Page 19 - ro membanes

2 1. INTRODUCTION TO SALINE WATER PRETREATMENT
FIGURE 1.1 Schematic of typical seawater desalination plant.
minerals from precipitating on the membrane surface, the RO membranes could operate and produce freshwater of consistent quality at a high rate without the need to clean these mem- branes for a very long time. Practical experience shows that for desalination plants with high source-water quality and well-designed pretreatment system, the RO membranes may not need to be cleaned for one or more years and their useful life could extend beyond 10 years (Jamaly et al., 2014).
In actuality, however, pretreatment systems remove most but not all of the insoluble solids contained in the saline source water and may not always effectively protect some of the sol- uble solids from precipitating on the membrane surface. The suspended solids, silt, and nat- ural organic matter (NOM) that remain in the saline water after pretreatment may accumulate on the surface of the RO membranes and cause loss of membrane productivity over time. In addition, because saline water naturally contains microorganisms as well as dis- solved organics that could serve as food for these microorganisms, a biofilm could form and grow on the RO-membrane surface, causing loss of membrane productivity as well. The types of foulants contained in saline source water are described in detail in Chapter 2.
The process of reduction/loss of active membrane surface area and subsequently of pro- ductivity of RO membranes due to accumulation of suspended solids and NOM, precipita- tion of dissolved solids, and/or formation of biofilm on the RO membrane surface is defined as membrane fouling. Excessive membrane fouling is undesirable and besides having negative impact on RO-membrane productivity, it could also result in an increased use of energy for salt separation, and in deterioration of product water quality (Henthorne and Boysen, 2015; Villacorte et al., 2015).
Most RO systems are operated to produce a constant flow of freshwater (desalinated) at target content of total dissolved solids (TDS). Productivity of RO membranes is typically