Locals trained to weld plastic media

When toilets begin to be flushed at year’s end, spare a thought for the Gisborne tradesmen who put together a vital component in the new wastewater treatment system.

The four workers from Electrinet have embarked on the massive and somewhat mind-numbing task of building 11,000 haybale-like, black plastic blocks to occupy nearly 5000 cubic metres. By the end of September, this structured plastic media will begin to be fitted into the new biological trickling filter tank on council’s Banks Street wastewater treatment plant. That process alone will take about a month.

The creation of the blocks is a significant milestone in the history of Gisborne’s wastewater system. Once the plant is commissioned at the end of the year, the city’s wastewater will flow through these blocks and undergo a biotransformation process before being discharged through the existing outfall 1.8km to sea. This will provide a much higher level of treatment than provided by the simple milliscreening process introduced 20 years ago.

Superchill Australia, which won the contract to supply and build the plastic media bales, wanted local people to create the bales and electricians to oversee the work. Electrinet was identified through a mutual acquaintance with Superchill’s New Zealand agent in Auckland.

Electrinet’s Jack Brass and Darren McKinley are operating the German-made welding machines brought in from Melbourne for the job, while managing director Barry Atkinson and general manager Stu Blair will oversee the electrical side.

Using heat and electrical current, each block is created by welding together 12 polypropylene sheets. The finished 300mm wide blocks measure 2.2m by 600mm and will consume more than 130,000 sheets. A third of these are on site, with the rest en route from Germany. Completed blocks will be stored either in or outdoors.

Superchill Australia managing director Bruno Saner has been on site for a week to help install the machines and train the Electrinet workers and Gisborne District Council project engineer Mike Yukich, as part of council’s quality assurance of the project.

Mr Saner said that with one man on each machine, about 120 bales will be produced each eight-hour day, with an estimated 100 days to complete the job. Four grades of plastic are being used. The heavier grade plastic bales are destined for the bottom of the tank, building to the lightest plastic bales at the top.

About two thirds of the bales need to be ready by the end of September for the start of the tank filling. When complete, the 10 layers of media blocks – each layer placed at right angles to the previous – will stand 6.1 metres high inside the 8m by 32m diameter tank.

Mr Saner will be back on site later in the year to oversee the installation of the media and the huge arms that distribute the wastewater over the media in the tank.

Council project manager Peter McConnell said structured rather than random media was chosen because it provided 25 percent more surface area with better distribution of water and airflow.

“Another advantage is that sections can be lifted out to see exactly how much biofilm is growing on the media. The information gained from this can be used to fine tune the operation of the biological trickling filter.”

The media material was initially developed for water cooling, in which Superchill Australia specialises, and has longevity of at least 30 years.

Napier piggybacked on Gisborne’s order to trial the structured media for its own treatment plant. The council will share its results with Gisborne.