ADVANCED WATER
TREATMENT
The effluent from a
typical secondary treatment plant still contains 20-40 mg/L BOD which may be
objectionable in some streams. Suspended solids, in addition to contributing to
BOD, may settle on the stream bed and inhibit certain forms of aquatic life.
The BOD if discharged into a stream with low flow, can cause damage to aquatic
life by reducing the dissolved oxygen content. In addition the secondary
effluent contains significant amounts of plant nutrients and dissolved solids.
If the waste water is of industrial origin, it may also contain traces of
organic chemicals, heavy metals and other contaminants.
Different methods are
used in advanced waste treatment to satisfy any of the several specific goals,
which include the removal of (1) suspended solids (2) BOD (3) plant nutrients
(4) dissolved solids and (5) toxic substances. These methods may be introduced
at any stage of the total treatment process as in the case of industrial
waterways or may be used for complete removal of pollutants after secondary
treatment.
PRINCIPLES
AND FUNCTIONS OF AERATION
Aeration is a unit
process in which air and water are brought into intimate contact. Turbulence
increases the aeration of flowing streams. In industrial processes, water flow
is usually directed countercurrent to atmospheric or forced-draft air flow. The
contact time and the ratio of air to water must be sufficient for effective
removal of the unwanted gas.
Aeration
as a water treatment practice is used for the following operations:
1. carbon
dioxide reduction (decarbonation)
2. oxidation
of iron and manganese found in many well waters (oxidation tower)
3. ammonia
and hydrogen sulfide reduction (stripping)
Aeration
is also an effective method of bacteria control.
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