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.

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