April 27, 2009

Wastewater Treatment

Annacis Island wastewater plant switches to chemical treatment system

Jean Sorensen

correspondent

Metro Vancouver is in the process of replacing the aging wastewater gaseous treatment system at the Annacis Island Wastewater Treatment Plant (WWTP) with a more modern chemical alternative.

“This process is part of the over-all treatment (at the Annacis Island plant),” said Ken Hui, Metro Vancouver engineer.

“It is at the end of the effluent treatment.”

There are five treatment plants for wastewater throughout the Metro Vancouver area.

The Annacis Island plant is the largest, serving one million residents and is a secondary treatment facility.

Treatment includes removing solids and materials from the wastewater as well as settling pond scum and then releasing the liquids into the Fraser. The released liquids are required to go through a final disinfecting treatment phase.

Metro Vancouver is currently using a gaseous disinfectant method for this treatment.

Metered chlorine gas enters the wastewater to achieve chlorination over a monitored contact time.

A sulfur dioxide gas is then introduced to remove the chlorine before discharge occurs.

The new system will deliver liquid sodium hypochlorite solution (SHS) into the wastewater to achieve the required chlorination, while a liquid sodium bisulphite solution (SBS) will be introduced to tame the chlorine.

The material disinfection standard will not change, said Hui.

Similar systems are in place in the Metro Vancouver area at the Lions Gate Wastewater Treatment Plant and the Lulu Island Wastewater Treatment Plant.

Hui said the replacement system is felt to be safer and easier to handle.

There are five construction stages. The first has been the pre-load of the site on Annacis Island with the E Mathers Bulldozing Company Ltd. of Richmond winning the contract.

Preloading of sand trucked to the site is being carried out so that construction can begin summer 2009.

The second contract that has been awarded is the pre-purchase of the chemical storage tanks, which will be on site and contain the solutions for chlorination and de-chlorination.

That contract has gone to Chemposite Inc. of Delta, which bid $579,516.

The contract was for the supply and delivery of seven storage tanks designed for outdoor concrete pad mounting and two insulated tanks designed for mounting on concrete as well for SBS containment.

The third contract phase is a pre-purchase of in-channel static mixers (for the chemicals introduced into the wastewater) with requests for proposals expected to be issued in the near future.

The Requests for Qualification for the main construction contract is currently underway.

Successful contractors will be short-listed and will respond to a tender call.

The main construction contract involves the construction of the two chemical containment areas.

It also includes the supply and installation of chemical delivery system, installation of pre-purchase chemical storage tanks and in-channel static mixers, utility trenches connecting the chemical delivery system to the point of chemical injections and supply and installation of associated electrical and instrumentation work.

The new system is designed to handle an average dry weather flow of up to 725 MLD and a peak instantaneous flow of 1,366 MLD.

The new system will not affect the contact time of the chlorination.

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