Silica rejection performance

Silica rejection performance of the cross-flow spiral-wound EDI test modules was also evaluated at various water temperatures and applied direct currents at a constant dilute flow rate in Figure 13.12. The dilute flow rate was kept constant at 8.5 gpm per EDI test module. 

The thin film composite membrane exhibited superior overall rejection performance in these tests, with ammonia and nitrate rejection showing an outstanding improvement. It has also been reported that silica rejection by the thin film composite membranes is superior to that of cellulose acetate. While the above data indicates a marginal improvement in the rejection of chemical oxygen demand (COD), which is an indication of organic content, other tests conducted by membrane manufacturers show that the polyurea and polyamide membrane barrier layers exhibit an organic rejection that is clearly superior to that of cellulose acetate. Reverse osmosis element manufacturers should be contacted for rejection data on specific organic compounds. ... 

Optimum performance is generally provided by a totally integrated but modular system with a high performance/low flux EDI design to provide for maximum rejection of silica and other troublesome elements such as boron. See Figure 9.4c for a schematic layout of a UF/RO/EDI train. 

Continuous electrodeionization systems can achieve 95% rejection of boron and silica, and 99+% rejection of sodium and chloride. This performance is possible due to voltage-induced dissociation of water that effectively regenerates a portion of the resin thereby allowing removal of weakly ionized species such as silica and boron.19 In fact, the boron in the effluent from a CEDI system can be lower than that in the effluent from a mixed-bed ion exchange system.13... 

A composite ceramic membrane was formed [77] by the graft polymerisation of a hydrophilic polymer, PVP, onto the surface of silica membranes (pore size = 3.0 pm). The flux of an unmodified UF membrane of an oil/water emulsion (4.7%) decreased with time as compared to the flux of the composite membrane. The dechne in flux was caused by fouling and/or the immediate formation of an oil gel layer on the surface of the unmodified membrane. The modified membrane, in contrast, was not only more resistant to adsorption of oil, but also had a higher oil rejection. The performance of the modified ceramic membrane depends upon the configuration of the grafted chains in response to solvent—polymer interactions. Thus, the hydrophific PVP polymer chains tend to expand or extend away from the surface in aqueous solutions, preventing oil adsorption on the membrane surface. Simultaneously, the hydrophific polymer allows the passage of water molecules preferentially over oil. 

Some compounds contain fillers such as silica and silicates that have a significantly abrasive effect on the screw and barrel. Special steels may be appropriate to minimise wear, and additionally, the amount of wear should be measured and recorded on a regular basis (at least annually). Changes in machine performance can then be avoided by planned replacement of eroded machine parts. If wear goes unchecked then the performance of the injection machine will change and affect product quality. The first sign of this could be an onset of moulding rejects. 

FIGURE 14.6 RO performances and chlorine resistances of composite RO membranes made from sulfonated poly(arylene ether sulfone) containing amino groups (aPES) and hyper-branched aromatic PA-grafted silica (HBP-g-silica) (a) salt rejection and (b) water flux. (Data from S.G. Kim et al.. Desalination, 325, 76-83, 2013.)... 

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