explain flocculation most important factor

Question

Explain Flocculation

• Most important factor effecting particle removal efficiency
• objective - to bring particles into contact so that they will collide, stick together and grow to a size that will readily settle
• enough mixing to provide contact and prevent settling in the flocculation basin but not too much to shear the floc particles making the floc small and dispersed
• Velocity gradient, G must be controlled in narrow range
• heavier the floc and SS Conc. the more mixing (high G)
• Softening floc is heavier and need more mixing (high G)
• Detention time ≥ 30 minutes
• Flow through velocity = 0.5 to 1.5 fps
• Paddle area = 10 to 25% of tank c/s area
• G, mean velocity gradient = 5 to 100 /sec
• Gt (also G_t) = 30,000 - 150,000