Engineering Design Considerations

The coagulation process starts with the rapid, high-intensity mixing of the coagulant with wastewater. Since complex reactions are involved, environmental engineers must use laboratory tests such as the zeta potential and jar tests to establish proper pH and dosage ranges.

Wastewater treatment facilities can rapidly mix inorganic coagulants with the wastewater either inline or in a separate flash-mixing basin. The detention time requirement ranges from 10 to 60 sec. Polyelectrolyte flocculants are added after the rapid-mix phase or downstream from the coagulant addition point because rapid mixing can break up the floc. Mixing during the flocculation stage should be gentle, with a mean velocity gradient of 50 to 175 determined as follows:

W = power per unit volume of fluid (lb/sec-sq ft) p = absolute viscosity of wastewater (lb/sq ft)

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