Author(s): Philip Roberts, Justin Taplin, Eric Zigas
Linked Author(s): Philip Roberts
Keywords: Seawater desalination; Diffusers; Brine disposal; Dilution; Mixing zones;
Abstract: Reverse Osmosis (RO), the most common seawater 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 did not specifically address desalination discharges. This changed in a 2015 amendment in which new regulations were adopted. These include the definition of a brine mixing zone (BMZ) and assessment of mortality of marine organisms due to the shear and turbulence resulting from the discharge. Predicting 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) 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 environmental impacts. In this paper we review and summarize the changes in the Ocean Plan. We give example applications to diffuser design and show the implications of designing to meet salinity requirement either at the jet impact point or at the end of the near field for the size of the BMZ and shear mortality using the recommended California design procedures. The velocity of the bottom spreading layer can be considerable and may need to be considered in design. We discuss the implications of hydrodynamic impacts on diffuser design.