Supply risk calls for new water treatment plant - Contractor Mag - Oct 2017 - Featured Image
Contractor

Supply risk calls for new water treatment plant

With an ageing water treatment plant at one of the Southern Hemisphere’s largest dairy processing sites, a solution was required – and fast.

BUILT BY KIWI DAIRIES in 1972, the 
Whareroa Fonterra site has the capacity to produce 13.8 million litres of milk per day.

However, its ageing treatment plant posed a serious risk to the manufacture of what amounts to nearly 20 percent of Fonterra’s production output.

The solution – a $17 million project of a new water treatment plant – was awarded to Beca and Fulton Hogan, and they were then faced with the task of meeting an extremely tight deadline.

The first water had to be supplied by August 1, 2016, giving the contractors 11 months to plan and construct a plant that was capable of producing up to 30 million litres of water per day.

In addition to this, there was also the inclusion of a new water intake in the Tangahoe River, and the difficult site location provided an extra set of challenges for the designers and engineers.

A concrete casting bed was constructed onsite to cast the precast wall panels for the clarifier and filter structure panels on. Plant casting on site allowed the manufacture of larger panels, thereby reducing insitu concrete ‘stitches’ and risks 
associated with leaks at joints. It was also more cost effective than precast sourced externally.

Setting up the project

For the treatment plant, the construction included a concrete clarifier structure and mixing chambers. The clarification was provided by stainless steel lamella plates – supplied by MRI Technologies in the USA – and the concrete filter structure is comprised of a Cadar Monolithic Filter Floor System with conventional media filters.

The plant is fully automated, including fire systems that tie into the systems at the main Fonterra Whareroa plant.

Aside from landscaping, fencing and a booster pump station upgrade, the final task for the treatment plant was the design and construction of a 4.5 million litre treated water reservoir. For this a stainless steel product was selected as it provided the best option in terms of capital and ownership costs.

For the Tangahoe river raw water intake, the task required the excavation of a three metre diameter, 12 metre deep wet wall. There was the construction of new intake screens – Johnson Screens Passive Intake Screen – and a stainless steel manifold in the river bed.

Pipework then needed to connect the intake screens to the wet well, including raw water pumps and controls, and an airburst screen cleaning system. Of significant note was the unusual site conditions, in that work in the riverbed, and the wet well (which was situated on a steep river bank), created a variety of constraints for both the designers and engineers.

Hand-dug by a team of specialists, the well holds two new submersible pumps, which pump water to two new hydro-cyclones that remove much of the silt, ahead of treatment in the new plant. The pumps are on guide rails, which improves safety for staff involved in maintenance or repair of the intake, and the provision of a 30mm grating at the clearwell is protecting fish life.

The water treatment plant is capable of treating up to 30 million litres of water per day.

Mitigating risks

The water treatment plant’s backwash pumps and UV units.

There were three key risks identified for this project, including procurement of imported long lead equipment, design progressing ahead of construction, and meeting the programme deadline (where there would be significant consequences if delivered late).

To deal with the issue of procuring a stainless steel pipe – as time did not allow for design to be completed enough to obtain subcontractor offers – a solution to engage subcontractors to procure pipe early and price packages of work as they became available was created.

A concrete casting bed was constructed onsite to cast the precast wall panels for the clarifier and filter structure panels on. The site manufacture of precast panels also provided several advantages such as direct control of production rates (timing) and quality, and plant casting on site allowed the manufacture of larger panels, thereby reducing insitu concrete ‘stitches’ and risks associated with leaks at joints. It was also more cost effective than precast sourced externally.

Once casting was complete, the slab was utilised as the foundations for the chlorine building, and it also provided paved areas around it – a further benefit was that the slab didn’t require demolition.

To improve speed of construction, bespoke steel formers were designed for the corner stitches. And while initially more expensive, these forms were significantly quicker to erect and strip than conventional forms.

Reducing environmental impact

A site-specific Environmental Management Plan detailed all environmental procedures – most specifically, sediment control.

For the work carried out on the river bed, which involved exposing the ground to install piping, the construction was isolated from the river itself by using a cofferdam, built from one tonne sandbags. Any water that got into the vicinity of work was directed away to a sediment site controlling the possibility of a contamination.

The new plant has been in use for more than a year now, and has improved the efficiency of the site’s processing plants by increasing onsite water storage by 50 percent (as the existing reservoir has been retained).

This article first appeared in Contractor October 2017.

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