Page 8 - ro membanes

At present, reverse osmosis (RO) mem- brane desalination is the predominant tech- nology for production of fresh water from saline water sources (seawater, brackish water, wastewater, etc.). The engineered semiperme- able membranes employed by this technology have the ability to allow transport of pure water molecules at an order-of-magnitude higher rate than they allow transport of salts.
However, the fine microstructure of the semipermeable membranes presently used for desalination by RO does not permit pas- sage of particulates contained in the saline source water or formed during the desalina- tion process. Therefore, if present in the feed water to the RO membranes in significant amount, these particulates may cause mem- brane fouling, which, in turn, may rapidly decrease membrane productivity and result in desalination plant-performance failure. Membrane foulants are typically organic and inorganic colloids and particulates, naturally occurring in the saline source water or gener- ated on the surface of the membranes by aquatic microorganisms or physicalchemical processes that occur during RO-salt separation and concentration.
The purpose of the pretreatment system is to adequately and effectively remove fou- lants from the saline source water and to secure consistent and efficient performance of the RO membranes that process the pre- treated water and produce desalinated wa- ter. The pretreatment system is typically located downstream of the desalination plant’s intake facilities and upstream of the seawater RO-membrane system.
Depending on the source-water quality, desalination plant’s pretreatment system may consist of one or more treatment processes including: screening, chemical conditioning, clarification by dissolved air flotation or gravity settling, granular media filtration, membrane microfiltration (MF) or ultrafiltra- tion (UF), and cartridge filtration. Prior to processing through sedimentation and/or filtration, the saline source water used for membrane desalination is conditioned by the addition of a number of chemicals (biocide, coagulant, flocculant, antiscalant, etc.) in or- der to improve performance of downstream treatment processes.
This book provides a detailed overview of key processes, technologies, and equipment used for pretreatment of saline water used for membrane desalination, and discusses their areas of application, past track record, advantages, and disadvantages. The work describes typical causes and mechanisms of RO-membrane fouling and presents most recent developments in pretreatment tech- nology and science. While the book makes reference, when applicable, to pretreatment of brackish waters, its content is mainly focused of pretreatment of seawater for RO desalination. It should be pointed out, how- ever, that practically all technologies and source-water conditioning methods described in this book are equally applicable to pretreatment of seawater and brackish water.
The book’s chapters address practically all aspects of pretreatment of saline source water used for production of fresh water by
Preface
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