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. To achieve ultrahigh-speed processing of a pretreatment process for seawater RO desalination, we combine traditional softening with ballasted flocculation (SBF) for Ca2+ and Mg2+ removal. An alkaline mixture of Ca(OH)2 and Na2CO3 was the most suitable softening agent for Ca2+ and Mg2+ removal with a reduced amount of generated sludge. This 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. Under the 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 that of softening without ballasted flocculation. The amount of sludge after standing for 3 min was 76.5% lower in SBF than in conventional softening. The silt density index of the treated seawater met the water-supply standard of RO membranes (i.e., <3.0). Furthermore, the SBF-generated sludge exhibited considerably improved dewatering property than 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.

Three-Dimensional Numerical Analysis and Operational Optimization of High-Efficiency Sedimentation Tank

“…As can be seen from Table 6, the high-efficiency settling tank shows great improvement in removal efficiency of hardness and a significant reduction in doses of flocculant, especially for the high-efficiency settling tank employing magnetite as the ballast material. The enhanced performances are associated with the recirculation of densified sludge and high settling velocity of the flocs in the ballasted high-efficiency settling tank [31]. The ballasted high-efficiency settling tank can reduce 73.3% of land footprint compared with the conventional clarifier for the treatment of the same amount of wastewater.…”