Desalination traditionally requires a “deep water outfall.” The purpose is to dilute the brine discharged to avoid harming the ecosystem. The 2.4 million gallons of recycled water Marin dumps into the bay daily could be used to dilute the brine to an acceptable limit. But other viable options are available for Marin.
The mixing of fresh water and salt water (brackish water) in Suisun Bay and the Delta provides essential estuarine habitat for migratory fish like salmon and resident fishes like delta smelt and striped bass. There is already a large variation in salinity. A well-planned discharge array would not significantly disturb the ecosystem and would indeed create a new ecosystem that would be quickly inhabited.
Nearly 400 billion gallons of water flow through the Golden Gate every day (https://sfenvironment.org/article/hydro/tidal-energy#:~:text=Nearly%20400%20billion%20gallons%20of,in%20the%20lower%2048%20states) In 2005, San Francisco took part in a study conducted by the Electric Power Research Institute (EPRI) on the technical and economic feasibility of tidal power in seven areas in North America, including San Francisco.
The study conservatively estimated that the Golden Gate site has 35.5 megawatts of total extractable average annual power, and that 15 to 17 average megawatts (MW) of this power could realistically be extracted by technologies currently in development. The cost of electricity generated, assuming incentives similar to those provided to other renewable resources, is estimated at six to nine cents per kilowatt-hour (kWh)—a cost competitive with current wind and natural gas generation, and about one-third the cost per megawatt of solar power.
A desalination discharge array at Point Bonita or Point Diablo would have a huge volume of water available for dilution of brine from the daily tidal flows. The tide could also be used to power the plant, in addition to solar and/or wind.
Instead of a 6-mile pipeline across the Richmond-San Rafael Bridge, a pipeline could be run from a sewage treatment plant to a desalination plant to provide water for dilution of brine prior to discharge into the bay.
The entire Carlsbad Desalination Plant project cost about $1 billion for the plant, pipelines, and upgrades to existing SDCWA facilities to use the water. The fresh water output from the plant is sent by a 10-mile pipeline. It can produce up to a maximum of 56,000 acre-feet per year. This is equivalent to 43 million gallons per day (mgd), or about 86% of the plant's output. Energy is a large part of the cost of producing the water so inexpensive, non-polluting sources should be fully utilized.
https://en.wikipedia.org/wiki/Claude_%22Bud%22_Lewis_Carlsbad_Desalination_Plant
