Coagulation/submerged hollow-fibre ultrafiltration for natural organic matter removal

Experiments were carried out to investigate the removal of humic substances from highly coloured raw water (50 mg Pt/L) by coagulation and outside-in hollow-fibre membrane filtration. Chitosan, a cationic biopolymer produced from crustacean shells, and polyaluminium chloride (PACl) were used as coagulants. The rate of permeability decline was determined for different backwash frequencies and different fluxes at a 95% water recovery of the membrane. Coagulation with PACl gave a good and consistent permeate quality with removals of 95.6%, 84.9%, and 66.1% for true colour, UV absorbers, and dissolved organic carbon, respectively. Longer filtration intervals with less frequent backwashing as well as lower specific aluminium dosages increased the rate of permeability decline. The particle size distribution in the retentate tank was not affected by the frequency of backwashing. Chitosan was less effective in coagulating humic substances and the removal efficiencies were notably lower. Preliminary results suggest that the permeability declines somewhat faster for chitosan compared to PACl-coagulated water.