DEFLUORIDATION OF DENITRIFIED WATER BY ALUM - CHARCOAL SLURRY ALONG WITH. ACID NEUTRALIZATION

Abstract

Nitrate and fluoride concentrations in drinking water are considered important due to their adverse health effects. Nitrates cause "Methaemoglobinaemia" in infants whereas fluorides cause dental caries or fluorosis, if not present in limits. In some areas, samples of ground water have been found to contain both nitrate and fluoride in unacceptable concentrations. Mekonen (2000) • suggested an integrated approach incorporating biological denitrification and physicochemical defluoridation. It was suggested that denitrification should be followed by defluoridation. However, during denitrification, alkalinity is produced. At higher nitrate concentrations, alkalinity produced adversely affects defluoridation. To overcome this problem in the present investigation, an effort was made to study the possibility of neutralization of denitrified water. Biological denitrification- w-as -studied using a sequencing batch reactor (SBR) while defluoridation by alum-charcoal slurry was studied using jar test apparatus. The parameters evaluated during denitrification were pH, alkalinity and residual nitrate concentrations while during defluoridation pH, residual fluoride and sulphate concentrations were measured. In the present study, during denitrification, reduction of nitrate-nitrogen was found to be in excess of 85%. An increase in alkalinity from 460 to 860 mg/L as CaCO3 from initial alkalinity of 280 mg/L and pH from 7.8 to 8.7 from initial pH of 7.4 were also observed. The use of acid neutralization of denitrified water before defluoridation resulted in an improvement in fluoride removal only in case of water samples of initial fluoride concentrations of 6 and 10 mg/L. The optimum pH range for defluoridation was found to vary from 6.7 to 7.1 when alum-charcoal slurry alone was used and from 5.9 to 6:3 when aluminium chloride alongwith alum-charcoal was used. Also, with the use of alum upto 900 mg/L, residual sulphate concentrations were found to be within permissible limits.