WATER TREATMENT CHEMISTRY BASIC INFORMATION AND TUTORIALS

The chemistry behind water treatment.

The treatment of water can be considered under two major categories: (1) treatment before use, and (2) treatment of contaminated water after it has passed through a municipal water system or industrial process.

In both cases, consideration must be given to potential contamination by pollutants and their removal from water to acceptable levels.

Several operations may be employed to treat water prior to use. Aeration is used to drive off odorous gases, such as H2S, and to oxidize soluble Fe2+and Mn2+ions to insoluble forms.

Lime is added to remove dissolved calcium (water hardness). Al2(SO4)3 forms a sticky precipitate of
Al(OH)3, which causes very fine particles to settle. Various filtration and settling processes areemployed to treat water.

Chlorine, Cl2, is added to kill bacteria. Formation of undesirable byproductsof water chlorination may be avoided by disinfection with chlorine dioxide, ClO2, orozone, O3.

Municipal wastewater may be subjected to primary, secondary, or advanced water treatment. Primary water treatment consists of settling and skimming operations that remove grit, grease, and physical objects from water.

Secondarywater treatment is designed to take out biochemical oxygen demand (BOD). This is normally accomplished by introducing air and microorganisms such that waste biomass in the water, {CH2O}, is removed by aerobic respiration of microorganisms acting on degradable biomass:

{CH2O} + O2 → CO2+ H2O (aerobic respiration).