NF Membrane Fouling

Tang, C. Y., and Leckie, J. O. (2007). Membrane independent limiting flux for RO and NF membranes fouled by humic acid. Environ. Sci. Technol. 41,4767 773. 

Membrane Limitations Chemical attack, fouling, and compaction are prominent problems with RO and NF membranes. Compaction is the most straightforward. It is the result of creep, slow cold flow of the polymer resulting in a loss of water permeability. It is measured by the slope of log flux versus log time in seconds. It is independent of the flux units used and is reported as a slope, sometimes with the minus sign omitted. A slope of—0.001, typical for noncelhilosic membranes, means that for every threefold increase in log(time), 10 seconds, a membrane looses 10 percent of its flux. Since membranes are rated assuming that the dramatic early decline in permeability has already occurred, the further decline after the first few weeks is veiy slow. Compaction is specific to pressure, temperature, and envi-... 

Chemical attack is often a result either of fouling prevention or cleaning in response to fouling. Chlorine and hypochlorite damage most RO and NF membranes, as do oxidants generally (see discussion of chlorine tolerance below). 

Membrane systems consist of membrane elements or modules. For potable water treatment, NF and RO membrane modules are commonly fabricated in a spiral configuration. An important consideration of spiral elements is the design of the feed spacer, which promotes turbulence to reduce fouling. MF and UF membranes often use a hollow fiber geometry. This geometry does not require extensive pretreatment because the fibers can be periodically backwashed. Flow in these hollow fiber systems can be either from the inner lumen of the membrane fiber to the outside (inside-out flow) or from the outside to the inside of the fibers (outside-in flow). Tubular NF membranes are now just entering the marketplace. 

Effluent pretreatment is necessary when RO is used as tertiary treatment in order to prevent membranes filters form being blocked or abraded. UF offers a powerful tool for the reduction of fouling potential of RO/NF membranes [57]. A typical pretreatment consist of a MF allowing the removal of the large suspended solids form the WWTP effluent followed by UF unit which removes thoroughly suspended solids, colloidal material, bacteria, viruses and organic compounds from the filtrated water. The UF product is sent to the RO unit where dissolved salts are removed. 

To counteract the typical misoneism of the world food industry, it is necessary to resort to appropriate scaling-up exercises in pilot- or industrial-plant scale to assess precisely the membrane process performance and reliability, as well as its economical feasibility. There are, however, a number of problems that have undoubtedly limited growth in ED membrane sales, like membrane-fouling problems, design considerations, cleanability, investment and membrane replacement costs, and competing technologies, such as NF and IER. 

In addition to the MF pretreatment, used to remove particulates and microorganisms from raw water, reducing membrane fouling in the successive steps, the NF step is able to reduce water hardness decreasing osmotic pressure of the RO feed, allowing operation at higher water recovery level. 

Nanofiltration membranes are "tighter" then either MF or UF membranes but "looser" than RO membranes. They can be used to remove dissolved species, such as hardness and color. Recent developments in NF membranes have made them applicable to de-color feed water without chlorination and with minimal membrane fouling (see Chapter 16.2). 

Concentration polarisation, as described in the previous section, can become irreversible if a gel is formed, which can be the case when solute solubilities are exceeded. Concentration polarisation depends strongly on solute concentration and operational conditions, such as pressure and stirring. Fouling of tight UF and NF membranes tends to occur more on the surface than in pores - similar to MF and loose UF. Cake formation is usually reversible and can, as in MF, form a second membrane. Surprisingly, 0degaard and Thorsen (1989) demonstrated that HS concentration and pressure, which influence precipitation and gel formation, had no influence on flux. The fouling layer thickness was calculated with pressure drop and flow. The film was soft, dark brown, and loosely connected to the surface. 

Nanofiltrarion membranes were received from Fluid Systems in San Diego, USA (now Koch Membrane Systems). Thin film composite membranes were chosen due to their low fouling characteristics compared to poly sulphone membranes used in other studies. The CA-UF membrane is, as the name suggests, classed as a UF membrane and the material is cellulose acetate. However, it is treated as a NF membrane here as it is often used for similar applications according to the manufacturer, and also because it exhibits some salt rejection. Membrane characteristics as given from... 

The presence of calcium and humic substances or natural organic matter (NOM) in surface waters can cause severe fouling of nanojittration (NF) membranes. Conditions offouling were studied as a function of solution chemistry, organic type, calcium concentration, hydrodynamic conditions, and membrane type. 

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