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Dome roof will help control odours

Thursday 21 October 2010, 3:48PM

By Gisborne District Council

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GISBORNE

A five metre high aluminium geodesic dome will next week become a shining beacon for a new way of treating the city’s wastewater.

Weather permitting, the 32.5 metre diameter dome will be fitted on top of the new biological trickling filter tank next Wednesday at the Banks Street wastewater treatment plant. This will bring the tank’s height at the apex to 15 metres.

By January next year, fine screened and degritted wastewater solids will be transformed into plant-like matter through the biological trickling filter process within the tank.

Colin Newbold, project manager for main contractor HEB Structures, said construction of the dome had been plain sailing, and completed within a few days. The challenge would be lifting the dome into position – a job that demanded no or very little wind.

“We are bringing in a 220 tonne truck-mounted crane from Tauranga to do this. Odour control ducting will be suspended from inside the roof before the dome is then lifted up and bolted down on to the tank through 24 ‘feet’.”

The roof, which comes from the United States with a $500,000 price tag, is a vital part in controlling odour and aerosols from the trickling filter process.

A network of odour control pipes built into the tank includes a grid of fibreglass pipes under the floor section. An air extractor system draws out odour from the bottom of the tank, recycles three-quarters of it and pushes the remaining quarter out into three specially designed bark and gravel odour beds.

The dome’s components were manufactured in Savannah, Georgia and supplied by United States company Temcor. Comprising 426 beams and 142 panels, the dome was assembled within a few days on site by three staff from Gisborne’s Universal Engineering and one from HEB Structures. By the end of the week, each set of bolts will be sealed with silicon rubber and covered by an aluminium disc. The bolts, known as Huck bolts, are similar to those used in aircraft technology.

Temcor consultant Raymond Stout, who has been involved with the company for 20 years, said the Gisborne dome was small in the scheme of Temcor creations.

“I have recently been involved in a couple of domes being built in Korea as covers for iron ore. Several had a diameter of 135 metres, and three were 120 metres.”

The domes have good longevity, only needing maintenance work on their silicon sealant parts after 20 years.

Temcor domes have been used for more than 40 years to cover water, wastewater and petroleum tanks, and for bulk storage and coal storage. One was even built to house Howard Hughes’ Spruce Goose plane in Los Angeles.

Once the roof is in place, the 11,000 black plastic media blocks with which the tank is now filled, will no longer be open for viewing. However, the roof provides two access points allowing what is expected to be six-monthly inspections of the media.

From January 2011, screened and de-gritted wastewater will be pumped up through the tank’s central column to a height of 8m, then distributed through six rotating arms to trickle slowly through many different channels in 10 layers of plastic media blocks.

Construction works have progressed steadily, despite days lost through high winds and torrential rainfall in recent months. More staff have been employed with about 60 now on site – 40 percent of them from Gisborne.