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Safe and clean drinking water is need of the hour. It is estimated that around 37.7 million Indians are affected by waterborne diseases annually. The contamination of groundwater and drinking water in rural areas by different chemicals such as arsenic, nitrates, fluorides, heavy or toxic elements, pesticides and fertilizers is a potential hazard.
A recent initiative by The Energy and Resources Institute (TERI) aims to address this problem with the development of a biological process for removing nitrates from water at a low cost, which can be useful for purifying contaminated water. Nitrates are one of the most common groundwater contaminants. Nitrate-contaminated water leads to eutrophication of waterbodies and affects human and animal health. The source of nitrates could be industrial, such as wastewater from the metal finishing and fertilizer industries, agricultural (run-offs from fields) or geological (weathering of rocks).
Nitrate removal using advanced physicochemical methods (ion-exchange, reverse osmosis and electrodialysis) are expensive and the concentrated waste brines require additional treatment. Conventional treatment processes like coagulation, filtration and disinfection are inadequate for nitrate removal.
TERI, in its study, Nitrate Removal In Membrane Bioreactors Using Indigenous Membranes, funded by the Ministry of Drinking Water and Sanitation, has employed biological process of converting nitrate to nitrogen gas by denitrifying bacteria. The process requires an external carbon source. Since commercial pure compounds, such as methanol, ethanol, glucose or acetate, add to the treatment cost, the researchers have opted for complex carbon sources derived from organic waste as a low-cost alternative. “The physicochemical processes to remove nitrates from water include reverse osmosis (RO), ion exchange (IE), electrodialysis (ED) and activated carbon adsorption in addition to pH adjustment. Though effective, these methods have high operation costs. Proper disposal of the concentrated rejects or waste brine is another key concern,” says Dr Malini Balakrishnan, Senior Fellow, TERI.
In contrast, the biological denitrification process developed by TERI leads to a complete conversion of nitrates to nitrogen gas. The process involves facultative anaerobes (micro-organisms that can live and grow with or without molecular oxygen) in the absence of oxygen, converting nitrates to molecular nitrogen while utilizing the substrates contained in water or wastewater as an electron donor.
The study focused on developing an indigenous membrane bioreactor for nitrate removal. The microbial biomass (sludge) in the bioreactor was separated from the treated water using ceramic membrane filters developed in TERI using waste sugarcane bagasse ash as the starting material. The effect of operation parameters, such as sludge concentration, carbon/nitrogen ratio and different waste based carbon sources was evaluated. Preliminary studies were conducted in batch mode and were followed by continuous studies in different configurations of membrane bioreactor (MBRs).
The MBR performance was monitored in terms of flux (flow rate per unit membrane area), trans-membrane pressure build-up across the membrane and the quality of the treated water (tested as per standard methods). Trials were also conducted with real groundwater sourced from Baliawas village, Gual Pahari, Gurgaon.The feed nitrate (as NO3-N) content was reduced from 20.4 mg/l to 1.7 mg/l after treatment and was well within the prescribed limits (10 mg/l). The method has not been tested commercially.
The technique, if implemented, is expected to help the NDA government in keeping its promise of providing safe and clean drinking water to the masses.