Environmental Engineering Reference
In-Depth Information
Water Treatment of organic and Inorganic Contaminants
Manmade compounds that contain carbon—synthetic organic chemicals
(SOCs)—are, from time to time, detected in U.S. water supplies. Some of
these are volatile organic compounds (VOCs), such as the solvent trichloro-
ethylene. The problem with VOCs in a water supply (i.e., any water supply
used by the public) is twofold. They are easily absorbed through the skin and
they volatize into gases that can then be inhaled by those taking a shower
or a bath or while washing dishes. How do water supplies become contami-
nated by organic compounds? Good question. Basically, sources of organic
contaminants are usually improperly disposed wastes, pesticides, industrial
effluents, and leaking fuel oil tanks (gasoline in particular).
Water supplies may also contain inorganic contaminants consisting mainly
of substances occurring naturally in the ground, such as sulfate, fluoride, arse-
nic, barium, radium, selenium, and radon. Metallic substances from industrial
sources can contaminate surface waters. The inorganic ion nitrate (from fertil-
izers and feedlot runoff in agricultural areas) occurs frequently in groundwa-
ter supplies. Another source of inorganic chemical contamination in drinking
water supplies is corrosion or deterioration of water supply equipment, such
as plumbing systems, which release metal and nonmetal substances into the
water, including lead, cadmium, zinc, copper, iron, and plumbing cement.
Inorganic contaminants can be treated by corrosion controls and removal
techniques. Corrosion controls reduce the presence of corrosion byproducts
(e.g., lead) at the consumer's tap. Removal technologies, coagulation and filtra-
tion, reverse osmosis, and ion exchange are used to treat source water that is
contaminated with metals or radioactive substances. The following sections
discuss processes for removing inorganic and organic dissolved solids from
water intended for potable use. Keep in mind that (with some modifications)
these same processes may act as tertiary treatment for wastewater.
Aeration
Aeration (
air stripping
) is a physical treatment process in which air is thor-
oughly mixed with water—a technique effective for removing dissolved gases
and highly volatile odorous compounds. Contact with air and oxygen can
improve water quality in a number of ways. When aeration is a first step in
processing well water, for example, it may achieve any or all of the following:
removal of hydrogen sulfide, reduction of dissolved carbon dioxide, and addi-
tion of dissolved oxygen for oxidation of iron and manganese (the oxygen in
the air reacts with the iron and manganese to form an insoluble precipitate—
rust). One of the most common uses of aeration is for taste and odor control.
Sedimentation and filtration are then necessary to clarify the water.
