Water Treatment: Coquitlam Ozone Treatment

SSBV Joint Venture

Engineers devised an unconventional large ozone treatment plant for unfiltered water that flows down from Coquitlam Lake towards Greater Vancouver.

Coquitlam Lake is one of three lakes that supplies potable water to the Greater Vancouver Water District, which in turn supplies 1.8 million people in Lower Mainland British Columbia. The Coquitlam source is closed off to the public and the water is of high enough quality that it is not filtered.

Some years ago the water district decided to switch from chlorine to ozone for its primary disinfection. Ozone, O3, a colourless gas consisting of three oxygen molecules, is a powerful disinfectant capable of killing bacteria and deactivating protazoa. It renders giardia, for example, inactive in under 10 minutes whereas chlorine takes an hour. Ozone also produces far fewer disinfection by-products (DBPs), which is good news for those who believe DBPs can cause cancer.

Adding ozone disinfection into the Coquitlam water supply, however, was not a simple matter.

Normally ozone is introduced into a water supply through fine bubble diffusers into a contact basin. The Coquitlam source supplies water by gravity, and moreover the lake level can vary by about 17 metres. As a result, introducing a conventional ozone treatment system and building a contact basin would have meant ultimately pumping 2,500 million litres of water a day.

To preserve the economies of the gravity supply the engineering team decided that they would ozonate the water in a pipeline under pressure. The in-line system was designed by a joint venture of Stantec, Amec E & C and Black and Veatch (SSBV) and has since been patented. The largest comparable system was 10 times smaller and did not have 17 metres of supply head variation.

The Coquitlam plant consists of two buildings, joined by a 1 kilometre long, 3 metre diameter stainless steel pressurized pipeline contacter. The Ozonation Facility at the upstream end houses the ozone generators capable of producing 2,800 kilograms of ozone a day. To the side of the structure is a tower containing the sidestream injection system. Fifteen per cent of the total water flow from the supply pipeline is routed into the tower. The ozone is injected at variable volume rates according to need, at a concentration of up to 12% by weight. A constant flow is maintained through pumps and valves so that the system accommodates and balances the varying lake levels, and meets the peaks and lulls in demand from customers downstream. Operations are automatically controlled and linked to a SCADA system.

From the sidestream injection system, the water passes into the stainless steel pressurized pipeline contactor to allow time for the ozone to do its work. The pipe allows a T10/T of 1.0 for CT (concentration x time) based on plug flow in the pipeline.

At the other end of the pipe is the Chlorination and Corrosion Control facility. Here the water chemistry is adjusted. Hydrogen peroxide is used to quench any unused ozone. Carbon dioxide and soda ash are added to adjust the pH levels, and chlorine in small doses is used as a residual disinfectant to prevent bacterial regrowth in the distribution system.

The finished water quality goal for the primary disinfectant is 99.9% (3-log) inactivation of Giardia. The $45 million project, commissioned in July 2000, provides immediate water in peak and low demand times. It is the largest plant in North America that uses ozone disinfection without filtration.–BPCCE

Project name: Greater Vancouver Water District Coquitlam Ozone Treatment Project

Consultants: Klohn-Crippen