Defluoridation Of Drinking Water Using A Combined Alum-Activated Alumina Process And Nanoscale Adsorbents

Abstract

Excess Fluoride in drinking water is a cause for concern in several countries in the world. Various techniques have been developed to mitigate the harmful effects of fluoride. In the present work, a combined alum and activated alumina (AA) process has been investigated. Tap water with sodium fluoride dissolved in it to produce a solution having a fluoride concentration of 5 mg/L was used as the feed. It was found that pretreatment with alum extends the time required for the regeneration of the AA column. The volume of water treated increased by 89% compared to AA process alone. Though the regeneration of the AA column has been well documented, subtle issues have ot been reported. The disposal of regeneration effluent is a concern in adsorption-based processes. This study aims to examine some of the issues involved in the regeneration of the AA column such as disposal of effluent, and the quantity of acid and alkali required. The regeneration effluent from the combined process, which had a fluoride concentration of 10-16 mg/L was treated in a solar still. The distillate from the still had a fluoride concentration of 2-3 mg/L, which is much lower than the concentration of the regeneration effluent. The cost of treatment decreased with each regeneration cycle and after four regenerations the cost was Rs 0.5/L of treated water. The volume of water treated after four regenerations was 307 L/Kg of AA. Studies were also done using field water from Banavara, Hassan district, Karnataka, which had a fluoride concentration of 3,0-3.5 mg/L. The combined process successfully produced treated water having an acceptable fluoride concentration. After one regeneration cycle, the operating cost was Rs. 1/L of treated water. Studies have also been conducted on a point-of-use water filter containing a bed of AA pellets. The filter was provided by an organization called TIDE. The present results appear to suggest that a column with a smaller diameter than the TIDE filter has a better removal capacity. Ceramic candles are widely used for water filtration as they are readily available and inexpensive. Hence they are suitable for household water treatment purposes. In the present work, ceramic candles have been impregnated with nano-size alumina and nano-size magnesium oxide and tested for their defluoridation capacity. The nanoparticles were generated in situ in the pores of the candle by solution combustion synthesis. It has been found the candle impregnated with nano-size magnesium oxide has a higher defluoridation capacity than nano-size alumina. Estimation of the particle size in the samples of treated water did not give conclusive evidence for the presence or absence of nanoparticles. The volume of water treated was low and the cost of treatment was high (Rs. 12/L for the candle impregnated with MgO). Hence such candles are unsuitable for defluoridation. Batch adsorption has been employed to measure the adsorption capacity of adsorbents. A model to capture the overall picture of the batch adsorption process, obtaining the kinetic and transport parameters involved has been developed. The mathematical model takes into account external mass transfer resistance, intraparticle diffusion, adsorption, and desorption. The equilibrium adsorption data was fitted using the Langmuir isotherm. The governing equations were solved using a finite difference technique known as the Laasonen method. The parameters were estimated by fitting two sets of data using a MATLAB function. The values estimated suggest that the adsorption process may not be diffusion-limited, in contrast to the assumption commonly used in the literature. The estimated parameter values were used to predict the concentration profiles for the other data sets. It was found that predicted and measured profiles agreed reasonably well.