Environmental Engineering Reference
In-Depth Information
1 billion viruses per
spoonful of unpolluted
lake water
Figure 3.2
Viruses in a teaspoon of unpolluted lake water. (Adapted from Ingram, C.,
The Drinking Water
Book: A Complete Guide to Safe Drinking Water
, 1991, Ten Speed Press, Berkeley, CA, p. 17.)
on testing water for evidence of any fecal contamination. Coliform bacteria
are used as indicator organisms whose presence suggests that the water is
contaminated. In testing for total coliforms, the number of monthly samples
required is based on the population served and the size of the distribution
system. Because the number of coliform bacteria excreted in feces is on the
order of 50 million per gram and the concentration of coliforms in untreated
domestic wastewater is usually several million per 100 mL, it is highly unlikely
that water contaminated with human wastes would have no coliforms. That
conclusion is the basis for the drinking water standard for microbiological
contaminants, which specifies in essence that, on the average, water should
contain no more than 1 coliform per 100 mL. The SDWA standards now
require that coliforms not be found in more than 5% of the samples examined
during a 1-month period. Known as the
presence/absence concept
, it replaces pre-
vious MCLs based on the number of coliforms detected in the sample. Viruses
are very common in water. If we removed a teaspoonful of water from an
unpolluted lake, over 1 billion viruses would be present in the water (
Figure
3.2
). The two most common and troublesome protozoans found in water are
Giardia
and
Cryptosporidium
(or
Crypto
). In water, these protozoans occur in
the form of hard-shelled cysts. Their hard covering makes them resistant to
chlorination and chlorine residual that kills other organisms. We cover micro-
organisms commonly found in water in much greater detail in
Chapter 6
.
Turbidity
Turbidity, a measure of fine suspended matter in water, is primarily caused
by clay, silt, organic particulates, plankton, and other microscopic organisms,
ranging in size from colloidal to coarse dispersion. Turbidity in the water is
expressed in
nephelometric turbidity units
(NTUs), which represent the amount
of light scattered or reflected from the water. Turbidity is officially reported
in standard units known as Jackson turbidity units, which are equivalent to
milligrams per liter of silica (diatomaceous earth) that could cause the same
optical effect. Turbidity testing is not required for groundwater sources.
Radionuclides
Radioactive contamination of drinking water is a serious matter.
Radionuclides (the radioactive metals and minerals that cause this con-
tamination) come from both natural and manmade sources. Naturally
