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1.2 MEMBRANE-FOULING MECHANISMS 3
measured by the volume of desalinated water that the membranes can deliver through a unit of membrane surface (square meter/square foot) over a certain period of time (day/h). This freshwater membrane productivity is defined as membrane flux. For example, most seawater reverse osmosis (SWRO) desalination systems at present are designed to operate at a constant membrane flux in a range of 13.5e18.0 L/m2 h (l/m2 h or lmh) and brackish water RO (BWRO) systems for 25e35 lmh. For a given source seawater salinity, temperature, and target freshwater TDS level, producing a constant volume of desalinated water will require the source water to be fed to the desalinated system at a constant pressure (typically in a range of 55e70 bars for SWRO desalination systems and 15e25 bars for BWRO facilities).
As RO-membrane fouling occurs, to maintain membrane productivity (flux) and water quality constant, the desalination system would need to be operated at increasingly higher transmembrane pressure (TMP), which in turn means that the energy needed to produce the same volume and quality of freshwater would need to be increased. The increase in RO-system transmembrane pressure over time is an evidence of accumulation and/or adsorption of fouling materials on the surface of the RO membranes (i.e., membrane fouling).
It should be pointed out that membrane fouling is not only dependent upon the source wa- ter quality and the performance of the pretreatment system, but also upon the RO membrane properties such as charge, roughness, and hydrophilicity (Hoek et al., 2003; Hoek and Agarwal, 2006), as well as upon the flow regime on the membrane surface (Wilf et al., 2007). Membranes with higher surface roughness and hydrophobicity usually have higher fouling potential.
Typically, compounds causing RO-membrane fouling could be removed by periodic clean- ing of the membranes using a combination of chemicals (biocides, commercial detergents, acids, and bases). In some cases, however, membrane fouling could be irreversible and clean- ing may not recover membrane productivity, which in turn may require the replacement of some or all of the RO membranes of the desalination plant.
Criteria most commonly used in practice to initiate membrane cleaning are:
1. 10%e15% increase in normalized pressure drop between the feed and concentrate headers;
2. 10%e15% decrease in normalized permeate flow; and/or
3. 10%e15% increase in normalized permeate-TDS concentration.
All RO membranes foul over time. However, the rate and reversibility of fouling are the
two key factors that have most profound effect on the performance and efficiency of the RO-separation process. These factors in turn are closely related to the saline source water quality and the performance of the desalination plant’s pretreatment system.
1.2 MEMBRANE-FOULING MECHANISMS 1.2.1 External and Internal Fouling
Depending on the location of accumulation of insoluble rejected matter causing decline of membrane performance, fouling can be classified as:
1. external or “surface” fouling and 2. internal fouling.