Some of the major physical processes in self purification of natural water systems are as follows: i Dilution ii Sedimentation and Re-suspension iii Filtration iv Gas Transfer. These processes are not only important but are also of significance in their relation to certain chemical and biochemical self-purification processes.
In this method relatively small quantities of waste are discharged into large bodies of water. Although dilution is a powerful adjunct to self-cleaning mechanism of surface waters, its success depends upon discharging relatively small quantities of waste into large bodies of water.
Growth in population and industrial activity, with increases in water demand and wastewater quantities, precludes the use of many streams for dilution of raw or poorly treated wastewaters. In suspension, solids increase turbidity and the reduced light penetration may restrict the photosynthetic activity of plants, inhibit the vision of aquatic animals, interfere with feeding of aquatic animals that obtain food by filtration, and be abrasive to respiratory structures such as gills offish.
Particles in the colloidal size range can stay in suspension for long periods of time, though eventually most of these will also settle out.
Re-suspension of solids is common in times of flooding or heavy runoff. Eventually they will again settle out, but not before their presence has increased the turbidity of the waters into which they have been introduced.
Self Purification of Natural Water Systems | Water Management
Small bits of organic matter or inorganic clays and other sediments may be filtered out by pebbles or rocks along the stream bed. The transfer of gases into and out of water is an important part of the natural purification process. The replenishment of oxygen lost to bacterial degradation of organic waste is accomplished by the transfer of oxygen from the air into the water.
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