Murupara Oxidation Pond Biosolids Disposal
Murupara Oxidation Pond Biosolids Disposal
A survey of biosolids levels in the Murupara oxidation ponds has recommended that ‘dewatering and disposal of Pond 1 sludge [be carried out] within three years’.
Issues
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When surveyed in 2007 the primary oxidation pond had an estimated volume of 6,138m3 of biosolids, 14.8 % of the pond capacity. |
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Dewatered this would comprise approximately 1,650 tonnes of biosolids. |
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This build up affects pond performance as treatment times are reduced and bacterial treatment is adversely affected. |
Figure 1; Murupara Ponds
Generally ponds require desludging every 15 – 20 years.
Options
Several options were assessed for removal and disposal of the biosolids and indicative costs prepared as follows;
• Dewater biosolids & dispose to landfill ($1,226,000) - receiving landfill would be in the Waikato
• Dewater biosolids & dispose to Kaingaroa Forest ($1,009,000). Two options are available for dewatering;
o Geotube dewatering ($604,000)
o Portable centrifuge dewatering ($568,000), but solids need additional storage.
• Dispose wet biosolids (6% solids) to Kaingaroa forest ($345,000)
The dewatering options are expensive because they require considerable man-hours, or the use of expensive equipment, or both. Logistically dewatering using geotubes is the easiest option, as both storage & dewatering happen on site. With centrifuging, storage is required elsewhere.
If biosolids is disposed to landfill it must be dewatered – landfills will not accept material that has not been dewatered.
If land in the Kaingaroa forest is made available for sludge disposal, there is considerable financial benefit to the population of Murupara in terms of reduced cost and therefore lower rates.
The approximate land area required for disposal of wet or dry bio-solids is 43 hectares, with an assumed one-off application rate of 300kg per hectare of total nitrogen.
There are a number of areas suitable for land disposal of solids, within economic trucking distance of the Murupara ponds. The topography is generally suitable, accessible and with significant areas available between waterways.
Application could be by truck or tractor or gun irrigator depending on the liquid content.
The harvest status of the areas shown is not known and discussion with the forest managers on these issues is required.
The next stage is to apply for a resource consent for the pond desludging, and land application of the resulting biosolids. Before doing this however, the approval in principle of the affected parties is being sought.