Environmental Engineering Reference
In-Depth Information
carbonaceous organic material, which is the most readily biodegradable. If a valid
relationship can be established between results obtained with the TOC test and the
results of the BOD test for a given wastewater, the use of the TOC test for process
control is recommended. Typically, the TOC content of surface and ground water will
vary from 1 to 20 mg/l and 0.1 to 2 mg/l, respectively.
TOC, BOD and COD reflect different fractions of the organic matter present in the
sample and for a typical untreated municipal sewage the following indicative ratios could
be mentioned:
Different types of water or wastewater samples would have different ratios. The lowest
the ratio, the easier the available organic matter would be degraded by biological means.
2.3
Biological characteristics
Microorganisms are commonly present in surface water, but they are usually absent from
most ground water because of the filtering action of the aquifer. A wide variety of
microorganisms could be found in surface water, including viruses, bacteria, fungi,
protozoa and nematodes. The most common microorganisms are bacteria. They vary in
shape and size from about 1 to 4 µm. Disease-causing bacteria are called “pathogenic”
bacteria. These bacteria, which require oxygen for their survival, are called “aerobic”,
while those, which thrive in an environment free of oxygen, are “anaerobic”. “Facultative
bacteria” are those that live either with or without free oxygen.
It would be extremely time consuming and would provide little useful information if
we try to identify all types of microorganisms present in polluted water. All water - borne
and many of the water - based diseases, are dependent on their transmission upon the
access of feces from a contaminated individual, which contain pathogenic
microorganisms, into a water supply source used for drinking or bathing purposes.
Because the number of pathogenic microorganisms present in wastes and polluted water
are relatively few and difficult to isolate and identify, the coliform bacteria, which are
harmless, more numerous and more easily tested for, are commonly used as indicator
organisms. Escherichia coli
(colon bacilli or coliforms)
are bacteria that inhabit the
intestines of warm-blooded animals. The intestinal tract of humans contains countless
rod-shaped bacteria known as coliforms. Each person discharges from 100 to 400 billion
coliform organisms per day in addition to other kinds of bacteria. Thus, the presence of
coliform organisms is taken as an indication that pathogenic organisms may also be
present, and the absence of coliform organisms is taken as an indication that the water is
free from disease-producing organisms. We could differentiate between “total coliforms”
(TC) and “fecal coliforms” (FC). FC are capable to survive at 44 ºC and originate
exclusively from human feces, while other coliforms may occur naturally in unpolluted
water and soils, as well as in feces. Their survival ambient temperature is 37 ºC. TC are
determined at this temperature and therefore include all types of coliforms, but only FC
are the indicator organisms, which would show the presence of fecal contamination.
However, Horan (1990) warns that in tropical conditions certain bacteria of non-fecal
