Pipiriki Wastewater Scheme

Septic Tanks

1. Primary Treatment

Primary treatment of wastewater in Pipiriki is undertaken in septic tanks.

The network is a STEDS (Septic Tank Effluent Discharge Scheme), which collects and treats effluent discharge from septic tanks located on the individual properties.

Three main processes take place in the septic tank:

(i) The heavier, solid particles settle to the bottom of the tank, forming a sludge layer;

(ii) Lighter materials such as fat and grease float to the surface, forming a scum layer, and

(iii) Within the septic tank there is little or no oxygen, and anaerobic bacteria (bugs that can live without oxygen) break down some of the solids. This helps reduce the build-up of sludge in the tank.

The clearer liquid (supernatant) lying above the solid 'sludge' layer in the septic tanks throughout the village has been treated to a Primary level and contains only smaller particles. This supernatant gravity feeds through pipelines to the Pipiriki Wastewater Pump Station (WWPS).

The Pipelines

The wastewater reticulation network is 2km long and is entirely gravity fed.

A map of the reticulation network is provided in the link at the bottom of this page.

Wastewater Treatment Plant

Wastewater is transported from the Wastewater Pump Station to the Pipiriki Wastewater Treatment Plant (WWTP), where it is further treated using two sand filters. The Pipiriki Wastewater Treatment Plant (WWTP) was commissioned in 1998.

The Pipiriki Wastewater Treatment Plant provides treatment of wastewater through the following steps:

2. Secondary Treatment

2.1 Sand Filters 1 and 2

Each sand filter is used for a two week period while the other is rested. Effluent from the pump station is discharged onto the top of the filter bed and then works its way through the sand bed at a high rate.

Sand acts as a very basic filter, removing algae present in the wastewater and also smaller solids which are suspended in the wastewater. Algae and solids get caught in the sand as the wastewater passes through the media and remain behind while cleaner wastewater continues on its way.

The Wastewater Pump Station collects effluent from the sand filters via a second pump sump and discharges to an irrigation field for land disposal.

3. Tertiary Treatment

3.1 Land Disposal

Treated wastewater is trickled through holes in a network of pipes lying just beneath the ground surface into a grassed area adjacent to the Whanganui River called an irrigation field'. The grass in the irrigation field acts as a very basic filter, removing colour caused by algae present in the wastewater and also smaller solids which are suspended in the wastewater. Algae and solids get caught in the grass as the wastewater passes through the irrigation field and remain behind while cleaner wastewater continues its way through the ground into the water table.

Land treatment is also thought to help remove pollutants present in wastewater, such as heavy metals (eg, mercury, lead and iron) and nutrients such as Nitrogen and Phosphorus. Scientists are still learning about this and understanding how it works.

Discharge of treated wastewater to the environment

Originally the Pipiriki Wastewater Treatment Scheme was designed to discharge treated wastewater to the Kaukore Stream. In 2014 the discharge into the Kaukore Stream ceased and treated wastewater is now discharged to land.

Pipiriki Wastewater Reticulation Network

• Pipiriki wastewater reticulation network (PDF, 204.7KB)