Water Reclamation and Desalination by Membranes

GE Water and Process Technologies, ZENON Membrane Solutions, Ontario, L6M 4B2, Canada 

Seawater can be desalinated using thermal processes and reverse osmosis (RO). While approximately half of the worldwide installed desalination capacity is based on thermal processes, there is a clear trend that favors RO, primarily because it consumes much less energy. 

In the last decade, the reclamation of effluents has developed rapidly as an altemative to seawater desalination for industrial uses, irrigation, and indirect potable water reuse. For water reclamation, contaminants that require treatment over and above what is provided by conventional biological treatment include suspended solids, microbes, nutrients (nitrogen and phosphorus, trace organic compounds (e.g., pesticides, endocrine disraptor compounds), and in certain cases dissolved salts. 

Microfiltration and ultrafiltration (MF/UF) membranes are flexible water treatment tools that can be used in a number of process configurations to meet advanced effluent treatment objectives. MF/UF membranes, when used by themselves, are limited to the removal of particulate and colloidal contaminants however, they can be combined with biological or chemical treatment to remove dissolved contaminants. Furthermore, they represent the ideal pretreatment to reverse osmosis by addressing their main weakness, fouling by particulate materials. 

Advanced Membrane Technology and Applications. W. S. Winston Ho, and T. Matsuura Copyright 2008 John Wiley Sons, Inc. 

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Wastewater reclamation for reuse by membrane filtration is more attractive than SWRO desalination due to much lower energy consumption and materials costs. For example the cost for producing RO product water firom secondary effluent and seawater is estimated to be 0.30/m and 0.70/m, respectively [9]. 

For relatively low solids content feeds, it is feasible to operate without continuous cross flow or surface shear, and this can reduce energy costs. Such applications include water treatment and pretreatment for RO desalination and reclamation. This mode of operation is called dead-end filtration or frontal filtration, and the key feature is that the deposition of retained species is allowed to grow. A t5q)ical cycle commences with a clean membrane (after backwash), and at constant flux the TMP rises according to Eq. (10.4). After a specified period (Q, or at a predetermined AP iax. the flux is stopped and the deposit is removed by backwashing and (usually) vigorous aeration. The contents of the membrane... 

A problem common to all membrane processes is that posed by the retentate in which impurities are concentrated. In some cases, retentate can be discharged with wastewater. Other options include evaporation of the water followed by disposal or incineration of the residue, reclamation of chemicals from industrial wastewater, and disposal in deep saline water aquifers. For the special case of desalination of seawater by reverse osmosis, the retentate is returned to the ocean, which has the potential to cause problems due to excess salinity. 

J. Kim, M. Wilf, J.-S. Park, J. Brown, Boron Rejection by Reverse Osmosis Membranes National Reconnaissance and Mechanism Study, Denver, CO Desalination and Water Purification Research (DWPR) Department of the Interior, Bureau of Reclamation, U.S., 2009. 

Reverse osmosis continues to be dominated by desalination, that is, by removing salt from water. The substantial range of applications includes seawater, brackish water, and reclamation of municipal waste water. Similar membranes can be used to pretreat boiler feed water, desirable because pretreated water forms scale in boilers more slowly. There... 

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