In higher flows, the discharge would occur at Tōtara Road.
As the river drops below intermediate flows and becomes more sensitive to nutrients, the discharge would be directed to below the Opiki Bridge. Here, the Manawatū River changes from a gravel bed to a soft sediment bed. As a result it becomes less sensitive to some of the nutrient levels in the treated wastewater.
In low flows, when the river is at its most sensitive, a portion of the discharge would be applied to land, with the remainder continuing to be discharged below the Opiki Bridge.
We expect the discharge would occur at Tōtara Road for approximately 30-40% of the time, and below Opiki Bridge for the remainder of the time, with a portion of the flow diverted to land during the driest periods of the year.
The discharge to land would likely be through large centre-point irrigators – similar to the large irrigation machines used on farms. We still need to identify where this would occur. Four hundred to five hundred hectares of land will be needed, which is equivalent to about 400 to 500 rugby fields.
The construction and operation of this option is well understood.
What we'd need to install or build
- pipes to move the treated wastewater from the treatment plant to the downriver discharge point and land application site
- storage basin
- rapid infiltration basin to buffer flows to the land application area
- wetlands and /or land passage systems at both river locations
- minor upgrades to the treatment plant on Tōtara Road
We’d also need to purchase irrigation infrastructure and purchase or lease the land application site.
Shifting a portion of the discharge away from Palmerston North may be opposed by downriver communities. Some landowners would also be directly impacted by the need to lease or purchase land.
- Lifetime cost: $264-380 million
- Potential rate increase per year: $300-450
- Total wastewater charge per ratable unit per year: $550-700