Seawater Quality vs. Product ...

Seawater Quality vs. Product Water Quality

The quality of the produced water will depend greatly on three factors: the type of desalination technology applied for salt separation; and if reverse osmosis is used, the salinity of the source water and the number of times the seawater is treated in sequence.  Thermal evaporation technologies such as multistage flash distillation (MSF), vacuum compression and multieffect distillation (MED) separate the water from the salts by evaporating it at boiling temperature and pressure.  Applying these technologies allows to produce very similar water quality irrelevant of thermal technology used and the actual salinity of the source seawater.

In seawater reverse osmosis (SWRO) desalination plants, salt is separated from water by applying pressure, which is proportional to the salinity of the source water.  Since the RO membranes are designed to provide certain near-constant salt removal efficiency, the higher the source water salinity the higher the product water salinity.  As a result, the desalinated produced by a one-time treatment of seawater is drinkable but it is of higher salinity (overall mineral content) than the water produced by thermal desalination.  In order to produce the same water quality as thermal desalination plants, SWRO plants need to have two sets of RO membranes in sequence.  The first set of SWRO elements produces water with salinity in a range of 150 to 350 mg/L and the second set further reduces salinity down to 10 to 50 mg/l.    The higher the source water salinity the closer the produced water will be to the upper level of the ranges listed above.  The Arabian (Persian) Gulf has the highest seawater salinity - around 42 to 46,000 mg/L and will produce water in the upper end of the water quality bracket.  Bay waters or the Pacific Ocean have salinity of 30 to 35,000 mg/L and will yield water in the lower end of the product water quality bracket.