Environmental Engineering Reference
In-Depth Information
the period was well under 1 percent, however. Most waterborne disease fatalities
occurred before 1940 and were attributable to typhoid fever.
17
The finding proba-
bly reflects the unavailability of antibiotics during the early time frame. Diseases
of a waterborne nature appear when disregard of known fundamental sanitary
principles occurs, hence, in most cases are preventable. As often occurs, very
young, elderly, immunocompromised, and critically ill persons with some other
illness succumb with the added strain of a water- or foodborne illness. These
groups of disease-sensitive people are thought to make up 20 to 25 percent of
the population of the United States.
18
Water- and foodborne diseases are sometimes referred to as the intestinal or
filth diseases because they are frequently transmitted by food or water contam-
inated with excreta. Raw drinking water and improperly protected and treated
surface and groundwater supplies may be polluted by excreta or sewage, which
is almost certain to contain pathogenic microorganisms with potential to cause
illness in consumers. In the United States, community waterborne outbreaks
during the period 1981 to 1990 predominantly associated with inadequately
treated surface water and deficiencies in the distribution system whereas untreated
groundwaters were the major source of waterborne diseases for persons utilizing
private water sources.
19
Survival of Pathogens
Survival periods for selected pathogens in surface and groundwater are given in
Table 1.1. The survival of pathogens is quite variable and affected by the type of
organism, the presence of other antagonistic organisms, the soil characteristics,
temperature, moisture, nutrients, pH, and sunlight. Table 1.1 is intended only as a
comparative measure of survivability among pathogens. The amount of clay and
organic matter in the soil affect the movement of pathogens, but porous soils,
cracks, fissures, and channels in rocks permit pollution to travel long distances.
Some organisms are more resistant than others. Soil moisture of about 10
to 20 percent of saturation appears to be best for survival of pathogens; drier
conditions increase die-off.
Nutrients may increase survival of some organisms, although elevated meta-
bolism in vegetative cells and the germination of spores may produce the opposite
effect. Typically, pH is not a major factor. As would be expected, survival of some
pathogenic bacteria at very low pH (e.g., pH 2.5 - 3) is poor in certain media.
20
,
21
When pH values are below the isoelectric point of both bacteria and viruses, sur-
face charge will be positive and, although controversial, may promote aggregation
and adsorption of cells to predominantly negatively charged particulate matter
and produce a protective effect against the potentially harmful effects of high
hydrogen ion. In addition, hydrogen ion may effect the solubilization of nutrients.
Viruses appear stable over the pH range of 3 to 9. Exposure to sunlight increases
the death rate. Low temperatures favor survival.
22
,
23
The survival of pathogens
in soil, on foods, and following various wastewater unit treatment processes, as
reported by various investigators, is summarized by Bryan
24
and others.
25
Most
