Study of seawater alkalization as a promising RO pretreatment method

Abstract

Inorganic fouling is a major challenge in seawater RO desalination due to its relatively high SO 4/HCO 3 molar ratios. In most of the recorded cases, if not all, calcium sulfate scale is dominating (>90% wt/ wt) other scale types such as CaCO 3, MgSO 4, MgCO 3, MgSiO 3, SiO 2 and CaPO 4. However, the standard pretreatment practice in RO plants depends on the acidification of seawater in order to remove the minor carbonate and bicarbonate. Meanwhile, solubility of the major sulfate species decreases considerably because of acidification. The present study investigated the application of NaOH alkalization on the Red Sea surface water instead. It was found that the method has many technical and economical advantages, such as removal of hard chemical species (i.e. hard carbonate, bicarbonate, sulfate, silicate, phosphate and iron), removal of suspended solids and colloids, bacterial disinfection, cleaning and wetting of a membrane surface, increasing permeate recovery, lower sludge volume of environmentally chemical nature, and the possibility of instant preparation of NaOH in-line by seawater electrolysis. The alkalization method is fast, clean and economic. The proposed model as well as chemical equations are presented in detail.