REMOVAL OF DISSOLVED ORGANIC CARBON (DOC) DURING RIVERBANK FILTRATION

Abstract

Riverbank filtration (RBF) has been extensively in use, throughout the world to supply safe and wholesome drinking water, in places of fluctuating surface water quality. The benefits of RBF in attenuating the various contaminants have been extensively studied and reported. Although RBF has been in use in India since, a long time, the water quality improvements due to bank filtration has been studied and reported only since 2005. The studies from across the world has shown that attenuation capacity of any bank filtration capacity is highly site specific. Attenuation capacity of RBF with respect to organics has been gaining momentum, in the recent years due to the stringent regulations which would be coming into effect with respect to DBPs. The most practical and the cost effective solution of control of DBPs is to control the source water quality. In developing countries like India, removing DBPs after its formation is not feasible. Hence the emphasis has to be on treatment schemes that would prevent or reduce the DBP formation. RBF schemes help in attaining the above said benefit. Several studies have shown the effectiveness of bank filtration in removing organics and trace organic compounds and thereby reducing the cost of treatment (Amiri et al. 2004; Börnick et al. 2001; Grünheid et al. 2005; Heberer et al. 2004; Kumar et al. 2012; Maeng et al. 2011; Ray et al. 2003; Ray et al. 2002; Schoenheinz et al. 2003; Worch et al. 2002). However, the results of these studies cannot be directly applied to Indian conditions due to the highly site specific nature of bank filtration mechanisms as well as due to the difference in operating conditions. Furthermore, most of the studies involving organics removal during bank filtration elsewhere have concentrated on rivers where DOC is < 6 mg/L. However in India most of the rivers in the urban areas have DOC > 6 mg/L. This may cause significant redox changes in the aquifer which may result in anaerobic conditions. Anoxic biodegradation is much slower than oxic biodegradation. Hence the effect of higher organic contamination on effectiveness of RBF to remove organics has to be investigated. Moreover, in Europe and the United States field tests are conducted at temperatures ranging from 5-25 °C, where in India temperatures vary from 5-40 °C. The observed water temperatures at Mathura ranged from 10-35 C. Higher temperature occurring predominantly affects biodegradation, adsorption and oxygen concentration in river water and hence the removal mechanism. Furthermore, the possibility of high DOC resulting in nutrient limitation condition for DOC degradation in the aquifer also needs to be explored. Moreover the Indian practice of placing the well in the riverbed is unique (Kumar et al., 2012) and the effects of this practice on the attenuation benefits of bank filtration have not been studied. One of the major disadvantages of this practice is that the travel time is relatively short and accordingly only limited purification in terms of organics and microorganisms can be achieved. Moreover RBF is, relatively untested in monsoon climates, i.e. locations dominated by strong seasonal rains followed by a prolonged dry season. Keeping these factors in view, it was proposed to evaluate the removal of dissolved organic carbon (DOC) during riverbank filtration. DOC, UV absorbance at 254nm and colour are kept as the main monitoring parameters keeping in view the stringent water quality regulations which may come into effect with regard to disinfection by products such as THMs, HAAs and other chloro-organics.