Twin Oaks Valley Water Treatment Plant, San Diego

What is it? A 100MGD (378,500m3/d) submerged-membrane water treatment facility producing high-quality water for San Diego County in California.

Who is responsible? CH2M Hill was awarded the $159 million design-build contract, and will operate and maintain the plant for the next 15 years. Zenon supplied its ZeeWeed UF membranes.

Why is it shortlisted?

• Before the construction of the plant, nearly half of San Diego County's drinking water was purified by the Metropolitan Water District of Southern California. In recent summers, the existing Skinner plant was pushed to its limit, requiring the Water Authority to call for increased conservation to avoid mandatory water use restrictions. The completion of the Twin Oaks Valley facility in 2008 will avoid future treated water shortfalls.

• By overlapping design and construction efforts and eliminating multiple redesigns and contract bidding, the DBO approach enabled the project to be completed approximately one year ahead of schedule.

• The Twin Oaks plant is part of the SDCWA's $3.2 billion capital improvement programme to diversify the region's water sources, and is the largest plant of its kind in the world.