Simultaneous removal of nitrate and fluoride from groundwater using Donnan dialysis coupled with denitrification and defluoridation systems

ABSTRACT

Integrated Donnan dialysis-denitrification-defluoridation system for nitrate and fluoride removal with chloride recirculation and treated water recovery.

This study presents an integrated framework for the simultaneous remediation of nitrate (NO3) and fluoride (F−) from contaminated groundwater by coupling Donnan dialysis (DD) with fixed-bed biological denitrification and chemical defluoridation. Two commercial anion exchange membranes – FAD PET 75 and FAS PET 75 – were evaluated in DD. Batch study optimization based on the limiting pollutant (fluoride) identified optimal NaCl driving ion concentrations of 125 mM for FAD PET 75 and 150 mM for FAS PET 75, while a Reynolds number (Re) of 4000 provided the best ion transport with low energy consumption. In thetransformation stage, the fixed-bed bioreactor achieved near-complete denitrification (>99%) using glycerol (C/N ratio 1.8–2 w/w), while the alum-bentonite module yielded 93.2% fluoride removal. Under continuous operation, the integrated system achieved a steady-state removal efficiencies of 89.8% (NO3−) and 86.4% (F−) with FAD PET 75, while the FAS PET 75 configuration exhibited 92.7% (NO3−) and 84.1% (F−) removal consistently meeting the World Health Organization (WHO) drinking water standards. With a specific energy consumption (0.36–0.48 kWh/m3) and treatment cost of 0.7–0.8 USD/m3, the proposed hybrid approach offers a competitive advantage over conventional reverse osmosis and advanced electrodialysis by eliminating secondary liquid waste generation and post-treatment.