Membrane applications in raw water treatment with and without reverse osmosis desalination

Abstract

Conventional raw water treatment methods are normally used for pretreatment in RO plants. The extent of pre-treatment depends on the raw water quality. The utilization of ultrafiltration [UF] instead of conventional pre-treatment stages before reverse osmosis was investigated from a technical-economical point of view by Ericsson and Hallmans [1]. Different types of raw water were included in the cost comparison, viz. surface brackish water and seawater with worse quality than normal groundwater. Troublesome raw waters may be of particular interest for pre-treatment with membrane methods [UF, NF and/or MF]. It was concluded from the investigation that the UF-RO concept is applicable on new desalination plants for reliable and stable operation.The aim of this paper is to investigate further the two alternatives with conventional pre-treatment and the membrane methods, which also are applied in water treatment plants without desalination. A literature review clearly indicates that membrane plants have advantages compared to conventional water treatment plants, but the costs are higher. On the other hand the treated water quality is usually much better, which is important if it otherwise is difficult to comply with some stringent quality limits. As an example it has been reported by Lyonnaise des Eaux [2] experiences of 6 UF-plants with hollow fiber membranes in France that the raw water quality is deteriorated considerably with respect to turbidity and organics. It has been shown that the required potable water quality could be attained easier with membrane plants compared to conventional treatment plants for surface water. The quality of the produced potable water was much better and uniform in membrane plants, but the water cost was doubled approximately.The applications of UF, NF and MF in different raw water treatment stages are discussed and compared with alternative treatment methods with respect to efficiency and economy. Recommendations are made concerning the use of these membrane methods under optimum conditions both with and without RO desalination.