Flotation with polyelectrolytes as a first step of a more sustainable wastewater treatment system

Within the framework of a research programme into more sustainable wastewater treatment systems based on physical-chemical pretreatment the application of dissolved air flotation (DAF) with polyelectrolytes as a primary treatment was investigated. Experiments with municipal wastewater were carried out in 2.5 l flotation batches and in a 100 l DAF pilot reactor. Two cationic polyelectrolytes (PEs) with molecular weights of 4×10⁶ g/mol and 8×10⁶ g/mol, were tested. The higher weight PE was more effective in removing turbidity and was used in the pilot experiments for coagulation/flocculation. Within the pilot system experiments the dry solids concentration of the float layer as well as the removal efficiencies of turbidity, COD_particulate, COD_dissoIved and nutrients, were studied. Typical results were that the obtained dry solids concentration of the floated sludge was 10%, when the float layer was removed after several hours. By using polyelectrolytes it was possible to remove more than 80% of COD_particulate and turbidity at dosages of 7-8 mg/g influent COD (i.e. 5 mg PE/l00 NTU). The removal efficiency of COD_dissolved was low. There was a proportional relationship between COD_particulate and turbidity. This led to the conclusions that by introducing a dosing control strategy based on continuous turbidity measurements a constant COD_particulate removal efficiency can be obtained.