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There are a number of other processes that may be employed to treat water, depending on the quality of the source water and the desired quality of the treated water. Processes that may be used to treat either surface water or groundwater include:

  1. lime softening, which involves the addition of lime during rapid mixing to precipitate calcium and magnesium ions;

  2. stabilization, to prevent corrosion and scale formation, usually by adjusting the pH or alkalinity of the water or by adding scale inhibitors;

  3. activated carbon adsorption, to remove taste- and odor-causing chemicals or synthetic organic contaminants; and

  4. fluoridation, to increase the concentration of fluoride to the optimum level for the prevention of dental cavities.

Compared to surface waters, groundwaters are relatively free of turbidity and pathogenic microorganisms, but they are more likely to contain unacceptable levels of dissolved gases (carbon dioxide, methane, and hydrogen sulfide), hardness, iron and manganese, volatile organic compounds (VOCs) originating from chemical spills or improper waste disposal practices, and dissolved solids (salinity). High-quality groundwaters do not require filtration, but they are usually disinfected to protect against contamination of the water as it passes through the distribution system. Small systems are sometimes exempted from disinfection requirements if they are able to meet a set of strict criteria. Groundwaters withdrawn from shallow wells or along riverbanks may be deemed to be "under the influence of surface water," in which case they are normally required by law to be filtered and disinfected.

Hard groundwaters may be treated by lime softening, as are many hard surface waters, or by ion exchange softening, in which calcium and magnesium ions are exchanged for sodium ions as the water passes through a bed of ion-exchange resin. Groundwaters having high levels of dissolved gases or VOCs are commonly treated by air stripping, achieved by passing air over small droplets of water to allow the gases to leave the water and enter the air. Many groundwaters-approximately one quarter of those used for public water supply in the United States-are contaminated with naturally occurring iron and manganese, which tend to dissolve into groundwater in their chemically reduced forms in the absence of oxygen.

Iron and manganese are most commonly removed by oxidation (accomplished by aeration or by adding a chemical oxidant, such as chlorine or potassium permanganate) followed by sedimentation and filtration; by filtration through an adsorptive media; or by lime softening. Groundwaters high in dissolved solids may be treated using reverse osmosis, in which water is forced through a membrane under high pressure, leaving the salt behind. Membrane processes are rapidly evolving, and membranes suitable for removing hardness, dissolved organic matter, and turbidity from both ground and surface waters have recently been developed.

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