Development of softening and ballasted flocculation as a pretreatment process for seawater desalination through a reverse osmosis membrane

Abstract

Efficient desalination through a reverse osmosis (RO) membrane requires the prior removal of blockade-causing substances from raw seawater. We achieve ultrahigh-speed processing by combining traditional softening with ballasted flocculation (SBF) for Ca2+ and Mg2+ removal. A mixture of Ca(OH)2 and Na2CO3 alkaline agents was most suitable for removing t Ca2+ and Mg2+ by softening and reducing the amount of generated sludge. In addition, the softening treatment simultaneously removed the suspended solids and bacteria from actual seawater. The settling velocity of the suspended solids generated via seawater softening was extremely low. In case of SBF under optimum conditions for desalinating actual seawater using an anionic polymer flocculant and microsand, the settling velocity exceeded 3.5 cm/s, 833 times higher than the settling velocity of softening without ballasted flocculation. The silt density index of the treated seawater met the water-supply standard of RO membranes (i.e., < 3.0). Furthermore, the dewatering property of the SBF-generated sludge was considerably improved compared with that of the sludge obtained via conventional softening. SBF can efficiently and quickly remove the causative substances of RO membrane fouling from seawater, thereby improving the treatability of generated sludge. SBF provides a new pretreatment process for seawater desalination using RO membranes.