other metalloids (or non-metals)Reading time:
The European Union has set parametric values for the following elements :
- antimony: 5 mg · L–1;
- boron: 1 mg · L–1;
- bromine: 10 mg · L–1 (as bromate ions);
- selenium: 10 mg · L–1.
The following processes may be considered: coagulation-flocculation, filtration through GEH (see section arsenic removal) or reverse osmosis. The effectiveness of these processes is not well known and preliminary tests are absolutely essential.
Conventional clarification treatments are ineffective unless sodium aluminate is used (partial removal). There are other potential solutions:
- filtration through GAC (the type of GAC must be selected after testing; phenomena involving competition with other adsorbable materials have a strong effect on GAC regeneration frequency);
- fixing on activated alumina or other metal oxides;
- carbonate removal using lime with manganese oxide precipitation;
- ion exchange over a specific resin (e.g. Amberlite IRA 743 from Rohm & Haas).
The removal of boron using reverse osmosis membranes primarily applies to seawater desalination treatment (see section desalination membrane application).
bromine: the problem of bromates
If the bromide ion concentration is not regulated, it will be the concentration of bromates (usually absent from raw water but formed mainly during ozonation) that is limited throughout the European Union and by several organisations (WHO, US-EPA, etc.).
Once formed, bromates are very difficult to remove and require expensive supplementary treatment (specific GAC, RO or NF membranes, etc.). Therefore, certain preventive measures are called for:
- pH optimisation during the ozonation stage, keeping the pH to below a limit value of approximately 7; the water’s ozone demand will then be lower;
- adding a small amount of ammonium ions upstream from ozonation (0.05 to 0.2 mg NH4+·L–1);
- optimising ozonation reactors and the way in which the ozone is injected into the water (using CFD);
- optimising clarification in order to reduce the ozone demand.
On the other hand, pre-ozonation will not form bromates under general utilisation conditions; in fact, pre-ozonation can even be beneficial when the raw water is rich in organic matter, due to the formation of by-products (aldehydes, organic acids) that partially inhibit the formation of bromates during the main ozonation stage.
Coagulation-flocculation with iron salts does remove selenites (Se[IV]) effectively at a pH of 6-7 (80 to 90%) but has no effect on selenates (Se[VI]), especially in the presence of sulphates ions. Ferric ions can affect the result by partially oxidising the selenites to form selenates, making it advisable to test other coagulants: Aℓ(III), Fe(II) …
Other potential treatments include:
- carbonate removal using lime (partial action on Se[IV] only;
- fixing on activated alumina, on GEH or on an ion exchange resin;
- reverse osmosis, nanofiltration, electro dialysis.