Membrane fouling sources

The main problem in membrane usage for water purification is the fouling layer that adheres to the membrane. The source of the fouling layer is the different species existing in the feed water and their increased concentration next to the membrane wall. When water permeates through the membrane, all rejected species accumulate next to the membrane wall, their concentration increases in comparison to the bulk concentration, and the motion away from the membrane is controlled by diffusion to the bulk of flow against the flux of the water flowing to the membrane. 

Figure 1.27 An example of surface treatment of a PVDF microfiltration membrane to minimise fouling. Source [75],...

Table 2.4 Sources of membrane fouling and scaling Substance Extent and/or mechanism...

Figure 2.15 Colloidal-organic matter complex membrane fouling mechanism. Source Schafer et al.

Figure 6.1 A suggested structure of humic acid. Humic materials are highly surface active, which is the basis for much of their fouling characteristics in membrane systems. Source C.E. Harland, Ion Exchange,...

Ultrafiltration can be introduced in the polysaccharide purification scheme instead of the alcohol precipitation step, but the membranes must be optimized in each case. Fouling of the membrane can completely change its selectivity, and the fouling properties of the solution depend on the way the extract is obtained. Twin-screw extrusion or stirred extraction leads to solutions with partially hydrolyzed molecules. The size spectra of the molecules is very large, and the small molecules produced can be a source of membrane fouling. The best performance is obtained with the 50 kDa membrane for the stirred extract and with the 10 kDa, as concerns the twin-screw extract. It can be concluded from these results that the twin-screw extract contains smaller molecules than the stirred extract. 

As discussed, NF process simulation offers some possible process optimization by predicting the membrane-fouling mechanisms and sources of performance deterioration. However, membrane modification and fabrication stiU ranains as one of the most attractive research strategies, offering a complete solution to the drawbacks associated with NF membrane applications. Development of new manbrane materials with special features such as antifouling, biocompatibility, and functionality has become the current trend in NF research. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

Reverse osmosis plants also are not immune from silica fouling, and where the raw water source naturally contains relatively high levels of silica, good pretreatment of the RO FW is a prerequisite. To reduce fouling of RO membranes by silica, pretreatment by acid adjustment, alum coagulation, and filtration usually is provided. 

Silt is formed by suspended particulates of all types that accumulate on the membrane surface. Typical sources of silt are organic colloids, iron corrosion products, precipitated iron hydroxide, algae, and fine particulate matter. A good predictor of the likelihood of a particular feed water to produce fouling by silt is the silt density index (SDI) of the feed water. The SDI, an empirical measurement (ASTM Standard D-4189-82,1987), is the time required to filter a fixed volume of... 

Other important future membranes properties include improved resistivity to extreme pH enabling better cleaning performance and resistance to oxidizers, organic solvents and particulate fouling. More important properties exist, yet the most important may be the resistance to fouling of the different sources. 

Membrane pretreatment includes microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF). Microfiltration and UF membrane processes can remove microbes and algae. However, the pores of MF and UF membranes are too large to remove the smaller, low-molecular weight organics that provide nutrients for microbes. As a result, MF and UF can remove microbes in the source water, but any microbes that are introduced downstream of these membranes will have nutrients to metabolize. Therefore, chlorination along with MF and UF is often recommended to minimize the potential for microbial fouling of RO membranes. The MF or UF membranes used should be chlorine resistant to tolerate chlorine treatment. It is suggested that chlorine be fed prior to the MF or UF membrane and then after the membrane (into the clearwell), with dechlorination just prior to the RO membranes. See Chapter 16.1 for additional discussion about MF and UF membranes for RO pretreatment. 

Tables 9.2 and 9.3 list the recommended feed water and concentrate flow rates, respectively, as functions of feed water source quality.1 Higher feed water flow rates result in water and its contaminants being sent to the membrane more rapidly, leading to faster rates of fouling and scaling. As Table 9.2 shows, an RO operating on a well water source can have a feed flow rate as higher as 65 to 75 gpm per pressure vessel, while a surface water source RO should not exceed 58 to 67 gpm per pressure vessel. The well water RO would require 12% fewer pressure vessels than the surface water RO.

Table 9.2 listed the recommended feed flow rates as a function of water source.1 At higher feed water flow rates, contaminants such as colloids and bacteria that may be present in the source water, are sent to the membrane more rapidly, resulting in faster fouling of the membrane. This is why lower flow rates are recommended for water sources that contain high concentrations of contaminants. 

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