Sewage treatment by anaerobic biological process associated with dissolved air flotation

This paper presents the results of a study performed with a lab-scale batch DAF unit fed with previously coagulated (with FeCl₃ or cationic polymer) effluent from a pilot scale up-flow anaerobic sludge blanket (UASB) reactor treating domestic sewage. The adequate coagulation/flocculation conditions – chemical dosage, time (Tf) and mean velocity gradient (Gf) in the flocculation step – and air requirements for flotation process were investigated. Best results were achieved for 65 mg.l−1 of FeCl₃ at Tf around 15 min and Gf of 80 s⁻¹. In the assays where only polymer was applied, 7 mg.l⁻¹ of cationic polymer dosage gave optimum removals with Tf around 15 min and Gf of 30 s⁻¹. Air requirements ranged from 9.5 to 19.0 g of air.m⁻¹ wastewater. Best TSS (95% and residual of 2 mg.l⁻¹), COD (85% and residual of 20 mg.l⁻¹) and total phosphate (95% and residual of 0.6 mg.l⁻¹) removals were obtained when applying FeCl₃, although the use of cationic polymer also produced good level of TSS (74% and residual of14 mg.l⁻¹) and COD (75% and residual of 45 mg.l⁻¹) removals. For the UASB-DAF (batch) system and FeCl₃, global efficiencies would be 97.2% for COD, 97.9% for phosphate and 98.9% for TSS.