The modern water treatment plan is often called upon to perform wonders with the water fed to it.  The clear, safe , even tasteful water that comes from a faucet may have started as a murky liquid pumped from a polluted river laden with mud and swarming  with bacteria. Or, its source may have been well water, much   too hard for domestic use and containing high levels of stain-producing dissolved iron and manganese. The water treatment plant operator's job is to make sure that the water plant product presents no hazards to the consumer.

A schematic diagram of a typical municipal water treatment plants is shown in fig. 2.1. This particular facility treats water containing excessive hardness and a high level of iron. The raw water taken from wells first goes to an aerator. Contact of water with air removes volatile solutes such as hydrogen sulfide, carbon di-oxide, methane, and volatile odorous substances such as methane thiol (CH₃SH) and bacterial metabolites. Contact with oxygen also aids iron removal by oxidizing soluble iron (II) to insoluble iron (III). The addition of limes as CaO or Ca(OH)₂ after aeration raises the pH and results in the formation of precipitates containing the hardness ions Ca⁺² and Mg⁺². These precipitates settle from the water in a primary basin. Much of the solid material remains in suspension and requires the addition of coagulants (such as iron III and aluminum sulphates) to settle the colloidal particles. The setting occurs in a secondary basin after the addition of CO₂ to lower the pH. Sludge from both the primary and secondary basins is pumped to a sludge lagoon. The water is finally chlorinated, filtered and pumped to the city water mains.