Opened in 1963, the Point Loma Wastewater Treatment Plant on Gatchell Road in San Diego, California, is one of the largest facilities of its type on the west coast, treating up to 190 million gallons of wastewater per day from an area of 450 square miles. Because of its location on the California coastline the plant is subject to typical marine-environment corrosion. The sedimentation basins are also subject to the severe corrosion environment typical of wastewater treatment plants with high hydrogen sulfide (H2S) levels.

As part of a concrete restoration program launched in 1994, V&A Consulting Engineers has been retained to perform inspection of many systems within the Point Loma facility. The initial work involved conducting a condition assessment to determine the extent of the deterioration of concrete at eight sedimentation basins built in 1963. Four of the basins were originally built using a T-Lock® PVC liner; the others had a vinyl ester coating.

V&A determined that the tanks’ integrity had not been compromised, and that the structures could be rehabilitated using a combination of concrete repair in certain areas and improved corrosion protection.

Project Approach and Outcome

V&A’s Coatings Department tested the extent of damage of the concrete in the eight, 40-year-old + sedimentation basins and measured the pH of the concrete before and after cleaning, in order to obtain a complete classification of the extent of hydrogen sulfide-induced deterioration of the concrete. Most of the observed damage was found above the water line and was caused by sulfuric acid produced by the wastewater. Changes in operational levels had caused the head space above the wastewater to come in contact with concrete that was not protected by the T-Lock® liner. In addition, the vinyl ester coating failed to protect the concrete. As a result of sulfuric acid attack, the pH of the concrete ranged from 2 to 3.5.

As part of the project, V&A evaluated more than 65 different coating and lining systems. Ultimately, only four of these systems were ultimately included in the project specifications.

Engineering drawings and technical specifications for concrete restoration and protective coating of the repaired concrete were prepared, and the project was put out for bid with an estimate of $2,200,000. The winning contractor submitted a low bid of $1,890,000. The low bid was based on application of a 125-mil high-solids polyurethane coating, which was determined sufficient to protect the concrete from the harsh environments and reduce maintenance costs over the years. The contractor was also paid an additional $350,000 to repair additional damage which became visible during the cleaning of the concrete.

A total of approximately 73,000 square feet of concrete was repaired and coated at a total cost of $2.24 million. V&A provided full-time inspection, with technical assistance, of the surface preparation and application of the coating. The average construction cost was $30.14 per square foot for surface preparation, concrete repair and coating application.

Point Loma’s 5-year Warranty

A critical and unusual, if not unique, aspect of the Point Loma concrete rehabilitation project was that—unlike almost any other job in this field—the client required a 5-year product and application warranty. As part of this warranty, 1-year, 3-year, and 5-year follow-up coating inspections by the engineers were included in the project specifications.

As of early 2003, six of the 1-year inspections have been completed. As expected, these resulted in several minor additional repairs; however, the overall performance of the coating system has met the intent of the project specifications.

The warranty period is providing the opportunity to detect and repair virtually all significant imperfections in the basins traceable to shortcomings in design, materials, and/or construction. The result is to give Metropolitan Wastewater and the taxpayers of San Diego a reliable, durable rehabilitated wastewater treatment facility which, with proper maintenance, is projected to have a useful life of anywhere from 60 to 100 years.

The photos below show a sedimentation basin (left) and a concrete surface (right) before and after repairs.