Environmental Engineering Reference
In-Depth Information
In surface water supplies, most turbidity results from the erosion of very
small colloidal material, including rock fragments, silt, clay, and metal oxides
from the soil. Microorganisms and vegetable material may also contribute
to turbidity. Wastewaters from industry and households usually contain a
wide variety of turbidity-producing material. Detergents, soaps, and various
emulsifying agents contribute to turbidity.
Turbidity measurements are normally made on “clean” waters as opposed
to wastewaters. In water treatment, turbidity is useful in defining drinking
water quality and is relatively easy to measure. Given that the total coliform
test is a very reliable routine test of drinking water quality, but not an actual
determination of pathogens in water, its use in combination with a turbidity
reading and their joint evaluation can provide an extra safety factor for judg-
ing water quality changes either at the source or during distribution system
sampling (De Zuane, 1997).
Note:
In the preliminary evaluation of raw water, when turbidity at
the source of supply is under 10 units, generally only disinfection is
required—with biochemical oxygen demand (BOD) at less than 1.0, coli-
form under 50 MPN/100 mL monthly average, and acceptable chemical
parameters. When turbidity at the source exceeds 40 units, conventional
treatment is considered necessary.
Turbidity is regulated by the 1996 Safe Drinking Water Act amendments
and Interim Enhanced Surface Water Treatment (IESWT) treatment optimi-
zation rules, which apply to large public water systems (those serving more
than 10,000 people) that use surface water or groundwater directly influ-
enced by surface water. The rules require continuous turbidity monitoring
of individual filters and tighten allowable turbidity limits for combined fil-
ter effluent, cutting the maximum from 5 NTU to 1 NTU and the average
monthly limit from 0.5 NTU (for conventional or direct filtration) to 0.3 NTU
in at least 95% of the daily samples for any two consecutive months.
Solids
All water contaminants other than gases contribute to the solids content. Solids
can be dispersed in water in both suspended and dissolved forms. Some dis-
solved solids may be perceived by the physical senses but fall more appropri-
ately under the category of chemical parameters (see
Chapter 8
). Classified by
size and state, by chemical characteristics, and by size distribution, solids in
drinking water may consist of inorganic particles (salts) with small concen-
trations of inorganic matter or of immiscible liquids. Contributory ions are
