Modeling and Simulation studies for Aluminium - Fluoride Interactions in Nalgonda Defluoridation Process

Abstract

A model simulator NALD-2 has been developed to study the interactions of fluoride and aluminium in the Nalgonda Defluoridation Process, which principally involves the preferential adsorption of fluoride ions onto insoluble aluminium hydroxides that are formed from alum hydrolysis reactions and undergo precipitation. This model represents the defluoridation mechanism taking into account the charged behavior of the amphoteric aluminium hydroxide colloids, charged site densities, and fluoride complexation reactions in order to predict the extent of defluoridation and concentrations of dissolved and colloidal aluminium in the treated water as functions of pH, alum dosage and raw fluoride concentrations. Model validations were carried out using secondary data of Selvapathy & Arjunan (1995) for total residual aluminium in Nalgonda treated water. Slight variations in pH into the acidic range cause substantial increase in colloidal aluminium and hence pH and alum dosages are important control parameters to limit residual aluminium in treated water. The NALD-2 simulator helps in predicting optimum alum dosages for minimum residual aluminium in the treated water. It was concluded that the dosages of alum recommended in the Nalgonda method cannot bring down the residual Al below the permissible limit of 0.2mg/L in the treated water.