Ohakune Wastewater Scheme

The pipelines

Wastewater from Ohakune township is transported through 6.12km of pipelines to the Ohakune Wastewater Treatment Plant. A map of the collection network is provided in the link at the bottom of this page.

Septage Receiver

Raw sewage is also received directly at the Wastewater Treatment Plant from septic tank trucks which empty into a Septage Receiver.

Wastewater Treatment Plant

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

1. Primary Treatment

1.1Step Screen: Raw wastewater arriving at the Wastewater Treatment Plant is passed through a screen with holes in it. Fluid wastewater and smaller solids pass through the holes and on to the next step in the treatment process. Larger solids, including wetwipes, rags, and other rubbish items that end up in our wastewater system, are retained by the screen as they are too big to fit through the holes (a bit like a large scale sieve).These items must be disposed of to landfill.

1.2 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.

1.3 Aerators

Four mechanical Aerators circulate air or oxygen through the water and help to make water flow around the lagoon. The chopping action of the Aerators also helps to physically break up blue algae on the wastewater preventing algal bloom.

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 curtain separating the lagoons is suspended under the water and is shown below.

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 removing 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 be disinfected by Ultra Violet irradiation.

3.2 Ultra Violet (UV) irradiation: Wastewater is further treated by exposure to Ultra Violet light. This kills any bacteria and viruses remaining in the wastewater by destroying their genetic core (DNA). It is also effective against many protozoa.