MWD awards contract to rehabilitate Skinner Treatment Plant ozone contactor walls

Apr 20, 2023

The walls of the ozone contactors at the Metropolitan Water District of Southern California's Robert A. Skinner Water Treatment Plant are experiencing cracks, and the March 14 MWD board meeting included the award of a contract to rehabilitate the structure walls of two of the contactors.

Slater Waterproofing Inc. was awarded a $394,534 contract for the work. Hydrophilic grout will be injected into the concrete cracks to inhibit the leaks at Contactor 1 and Contactor 2 along with the inlet channel, and a finish mortar coating will be placed on the concrete walls.

The Robert A. Skinner Water Treatment Plant treats both Colorado River Aqueduct and State Water Project supply and delivers that treated water to the Eastern Municipal Water District, the Western Municipal Water District and the San Diego County Water Authority. When the plant began service in 1976 its treatment capacity was 150 million gallons per day, and subsequent expansion of the plant now allows for a capacity of 350 million gallons per day.

Ozone is the primary disinfectant used at each of MWD's treatment plants. The control of toxic algae and compounds which cause undesirable taste and odor allows MWD to treat any blend of Colorado River Aqueduct and State Water Project supply. The ozone treatment process equipment includes liquid oxygen storage tanks, ozone generators, high-voltage power supply units, a cooling system, ozone contactors, destruct systems and safety and water quality monitoring equipment. The systems are controlled by a series of networked programmable logic controllers which interact with the plant's supervisory control and data acquisition system.

The ozone contactors at the Skinner treatment plant consist of large concrete basins where the plant's influent water is mixed with ozone to disinfect the water. The Skinner plant's ozone contactor structure was placed into service in 2010. The system consists of six contactors which are each 120 feet long, 38 feet wide and 30 feet tall along with an inlet channel and instrumentation galleries. Ozone and water are mixed within the contactors for a calculated duration to meet state and federal disinfection requirements. Contactors 1 through 4 are actively in service, while Contactor 5 and Contactor 6 were decommissioned when the plant's treatment capacity was reduced from 630 million gallons per day to 350 million gallons per day in 2017. The concrete structure totals 62,700 square feet.

Regular inspections of the structure are conducted by MWD staff members. In recent years, expansion cracks have developed in the walls. The cracks vary in length from 2-12 feet and are estimated to total 2,400 feet in length. The width of the cracks is typically less than 1/16 of an inch. Expansion cracking of reinforced concrete occurs naturally throughout the lifespan of a water-retaining structure, and in many cases the cracks will seal

themselves over time as the mineral content in the water calcifies. In some cases, the cracks do not self-seal, which could lead to leakage of ozonated water from the contactor basins into the instrumentation galleries. In those cases, MWD staff has developed a methodology to repair the concrete walls proactively. The methodology includes the use of hydrophilic grout injection into the concrete cracks to inhibit the leaks. The concrete walls of Contactor 3 and Contactor 4 were rehabilitated with that method in July 2019, and periodic inspections performed over the following three years confirmed the effectiveness of that approach.

The contract for Contactor 1 and Contactor 2 was advertised for bid Dec. 14. Five bids were received by the Jan. 26 bid opening date. Slater Waterproofing Inc., which is based in Montclair, submitted the low bid of $394,534. The second-lowest amount of $498,776 was the bid of Tharsos Inc. of La Mesa. The engineer's estimate was $591,000.

While the hydrophilic grout is being injected into the walls equipment will be protected in place. MWD staff will clear the work area and provide a contractor work staging area.

The $598,000 total budget also includes $56,000 for construction management and inspection, $53,000 for MWD staff labor, $50,000 for submittals reviews and preparation of record drawings and $44,466 for contingency. The work is expected to be complete by January 2024.

Joe Naiman can be reached by email at [email protected].