Appendix I - Interim recommendations of design changes for two-pond systems for dairy shed wastewater.
Anaerobic Pond
- Determine the size (volume) of pond required for the waste load calculated using Table 2.3 in the Agricultural Waste manual.
- In areas where experience has shown excessive crusting to be a problem increase the surface area of the pond by 50% by either making the pond shallower than the usually recommended 4 m or making the whole pond larger.
- The length to width ratio of the anaerobic pond should 2:1, with the long axis perpendicular to the prevailing wind.
- Desludge the pond when necessary. Do not presume that the five year average desludging interval will apply to any particular pond.
- The inflow should be at one end of the pond with the inflow pipe discharging into the pond about 6m from the end of the pond.
- The outlet should he at the opposite end from the inlet, about 1.5 m from the edge, and should be baffled so that effluent is taken from about 250 mm below the surface (a "Tee" outlet is suitable).
Second Pond
- The second or "aerobic" pond should he sized initially using the recommended loading rate of 84 kg BOD/hectare and assuming 70% removal of BOD5 through the anaerobic pond. The length to width ratio should be at least 2:1, with the long axis perpendicular to the prevailing wind.
- If the effluent from the second pond is not of the desired quality for a pond system, i.e. unfiltered BOD5>100g/m3 and ammonia-N>70g/m3, then add another pond in series. This third pond in the system should be at least 50% the size (area) of the second pond. Duckweed (lemna or spirodella) to establish and totally cover the third pond. This will act as a "brake" on convection currents in the pond and thereby improve settling of fine suspended solids resulting in lower BOD5 in the effluent.
In some pond systems algae in the effluent may add significantly to the BOD5 and Suspended Solids of the effluent. The duckweed-covered pond will eliminate this. Further reduction of ammonia N will also be achieved by adding a third pond by virtue of the additional area that is provided for ammonia volatilisation.
Extending the pond system to accommodate an increased load (increased herd size)
- The anaerobic pond may he left unaltered but more frequent desludging and more attention to crust control may he needed.
- Additional "aerobic" pond area should be provided to maintain the designated loading rate "aerobic" ponds. The additional area should be added as another pond, rather than by enlarging an existing pond.
- Increase "aerobic" pond area beyond the prescribed area for 84 kg/ha. loading as per the recommendations under the section, Second Pond, above.
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