Water Treatment "Part 10": The importance of the aeration stage during anaerobic groundwater treatment.

Under the impermeable layers, anaerobic groundwater is extracted. While this water contains dissolved organic materials, iron, manganese, hydrogen sulfate, methane, and ammonium, it lacks oxygen and nitrates. As a result, producing drinking water requires numerous stages of aeration and filtration.
Aeration attempts to improve the oxygen content while lowering the levels of carbon dioxide, methane, hydrogen sulfate, iron, manganese, and ammonium in this groundwater. According to the so-called ammonium oxidizing bacteria, NH4+ are first converted to nitrite during the filtration process, and then to nitrate.
Biological changes can take several weeks to complete in the so-called ripening period, and occur all at once. The removal of ammonium requires good biomass management, as its amount can be reduced if the backwash is repeated too much. While biomass can accumulate if backwashing does not occur, this accumulation leads to the growth of bacteria (such as Aeromonas).

Empty aeration tank for iron precipitation

3.55 mg/l of oxygen is consumed during the elimination of ammonium, while 3.44 mg/l of nitrates are generated. When the ammonium concentration is higher than 3 mg/L, the amount of oxygen needed to remove the ammonium exceeds the amount of oxygen that can be dissolved in the water as a whole. Depending on the water's temperature, the accessible oxygen concentration in immersed filters is restricted to between 9 and 12 mg/L. Dry filters should be used if the process requires a higher concentration of oxygen. In this case, in order to make the water saturated, oxygen is continuously added during filtration. Dry filters do not contain a layer of water, and at the same time that the air passes through it, the water flows through it passing through granules with a diameter of between 0.8 and 4 mm. The oxygen in the water is fed by the oxygen in the air and is finally utilized by the bacteria.

The aeration phases cause the pH to rise, the oxygen concentration to rise, and the carbon dioxide concentration to fall. In addition, due to chemical and biological transformations, the concentration of nitrates increases, and the concentrations of Mn2+, Fe2+, and NH4+ decrease.

References:

• Taparhudee, Wara (2002). "Applications of Paddle Wheel Aerators and Diffused-Air System in Closed Cycle Shrimp Farm System" (PDF). Witthayasan Kasetsart (Sakha Witthayasat). 36: 408–419. Retrieved 26 April 2020.
• Unsafe water kills more people than war, Ban says on World Day". UN News. 22 March 2010. Retrieved 10 May 2018
• Raymond Desjardins- Livre: Le traitement des eaux- 2éme edition- Ecole Polytechnique de Montréal- 1997- ISBN 2-553-00643-8
• Drinking Water Treatment- EDX- Delft University of Technology.
• Book- Drinking Water: Principles and Practices- by Hans J C Van Dijk (Author), Jasper Q J C Verberk (Author), Peter J De Moel.