|
To overcome a bacterial re-growth problem in the
distribution system of the Central Fraser Valley communities including the
City of Abbotsford and District of Mission and to protect against
waterborne giardiasis under a non-filtration mode of operation, the
Norrish Creek chlorination station was constructed. Located at the intake,
the scheme utilizes the 10 km 900 mm diameter supply main to achieve
chlorine contact suitable to achieve 3 log giardia cyst reduction.
The station, sized for a flow of 91,000 m3/d
(20 Imgd), includes two independent chlorine injection systems to ensure
continuous uninterrupted disinfection and 100% backup capability. Wallace
and Tiernan V2000 chlorination units are used. The station includes an
auxiliary power unit and compound loop chlorine (flow and chlorine
residual). A SCADA system monitors chlorine residual, turbidity, pH,
pressure and various mechanical functions.
Chloramines are formed as a secondary disinfectant by the
addition of ammonia at a point about 10 km from the chlorine addition
point.
|
|
The Greater Vancouver Water District provides bulk water
to nineteen municipalities and about 2 million people.
At present the only
form of treatment at the three surface water sources is disinfection by
chlorination. Bacterial regrowth in the extensive distribution system was
at level that exceeded the Health Canada guidelines.
Dayton & Knight
Ltd. undertook the pre-design of twenty-one strategically located
secondary disinfection stations which, for public safety reasons, was based
on liquid sodium hypochlorite. The effort included locating the stations
on the GVWD transmission system and addressing such design issues as the
method to store sodium hypochlorite, the operation status after a design
earthquake, remote operation from the GVWD central control and public
safety during bulk delivery of sodium hypochlorite. A detailed total cost
estimate was determined for each station.
|
