Ferric Chloride Pretreatment

MF with ferric chloride pretreatment increased NOM removal substantially. However, the process efficiency (towards organic removal) is very dependent on the nature of the organic matter. In a surface water the composition of organics changes constantly, and thus the treatment efficiency cannot be readily predicted. Careful monitoring and control would be required to maximise organics removal. 

Ferric chloride pretreatment could achieve organics removal up to 90%. The removal depends strongly on solution chemistry , organic ty pe and concentration, and the characteristics of the formed floes. Performance is difficult to predict if the number of parameters change rapidly as in a normal surface water. Iron rejection is low at high dosage when the floes are very small. These floes penetrate into the membranes and cause most detrimental flux decline. 

Figure 7.47 LC-OCD analysis of permeate samples of ferric chloride pretreated NF and with feedwater containing OPS colloids compared to feed HA characteristicsfor TFC-SR membrane.

These phenomena explain the changes in rejection observed due to ferric chloride pretreatment Calcium and sodium rejection increase when small (lOnm, Lo and Waite (1998)) are deposited on the membrane. However, this barrier decreases the rejection of DOC. This can be explained with the increased concentration polarisation effect. The concentration of DOC in the boundary layer would increase and, due to the organics being a mixture of compounds of different sizes, the small organics permeate through the membrane at a higher rate. 

Ferric chloride pretreatment of the solutions which also included hematite colloids confirmed the trends of low organic, but high cation rejection at the high dosage. 

Further, rejection of organics will be investigated as a function of membrane pure water flux, MWCO or pore si e. This is followed by fouling under fouling conditions where resistances of the fouling layers will he compared. Then particulate fouling is investigated and similar comparisons of resistances and rejection are made. The next section is dedicated to the effect of ferric chloride pretreatment on flux and rejection of a number of membranes. 

Ferric chloride pretreatment leads to a higher WQP due to the higher organic and colloid rejection. While the value is stable in NF (although minor changes can occur as shown in Chapter 7), a correction of WQP due to the improved rejection is carried out for MF and UF. 

Combining flocculation and coagulation in a pretreatment process has also been smdied. In a key paper by Lopez-Ramirez et al. [64], the secondary effluent from an activated sludge unit was pretreated, before RO, with three levels intense (coagulation-flocculation with ferric chloride and polyelectrolite and high pH sedimentation), moderate (coagulation-flocculation with ferric chloride and polyelectrohte and sedimentation), and minimum (only sedimentation). The optimum for membrane protection, in terms of calcium, conductivity, and bicarbonates reduction, was the intense treatment. Membrane performance varied with pretreatment but not reclaimed water quahty. The study recommended intense pretreatment to protect the membrane. 

The treatment of bilge water and emulsions resembles that of the treatment of oil field brines and produced water. Chen et al. [25], using ferric chloride and other chemicals to enhance the performance of Membralox 0.2, 0.5 and 0.8 pm membranes, describe permeate fluxes between 1400 and 34001/m h. Without pretreatment however severe fouling occurred as well as break-through of oil. Zaidi et al. [26] report about the continuation of this work. They quote fluxes between 800 and 12001/m h, but also mention substantial lower fluxes in long term pilot tests using 0.8 pm membranes. In addition they indicate a drop in permeate flux caused by conditions of low pH, the presence of sea water, corrosion inhibitor, oil slugs or flow variations. 

Inorganic colloids (hematite, 75 nm) did not cause irreversible flux decline. Pretreatment of the solutions using ferric chloride not only prevented flux decline under criticalfouling conditions (high calcium concentration and IHSS HA), but also influenced rejection. The latter depends on the charge of the ferric hydroxide precipitates. Cation rejection increased when positive ferric hydroxide colloids were deposited on the membrane, which the organic rjection decreased. 

Ferric chloride is used in MF and UF to increase rejection. In Chapter 7 it has been demonstrated that ferric chloride can also be used to reduce fouling in NF. In this section the effect on rejection, flux, and the cost of such a pretreatment are compared. 

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