High-rate dissolved air flotation for water treatment

This paper presents the results of an experimental investigation about the performance of a horizontal flow high-rate pilot scale Dissolved Air Flotation (HRDAF) unit containing inclined parallel plates for treating a coloured and low turbidity raw water. Experiments were performed with the DAF unit in order to verify the influence on flotation of : (i) the water velocity (Vh) between the plates, in the range 18 to 96.5 cm.min⁻¹ with corresponding Reynolds numbers between 240 and 1060; (ii) the supplied air (S*) value ranging from 2.2 to 8.5 g of air/m³ of water ; (iii) the angle of the plates (60° or 70°). The best pilot plant operational condition was obtained applying only 4.0 g/m³ (S*) with Vh around 18 cm.min⁻¹ for treatment of water coagulated with a Al₂(SO₄)₃ dosage of 40 mg.l⁻¹. In these conditions, the unit presented very good removal efficiencies of colour (90%, residual of 10 uC), turbidity (88%, residual of 0.8 NTU ) and TSS (94%, residual of 1.8 mg.l⁻¹). Furthermore, the unit could operate at higher Vh values up to 76 cm.min⁻¹ and still present good results. The DAF unit thus behaved as a high rate unit presenting good performance with low air requirement.