All about Water: Basic Concepts - The Drinking Water - page 21

Reference dose (RfD) —An estimate of the amount of a chemical that a
person can be exposed to on a daily basis that is not anticipated to
cause adverse systemic health effects over the person's lifetime.
Representative sample —A sample containing all the constituents present
in the water from which it was taken.
Reverse osmosis (RO)— Solutions of differing ion concentration are sepa-
rated by a semipermeable membrane. Typically, water flows from
the chamber with lesser ion concentration into the chamber with the
greater ion concentration, resulting in hydrostatic or osmotic pres-
sure. In RO, enough external pressure is applied to overcome this
hydrostatic pressure, thus reversing the flow of water. This results in
the water on the other side of the membrane becoming depleted in
ions and demineralized.
Safe Drinking Water Act (SDWA) —A federal law passed in 1974 with the
goal of establishing federal standards for drinking water quality,
protecting underground sources of water, and setting up a system
of state and federal cooperation to ensure compliance with the law.
Screening— A pretreatment method that uses coarse screens to remove
large debris from the water to prevent clogging of pipes or channels
to the treatment plant.
Secondary drinking water standards— Regulations developed under the
SDWA that established maximum levels of substances affecting the
aesthetic characteristics (taste, color, or odor) of drinking water.
Secondary treatment —The second step of treatment at a municipal
wastewater treatment plant. This step uses growing numbers of
microorganisms to digest organic matter and reduce the amount of
organic waste. Water leaving this process is chlorinated to destroy
any disease-causing microorganisms before its release.
Sedimentation —A physical treatment method that involves reducing the
velocity of water in basins so the suspended material can settle out
by g rav it y.
Solvated —When either a positive or a negative ion becomes completely
surrounded by polar solvent molecules.
Surface tension —The attractive forces exerted by the molecules below
the surface upon those at the surface, resulting in them crowding
together and forming a higher density.
Surface water —All water naturally open to the atmosphere; all springs,
wells, or other collectors that are directly influenced by surface water.
Surface Water Treatment Rule (SWTR) —A federal regulation established
by the USEPA under the SDWA that imposes specific monitoring
and treatment requirements on all public drinking water systems
that draw water from a surface water source.