity fed out. Excavated ponds are not suitable in areas where the watertable is close to the land surface.
Turkey nest ponds or above ground storage's can be built by using the soil which is obtained from digging the hole to build the embankments. Effluent generally has to gravity through an outlet pipe. Turkey nest ponds are ideal the watertable is close to the land surface and shandying effluent into irrigation channels.
Ponds do not have to be regular shapes and can be blended into the topography or surroundings. Rectangular ponds are easier to de-sludge, when necessary, centre.
Rectangular ponds compared to square ponds are also cheaper to construct on hill country with the long side on the contour.
Long narrow ponds should be avoided, as they tend to crust and silt at the end where effluent is entering. This affects anaerobic functioning.
There are many types of machines that can be used to construct ponds. However equipment which can spread and compact excavated material is best. The more compacted the embankments and pond surface are, the less likely it is that the pond will leak or crack.
Bucket type earthmoving equipment cannot spread or bulldozers, whilst scrapers are even better compacters but rollers in association with water carts provide even better
Payment of contractors is generally determined by the amount of earthworks required to construct the system. require removal the contractor may charge for further time. this work as possible prior to the contractor commencing.
All topsoil should be cleared and stockpiled, and replaced erosion and providing wall stability. Dams in which topsoil is incorporated in, or left under the clay material that forms the bank, are more likely to leak.
Ponds should be impermeable to water and a soil investigation should be undertaken. Refer to Agriculture Note; AG0424: Dairy effluent: Pond site selection. The organic matter from the cow manure entering the system will eventually form a biological seal.
Ponds constructed using cracking clay soils, or soils containing less than 20 per cent clay require sealing. Clay from another area can be placed 150 mm thick and compacted with 150 mm of existing soil on the pond floor and walls. Alternatively there are additives, which can be reduce soil permeability.
To prevent the clay seal from drying out and cracking, the pond should be filled with water as soon as possible after maintained in the pond.
Polyethylene liners are also available to seal ponds in light soils, however good management is essential to prevent liner damage. Placing tyres in the base of the pond on top the liner will assist contractors in locating the liner and possibly avoid puncturing.
Side slopes and embankments
The crest of embankments should be a minimum width of 3.5 metres. This will allow for vehicular movement during batters should have at least a 2:1 slope (2 metre in the horizontal direction for every 1 metres in the vertical direction) to prevent batters slumping, and to allow free machinery movement. Bank batters of 3:1 are preferable especially for the external batter and where ponds are to be located in sites with unstable soils, or when optimal compaction is not possible.
When desludging, ponds with steeper banks may clean more easily, but an adequate slope is needed to prevent the banks from slumping or breaching.
Pipe sizing and type
UPVC and HDPE (polyethylene) pipes are suitable for gravity or pressure pipelines. Rubber-ring-jointed PVC pipe should be installed in preference to solvent-welded pipe to avoid corrosion from the effluent.
If concrete pipes are used, sulphate-resistant cement is recommended with all exposed starter bars being galvanised to avoid decay. Steel cast iron and ductile iron pipes should be given a protective coating to avoid corrosion from the effluent.
For gravity pipelines, sewer-class pipes should be used rather than stormwater-class pipes. The main consideration for gravity conveyance is the presence of solids in the liquid. Table 1 gives recommended grade requirements of pipelines.
Table 1: Minimum grades for gravity pipe drains conveying effluent.
|Inside diameter (mm)||Minimum grade Without solids||Minimum grade With Solids|
|75||0.2% (1:500)||3.3 % (1:30)|
|100||0.1% (1:1000)||2.5 % (1:40)|
|125||0.07% (1:1450)||2.0% (1:50)|
|150||0.05% (1:2000)||1.7% (1:60)|
At least 150mm pipe is recommended for transferring effluent between ponds and for the outlet from ponds to the land application area or irrigation channel. Larger diameter pipes will have the advantage of higher discharge rates and less likelihood of blocking.
At least 50mm piping is needed for pumping effluent. For pipeline lengths exceeding 100 metres or where higher heads (the distance between the effluent level and the pump outlet) are involved, at least 75mm pipe is preferable. When choosing pressure classes for pipes some extra strength needs to be allowed for water hammer. HDPE pipe has greater flexibility to withstand water hammer than UPVC pipe.
Table 2: Common pipe classes recommended for pumping effluent.
|Imperial K119||Metric AS1159|
|Class A - 150 ft head 65psi||Class 4.5 - 45 m head 65psi|
|Class B - 200 ft head 87 psi||Class 6 - 60 m head 88 psi|
|Class C - 300 ft head 135 psi||Class 9 - 90 m head 131 psi|
|Class D - 400 ft head 175 psi||Class 12 - 120 m head 176 psi|
The effective storage capacity of a pond is the difference in the effluent level at the start of the winter or storage period and the effluent level when the pond is full.
Figure 1. Storage capacity of a pond
However it is recommended that the outlet pipe or pump suction line are not within 0.5m of the bottom of pond. This will prevent sludge and solid material from blocking the pipes and pumps.
Figure 2. Simple baffle to prevent pipe blockages
Figure 3. Configuration of pipes and earthen banks
If the pond is to be emptied using gravity, flows can be controlled by a gate valve, slide or a similar structure, which can be placed on the downstream or upstream end of the pipe.
Figure 4. Position of valves/controls on outlet pipes
A downstream control is convenient to operate but the pipe is exposed to constant water pressure, which may cause leakage. The main disadvantage of the upstream control is the inconvenience of operating and maintaining it. If it is laid in a sloping position it requires a very long spindle supported on concrete cradles, and if set in a vertical position it needs a catwalk with a safety fence to reach it.
Pipelines should be buried at least 500 mm below the earth surface and at least 600 mm below where there is vehicle movement. The larger the pipe, the greater the cover required.
Rainfall run-off diversion
There should be a diversion or cut-away ditch around excavated or hillside ponds so that rainfall run-off is diverted away.
Starting and filling new ponds
The organisms in the dairy effluent are generally sufficient to start the pond functioning, and therefore the ponds will usually not require 'seeding'.
However if 'seeding' is required then effluent from an active pond (approx. 200 litres) is recommended Ponds should be filled with clean water before effluent is discharged into them. This helps seal the bed and banks, decreases odour and reduces the concentration of effluent components.
Grassing and trees
Once the earthen banks are formed, the stockpiled topsoil can be placed on the crest and batters. Grass cover on the embankments will help stabilise the banks, prevent cracking walls and potential leaks, and help to protect the banks from wave damage and rainfall erosion.
Trees should not be allowed to establish on embankments since they can disturb the stability of the batters.
Fencing and safety
Ponds must be fenced to keep out children and stock. Stock can cause extensive damage to batters and structures.
A sign should be erected on ponds warning of deep water which may be hidden by a surface crust or scum.
Managing Dairy Shed Wastes Technical Bulletin; Vol. 2, Dairy Research and Development Corporation 1994. Published by Department of Agriculture, Victoria.
This publication may be of assistance to you but the State of Victoria and its officers do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.