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Adjusting the pH-value

Process description

The first step in the treatment of surface waters to drinking water is the removal of suspended solids and colloids (particles <1 μm in diameter) from the water. For this purpose, the surface water is subjected to so-called coagulation and flocculation with the aid of chemicals. The optimum pH for this treatment is often below pH 7.4. Raw waters of medium to high alkalinity therefore benefit from a pH adjustment with acids in order to reduce the consumption of expensive chemicals and the associated production of waste sludge. In addition, when coagulating with aluminum salts, a well-adjusted pH value helps to keep the residual aluminum content low. This also reduces the aluminum load of any downstream activated carbon filter.

Gas application

For pH adjustment before coagulation, CO2 is the acid of choice. Dissolved in water, CO2 reacts as a weak acid which is excellently suited for the precise adjustment of the pH value in water. The figure below shows the typical flat neutralisation curve with weakly acidic carbon dioxide compared to a strongly acidic mineral acid.

The flatter curve with CO2 shows that even in the vicinity of the neutral range, carbon dioxide addition only causes gradual, relatively small pH changes. As a result, hyper-acidification, such as occurs frequently with the use of mineral acids, is excluded. Elaborate control technology can also be dispensed with.

Advantages:

  • Safe and simple pH control; hyper-acidification is not possible
  • No handling of dangerous, aggressive acids
  • No corrosion of surrounding plant technology
  • No increase in salt content; relevant for the chemical corrosion properties of the drinking water 
  • No investment costs for acid storage
  • Low operating costs

Messer solution

For the rapid and complete dissolutionof CO2, Messer supplies diverse, widely tested technologies, varying from fine-bubble injection directly into the main water stream to complete dissolution in a side stream under hydraulically defined conditions. In each project, we specify and offer the injection and control technology. Additionally, we calculate your CO2 demand.