Silica rejection

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. ... 

Silica rejection across any EDI module strongly depends upon the applied current, dilute flow rate, water temperature, and inlet silica concentration. Pilot data can be easily fitted into the following empirical correlation (Dey, 2005b) ... 

Figure 13.11 Silica rejection versus applied current at constant temperature of 23°C (Dey, 2005b).

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. 

Procedure. Allow the whole of the sample solution (1 L) to flow through the resin column at a rate not exceeding 5 mL min . Wash the column with 250 mL of de-ionised water and reject the washings. Elute the copper(II) ions with 30 mL of 2M nitric acid, place the eluate in a small conical flask (lOOmL, preferably silica) and evaporate carefully to dryness on a hotplate (use a low temperature setting). Dissolve the residue in 1 mL of 0.1 M nitric acid introduced by pipette and then add 9 mL of acetone. Determine copper in the resulting solution using an atomic absorption spectrophotometer which has been calibrated using the standard copper(II) solutions. 

Softening and raising the pH level of the RW tends to increase the rejection rate of silica, boron, and total organic carbon (TOC). It also tends to increase silica solubility, thus reducing the risk of silica deposition on the membrane. 

Thin film composite (TFC) is an ultrathin barrier membrane on polysulfone support layer, of good chemical stability. It has a wide operating pH range of 2.0 to 12.0 at 0 to 40 °C, but cannot tolerate chlorine. TFC membranes are better at rejecting silica than CA membranes. 

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. 

The elements of the multi-residue method should be used as needed. There is no requirement, for example, to test the full version of the German method DEG 19 without any deviation. This full method combines GPC and silica gel cleanup. A poor recovery of compounds from the silica gel is not a reason to reject the multiresidue approach, provided that the chromatograms of GPC eluates are free from interference. 

For desorption the vapor desorbed from the silica gel has to be condensed. For this reason a low temperature heat sink is required. The hydraulics of the plant provides two heat sinks a 10 m3 rain water cistern and the thermal solar plant. With these heat sinks two desorption modes could be carried out a desorption with simultaneous condensation of the vapor an a second mode in which the desorption and condensation were not done simultaneously. If desorption and condensation of the vapour occur at the same time, then the condensation heat is rejected via the rain water cistern. The condensation heat can also be removed by the solar plant when the desorption and condensation operation are discontinuous. In this case the solar plant heats up the adsorber during daytime but no condensation is done. The condensation take place through the solar system during the night given correspondingly low outside temperatures. 

In these tests, the mixture of Na2Si03/tall oil was added to the scrubber before desliming. Collector used in the bulk circuit was sulphosuccinate. In the rutile circuit, phosphoric acid ester was used. Silica was rejected in a bulk talking. The overall metallurgical results obtained in the continuous operation are shown in Table 25.12. 

Ballou, E. V., M. I. Leban and T. Wydeven. 1973. Solute rejection by porous glass membranes. III. Reduced silica dissolution and prolonged hyperfiltration service with feed additive. J. Appl. Chem. Biotechnol. 23 119-30. 

The first group comprises both dry absorbent (molecular sieves, activoted olumino and silica gel) and continuous absorption processes. The dry obsorbent processes will remove virtually oil of the free ond dissolve water from the condensate streom, however they were rejected on the basis that salt deposits in the absorbed water could contomlnote the porous moteriol during regeneration. 

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