By Frank DeSilva
The US EPA limit for fluoride in water is four mg/L, with a secondary (recommended) level of two mg/L to protect against cosmetic dental effects linked to excessive fluoride consumption. The State of California limits fluoride to two mg/L in drinking water. Excess fluorides can cause mottling of tooth enamel in children under the age of nine. In adults with weakened bones, the potential for fractures is increased.
It is estimated that there are about 200,000 Americans with drinking water in excess of the US EPA limit of four mg/L. Roughly 1.4 million Americans have fluoride in their drinking water at a level between two and 3.9 mg/L. Two-thirds of Americans drink fluoridated water supplies that contain fluoride at approximately one mg/L.
Anion exchange
High levels of fluoride ions can be present in some industrial waters as a result of glass manufacturing, electroplating, steel, aluminum, pesticide and fertilizer manufacturing.
Fluoride at these higher concentrations can be removed by lime precipitation as calcium fluoride, with effluents expected in the range of 10 to 20 mg/L.
This treated wastewater can then be polished with an ion exchanger. Low concentrations of fluoride ions can be removed by ion exchange using a strong base anion resin.
The use of a type 2 strong base anion resin in low concentration removal is preferred for POE potable water applications to minimize the introduction of taste and odor to the potable water that might occur from type 1 strong base anion resins. Effluent levels in the range of much less than one ppm can be expected from a typical fluoride influent.
Regeneration of anionic resins is accomplished by the use of sodium chloride applied at a dosage of five lbs. (2.26 kg) per cubic foot at five percent solution strength with a minimum contact time of 30 minutes. The operating capacity is 12 kilograins per cubic foot for all anions except silica and CO2.
Of importance is that the anionic resin is not selective for fluoride. Rather fluoride is the least strongly held anion. This means that the anion unit, if over-run, can dump the collected fluoride into the drinking water at levels higher than the inlet.
When sizing a system for fluoride removal using ion exchange as the removal mechanism, put plenty of safety into the design, discount the predicted throughputs and consider the installation of a lead–lag system with a working bed and a polishing bed. County and state regulators usually like to see the lead-lag system that contributes to the inherent safety.
The strong base anionic system should be especially considered for this application when removing other anions, such as nitrate, arsenic or uranium. This will hold the contaminants more strongly than fluoride and can be easily removed by the resin.
Brine concentration and dosage may have to be increased, depending on which of these contaminants is being addressed; for instance, uranium applications call for brine dosages of 15 to 20 pounds (6.80 to 9.07 kg) per cubic foot. When looking at multi-ion removal systems, be sure to get the complete inlet water analysis and submit these numbers to the resin manufacturer for a performance projection.
As with any chloride cycle anion exchange service, pH will drop during the run as alkalinity is reduced.
Sizing the anion unit is simple. The recommended service flow rate is two to four gpm per cubic foot (7.57 to 15.14 L/m). For capacity estimates, sum up the anions that are removed by chloride form anionic resin, calculate the loading in grains per gallon and divide the capacity of the resin by the grains per gallon. (See examples in Tables 1 and 2.).
Activated alumina
Fluoride is also effectively removed by activated alumina, which is a synthetic aluminum oxide that removes fluoride more selectively than anion exchange resin. Capacities of up to two kilograins per cubic foot can be achieved with the use of this media. Service flow rate should be about one gpm per cubic foot (3.78 L/m) for best results. Capacity is significantly influenced by the alkalinity, so be sure to have an accurate water analysis performed.
The best removal is seen when raw water pH is in the range of 5.5, where the attraction of the fluoride ions is the greatest and the interference of competing ions is the least. At the beginning of the service run, the effluent pH will be high and the fluoride removal will be minimal. The bed will begin removing the fluoride effectively as the pH within the media equals the lower pH of the influent raw water.
As the effluent pH drops, fluoride removal becomes more effective and the water is suitable for use. (See Effluent pH Curve). Many larger fluoride removal systems use pH adjustment with acid to optimize throughput capacity.
Regeneration process
Regeneration of activated alumina requires an upflow and a downflow rinse of one percent sodium hydroxide followed by a bed-neutralization step with low pH influent water. This somewhat complex regeneration, plus the required handling of acid and caustic, makes this a somewhat undesirable procedure for the typical POE application, with most such systems using activated alumina on a disposable basis.
Begin by backwashing the bed at eight to nine gpm per square foot (30.28 to 34.07 L/m) for approximately 10 minutes. This should expand the bed approximately 50 percent. Be careful not to backwash the material out of the vessel. This backwash step is very important to remove any suspended solids that have been filtered from the inlet water and also to alleviate any packing the bed experienced during the downflow service run.
Regenerate the bed with one percent sodium hydroxide (NaOH). Upflow regeneration at 0.25 gpm per cubic foot (0.95 L/m) is preferred for best results, followed by a downflow caustic dose. The caustic dose should be approximately three pounds (1.36 kg) per cubic foot and the contact time should be at least one hour.
Slowly rinse the volume of one tank to displace the caustics. Then fast rinse it with raw water through the bed that has been adjusted to a pH of approximately 2.5 downflow, until the effluent water pH reaches 9.0 to 9.5.
Adjust the influent raw water to a pH of 4.0.When the effluent water pH reaches 8.5 or less, it is safe to return to service. Adjust the influent raw water to a pH of 5.5. A pH of 5.5 should be maintained throughout the remainder of the service cycle to maximize throughput capacity.
POE applications
There are two viable methods of removing fluoride with media for POE applications–ion exchange and activated alumina. The capacity of the ion exchange method is determined by the TDS of the influent water because the media is non-selective.
The capacity of the activated alumina method is determined by the water chemistry, primarily the pH of the influent water. It is of utmost importance that the water treater collect the necessary water chemistry data and submit them to the media supplier so an accurate capacity throughput calculation can be made.
The analytical parameters needed for this include fluoride, sulfate, nitrate, chloride, alkalinity and pH (measure on-site for best accuracy). Other contaminants of interest include arsenic, uranium and lead.
About the author and company
Francis J. ‘Frank’ DeSilva is the National Sales Manager of ResinTech, working from the company’s Los Angeles, CA office. He has been employed in the water treatment industry for over thirty years. DeSilva has a Master of Science in environmental engineering from New Jersey Institute of Technology and a Bachelor of Science in environmental science technology from Florida Institute of Technology. He has authored numerous articles on ion exchange and related topics and is currently on The Board of Directors of the Pacific Water Quality Association. DeSilva also serves on their Technical Committee. He can be reached at (760) 809-4864 or [email protected].
ResinTech, Inc. is a manufacturer/supplier of ion exchange resins, activated carbon, and specialty medias to the water and wastewater marketplace.
Index page: Photo courtesy of ResinTech and Pureflow Filtration Division, and Don Pollard Photography and Imaging