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Waitara Wastewater Treatment Plant

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Each year we treat approximately 1.1 million cubic metres of wastewater at the Waitara Wastewater Treatment Plant on Queen Street, Waitara. The average daily volume is around 3,000 cubic metres, but this can increase more than six-fold after periods of heavy or extended rainfall.

Waitara township sewage is relatively low in BOD5 (Biochemical Oxygen Demand - a measure of the amount of organic material) and suspended solids, but high in per capita volume because of serious infiltration and stormwater inflow problems, both of which are being progressively addressed.

The treatment process carried out at the Waitara Wastewater Treatment Plant is a chemical process known as a ‘High-Lime’ process.

Process steps include;

• Screening the wastewater through a 0.5mm screen to remove larger solids. 
• Batch dosing of wastewater with lime to elevate the pH and kill pathogens.
• Holding the dosed wastewater in one of the eight quadrants of the two balancing tanks, and stirring gently for at least four hours for disinfection to significantly reduce faecal coliform bacteria levels.
• The treated wastewater being released at a controlled rate and pumped to and discharged via the Waitara Marine Outfall into the Waitara Embayment via a 1630m long, 600mm ID (Internal Diameter), HDPE (High Density PolyEthylene) lined steel pipeline.

The milli-screened solids are contained within the treatment plant building to prevent odour dispersion, and are disposed to landfill weekly. Operation of the plant is mostly automatic with only part-time attendance of operational staff on week days. A telemetric alarm system is included in the control system to alert our supervisory staff of any equipment or process failure, and to allow a rapid response. 

Powdered lime is brought to the plant by road on a weekly basis and is contained in the fully enclosed and bunded storage facility alongside the treatment plant building. Each year the plant consumes about 300 tonnes of lime.

After lime addition the screened wastewater (effluent) is continuously stirred and batch-mixed in large tanks on the Queen Street site for three to four hours to allow for up to 99.996% die-off of bacteria, prior to controlled discharge to sea at 1.3km offshore, via the gravity trunk sewer, outfall pumping station (within Marine Park) and outfall itself. At Queen Street there are two large pre-stressed concrete tanks, each of 4.5m litre capacity, and divided into quadrants to give a total of eight separate holding compartments. The final plant discharge is controlled by a flow-modulated valve on the outlet pipe in Queen Street.

Air extracted from the treatment plant building passes through a below-ground soil bio-filter (also in Queen Street) for natural bacterial purification before atmospheric release to prevent odours.

One of the two storage tanks (of four quadrants each).


View of the Waitara wastewater treatment plant from the adjacent Queen Street,
including bio-filter (middle ground).

 

Waitara Wastewater Treatment Plant History

Until the 1970s, the freezing works and town wastes went into the Waitara River either directly or via riverbank communal septic tanks (in 2006 we uncovered a decommissioned tank while carrying out stormwater works in Leslie Street).

In the mid-1970s this situation was improved with a new scheme of riverbank interceptors and sewage pump stations, as well as a combined gravity trunk sewer from the freezing works to a new outfall pumping station adjacent to the Waitara River mouth west bank (today’s Marine Park).

A new outfall was proposed of 1,630m in length extending from the outfall pumping station into the Waitara embayment, with the diffuser (where the effluent is discharged) sited midway between the two reefs (Tuaranga to the west and Airedale to the east).

The outfall was constructed (with some difficulties) in 1977. During construction a 300m section of outfall broke off and washed ashore. This section was rejoined at the sea-joint but questions remained over the structural integrity of the pipeline. The outfall was subsequently refurbished from 1991 to 1994 by sleeving the existing outfall with a plastic liner, additional stabilisation, and extended by the addition of a new diffuser section (at the end where the discharge occurs at about 1,230 metres off shore). The upgrade has a design life of 25 years. The diffuser was designed to achieve dilution of 100:1 at the landward port at a flow of 223 litres/second (19,267 cubic metres/day).

The unique nature of the combined wastewater streams in Waitara made identification of a suitable treatment method difficult. The primary objectives were to remove gross solids and to reduce coliform levels, for which a chemical disinfection process was deemed the most suitable treatment method for the milli-screened effluent.

At the time there were four separate dischargers to the Waitara outfall being;

• The Petralgas methanol plant (now Methanex Waitara Valley).
• The Synfuels synthetic gasoline plant (now Methanex Motunui).
• The AFFCO freezing works (no longer in operation, site owned by ANZCO).
• Waitara Borough Council (now New Plymouth District Council).

The two Methanex plants treat their wastewater (mostly process wastewater) before discharge to the town sewerage reticulation and outfall respectively.

If you would like to find out more information about the Waitara Wastewater Treatment Plant, please contact us.