Science of Water : Concepts and Applications

Water Chemistry 115
organic resins. These clear, BB-sized resins are sphere shaped and have the advantage of providing
a greater number of exchange sites. Each of these resin spheres contains sodium ions, which are
released into the water in exchange for calcium and magnesium. As long as exchange sites are available,
the reaction is virtually instantaneous and complete.
When all the exchange sites have been utilized, hardness begins to appear in the infl uent (breakthrough).
When breakthrough occurs, this necessitates the regeneration of the medium by bringing
it in contact with a concentrated solution of sodium chloride.
Ion exchange used in water softening has both advantages and disadvantages. One of its major
advantages is that it produces softer water than does chemical precipitation. Additionally, ion
exchange does not produce the large quantity of sludge encountered in the lime-soda process. One
disadvantage is that although it does not produce sludge, ion exchange does produce concentrated
brine. Moreover, the water must be free of turbidity and particulate matter or the resin might function
as a fi lter and become plugged.
SCALING AND CORROSION CONTROL
Controlling scale and corrosion is important in water systems. Carbonate and noncarbonate hardness
constituents in water cause scale. It forms a chalky-white deposit frequently found on teakettle
bottoms. When controlled, this scale can be benefi cial, forming a protective coating inside tanks
and pipelines. A problem arises when scale is not controlled. Excessive scaling reduces the capacity
of pipelines and the effi ciency of heat transfer in boilers.
Corrosion is the oxidation of unprotected metal surfaces. Of particular concern in water treatment
is the corrosion of iron and its alloys (i.e., the formation of rust). Several factors contribute
to the corrosion of iron and steel. Alkalinity, pH, DO, and carbon dioxide all can cause corrosion.
Along with the corrosion potential of these chemicals, their corrosive tendencies are signifi cantly
increased when water temperature and fl ow are increased.
DRINKING WATER PARAMETERS: CHEMICAL
Water, in any of its forms, also … [has] scant respect for the laws of chemistry.
Most materials act either as acids or bases, settling on either side of a natural reactive divided. Not
water. It is one of the few substances that can behave both as an acid and as a base, so that under certain
conditions it is capable of reacting chemically with itself. Or with anything else.
Molecules of water are off balance and hard to satisfy. They reach out to interfere with every other
molecule they meet, pushing its atoms apart, surrounding them, and putting them into solution. Water
is the ultimate solvent, wetting everything, setting other elements free from the rocks, making them
available for life. Nothing is safe. There isn’t a container strong enough to hold it
—L. Watson (1988)
Water chemical parameters are categorized into two basic groups: Inorganic and organic chemicals.
Both groups enter water from natural causes or pollution.
√ Note: The solvent capabilities of water are directly related to its chemical parameters.
In this section, we do not look at each organic/inorganic chemical individually. Instead, we
look at general chemical parameter categories such as dissolved oxygen organics (BOD and COD),
(DO), synthetic organic chemicals (SOCs), volatile organic chemicals (VOCs), total dissolved solids
(TDS), fl uorides, metals, and nutrients—the major chemical parameters of concern.
ORGANICS
Natural organics contain carbon and consist of biodegradable organic matter such as wastes from
biological material processing, human sewage, and animal feces. Microbes aerobically break down