Design of Seawater Desalination Brine Diffusers

Seawater desalination is growing in California with many plants currently underway and under design. Reverse Osmosis (RO), the most common desalination process, generates brine waste whose salinity is about twice that of the intake seawater. This level of salinity would be toxic to many marine organisms, so it must be quickly diluted to safe levels. Until recently, the California Ocean Plan (Plan) did not specifically address desalination discharges. This changed in a 2015 amendment to the Plan in which new regulations were adopted. These include the definition of a brine mixing zone (BMZ), beyond which salinity limits must not be exceeded, and assessment of mortality of marine organisms due to the shear and turbulence resulting from the discharge. Prediction of the entrainment of marine organisms into the diffuser jets and the effect of turbulence on them is difficult. To unify the process, the California State Water Resources Control Board (SWRCB) recently commissioned two studies for diffuser design and prediction of entrainment mortality. In addition to this hydrodynamic effect, recent field studies in Sydney, Australia, suggest that the dynamics of the dense bottom spreading layer can also result in a significant environmental impact. These hydrodynamic effects suggest that they should be accommodated in environmental regulations in addition to the usual salinity limitations.

In this paper we review and summarize the changes in the Plan and the diffuser design studies. We give example applications to brine diffuser design and show the implications of different designs for the size of the BMZ and shear mortality using the recommended California design procedures. We also discuss the more recent field studies and implications of hydrodynamic impacts on diffuser design and regulations.