Pretreatment for Desalination

Pretreatment for Desalination

Desalination facilities for conversion of sea water or brackish water, to potable water or industrial process water typically rely on nanofiltration (NF) membranes or reverse osmosis (RO) membranes. Evaporators are also used to treat difficult waters with high TDS and organics.
RSL Membranes™ are an excellent method to pretreat water for this application. RO membranes and evaporators are sensitive to high levels of silica, which will cause fouling in both RO membranes and evaporators. Parameters such as aluminum, barium, iron and zinc can create fouling issues as well. A significant benefit of the RSL Membrane™ is its ability to absorb certain metals during the filtration process. Testing and ICP analysis has provided a data base which confirms that RSL™ Powder, used as the membrane skin layer, has consistently adsorbed:

1. Aluminum >85%
2. Barium > 70%
3. Boron >40%
4. Fe >85%
5. Si >65%

Other metals, to a lesser extent are also being adsorbed.

Table 1 summarizes a study that evaluated the adsorption capability of RSL™ Powders compared to conventional media (Activated Alumina and Diatomaceous Earth- DE) used for filter cake/dynamic membrane filtration processes. With this level of adsorption capability, specifically for barium and silica, RSL Membranes™ are an excellent choice for pretreatment of water prior to RO membranes in pure water applications.

Because the RSL Membranes™ can handle a high suspended solid load (as high as 5000 ppm), the technology replaces conventional clarifiers or DAF’s. If softening of the water is required through the use of lime or NaOH addition, calcium and magnesium can be precipitated and removed via the RSL Membrane™. Furthermore, the RSL Membranes™ have been used extensively to remove high amounts of silica with the addition of poly aluminum chloride to cause a crystallization of the silica.

RSL Membrane™ were compared to Veolia Ceramem membranes in the first assessment of the RSL Membranes™. The technologies were evaluated for pretreatment prior to an RO membrane. RSL Membranes™ outperformed the Veolia Ceramic UF membranes. Flux rates were 10 times the ceramic membranes and produced similar quality water (SDI < 3 and typically < 1). In the above photo, the white trailers contain Veolia’s ceramic membranes and the small blue trailer to the left contain the RSL membranes™. As you can see the footprint impact is significant. Both systems had the same flow rate and were operated for a year, side by side, with full time operation over a four-month period. The Ceramic membranes had fouling issues and were limited to a TSS concentration < 100ppm in the feed water. RSL Membranes™ could operate over a large range of TSS levels.