Electrocoagulation process considerations during advanced pretreatment for brackish inland surface water desalination: Nanofilter fouling control and permeate water quality

Pretreatment with coupled aluminum electrocoagulation (EC) – microfiltration (MF) successfully mitigated colloidal and organic fouling during nanofiltration (NF) of an inland natural brackish surface water. Natural organic matter (NOM) removal by EC was enhanced due to (i) greater degree of NOM protonation by maintaining a slightly acidic pH during electrolysis, compared with that of minimum aluminum solubility (pH of 5.5 compared to 6.2), (ii) providing sufficient contact time during electrolysis and flocculation by utilizing intermediate current density (10 mA/cm2), and (iii) combination of charge neutralization and sweep coagulation by increasing aluminum dosage. Direct evidence for removal of the hydrophobic fraction of NOM along with a portion of the hydrophilic moieties by EC-MF compared to MF-only was provided by attenuated total reflectance-Fourier transform infrared spectroscopy. NF flux profiles with EC-MF pretreatment of natural saline water were nearly identical to that of a model solution with similar ionic composition with no added NOM suggesting insignificant role of organic fouling. X-ray photoelectron spectroscopy showed trace amounts of CaCO3 precipitates after EC-MF pretreatment, confirmed by electron microscopy, that caused only minor flux decline. Finally, NF achieves excellent strontium removal as well as other divalent ions and NOM.