Case Studies

Shannon Wastewater



Significant upgrade of the Shannon Wastewater Treatment Plant includes 10,000m3 storage capacity and land irrigation system designed and installed by Irrigation Services.  


Problem that needed solving

All treated wastewater was previously being discharged into waterways via Stansells Drain, feeding into the Mangaore Stream and on into the Manawatu River.  The new design improves the quality of local waterways, something that the Horowhenua District Council is leading the country in.


Size/scale of project

With the completion of the Shannon land treatment scheme and intended changes to the Foxton discharge, the district will see 100% of the district’s wastewater passing into or over land.

On a volume basis, about 95% of the district’s total wastewater volume will be irrigated to land in a way that not only avoids waste water discharging to water, but also provides for treatment of nutrients and pathogens, and in a way where the vegetation grown can utilise the applied water.  This compares to only 4.5% of wastewater generated at the national level being beneficially used for irrigation.


Irrigation Service’s role in project

Irrigation Services tendered for the irrigation work, worked with the main contractor (Downers) and managed sub-contractors related to the irrigation work (electrical and roading).  The completed system was commissioned by Minister for the Environment, Dr Nick Smith, with Irrigation Services providing comprehensive handover documentation, operating manuals and user training.


Complexities and challenges

The control system is very high tech, enabling individual flow rates for each of the 6 zones to be monitored and operated from Council computers.

The project was complex and involved consultation with local iwi, engineering irrigation crossings to span waterways and a road bridge, and installing a large suction wet well with a 600mm poly intake integrated with concrete liners.


Description of outcome/solution

The new system sees treated wastewater being collected and stored in an oxidation pond before being discharged at low rates on to purpose-bought council-owned pasture.  The irrigation system is designed to operate on average for 355 days per year, allowing for an average of 10 days of high flow in the Manawatu River, at which time wasterwater is discharged through mesh cages containing small rocks that further filter the wastewater.


  • Previously 100% of treated wastewater was discharged to water, this new land based irrigation system will remove 80% of this discharge from water.
  • The remaining 20% will travel through two land-passage systems, an artificial wetland and a gently sloping channel planted with grass and flax leading to the Manawatu River.  It is important to note that this would happen only during high river flows.  Council is undertaking a 5-year trial to explore if more than the 80% can be irrigated.




Shannon, Horowhenua


Horowhenua District Council