Critical Fouling Conditions

The above studies now allow the definition of critical fouling conditions in NF. Further studies of parameters at these conditions will enable to understand the underlying mechanisms of deposition and fouling. 

The increased flux decline at higher surface roughness was verified, except for the TFC-ULP membrane. The discrepancy can be explained with the lower flux of the TFC-ULP membrane compared to the other membranes, which counteracts deposition and thus flux decline. 

The TFC-SR membrane, which is the membrane with the most negative surface charge and exclusively negative functional groups (see Chapter 4 for details), showed greatest deposition, but only moderate flux decline. However, the TFC-S membrane exhibited greatest flux decline and irreversibility, possibly due to compaction effects during the experiments (see Baseline at the start of this chapter). 

Membrane After After After water DOC Calcium 

For the TFC-S membrane the deposit is very rough, indicating some crystalline precipitation, For the TFC-ULP membrane the deposit is even more porous and the membrane structure still seems visible to 

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In this book pressure-driven membrane processes are compared on the basis of their potential to remove natural organic matter (NOM), critical fouling conditions, and the structure of the deposit formed in and on the membranes. 

An understanding of critical fouling conditions and examination of fouling morphology will lead to improved performance, which wiU reflect in a reduced energy and/or chemicals requirement of existing membranes. It may also explain why membranes do not perform ideally and how this can be improved. 

Table 7.19 Rejection and jinaljlux to initial pure water flux ratio of membranes at critical fouling conditions (12.5 ntgC DOC HA, 2.5 mM CaC/, 1 mM NaHCOj, 20 mSem, pH 8).

In this section, the critical fouling conditions, which lead to the establishment and verification of fouling mechanisms, will be determined. Experiments were carried out under enhanced fouling conditions, namely 12.5 mgL" organic carbon and, for most experiments, calcium concentration of 2.5 mM. These conditions are extreme for surface waters but help understand and accelerate fouling, and are not unrealistic if one considers the conditions of modules in a filtration plant operating at 80 to 90% recovery (i.e. feed concentrated 5 to 10 times). 

As described above, at the selected critical fouling conditions, of high calcium concentration in the presence of HA, a deposit was formed on the membrane surface. Further studies were undertaken to examine if this flux-decline-causing deposit also formed in the presence of other organics. 

Effect of Ferric Chloride Addition at Critical Fouling Conditions... 

Figure 7.46 Ferric chloride addition at critical fouling conditions (TFC-SR membrane, 2.5 mM CaCf, 12.5 mgL DOC HA).

Electronmicrographs of clean membranes were shown m Figure 7.10. The deposits formed at critical fouling conditions were shown in Figure 7.27. In this section further electronmicrographs as a function or organic type, pH and calcium concentration are shown. 

At critical fouling conditions ferric chloride successfully prevented fouling at any dosage. At the higher organic concentration the iron oxyhydroxide precipitates are neutralised and the osmotic effects observed are smaller. The impact on rejection of these less positively charged colloids is also reduced. 

It t as found that the flux decline, even at critical fouling conditions is completely avoided by addition of ferric chloride. For large dosages (100 mgL FeCh), an osmotic pressure builds up and this reduces flux reversibly. The positive ferric hydroxide colloids deposit on the membrane and their charge appears to govern rejection. Cation rejection increases considerably, while the rejection of organics decreases. This demonstrates that the deposit on a fouled membrane can change rejection characteristics. 

Results of OPS systems with 75 nm primary colloids are shown in Table 8.4. Membrane resistance is reduced in the presence of colloids compared to the critical fouling conditions (Table 8.3). The change in resistance possibly resulted from decreased rejection. For the 10 kDa membrane resistance and rejection increase in the presence of colloids. 

For the SPO system, resistances are also lower compared to critical fouling conditions. Results j are... 

Figure A5.5 gives the concentration of all calcium species present as a function of pH for a calcium concentration of 2.5 mM, which was the concentration of the critical fouling condition in some experiments.

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