To achieve this goal, softening with IX as pretreatment and inter-stage treatment for a three stage, single pass RO unit is one option.
Recovery Processes in RO Units
In order to achieve high recovery, softening down to ppb levels of hardness is required. To achieve this reliably, particularly for high salinity water, a selective IX process is used. One example of this is the weak acid cation resin type (for example, Lewatit CNP80), which is typically used prior to the RO treatment for brackish waters. Another example is chelating resins, which are able to effectively soften to ppb levels, even from saturated brine solutions.
This type is typically used to soften the concentrate from an RO plant, ahead of further RO treatment. An iminodiacetic acid chelating resin (for example, Lewatit MonoPlus TP208) is typically selected when strontium and barium removal is important; otherwise an aminophosphonic acid resin (for example, Lewatit MonoPlus TP260) is preferentially used.
In most applications a chemical treatment with acids or antiscalant is used as pretreatment for an RO process. The softening process with IX has advantages if, like in the example above, the discharge of salts is difficult, or if the solubility products (Ksp) of the salts are far above the saturation limit so that an antiscalant cannot be used for the application.
Boron Removal in Water Treatment Plants
In contrast to the produced water treatment example above, IX is used as a post-treatment for boron removal. Boron removal with RO alone is done at pH 9. At this pH, the Boron is partially negatively charged and the rejection can be up to 90 per cent with Seawater RO, and 75 per cent with Brackish water RO elements. To achieve a limit of below 0.5 mg/l Boron in the permeate, an additional RO treatment of the first permeate is necessary (via 2 pass, or partial two pass system).
This article is protected by copyright. You want to use it for your own purpose? Contact us via: support.vogel.de/ (ID: 42332047 / Water Treatment)