OCEAN OUTFALL REBALLAST PROJECT, WHEN PROJECTS EXTEND TO THE DEEP UNKNOWN

The Orange County Sanitation District (OCSD) initiated a project for engineering services to evaluate, design, and construct various improvements on their 120-inch and 78-inch (3.0 m and 2.0 m) outfalls. The project locations are identified in Figure 1. OCSD processes an average daily flow of approximately 243 mgd (920,000 m3/day). Although OCSD is beginning the development of water reuse programs, their outfall systems will remain as an integral part of their disposal program. In addition to providing safe and economic methods to dispose of salts, the outfall will still be needed during peak flow periods and for disposal of treated wastewater. At the present time, enhanced primary and secondary effluents are diverted to the outfall. Following future improvements to OCSD's treatment facilities, the outfall will carry effluent from the new secondary treatment systems.OCSD's primary outfall extends a distance of approximately 5.5 miles (8.9 km) offshore and diffuses treated wastewater into the ocean through a diffuser. The diffuser section is over a mile (1.6 km) long and includes over 500 2.96-inch (72-mm) diffuser ports. The ports diffuse the treated wastewater at a depth of 180 to 190 feet (55 m to 58 m) below the ocean surface. The plan and profile of the 120-inch (3.0 m) outfall is shown in Figure 2.OCSD's 78-inch (2 m) outfall was used as their primary outfall until the second outfall was placed in service in 1972. At that time it was placed in standby mode and is presently maintained to be used only for emergency disposal. To date no emergency has required its use. It extends approximately 1.5 miles (2.4 km) offshore and discharges at a depth of about 65 feet (20 m) below the surface. This paper will discuss the unique engineering concepts that were developed to accomplish both the design and construction challenges for this project. The 4.95-million construction portion of the OCSD Ocean Outfall Reballast project represents a first of its type and was completed on October 24, 2002.