Outfall and intake problems hit OC desalination

The pilot plant for the South Orange Coastal Ocean Desalination Project in California has been forced to close down due to relocation of its outfall pipe due to heavy storms . The project has also run into problems with its intake water.

The pilot plant is operated and funded by Municipal Water District of Orange County (MWDOC) and its partners South Coast Water District, Moulton-Niguel Water District, San Clemente, Laguna Beach County Water District and San Juan Capistrano.

If the pilot is successful, the agencies could decide to go ahead with a 15 MGD (56,700 m³/d) project that would decrease the area's dependence upon imported drinking water supplies. South Orange County depends on water imported from northern California and the Colorado river to meet approximately 95% of its local demand.

Karl Seckel, assistant general manager of MWDOC, told D&WR that recent heavy storms had shifted a berm, which created a lagoon from an inflowing creek and was originally on the shore side of the outfall. Now the outfall ends within the lagoon, he said, creating a danger to bathers if it filled the lagoon with outfall water and threatened the berm. It was shut off on 9 July 2011.

While the outfall can be fixed with a 90 ft (27 m) extension, the plant will still have problems with its 350 ft (106 m) slant-drilled intake beneath the seabed. The intake is picking up 7,500-year-old groundwater from the San Juan Basin underneath the lagoon.

The designers had expected more of the water to be ocean water with only low levels of iron (Fe) and manganese (Mn). Instead, the pumping has continued to bring in water with high levels of Fe and Mn, which need to be removed.

Incorporating Fe and Mn removal might involve expenditure of US$ 50 million onto the cost of the full-scale facility. Part of the pilot plant testing work includes testing of several options for Fe and Mn removal technologies to determine which might be the most cost-effective, if they are needed.

Otherwise the test slant well has produced a very high quality water that has been fully filtered by the natural sands and gravel materials beneath the ocean.

Investigating the Fe/Mn problem will require a longer test and/or more wells at a cost of up to US$ 3 million. The partner agencies now have to decide what price they want to pay.

"An extension of the pumping test will help to tell us about how much iron and manganese may remain in the water being pumped, or if it will decline over the long run as originally expected," says Seckel.

A 2011 cost update estimated the cost of the facility at about US$ 215 million in 2017 dollars (midpoint of construction and assuming Fe and Mn removal is included) with an overall cost of water estimated at US$ 1,830 per acre-foot (US$ 1.48/m³). The current cost of imported water sources is about US$ 800 per acre-foot (US$ 0.65/m³).

The water from the facility will receive an incentive of US$ 250 per acre-foot (US$ 0.20/m³) from MWDSC once operations begin. With this incentive, the cost of ocean water is estimated to drop below the cost of imported water sometime between 2022 and 2030.

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