The use of microfiltration membranes for seawater pre-treatment prior to reverse osmosis membranes

Since the early 1980s, microfiltration and ultrafiltration have been widely used for drinking water applications. More recently, membranes have started to be used in membrane bioreactors or on secondary wastewater effluents for reuse application. Until recently, in desalination treatment plants seawater was pre-treated with a combination of conventional techniques such as air flotation, clarification, sand filtration and cartridge filtration prior to spiral wound reverse osmosis (RO) desalination membranes. Microfiltration or ultrafiltration membranes are now considered as an alternative solution for seawater pre-treatment. This paper describes pilot test results obtained on Mediterranean seawater using a 0.1 mum hollow fiber Microza(R) module. A pilot rig was operated at different fluxes ranging from 80 to 140 L/h.m(2) with and without ferric chloride addition. Run cycles between chemical cleanings varied between 10 days and 30 days depending on the operating conditions and seawater quality. During the tests, seawater silt density index (SDI) ranged between 6 to non-measurable while the microfiltered water SDI remained below 2 under the optimized operating conditions. With low and constant microfiltered water SDI values, it is possible to design a full-scale plant with fewer RO membranes and to expect longer RO run cycles.