Process modeling for estimation of colloidal aluminium in alum treated water during defluoridation

Fluoride contamination is present in most groundwater resources and aluminium (Al) based techniques are mostly adopted for defluoridation of water because of the high affinity between aluminium and fluorides. The presence of residual aluminium in treated drinking water has become a cause of worldwide concern and aluminium based coagulants have come under scrutiny with findings of neuro-toxicity when aluminium is ingested in different forms. This paper presents the theoretical model NALD-2 developed to study for interactions of fluoride and aluminium in the alum based Nalgonda Defluoridation Process and that can predict for extent of defluoridation as well as concentrations of dissolved and colloidal aluminium in the treated water with respect to changes in pH, alum dosage and initial fluoride concentrations. This model represents the defluoridation mechanism taking into account the charged behavior of the amphoteric aluminium hydroxide colloids, available charged site densities and fluoride complexation reactions. Colloidal aluminium concentration is a function of alum dosage and pH of the water. The NALD-2 model helps in predicting alum dosages for minimum residual aluminium in alum treated water, as the fluoride removal is dependent on the available charged sites on the aluminium hydroxide particles.