Flash mixing of aluminium sulphate, ferric chloride, or ferric sulphate into raw water requires extremely rapid mixing within the first fraction of a second of contact. The Velocity Gradient (G-value) (velocity gradient) governs destabilisation efficiency.
Velocity Gradient (G-value) (velocity gradient) 300–1000 s¹ at mixing zone. Contact time <1 second for effective particle destabilisation. Coefficient of Variation (CoV) ≤0.05 across pipe cross-section before flocculation zone. Mixer must maintain Velocity Gradient (G-value) across flow turndown range. KE preferred for flash mixing (higher K-factor generates greater Velocity Gradient (G-value), essential for particle destabilisation). VN applicable only on large-bore low-velocity mains where ΔP budget is very tight.
Preferred MOC: FRP or 316SS for alum (Al₂(SO₄)₃). FRP or rubber-lined for ferric chloride. Carbon steel not suitable for acidic coagulants.
VN preferred — generates higher Velocity Gradient (G-value) at lower pressure drop (ΔP) than helical. Injection nozzle at centreline in highest-turbulence zone. Immediately upstream of flocculation basin (no settling between mixer and basin).