On October 11, the Metropolitan Water District of Southern California will open the bids for the final contract for a $241-million retrofit meant to combat the fast-spreading algae. It produces a chemical that gives water a bad taste and odor.
One of the bidders for the $100-million chemical-stage contract will be the joint venture of San Francisco-based Obayashi Corporation and Hayward, Calif.-based Shimmick Construction Co., Inc., which was awarded the construction design/bid/build contract in July, says Shimmick chief estimator Mike Kenney.
MWDSC Officials hope that when on line in 2009, the new disinfectant step could help screen out a particularly prolific algae strain that recently moved into the reservoir that delivers drinking water to San Diego and southwest Riverside counties. (http://www.mwdh2o.com/mwdh2o/pages/business/construction/PDF/1498NIB.pdf)
Metropolitan Spokesman Rob Hallwachs says Plaktothrix perornata, a potent producer of 2-methylisoborneol (MIB), was discovered after the district applied 5 tons of copper sulfate granules August 9 and another 6 tons on September 9 to reduce native MIB and geosmin producers. "The Plaktothrix that survived the treatment recovered in a few days and its population and MIB production then increased very rapidly," says Hallwachs. MIB was found to be as high as 1,800 ng/L in mid-September and Planktothrix filament averages as high as 1,200 per 100 mLs. Hallwachs says consumers can often detect a taste and odor problem at MIB levels as low as 5 ng/L.
In order to avoid the offensive odor and smell in the otherwise safe drinking water, Metropolitan stopped using the lake as a source of water by drawing from the Colorado River Aqueduct, Lake Perris and Diamond Valley Lake through a by-pass canal.
Dr. Mic Stewart, Metropolitan water quality manager, theorizes that the algae, a member of the Cyanobacteria photosynthetic bacteria family, made its way south through the State Water Project aqueduct system after a 2004 levee failure in the Sacramento-San Joaquin River Delta and flourished in the Skinner runoff.
Patty Friesen, spokesperson for Contra Costa Water District, says the algae is present in Delta water sources in small numbers, "but it hasn't developed into a problem."
On September 17, 8 tons of rice-size copper sulfite - the predominant method of dealing with algae in the water supply, according to Hallwachs -- was dropped from a helicopter to control planktonic or floating algae. One of the challenges of combating Plaktothrix is that it tends to float in the water column rather than attach to the bottom so granular applications don't have an impact. A September 24 follow-up treatment of 8 tons was designed to eliminate the remaining non-native algae.
Plaktothrix is not affected by the conventional coagulant, sedimentation, filtration, disinfection process now used at Skinner. Even an oxidator - something that was planned for a number of MWDSC facilities before the algae was discovered--can't screen out all of the algae. "It will eliminate 90 percent to 95 percent of the MIB," says Stewart. That still leaves as much as 10 percent, enough to leave a bad taste in the mouths of consumers and water quality managers.
Jason Allen, an engineer at American Water Works Association, says the addition of powder activated carbon can be effective for dealing with Plaktothrix. But is not easily ramped up for intermittent episodes and can be expensive if a time chamber has to be added. "You have to ask if it is cost effective if it is not a safety concern," he says.
his fall, the battle against Plaktothrix perornata begins in earnest at the Robert A. Skinner Water Treament Plant near San Diego.
