Treatment of Surface Water By a Hybrid Coagulation and Immersed Ultrafiltration Membrane for Potable Water Production

In recent years, surface water purification with ultrafiltration membrane has become an attractive alternative to conventional treatment. Some of the advantages of ultrafiltration membrane compared to conventional water treatment process include none or less use of chemical agents, good quality of produced water independent of feed water quality, good removal efficiency towards microorganisms, less production of sludge, compact process, and easy automation. Originally, ultrafiltration was used to remove turbidity and microorganism from good-quality surface water. Presently the main goal of this technology is for the removal of microorganisms, natural organic matter (NOM), dissolved organic matter (DOC) and other components. This trend can be attributed to the improved cost effectiveness of membrane systems as compared to conventional treatment technologies such as filtration and to progressively more stringent water quality regulations.

This study examined the effect of hybrid process coagulation and ultrafiltration membrane performance for potable water production. An ultrafiltration system with a capacity of 24m³/d was constructed to treat raw water taken from the impounded source water supplying the largest water utility in South Africa. A polyvinylidene (PVDF) hollow fibre ultrafiltration membrane was immersed in the process tank as the main component in the filtration system. The membrane area was 37.6 m² with a design membrane flux of 25 L/ m².h and a maximum membrane flux of 45 L/ m².h. It was operated on the outside – In mode with a pore size of 0.04 μm nominal. Surface water contained turbidity of approximately 100NTU. Ferric chloride was used as the primary coagulant. The system performance was evaluated by measuring dissolved organic carbon (DOC), ultraviolet absorbance at 254 nm (UV₂₅₄) and membrane permeate flux. Also residual concentration of metal ion (Fe) in permeate was determined.

The results showed that regardless of the raw water turbidity, the membrane permeate water turbidity was always below 0.1 NTU. The hybrid system removed DOC more efficiently than the conventional water treatment process, and the removal percentage by the hybrid system was in the range of 34–54%. The membrane permeability improved greatly with the inclusion of citric acid as part treatment chemical for chemical cleaning in place (CIP). Pre-treatment with coagulation not only improved the treated water quality, but also enhanced membrane performance, that is, the permeate flux increased and flux decline was reduced.

Ultrafiltration of surface water with PVDF membranes was proven very useful for drinking water production. The results of this study have shown that ultrafiltration membrane can be applied to impounded water treatment for public supply from the viewpoint of treatment efficiency and effectiveness of contaminant rejection.