Mineral anions are among the most important toxic substances harmful to humans and animals even in low concentrations. Nitrate ions are considered important surface and groundwater pollutants for their high solubility in water. Nitrate is converted to nitrite and N-nitroso in the human body through certain reactions. N-nitroso combinations are considered carcinogenic and the primary cause of methemoglobinemia diseases. The existing nitrate in water resources is not easily separated for it is highly soluble and therefore the methods usually used for this purpose are very costly. Various methods can be used to remove or reduce the concentration of nitrate. Reverse osmosis, electrodialysis, electrocoagulation, ion exchange, and membrane processes are among the physicochemical methods of nitrate purification. Among the types of nitrate purification methods, biological processes with relatively high efficiency, the possibility of complete removal of the pollutant, and less harmful effects on the environment, are some of the most suitable options for the decomposition and removal of nitrate from water and groundwater. In a situation where the contaminated area is wide and it is not possible to pump water due to economic reasons or the large volume, treatment using in-situ methods will be a more suitable option. One of the types of in-situ biological treatment processes is the use of the permeable reactive bio-barrier (PRBB) method. PRBB is one of the novel and reliable methods used for in-situ groundwater remediation.  A PRBB is an emplacement of reactive media in the sub-surface designed to intercept a contaminated plume, provide a flow path through the reactive media, and transform the contaminant(s) into environmentally acceptable forms to attain remediation concentration goals down the gradient of the barrier. PRBB can degrade nitrate at a high rate under anaerobic conditions. In this research, different concentrations of four chemical substances including ZVI, nZVI, NaS2O3, and Na2S2O5 were used as Oxygen Capturing Compounds (OCC) in the removal of nitrate from polluted water in the PRBB process on a laboratory scale. Based on the obtained results, these substances affect the parameters of DO, pH, MLSS, turbidity, nitrate concentration, durability of DO, and ORP. After laboratory tests and taking into the opinion of experts, the qualitative effect of each parameter was calculated quantitatively and through the analytical hierarchy process (AHP), the best oxygen-capturing compound was selected. The results obtained from AHP (with an inconsistency ratio of 0.063), revealed that the priority of choosing OCC from three technical, economic, and environmental aspects is related to the concentrations of 750 mg/l ZVI, 240 mg/l Na2S2O3, 85 mg/l Na2S2O5 and 550 mg/l nZVI respectively. The concentration of 750 mg/l ZVI (the best OCC) during the experiments was able to reduce the DO from 7 to 0.05 mg/l in approximately 500 minutes and within the ideal range of the denitrification process (DO<0.05 mg/l). In addition, after the durability of DO tests, it was found that this substance has maintained its deoxygenation properties for longer than other used compounds. Also, based on the results, the concentration of 750 mg/l ZVI had a positive effect on the increase of MLSS and removal of nitrate. 

Keywords: Analytical Hierarchy Process, Oxygen Capturing Compound, Nitrate, Permeable Reactive Bio Barrier

     

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