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This plant, formerly known as the Dunedin Water Pollution Control Plant, was constructed in 1981. Since then, it has undergone a number of improvements.

A brief history

The Lawyers Head wastewater outlet was commissioned in 1908. This was a huge improvement from previous methods of wastewater disposal  dispersing the waste into the upper Otago Harbour.

Discharged wastewater was untreated until the mid 1950s when coarse screens were installed at the Musselburgh Pump Station. In 1966, additional treatment was provided with the installation of comminutors at the Tahuna Waste Water Treatment Plant (WWTP).

The Tahuna WWTP, and the associated Lawyers Head discharge, has the largest wastewater catchment within the Dunedin City area, referred to as the Musselburgh Catchment. This catchment expanded when we decommissioned the Sawyers Bay, Burkes, Company Bay, Macandrew Bay and Ocean Grove treatment plants and piped wastewater into Dunedin for treatment.

Since 2000, we no longer discharge wastewater into the Otago Harbour. The Tahuna WWTP treats, on average, 35 million litres of wastewater a day from around 80,000 residents and industry.

Tahuna is currently undergoing a major upgrade including 1.1 km sea outfall and improved treatment. 

How the Tahuna plant works

Screened wastewater from the Musselburgh pump station enters three primary sedimentation tanks. Here around 65.7% of the suspended solids, 61.2% of oils and greases and 33.8% of BOD is removed.

Oil and grease, which float to the surface, and sludges that have settled on the bottom are collected by moving bridges with surface and bottom skimmers. The scum and sludge is transferred to a thickening tank. Oil and grease skimmed off the surface of the thickening tank are concentrated and sent to the scum tank. Sludges are concentrated in the bottom of the thickening tank.

Sludge is dewatered and is pumped to one of three belt presses where, aided by polyelectrolytes, more water is removed. The dewatered sludge, at between 30% - 35% dry solids, is fed into the incinerator. The fluidised bed incinerator burns the sludge, oil and grease at around 825 degrees C.

The gases produced are mechanically washed with water to remove ash and to cool the gases. The gases from incineration and ventilation air from other areas of the plant are collected and passed through a biofilter to remove odours before being discharged to the atmosphere.

Treated wastewater flows by gravity to Lawyers Head where it discharges into the sea.