Acid Attack - Case Studies

A.    Barceloneta Trunk Sewer (Baceloneta, Puerto Rico) (ACI 210.1R-94, 1996)

• Background: The Barceloneta Trunk Sewer, operational in 1976, collects sewerage from several pharmaceutical plants as well as domestic flow. Cast-in place reinforced concrete manholes at 280ft intervals and reinforced concrete pipes were used in construction. The depth of the pipe invert below the ground varies from 5ft to 25ft. The pipes are subject to a wide range of temperatures, pH levels, and chemical compositions.

• Problem: Ground subsidence of the pipeline back fill appeared along the sewer alignment. When investigated the pipe was found to have seriously deteriorated and in many cases the concrete was almost nonexistent.

• Solution: Many options including total replacement were considered. Because the sewer  ran adjacent to a major road, replacement was found to be cost prohibitive. The installation of a pipe lining system was selected as the method of rehabilitation. The process consists of cleaning the existing pipe interior and installing a flexible plastic-lined fiberglass hose impregnated with a polyester resin. The resin is activated by circulating hot water through the hose for a period of time. The water pressure acts to both install the hose and to place it in intimate contact with the existing sewer walls.

• Performance: Performance of the replacement liner after several years has been excellent, showing no signs of deterioration.

B.   Los Angeles Sanitary Sewer System and Hyperion Sewage Treatment Facility (ACI 210.1R-94)

• Background: The sewer system of Los Angeles consists of over 6000 miles of sewers servicing an area of 600 square miles. Two upstream reclamation plants, the D.C. Tillman and the Los Angeles-Glendale plants serve the San Fernando Valley. These plants treat sewage to advanced secondary standards discharging effluent into the Los Angeles river and returning the solids back to the sewers for transport to the Hyperion regional treatment plant located on the cost of Santa Monica Bay. All other sewers connect to one of four major interceptors for sewage transportation to the Hyperion treatment plant.

• Problem: The life span of concrete sanitary sewers in Los Angeles are severely shortened due to the bacterial acid attack. Long distances traveled to reach the Hyperion plant cause influent to remain in the sewers up to 72 hours. Point source control of toxin producing industry has also served to aggravate the existing situation. Previously produced toxins were directly fed into the sewer systems and actually help reduce the growth of H₂S producing bacteria. With the EPA requirement to pre-treat plant effluent the toxin levels in the sewer decreased causing an increased growth of the destructive bacteria and an accelerated deterioration of the concrete sewers (Fig. 1 and Fig. 2). These unprotected pipes exposed to an increasingly aggressive acid environment have failed within 5 years.

• Solutions: The city of Los Angeles has developed a number of engineering polices to combat this problem.

For all new concrete construction, protect the inside of manholes and the inside crown of pipes above the waterline with a sheet of acid-resistant PVC, mechanically anchored to the concrete.

For the repair of old concrete construction, restore the concrete surface and then protect it with an applied coating or lining. Current methods used to protect concrete include the application of roll-on plastic sheets which are cemented to the concrete. Spray-on coatings are not considered a long term solution

• Primary Settling Tanks

For existing concrete above the waterline, remove damaged concrete back to sound concrete and restore the surface with shotcrete. Some bad experiences have been found with spray-on coatings , resulting from pinhole holidays and poor quality control (Fig. 3). Roll on plastic sheets have been used successfully, but the physical situation in sewers often prohibits their use.

For new construction above the waterline, provide 100 percent protection using PVC lining systems.

• Background: The Dworshak national fish hatchery is located at the junction of the Clearwater and North Fork Clearwater Rivers in Idaho. The facility was design to reuse water needing only a small amount of water need to supplement the flow and distribution system.

• Problem: Chemical attack of the concrete exposed to hatchery water has caused the removal of the Portland cement paste from structural surfaces. The Dworshak reservoir collects snow melt runoff and releases the pure water during the seasonal incubation and rearing phase of the hatchery production. The following table summarizes a typical chemical analysis of the Dworshak reservoir water.

• Solution: The cause was identified as a form of carbonic acid or aggressive CO₂ attack. Typical solutions include either a water treatment approach or coating of the concrete. Because of the possible effects on the fish population the option of water treatment was not considered. Coatings of epoxy mortar and a urethane coating were applied to 500 square feet of the damaged surface area for evaluation. After two years of exposure the repaired area seems to be performing well other than adjacent to cracks and joints (fig. 4)