Defluoridation

Defluoridation refers to methods of water treatment that reduce the concentration of fluoride in the water, normally, in order to make it safe for human consumption. Some water treatments that have the capacity of reducing the fluoride concentration along with most other anions, or anions and cations, in the water, are not considered as defluoridation methods. Thus general demineralising methods like distillation, reverse osmosis, electrodialysis and resin de-anionisation, which are able to remove fluoride fully or partly from the water, are not considered as defluoridation methods. On the other hand methods that only remove fluoride without any addition or reduction of other parameters are not yet discovered. That's why the expression “fluoride removal” lacks precision. Defluoridation is used to characterise methods that reduce the fluoride ion specifically, without major other changes to the quality of the treated water.

Defluoridation of water differs from normal piped water treatments:

• Because fluoride does not deteriorate the piped water quality neither technically nor oganoleptically, it is normally only required for the small part of the water that is used for drinking and cooking.
• Defluoridation of large quantities of water, out of which only small part is used for human consumption, is environmentally unsound. This is because of accumulation of correspondingly large quantities of toxic sludge that would create a new problem, more severe to deal with.
• Most large cities and towns do not need defluoridation, because they are historically built in sites privileged to have good water resources.
• Defluoridation is easily avoided in industrialised countries, as they have no difficulties in prospecting and transporting low fluoride water.

Defluoridation, is only required in small scale, mostly in rural areas in developing countries. As such, many defluoridation approaches have been launched, proclaiming success, without unbiased field proof and thorough optimisation. Further confusion comes with the fact that the process or the technical set-up that may work in one context of socio-economic and environmental conditions may fail in another. Local availability and acceptability of the required materials, fluoride contamination level and water quality are major factors to be considered when selecting the process and the design that minimises the capital and running costs.

Defluoridation technology has to be simple, affordable, reliable and operational at least at three different levels

• Domestic level, e.g. in the kitchen where piped water may or may not be available.
• Institutional level, e.g. in schools or a working place where piped water may be available, and the defluoridator is protected from theft or misuse.
• Community level, e.g. in a village or a market where the defluoridator is supplied with water often intermittently through a pipe or a hand or power pump.

Three treatment processes

Sorption

A sorption process is normally designed in a plug flow filter column, including a medium that has a certain capacity of absorption, adsorption or ion exchange of the fluoride. This process requires recharge or regeneration of the medium upon saturation. Numerous media are known to have defluoridation properties, as listed below.

However, due to capacity, availability and subsequent water quality limitations only bone char and activated alumina are worth mentioning. The article discussed plants based on bone char sorption, which is the process of choice if acceptable by the community.

Alternatively, similar set-ups can be utilised in the activated alumina process. This is often preferred e.g. in strictly vegetarian societies that consider the use of (cow) bone char as unethical.

Sorption design

The original discussion considered daily water demand, media capacity, raw-water fluoride concentration, operating period and bulk density. The equation images did not survive in the available page capture. A detailed treatment of small-community systems is available in The State of Art of Small Community Defluoridation of Drinking Water.

Co-precipitation

A co-precipitation process is often carried out in batch containers, where the precipitating chemicals are totally mixed with the raw water. This process requires filling of water and chemicals, mixing and settling and subsequent withdrawal of the treated water and the produced sludge. In the so-called Nalgonda technique high dosage of alum and lime are used in a coagulation/sedimentation process, where the precipitation of aluminium hydroxide instantly binds a part of the water fluoride.

Nalgonda process design

The original article emphasized that design depends on the source water, chemical quality, pH, alkalinity and verified performance. Its equation images were not preserved. See the complete workshop paper on small-community defluoridation for the technical discussion.

Contact Precipitation

Contact precipitation is a process where the water is mixed with the precipitating chemicals, being calcium and phosphate ions, flowing into a catalytic filter column. So far this process is only known to operate in a fluoride-saturated bone char, being the catalyst, dosed with calcium chloride and sodium di-hydrogen phosphate. Contact precipitation is considered to be the process of choice in the future, when the local availability of bone/bone char reaches its limit.

Testing or metering

The co-precipitation technique, even though arbitrary in nature, is reliable in the sense that the same removal is obtained if the procedure followed exactly.

On the other hand the process results in poor removal efficiency and it demands burdensome daily preparation.

The sorption process is less burdensome. It is also able to provide high removal efficiencies, even at high fluoride levels in the raw water. However, the operator has to watch for the breakthrough the column at saturation. As fluoride is oganoleptically neutral, this cannot be done without either regular testing of the treated water fluoride or checking the accumulated water flow.

Testing the fluoride concentration in the treated water used to be impossible without expensive laboratory equipment. Now simple, reliable and cheap test kits are available.

The original article described a user-friendly set-up for household defluoridation. The secondary cartridge filter ensures that no toxic aluminium can escape with the treated and the water meter ensures that accumulated water flow can be compared with declared capacity.