Groundwater denitrification
Loading...
Date
2024-10-30
Authors
Μπενέκος, Ανδρέας
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Consumption of nitrate-contaminated water has been reported to be hazardous for human health, contributing to the development of several diseases such as various forms of cancer and thyroid problems.Pregnant women and infants face a significantly higher danger. In the pregnancies case, relevant studies have revealed that nitrate consumption is associated with several birth defects and occasionally abortions, while under 12-month old babies are exposed to the disease called methemoglobinemia (also called as “blue baby syndrome”), due to the inhibiting effect of nitrates on the red blood cells’ function as oxygen carriers. Among the numerous processes being proposed as potential nitrate-removal technologies, electrochemical nitrate removal and autotrophic hydrogenotrophic denitrification were studied throughout this dissertation. Concerning the electrochemical nitrate removal, the electrocoagulation process was evaluated under various conditions. In the first set of experiments, the process was based around the usage of aluminum electrodes, studying the effect of several other parameters; initial pH, initial nitrate concentration and current density. After concluding how these parameters affect the efficiency of the process, the next step was to face the ammonium generation problem, which is the main by-product of the nitrate electro-reduction process. Ammonium cations are extremely toxic for human health (as well as other mammals) and consuming ammonium-contaminated water exposes organisms to serious dysfunctions in internal tissues, especially inside the liver and the lung. To handle this situation, an electrochemical oxidation process was applied to the electrocoagulation-treated solution, aiming to remove the extremely toxic to human health ammonium cations through their oxidation to other nitrogenous forms (either nitrates or nitrogen gas). The anode electrode was a titanium plate coated with IrO2 (Ti/IrO2), while the effects of cathode electrode, current density and anode electrode active surface on the ammonium removal efficiency were evaluated experimentally. As a developed stage of the hybrid electrochemical process being described above, a continuous flow treatment of real nitrate-polluted groundwater from Neo Vouprasio was established, using iron electrodes instead of aluminum. A continuous flow system was explored to completely eliminate both nitrates and EC-generated ammonium cations. This system included an electrocoagulation reactor, a settling tank, and an electrochemical oxidation reactor connected in series. The current densities applied in the electrocoagulation process were the same as those in the batch experiments, while the volumetric flow rates were set at 4, 6, and 8 mL/min. Nitrate removal efficiency during the electrocoagulation process ranged from 52.0 to 100%, and the ammonium concentration at the electrochemical oxidation outlet significantly decreased (53–100%), depending on the applied current density and volumetric flow rate. The concept of autotrophic hydrogenotrophic denitrification was also experimentally evaluated in this dissertation. Specifically, a continuous flow hydrogenotrophic denitrification system with attached growth was investigated for groundwater treatment. Two bench-scale packed-bed reactors were used in series without external pH adjustment or carbon source addition, relying solely on inorganic carbonate salts present in the groundwater as the carbon source for denitrifying bacteria. Hydrogen was produced by water electrolysis using renewable energy sources, minimizing resource consumption in the treatment process. The biofilter was tested with three groundwater retention times (13.5, 27, and 54 minutes, corresponding to 20, 10, and 5 mL/min inlet water flow rates) and two hydrogen flow rates (10 and 20 mL/min) to evaluate its efficiency under varying conditions. Significant nitrate removal percentages, ranging from 64.1% to 100%, were achieved in all cases. When nitrite accumulation was detected, a continuous flow electrochemical oxidation process was tested as a post-treatment step to remove toxic nitrite anions completely. Finally, an advanced mathematical model of the attached growth hydrogenotrophic denitrification process was developed to predict the concentrations of all substrates examined in the biofilter (nitrate, nitrite, inorganic carbon, and hydrogen).
Description
Keywords
Nitrate removal, Autotrophic hydrogenotrophic denitrification, Electrocoagulation, Electrochemical oxidation