Activated Alumina

Using granulated activated alumina to remove excess fluoride from drinking water is a widespread method and was first implemented in the United States in the 1930’s. The raw material, Al₂O₃ is produced as a by-product during aluminium smelter. Activated alumina has a very high surface area and can bind inorganic ions such as fluoride on its surface sites.

Al-OH + F^- ↔  Al-F + OH^-

This reaction is pH-sensitive and works best in the optimal pH-range of 5-6. Above a pH of 8 the method becomes ineffective. When all surface sites are occupied the filter cannot take up more fluoride and needs to be regenerated. This is done by treating the activated alumina with a strongly alkaline solution (e.g. NaOH) to reverse the reaction, followed by a strong acid (H₂SO₄) to re-establish a positive surface charge.

Advantages:

• high adsorption capacity
• implementation experience from many different countries
• well accepted amongst users
  
  

Disadvantages:

• Fluoride removal efficiency decreases with increasing pH
• potentially expensive if chemicals/filter material are not locally available and have to be imported
• regular maintenance needed to regenerate or replace filter material
• adsorption capacity deteriorates with each regeneration cycle
• acidic/alkaline waste that needs disposing

 

Bone Char / Hydroxyapatite
Nalgonda Technique
Contact Precipitation
Reverse Osmosis