Pre-dilution of desalination reject brine: Impact on outfall dilution in different water depths

Abstract

High discharge salinity of reject brine from desalination plants necessitates the use of submerged jets which induce mixing with ambient water and lead to reduction in concentrations of salt and other contaminants. Pre-dilution, in which brine is blended with a lighter effluent prior to discharge, can also cause reduction in contaminant concentrations. Condenser cooling water from a co-located power plant, treated wastewater effluent from a treatment plant and seawater can be used for pre-dilution. The effect of pre-dilution on shallowness and contaminant concentrations in the diluted effluent is examined for discharge using single port and multiport outfalls. The density difference between the effluent and ambient seawater is lower for the discharge of pre-diluted brine, which leads to shallow conditions. Pre-dilution also results in lower concentrations of contaminants in the diluted effluent by reducing the discharge concentrations as well as increasing outfall dilution in deep water by increasing the densimetric Froude number. This helps in satisfying regulatory requirements on contaminant concentrations with small discharge velocity or using outfalls with small number of ports. While discharge of pre-diluted brine using an existing outfall helps save on the capital cost, it can cause a significant increase in pumping cost. For a new outfall, design parameters can be adjusted such that pumping cost stays constant.