Raetihi Wastewater Scheme

The pipelines

Wastewater from Raetihi township is transported through 6.12km of pipelines to the Raetihi Wastewater Treatment Plant. There are three wastewater pumping stations in Bridge, Highway and Seddon Streets.

The Raetihi wastewater collection network is partly made up of clay and asbestos pipes which were installed in the 1970s. An ongoing relining program has seen quite an extent of the town re-lined with PVC (plastic) pipe which achieves the following:

1. Extends the life of the pipe by 30 years.
2. Reduces root intrusion and inflow of groundwater from unsealed joints.

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

Wastewater Treatment Plant

Wastewater is transported through the Raetihi Wastewater Network to the Raetihi Wastewater Treatment Plant, where it is treated prior to discharge.

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

1. Primary Treatment

1.1 Primary Oxidation Lagoon: Wastewater is piped into a Primary Oxidation Lagoon where heavier solids separate from the wastewater through settling as sludge to the bottom of the lagoon. This sludge is colonised by anaerobic bugs (bacteria that don't need oxygen to survive) which help to break it down by using it as a food source. Bugs also live suspended in the wastewater and use up organic matter food throughout the lagoon.

2. Secondary Treatment

2.1 Secondary Oxidation Lagoon: Wastewater remaining at the top of the Primary Lagoon contains less solids and is piped through a curtain to the Secondary Oxidation Lagoon where further settling and removal of organic solids takes place. Bugs are present in this lagoon in lower numbers and need oxygen to survive. The below figure shows the Primary and Secondary Oxidation Lagoons sitting adjacent to one another.

3. Tertiary Treatment

3.1 Stone Media Lagoon: Wastewater leaving the Secondary Oxidation Lagoon travels through a Stone Media Lagoon. Stones (or gravel) act 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 stones as the wastewater passes through the stone media and remain behind while cleaner wastewater continues on its way to discharge. Some denitrification (removal of nitrogen) also occurs in the media.