This paper presents the results of a study performed with a lab-scale batch DAF unit fed with previously coagulated (with FeCl3 and/or cationic polymer) effluent from a pilot-scale expanded bed anaerobic reactor treating domestic sewage. The association between ferric chloride and polymers was studied, aimed at sludge reduction. Ferric chloride dosages ranging from 15 to 65 mg.l−1 and polymer dosages from 0.25 to 7.0 mg.l−1 were investigated. Flocculation conditions were kept constant: 20 min of time (Tf) and 80 s−1 of mean velocity gradient (Gf). Air requirement was kept to 19.0 g of air.m−3 wastewater, using 20% recycle ratio and saturation pressure at 450 kPa. When the anaerobic reactor was operating at steady state conditions, it was possible to reduce the FeCl3 dosage from 65 to 30 mg.l−1 after applying 0.4 mg.l−1 of non-ionic polymer, before the DAF process. For these dosages, 79% COD removal (residual of 23 mg.l−1), 86% total phosphate removal (residual of 0.9 mg.l−1) and 98% turbidity removal (residual of 2.6 NTU) were observed. Furthermore, the use of adequate polymer together with 30 mgFeCl3.l−1 leads to the production of high rising rate flocs.

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