Wastewater Treatment Facility

This page outlines the history of the waste water treatment plant in Porirua City pictured below, and how it operates.

Wastewater treatment plant view looking north.

Prior to this treatment facility being commissioned the wastewater from Titahi Bay was "chopped up" with a comminutor and then discharged to sea at the same site as it is today. This, in effect was discharging raw wastewater directly into the sea. The effect of that discharge was to have raw wastewater floating in the recreational swimming area of Titahi Bay.

In March 1990 this facility was commissioned and since then there has been no conclusive evidence that raw wastewater from this plant has been found on the Titahi Bay beach. There have been instances of high faecal coliform counts on the beach however, these have been attributed to animal wastes washed out in storm water. Tests are still carried out each time this facility has a discharge that exceeds the allowable limits.

Our current discharge consent under the Resource Management Act (RMA) allows us a 30 parts per million count for Suspended Solids (SS) and Biological Oxygen Demand (BOD). We are currently well within those criterion. As of 1 October 2003 we have a consent condition placing a maximum of 1000cfu/100ml sample. This is tested by an independent source and at present with the UV treatment is less than 700cfu/100ml.

The other environmental issue is the disposal of the screenings and the final product sludge.

The issues of screenings and sludge disposal are very real as both are disposed of in the landfill on Broken Hill road, that is a public access area. The screenings and the sludge are mixed with domestic waste on a ratio of 6 parts domestic to 1 part of our waste. This also is a requirement of the resource consent under the RMA. This criteria is strictly monitored on a daily, weekly, monthly basis by the manager of the landfill and on an annual basis by the Greater Wellington Regional Council.

Processes within the wastewater treatment facility

The processes within the plant are, from the beginning:

  • Screening to remove gross solids and grit
  • Extended aeration activated sludge (the biological treatment process)
  • Clarification
  • Disinfection
  • Thickening
  • Dewatering and disposal


This process is via 4 rotating screens (milliscreens) with 3mm appurtures. The gross solids and grit removed from the process are compacted to remove excess water and then deposited in a bin for transport to the local landfill where they are disposed of in accordance with the resource consent conditions.

The liquid and finer solids that pass the screens carry on to the Aeration basin. Solids make up about 0.5% of raw sewage and about half of these are in the dissolved state.

Extended Aeration Activated Sludge

The aeration basin is made up of 4 connected channels being:

  • aeration
  • anoxic
  • anoxic
  • aeration

In the aeration zone naturally occurring bacteria, which have the ability to change between aerobic, anaerobic, or anoxic respiration, consume the dissolved and suspended solid organic wastes in the screened effluent. Using air that is mechanically pumped into the liquid, they oxidise carbonaceous matter to provide energy and cell growth, and ammoniacal substrates to form nitrates and nitrites – which are good fertilisers.

In the anoxic zone denitrification occurs. There is no free oxygen supplied in this zone so the bacteria are forced to utilise the chemically bound oxygen from the nitrates and nitrites, releasing harmless nitrogen gas to the atmosphere.


Clarification, as the word suggests, is the making of the treated wastewater (mixed liquor) clearer. This is achieved by gravity. The mixed liquor is gravity fed into the centre of the two clarifiers where the circular design forces the wastewater to the outsides. In the process of the liquid travelling from the centre to the outside the solids settle to the bottom leaving a very clear effluent to overflow the weir.

The settled solids are critical to the treatment process as they contain about 30% bacteria. They are returned to the aeration basin at a rate of about one volume of sludge to one volume of raw influent.

Surplus sludge must be removed from the process and is pumped to the thickeners.


Disinfection is achieved by the use of Ultra Violet radiation. After the wastewater has been “clarified” the treated wastewater is then sent out to sea at Rukutane Point. Prior to this happening it is passed through two banks of Ultraviolet Radiation. These are designed specifically to kill pathogens.

The activated sludge process reduces the faecal coliforms in raw sewage from six to eight million per 100ml to about of 50,000 per 100ml sample. After passing through the UV plant the remaining coliform count is in the order of 500 – 700 per 100ml sample. On numerous occasions the faecal coliform levels have been less than 4 per 100ml of sample.


The process of thickening is short term (12 hours). The surplus sludge is pumped from the clarifiers into round tanks called “thickeners” (strange that) where the sludge is allowed to settle to the bottom and the liquid content is returned to the Aeration Channel. To assist the thickening process there is a ‘picket fence’ in the thickener which gently stirs the solids and helps them to ‘pack down’. This thickened sludge (at about 2% solids) is then pumped to the two centrifuges.

Just before the sludge enters the centrifuges, it is blended with polyelectrolyte solution which causes flocculation and helps aid dewatering.

De-watering and disposal

Dewatering is via two centrifuges which spin at 3000rpm (like a spin drier) forcing the solids to the periphery allowing the water to drain away. These have a capacity of approximately 30-40 tonnes of dry product per day. The final product at about 18 – 20% solids is trucked to the landfill where it is covered in accordance with the resource consent conditions.

Plant Tours for Visitors

One of the things that is really important to the Wastewater Treatment Facility is the continued tours by schools, WelTec, Whitireia Polytechnic and private groups. A phone call to the Plant Supervisor on (04) 236 6667 will ensure a warm welcome and a tour of the facility.