A study of hybrid clarification-filtration processes for potable water treatment
This research investigates the removal of natural organic matter (NOM) from low turbidity waters by optimising an integrated coagulation and membrane filtration process. In conjunction with a regional water utility, the feasibility and operability of pre-coagulation with ultrafiltration membrane units, is investigated at both the fundamental and applied levels. The pH of coagulation greatly affects the growth of flocs. Although flocs are produced over the range of pH values, there is a significant improvement in performance at the optimum pH. The results show that for ferric sulphate, the optimum pH value for coagulation was 4.8; for ferric chloride the optimum pH value was 5.0; and for alum the optimum pH value was 6. Manipulation of the mixing regime during orthokinetic flocculation allows control of the final floe size. A high shear rate (447 s-1) induces greater particle collision and micro-floc formation, allowing an increased rate of growth during slow mixing. A mixing regime of 60 seconds rapid mixing (shear rate G= 447 s-1) followed by 3 minutes slow mixing (shear rate G= 20 s-1) is required for flocculation of the NOM and adequate enmeshment and removal of excess iron. Experiments conducted with the optimised coagulation regime and a Norit hollow fibre ultrafiltration membrane, operated in dead-end mode, leads to significant fouling. The results for the submerged Zenon ultrafiltration membrane also indicated membrane fouling, when the coagulation regime was optimised for NOM reduction and iron removal. Changing the coagulation regime altered the level of membrane fouling. It can be concluded that the optimum coagulation conditions in conventional water treatment are not always the optimum conditions for coagulation with ultrafiltration treatment and the unconventional coagulation conditions can be much more effective, than the conventional ones, in the context of membrane filtration, depending on the raw water characteristics.