Canada's first gravity-fed water processing plant being built

The City of Nanaimo has broken ground on the South Fork Water Treatment facility, Canada’s first gravity-fed water processing plant and the largest infrastructure project in the city’s history.

The three-storey, 116 million litre per day water facility includes raw water screening, coagulation/flocculation, a two-stage GE siphon membrane system and chemical feed systems.

Ancillary facilities include a 7.5 million litre reinforced concrete clearwell, valve/chamber and pump station, and more than three kilometres of 1,350 mm diameter supply mains.

The site’s location at an elevation above the City of Nanaimo, but below the elevation of the watershed’s dam, takes advantage of a natural slope enabling siphoning to pull the water through the membrane filtration system, eliminating the need for pumps.

Bill Sims, manager of water resources for the City of Nanaimo, said the system, Canada’s first, saves $62,000 per year in electricity over a pump-fed system.

Water is siphoned through hollow fibre membranes that filter water to the bacterial level through microscopic pores on the membranes’ polymer surfaces.

Thousands of hollow fibres are fabricated into modules, and cassettes are formed by sliding modules into guide slots and locking them into place.

At South Fork, each cassette houses 84 modules, and five cassettes are immersed in each concrete filtration basin.

The concrete tanks are 3.7 metres across and had to meet tolerances of plus or minus 2.0 mm to allow the cassettes to set in without rubbing against the tank walls.

“Add to the fact that the walls are a half a metre thick and have an incredible amount of rebar in them because it is a post-disaster structure, there is no room for error,” said Sims.

Kenaidan Contracting Ltd., the general contractor on the project, completed the challenging installation with proprietary prefabricated aluminum, composite and steel formwork systems.

Less intricate concrete elements, such as the plant’s square and straight walls, used aluminum formwork, while more complex components called for steel.

“The tie spacing can be further apart due to the increased capacity of the steel to span longer distances when under load,” ,” said David Simpson, project manager for Kenaidan.

The concrete walls for the clearwell were formed using a steel radius formwork system.

“The complexity of the high walls (7m+) combined with the radius of the walls were such that the steel forming system was the preferred choice,” said Simpson.

“There is a large dividing wall within the clearwell that is straight, for this wall we will use the aluminum.”

Kenaidan also ensured the concrete tolerances with additional quality control inspections before, during and after the formwork and rebar installation, concrete pour and formwork removal.

The plant uses a Profibus communication system to run its instrumentation in a plug and play format that is less intense on calibration.

“It provides a larger degree of autonomy than older, historically used communication systems,” said Simpson.

“Profibus provides the plant operators with a greater ability to communicate and control equipment and operations.”

South Fork’s state of the art design means the facility will send 99 per cent of the water it processes out to the city lines.

Older plants see as much as 10 per cent water waste.

One of the project’s biggest challenges, said Sims, was the transition from the HST back to PST.

Under the HST, the cost was transparent, with the city expecting a refund for most of the tax under that system.

“We put it out to tender before the rules about how the transition would apply were announced,” he said.

“We awarded the contract and then the rules were introduced, so we had this $2 million additional burden to cover material costs.”

Combined with a higher than expected construction cost, the city’s budget was overdrawn, meaning officials had to seek additional grant funding and increase their debt load.

Kenaidan and the city completed a scope reduction exercise that value engineered $1.7 million off of the original $47.5 million bid, said Sims.

“It was greatly appreciated and set the collaborative tone for the entire project,” he said.

A 160-square-metre maintenance and storage building, which erased half a million from the budget, will be constructed at a later date.

Co-ordination was key for Kenaidan, with more than 20 subcontractors involved at various stages of work.

“The complexity will continue to grow throughout the next few months as additional subcontractors are brought to site,” said Simpson.

“Scheduling the work is ongoing and requires daily attention.”

The plant upgrades Nanaimo’s single process water treatment to a multi-stage process as mandated by the local health authority in 2008 for all water treatment systems on Vancouver Island serving more than 500 people. Construction started in May 2013 and it is expected to be operational in the second quarter of 2015.