Flotation technique with coagulant and polymer application applied to the post-treatment of effluents from anaerobic reactor treating sewage

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.