Wastewater is called strong or weak depending upon the amount of oxygen that is
required to oxidize and stabilize it.
(1) Dissolved oxygen. Fresh water normally contains dissolved oxygen
(DO) in amounts ranging from about six to about 12 parts per million (ppm) parts of
water. Fresh wastewater normally will have a DO content within or just below this
range. As wastewater becomes stale, the DO is consumed. Thus, the DO of
wastewater is a measure of its freshness. Oxygen in wastewater is necessary to
support aerobic bacterial action.
(2) Biochemical oxygen demand. The organic matter in wastewater,
primarily of human or food origin, is unstable and readily decomposed and oxidized by
biological or chemical agents to form more stable substances. This process requires
oxygen. The stronger the wastewater, the greater the amount of oxygen that is
required. The biochemical oxygen demand (BOD) is defined as the quantity of oxygen
required for the biochemical oxidation of the decomposable (organic) matter at a given
temperature within a given time, usually 5 days at 20C (68F). It is normally expressed
as the 5-day BOD in milligrams per liter (mg/l) or ppm. Average domestic wastewater
varies from approximately 150 to 250 mg/l (ppm) 5-day BOD.
c. Biological Characteristics. Bacteria, viruses, and parasites make up the
biological characteristics of wastewater. Wastewater contains vast quantities of
bacteria and other organisms that originate in discharged wastes. The feeding activities
of these organisms assist in decomposing wastewater. Aerobic bacteria decompose
organic matter in the presence of free oxygen. Anaerobic bacteria decompose organic
matter that is shut off from free oxygen, such as in the interior of a mass of feces or a
dead body. The products of anaerobic decomposition have an extremely unpleasant
odor. Matter in which this condition exists is said to be septic. A large number of the
bacteria in wastewater are coliform bacteria -- those found in the digestive tract of
normal humans. While most of these bacteria are harmless, pathogens will usually be
present in wastewater containing the discharges of many persons. It is these relatively
few pathogenic organisms that pose the greatest public health hazard. Wastewater that
is not properly treated may eventually find its way into a community water source and
spread waterborne diseases.
d. Composition of Wastewater. The composition of wastewater varies from
hour to hour, day to day, and season to season; but its average composition can be
determined for a given period. Normally, wastewater is 99.9 percent water by weight.
The remaining 0.1 percent (1,000 ppm) is organic and mineral matter (dissolved,
suspended, organic, and inorganic solids). Most of the mineral matter consists of salts
from the water supply, urine, meat and vegetable extracts, and permissible acids and
alkalies from industries. The organic matter, primarily from human or food origin, is
unstable and readily decomposed and oxidized by biological or chemical agents to form
more stable substances. The total organic and mineral matter in wastewater comprise
about 0.1 percent by weight. This matter is further classified as filterable and
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