The State Water Resources Control Board has released a proposed amendment for a 45-day public comment period that would amend the Water Quality Control Plan for Ocean Waters (Ocean Plan) to address desalination facilities.
Desalination is one element in California’s diverse water supply portfolio, and can be a reliable source of water at the regional or local level. Several coastal communities are considering desalination as a way to develop additional reliable supplies and serve as a buffer in times of drought. The process involves removing the dissolved salts and minerals from seawater to produce water that is fit for human consumption. In order to produce this water, desalination facilities take in large amounts of seawater; the output of the plant is both potable water and a salty brine that is then discharged to ocean waters. However, how the facilities draws the water in and how it discharges the brine can have significant impacts on marine life as well as ocean water quality.
Currently, there are ten small existing desalination facilities in operation on the coast of California with a combined production capacity of 6.1 million gallons per day (MGD), many of them only operating intermittently when existing water supplies need to be supplemented. These facilities are regulated on a project-specific basis by the regional boards. However, with fifteen desalination facilities currently under consideration with a combined production capacity of 250 to 370 MGD, updating the Ocean Plan to provide statewide consistency to the permitting process and to ensure that impacts of facilities are fully addressed has been identified as a priority for the State Water Board.
The proposed amendment addresses the following:
The type and design of the intake structures used for the facility can significantly impact marine life. Surface intakes draw water directly from a water body, potentially drawing in aquatic organisms into the facility’s water processing system. Even though intakes can be screened, larger organisms can become trapped on the screens, while other aquatic organisms are very small and can pass through screens with smaller slot sizes. Subsurface intakes, such as beach wells and infiltration galleries, do not have the impacts on marine life as they collect water through sand sediments which acts as a natural barrier to organisms. However, subsurface intakes may not be feasible in all areas due to the local geological conditions.
While surface water intakes are less expensive to build, they have higher operational costs than subsurface intakes, and water drawn from surface water intakes must undergo pretreatment to remove biological materials and suspended solids whereas the natural filtering provided by subsurface intakes eliminate the need for such pretreatment.
The proposed amendment would establish subsurface intakes as the preferred technology for seawater intakes for any new or expanded facilities, although surface water intakes would be allowed if subsurface intakes are shown to be infeasible.
Brine is a waste by-product of the desalination process and is typically discharged back into the waterbody. Brine is saltier, denser, and heavier than the surrounding ocean waters, and if improperly discharged, it can form a dense plume that accumulates on the seafloor, significantly impacting marine life.
When discharging brine, it’s important that the brine diluted as quickly and as close to the point of discharge as possible. Passive discharge of brine is discouraged because of how slowly it mixes in the receiving waters, if at all. There are several different methods for discharging brine, such as commingling brine with an existing wastewater stream, discharging brine through multi-port diffusers, or diluting brine through flow augmentation.
Desalination plants would be required to use the most protective brine discharge method after a facility-specific evaluation, with guidelines for brine discharges set at a limit of 2 ppt above the natural background salinity at 100 meters from the point of discharge. There is an option to allow facility-specific receiving water limits for salinity on a case-by-case basis if the facility owner or operator is willing to undergo the additional studies.
Siting of new facilities
The proposed amendment would provide a consistent framework for determining the best sites for desalination facilities by establishing specific limits and factors that must be assessed, such as the best location for intakes, the feasibility of subsurface intakes, and the availability of wastewater for mixing. The proposal would not limit facilities to a specific site or prohibit co-location with a power plant.
The proposed amendment would address mitigation calculations and options for new and expanded facilities, eliminating current inconsistencies and ensuring that impacts from desalination facilities are fully measured and mitigated.
In developing the amendment, the State Water Board seeks to balance the water supply and reliability benefits of desalination with the need to mitigate the potential impacts of the facilities. While there are other impacts related to desalination facilities, such as energy use, infrastructure considerations, and community concerns, those are outside the purview of the State Water Board and therefore not part of the amendment.
The amendment has been under development since 2007, with expert review panels commissioned to study the impacts and effects of brine discharges and salinity toxicity, the impacts of different intake structures and how to determine mitigation. You can find the reports of the expert review panels, the proposed amendment, the Supplemental Environmental Document, and more information by clicking here.
The proposed amendment is being circulated for public comment with a public workshop on August 6, and a public hearing on August 19 that coincides with the close of the public comment period. The proposed amendment could be considered for adoption by the State Water Board as early as fall of 2014.