Gastric acid
The main constituent of gastric acid is hydrochloric acid which is produced by parietal cells (also called oxyntic cells) in the gastric glands in the stomach. Its secretion is a complex and relatively energetically expensive process. Parietal cells contain an extensive secretory network (called canaliculi) from which the hydrochloric acid is secreted into the lumen of the stomach. The pH of gastric acid is 1.5 to 3.5 in the human stomach lumen, the acidity being maintained by the proton pump H/K ATPase. The parietal cell releases bicarbonate into the bloodstream in the process, which causes a temporary rise of pH in the blood, known as an alkaline tide.
Hypolimnetic aeration
For the pH of drinking water, the threshold value is 6.5 - 9.5. pH values outside the neutral range (pH 6.5 - 7.5) are principally critical, since they provide an indication of the corrosion behaviour of water. Slightly acidic water (pH 4 - 6.5) usually corrodes galvanized iron pipes, but also copper and asbestos cement pipes. This process is known as acid corrosion. Practical experience has shown that the use of unprotected steel pipes is only possible at neutral pH values. By lower pH values, the removal of the pure zinc layer is promoted. As a result of the dissolved salts and gases, natural cold waters usually show a slightly alkaline reaction. These properties are created by setting the equilibrium concentrations of dissolved carbon dioxide in the form of bicarbonate ions and carbonate ions. Higher alkaline pH values (pH 9 - 14) in the presence of oxygen as an oxidant lead to so-called oxygen corrosion. To avoid the described acid or oxygen corrosion, buffer solutions are added to the raw water for drinking water production. By the pH-stabilizing effect of hypolimnetic aeration, the application of these buffer solutions can be reduced, thus reducing operating costs.