Comparison of Ultrafiltration and Dissolved Air Flotation Efficiencies in Industrial Units during the Papermaking Process

The efficiency of an ultrafiltration unit has been studied and compared with a dissolved air flotation system to obtain water with a suitable quality to be reused in the process. The study was carried out at a paper mill producing light weight coated paper and newsprint paper from 100% recovered paper. Efficiency was analysed in terms of removal of turbidity, cationic demand, total and dissolved chemical oxygen demand, hardness, sulphates and microstickies. The performance of the ultrafiltration unit and the membranes were studied in depth, particularly their variability during on-going operation. As expected, the ultrafiltration gave higher removal efficiencies than the dissolved air flotation cell in parameters like turbidity, cationic demand, dissolved chemical oxygen demand and microstickies. The greatest difference in performance between the units concerned cationic demand and dissolved chemical oxygen demand. Ultrafiltration was severely influenced by the operating time, with removal efficiency of the dissolved fraction decreasing by 75% and that of the colloidal fraction decreasing by 30% after 312 h of continuous operation. Membrane analysis, carried out to identify the cause of poor membrane performance, showed that the active layer was degraded, probably due to the effect of suspended solids.