Using Modern Technology to Solve Problems Tauranga's Wastewater Ocean Outfall

M. Mooney (Bay Dynamics NZ), C. Thomas (Tauranga City Council)

This paper will provide insight into some of the problems facing TCC with its existing Ocean outfall asset and some of the mitigations currently underway to investigate these.

Tauranga’s Te Maunga outfall system discharges treated wastewater effluent from Tauranga City to the Pacific Ocean approximately 1km from the shore. The outfall system comprises a number of components: ponds, wetlands, outfall pump stations, UV treatment, and landward and ocean outfall pipeline sections. The outfall is consented to discharge up to 50,000 m³/day at a maximum rate of 900 litres/sec. The outfall pipeline was designed to deliver a maximum flow of 900 l/s and was constructed in 1976/77 using 600mm ID concrete pipes. However, the ocean section of the pipeline was damaged during construction. Grouting of cracks and defective joints was undertaken at the time. In 2010, following dye testing, further repairs were carried out and bands were installed at identified leak locations. The operational capacity is currently capped at about 600 l/s to protect the ocean section of pipeline. 

Tauranga City is experiencing rapid growth and needs to increase the peak capacity of the outfall to 820 l/s by 2029. The condition of the ocean section has a major influence on how this is accomplished. 

In January 2022, Tauranga City Council approached Bay Dynamics NZ with a view to creating a customised ROV that could clear pipeline blockages and carry out a CCTV inspection and sonar survey. Bay Dynamics accepted the challenge and agreed to co-fund the development of an ROV on the basis that they could use it commercially.

Bay Dynamics’ approach was to create a universal system for carrying out a number of different tasks inside the entire length of the pipe. Initial tasks were to remove the grout debris pieces, visually inspect the internal lining, and 3D sonar scan the pipe for defects. Future tasks would include repairing the pipe. The ROV was designed with a modular front payload system to allow the change out of specialist tools, on the job, as required. This also allows for future job-specific payloads such as injectors, cutters, grinders, and other repair options to be realized without needing to change the ROV itself. Available reports provided limited information on the internal condition of the pipeline so a "worst case scenario" was prepared for. While debris pieces weighing 10kg had been recovered by divers in the past, the debris removal system was designed to lift 30kg of deadweight. Visuals from previous cameras showed poor visibility so a special machine vision enhancement camera was used to assist in visual inspections. The system is battery-powered and uses a fibre optic cable to feed all data live to the surface and allow real-time piloting. The ROV is capable of handling up to 6 cameras and 8 survey accessories simultaneously to cover as many tasks as possible while also being easily adapted for unforeseen tasks if needed. The ROV was used successfully in February of 2023 removing nearly 120kg of concrete grout pieces.