Contact Precipitation

The idea of developing calcium phosphate pellets to prolong the life of bone char filters was first pursued by the Water Quality section of the Catholic Diocese of Nakuru (CDN) in Kenya. The pellets are produced locally by CDN and mixed with bone char in household and community filter systems. The principle of contact precipitation is based on the following processes:

1) fast uptake of fluoride on bone char

2) slow release of calcium and phosphate from the pellets

3) subsequent precipitation of hydroxyapatite (Ca₅(PO₄)₃OH) and fluorapatite

   (Ca₅(PO₄)₃F) on bone char

The fluoride uptake capacity is considerably increased when compared to a filter using just bone char. Once the pellets have released the available calcium and phosphate, fluoride values in the water rise again and the filter material needs to be replaced. A regeneration of the pellets is not possible.

Advantages

• enhanced uptake capacity and filter life
• locally available materials, local production
• low cost
  
  

Disadvantages

• the pellets cannot be regenerated
• chemical processes are not fully understood yet

 

Bone Char / Hydroxyapatite

Activated Alumina

Nalgonda Technique

Reverse Osmosis