Experience in reverse osmosis pretreatment

To prevent reverse osmosis (RO) membrane fouling, all the organic, colloidal and biological matter needs to be removed from the feed water to the RO system. A proper pretreatment is therefore considered to be the key to successful RO plant operation. A key component of the RO pretreatment process is the deep bed filter. The purpose of this filter is to remove suspended matter such as the precipitates of coagulation and flocculation and various microorganisms. This paper outlines the pretreatment steps involved in preparing the water to enter the RO membranes, with particular emphasis on filtration, which is considered the heart of the pretreatment process. Case studies are presented from over 30 years of experience in the field of pretreatment. Filter media selection with regard to their type, size, size distribution and specific gravity (SG) are discussed. What causes filter media losses during backwash, short filter runs, free board requirements are explained and field examples are presented. Optimum packing layer configurations to avoid percolation of filter media into the filter under drain that can cause nozzle blockages and contamination of filtrate water are presented. Optimum backwash velocities to keep the filter bed clean and avoid mud balling leading to channeling and filter media solidification are also discussed.