Design of baffled hydraulic channels for turbulence-induced flocculation

A correlation between the turbulence generated in a channel flocculator and the efficiency of flocculation is presented. Two test units were used, a horizontal flow flocculator of 85 channels, 47 mm wide, in series totalling 102 m in length which was adjustable in longitudinal slope and into which different grid-type baffle arrangements were inserted and a clear-sided channel for the measurement of flow velocity and turbulence levels using laser-Doppler anemometry (LDA). For flocculation a synthetic river water was coagulated with aluminium sulphate and efficiency assessed by turbidity change after 30 min settling. Conclusions reached were that a flocculator retention time of 15–20 min is adequate with G×t of 25,000–35,000 and a longitudinal bed slope of 1 in 48. Sixty percent of G×t should occur in the first half of the system with the time between turbulence surges to be around 8–10 s induced either by a channel bend or by inserting baffle(s) in the first third or so of the flocculator. Derived head loss coefficients are 3.2 for a channel bend and 6 for a grid baffle with effective obstruction of the flow area of up to 45%.