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Full biological transformation will take time

The biological process that will soon be responsible for treating the city’s wastewater will take time to settle down once commissioned.

Peter McConnell, project manager for the Gisborne Wastewater Project, says the biological trickling filter process will run smoothly but it could take up to three months for the biofilm – similar to the slime that grows on river stone – to grow within the tank.

“It will, therefore, take at least that time for full biological treatment or biotransformation to occur.”

Gisborne District Council chief executive Lindsay McKenzie, who started his career in the laboratory of a wastewater treatment plant, says that the treatment process will take time to reach full potential depending on how fast the biofilm on the media grows.

“The breakdown and transformation of the screened wastewater into plant-like matter will be done by a living biological system that will grow and metabolise the waste on the surface of the black plastic media blocks within the trickling filter tank.

“Development of the biofilm in the short term may be affected by the nature of the waste the plant receives during the holiday period including the large proportion of portaloo waste from events such as the Rhythm and Vines festival.”

Peter McConnell says council staff, with the assistance of Superchill – which supplied the media and the rotating distributor arms – will monitor the trickling filter process. Sections of the black media blocks have been placed within a “sampling tower” that can be lifted out of the tank for visual inspection. Each block in the tower is numbered to provide a record of its position within the tower and how the biofilm is growing at the different levels.

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The blocks will initially be checked weekly and then, once the biofilm is well-established, monthly.

“Superchill has provided operating procedures and has staff in Australia and Germany we can call on for advice. We can also talk to council staff in Hastings which is also running a biological trickling filter process.”

How does the biological trickling filter system work?

• Screened and degritted wastewater is pumped to the top of the biological trickling filter tank and distributed through six rotating arms.

• Wastewater slowly trickles through hundreds of channels created by 10 layers of large black plastic media blocks within the biological trickling filter tank. Each layer of blocks is at right angles to the previous layer. The media is designed to have a large surface area and lots of air space.

• Friendly bugs catch and convert waste to food.
• Bugs grow on the surface of the media like slime on river stone.
• Biological growth (biofilm) eventually falls off the media surface and flows out with the water.
• Treated water is pumped to sea via the existing 1.8km outfall.

What will the wastewater look like once it is treated?

Colin Newbold, project manager for main contractor HEB Structures, has also built biological trickling filter tanks in the UK. He says the treated wastewater looks like water with fluff in it.

“If a bucket is filled with the treated wastewater, the fluffy component settles to the bottom and the water is clear.”

Within 18 months, a UV process will be added to the treatment plant which will greatly reduce the bacteria levels in the treated wastewater.

The cost of this is included in the total $39.5 million wastewater project budget approved by council.


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