Loss of Salt Rejection

Membrane scaling and degradation can lead to a loss in normalized salt rejection as can breaches O-rings and permeate tube. 

Membrane degradation involves the loss of membrane integrity (see Chapter 12.1.2.1). Because of the loss in integrity, feed water is allowed to pass into the permeate leading to an increase in permeate concentration. Thus, salt(s) passing into the permeate increases while the apparent salt rejection decreases. 

Breaches in hardware can allow feed water to mingle with permeate. As is the case with an increase in water flux, breaches in O-rings and permeate tubes will allow high concertration feed water to mingle with low concentration permeate, thereby increasing the concentration of the permeate (see Chapter 12.1.2.2). The overall salt rejection will appear to decrease and salt passage appears to increase. 

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High productivity can also be achieved with brief, measured exposure to free chlorine (see Chapter 8.2.1). Membrane manufacturers will sometimes treat their membranes with a very short exposure to free chlorine. This results in membranes that exhibit higher flux with no change in salt rejection. Longer exposure to free chlorine will result in a permanent loss of salt rejection. Note that exposure to free chlorine by the end user is a violation of the membrane warranty and should not be attempted to increase flux. 

The main mechanisms of membrane fouling are adsorption of feed components, clogging of pores, chemical interaction between solutes and membrane material, gel formation and bacterial growth. Fet us first consider bacterial growth on membranes. Microbiological fouling of reverse osmosis membranes is one of the main factors in flux decline and loss of salt rejection [25-29] (Table 11.1). 

The FT-30 membrane was found to be resistant to swelling or salt rejection losses at high feedwater temperatures. In simulated seawater tests, the membrane had stabilized at about 99 percent salt rejection at temperatures of 40°C and higher. 

Normalized salt rejection is a function of the concentration driving force across the membrane, as shown in Equation 11.3. Factors that lead to loss or increase in salt rejection are discussed below. 

Increases in salt rejection are typically due to membrane compaction (see Chapter 12.1.1.4). As the membrane becomes denser due to compaction, the passage of salts through the membrane is reduced, leading to a loss in salt passage and in increase in salt rejection. 

Data, particularly normalized data, is evaluated to determine the nature of the loss in membrane performance (see Chapter 11.3 for a complete discussion on data normalization). Normalized permeate flow, salt rejection, and differential pressure should be evaluated to determine trends in performance. 

Monomeric amines have two advantages over polymeric amines in interfacial composite membrane fabrication. First, monomeric amines can be obtained in most cases as pure crystalline compounds, identical in lot after lot. Polymeric amines, on the other hand, will show variations in purity, molecular weight, chain branching and viscosity from lot to lot. This adds an element of variability to the membrane fabrication process. Second, monomeric amines lead to thicker barrier layers, which consequently tend to show better abrasion resistance and greater tolerance to chemical attack. By contrast, a membrane such as PA-300 is normally overcoated with a protective layer of water-soluble polyvinyl alcohol to minimize abrasion and salt rejection losses during spiral element assembly. 

In applying reverse osmosis for NOM extraction, the choice of an appropriate river water, preferably high in DOC and low in salt content, is important. Also the choice of a suitable membrane with maximised rejection is critical to avoid the loss of small organics with the permeate. The drawbacks of this method are the concentration of salts and other contaminants. This generally results in a product of high ash content and the salt can lead to precipitation in the concentrate. Particles in the surface water also need to be removed prior to concentrations however this can be achieved with microfiltration (MF). 

A dramatic effect on Pseudomonas fouling was observed when the silver nanoparticles were immobilized on a thin-film composite PA membrane (Lee et al. 2007). SEM measurements confirmed that all Pseudomonas cells were made inactive on the modified-membrane surface, while water fluxes and salt rejections remained unchanged. High antibacterial activity toward E. coli and 5. aureus was also found with CA membranes modified with Ag nanoparticles (Chou et al. 2005). However, a significant loss of silver was found as a result of water permeation, and the antibacterial activity of the membranes disappeared after 5 days (Son et al. 2004). The loss of the entrapped silver nanoparticles was also reported for modified PS membranes, which have a high antimicrobial activity toward E. coli, P. mendocina, and the MS2 bacteriophage (Zodrow et al. 2009). 

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