Flouride Removal

Q: We just received our new wastewater discharge permit from the city, and it contains a new fluoride limitation of 10 mg/L. We have six months to comply. We called up our pretreatment inspector and asked why the new limit and if we are being targeted. She responded that all significant industrial users will be receiving this limitation as their permits are renewed, and it’s driven by the city’s plan to reuse a portion of its wastewater treatment plant’s effluent for non-drinking water uses. After reviewing our Material Safety Data Sheets and finding two products containing fluoride, we sampled our wastewater over a five day period and found our fluoride concentration ranged from 20–50 mg/L. We already have a wastewater pretreatment system that uses sulfuric acid, 25% sodium hydroxide to adjust pH and polymers to adjust pH and remove zinc, chrome, nickel, and copper. Any recommendations on how we can easily remove fluoride without major modifications to our pretreatment system? E.G.

 

A: The major reason fluoride is difficult to remove from wastewater is that it forms so many more soluble compounds as compared with other contaminants, such as heavy metals.

The most common method for the removal of fluoride is precipitation with lime as calcium fluoride (CaF2), but this technique may or may not be able to achieve compliance with your permit. Most lime-based systems achieve fluoride effluent concentrations in the 10–20 mg/L range, although I have read one report that achieved fluoride concentration just below 10 mg/L. This would obviously require a change in your chemical feed system as well as a larger pH adjustment tank (15-min retention time or longer) due to the slower reaction time of lime. You can enhance the fluoride removal with the addition of magnesium hydroxide (Mg(OH)2), resulting in an effluent fluoride concentration down near 1 mg/L; this is attributed to the adsorption of the fluoride ion into the magnesium hydroxide floc. Again, this would require a second slurry feed with all its challenges as well as an even larger pH adjustment tank (at least 20-min retention time).

Since lime provides a source of calcium, an alternative is calcium chloride (CaCl2). This can be much more easily adapted to your system, because a calcium chloride solution can be made in a day tank equipped with mixer and cover and fed into your pH adjustment tank. But again, you may be challenged with the 10 mg/L limitation. The addition of a second chemical, either phosphoric acid to provide the phosphate ion (PO43-) or aluminum sulfate (Al2(SO4)3), also known as alum, has been reported to improve the precipitation of CaF2 sufficiently that it should meet the 10 mg/L limit.

If these precipitation options are found to be inadequate, packed beds of activated alumina can be used to remove fluoride after your clarification step. Activated alumina processes are pH-sensitive and fluoride is best absorbed below pH of 8.2, so you may need a pH adjustment stage prior to entering the activated alumina beds.

As a post-precipitation process with industrial wastewaters, activated alumina has been reported to reduce fluoride concentration down to 1 mg/L. The activated alumina can be regenerated with dilute sodium hydroxide and sulfuric acid with this regenerant solution being sent back to the pretreatment system for neutralization and fluoride removal by precipitation.
My final recommendation is that you engage the assistance of your chemical supplier to perform bench-top treatability studies in order to determine the most cost-effective treatment to consistently achieve compliance. I would start with calcium chloride addition and its variations, moving next to lime and then magnesium hydroxide. If none of these produce your desired results, the likely successful combination will be calcium chloride addition followed by pH adjustment/activated alumina after clarification.