Pure Water Branched Subaqueous Diffuser Pipeline

The City of San Diego Pure Water Program will treat 34 mgd of raw sewage and convey the water to the Miramar Reservoir for blending with surface water for direct potable reuse. The sewage is treated in secondary and advanced processes and conveyed 7.5 miles in 48-inch ID cement lined welded steel pipe to the reservoir. The last portion of the conveyance system is a HDPE subaqueous pipe laid on the bottom of the reservoir with depths ranging from 50 to 110 feet. The subaqueous conveyance systems consist of 0.63 miles of a 54-inch mainline with step down sizes of 48, 42, 36, and 30 inches. Eleven branches totaling 0.4 miles with pipe sizes ranging from 8 to 24 inches with duckbill type diffusers to distribute the PW within the reservoir. The reservoir contains invasive quagga mussels creating significant potential for plugging the diffusers and pipes. The last portion of the landside pipe is tunneled into the reservoir and must be protected from the intrusion of reservoir water. This paper discusses the reservoir modeling effort to ensure adequate dilution is achieved and focuses on the design aspect of the subaqueous pipeline to facilitate uniform flow and mixing. Construction sequence and procedures for connecting the land and subaqueous segments was elaborated as it a crucial part of this work.