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

Within the framework of a research programme into more sustainable wastewat er treatment systems based on physical-chemical pretreatment the applicatio n of dissolved air flotation (DAF) with polyelectrolytes as a primary treat ment was investigated. Experiments with municipal wastewater were carried o ut in 2.5 l flotation batches and in a 100 l DAF pilot reactor. Two cationi c 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 removi ng turbidity and was used in the pilot experiments for coagulation/floccula tion. Within the pilot system experiments the dry solids concentration of t he float layer as well as the removal efficiencies of turbidity, CODparticu late, CODdissolved and nutrients, were studied. Typical results were that t he obtained dry solids concentration of the floated sludge was 10%, when th e float layer was removed after several hours. By using polyelectrolytes it was possible to remove more than 80% of CODparticulate and turbidity at do sages of 7-8 mg/g influent COD (i.e. 5 mg PE/100 NTU). The removal efficien cy of CODdissolved was low. There was a proportional relationship between C ODparticulate and turbidity. This led to the conclusions that by introducin g a dosing control strategy based on continuous turbidity measurements a co nstant CODparticulate removal efficiency can be obtained.