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 I flotation batches and in a 100 I DAF pilot reactor. Two cationic polyelectrolytes (PEs) with molecular weights of 4 x 10(6) g/mol and 8 x 10(6) 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, CODparticulate, CODdissolved 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 CODparticulate and turbidity at dosages of 7-8 mg/g influent COD (i.e. 5 mg PE/100 NTU). The removal efficiency of CODdissolved was low. There was a proportional relationship between CODparticulate and turbidity. This led to the conclusions that by introducing a dosing control strategy based on continuous turbidity measurements a constant CODparticulate removal efficiency can be obtained.