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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