DESIGN OF AN INTERMITTENTLY OPERATED OUTFALL

Abstract

The City of San Francisco, California, has a combined sewer system that presently overflows into the bay or ocean when it rains. Under the current wastewater management program being implemented, this combined flow of storm and sanitary wastes will be treated and discharged into the Pacific Ocean through a three-pipe outfall system. The system will dispose of the treated sanitary wastewater (dry weather flow) through a 2740-millimetre-(mm)-diameter, 6-kilometre (km) ocean outfall, and the mixture of sanitary wastewater and stormwater (wet weather flow) will be disposed of through two 2740-mm-diameter outfalls, 4 km long. The depth of the dry weather diffuser is approximately 24 m and the depth of the wet weather diffuser is approximately 17 m. The wet weather outfall will operate only when the rain exceeds 0.5 mm per hour. Rain exceeds this intensity about 4 percent of the time, or 350 hours per year. The peak wet weather flow is approximately 1800 megalitres per day (Ml/day)3. The average dry weather flow is 400 Ml/day and the peak dry weather flow is approximately 700 Ml/day. This paper reviews the unique problems of an intermittently operated outfall located in an area of shallow water with loose bottom sediments on a coast subjected to high wave energy. The circulation of seawater through the dormant diffuser and the resulting reduction in hydraulic capacity by sediment intrusion and biofouling are identified as the most severe problems for the intermittently operated outfall. The features incorporated to reduce these problems are bottom-exiting risers, diffuser ports elevated above the seabed, four ports per riser with a dual valve system, and use of antifouling materials and a flushing system.