Metal salts are used extensively in water treatment to remove suspended and dissolved organic matter from raw water sources. Industrial wastewater treatment systems like those found in pork, poultry and other food processing facilities also rely heavily on metal salts. This class of chemical is also becoming more common in biological wastewater treatment plants. In biological plants they often do double duty by aiding with settling as well as removing phosphorous.
There are several metal salts that are commonly seen:
Aluminum and ferric sulfate are typically used as primary coagulants in conventional surface water plants. The chloride versions have been known to increase corrosion issues in water distribution systems as well as in industrial treatment systems like dissolved air flotation (D.A.F.) units. As a result, sulfates are more common. Ferric sulfate has the advantage of removing more dissolved organic material. Plants who convert to ferric typically do so because of high TOC levels.
Polyaluminum chloride is becoming more common as a replacement for aluminum sulfate. It is also seeing more use in wastewater treatment plants for phosphorous removal. It uses less alkalinity than alum or ferric. This is important in wastewater treatment plants because alkalinity is lost during the nutrient removal process.
Sodium aluminate is not normally seen as a primary coagulant. However, it is popular for phosphorous removal in wastewater treatment plants. Sodium aluminate will actually increase the pH and this contribute to alkalinity.
These topics are covered more in-depth in the chemical aids for settling class. This continuing education class is currently 2 contact hours. However, an expanded version is in the works.