15
Activated carbon
AFM® is used with activated carbon in combination with chlorine or other oxidising agents, the filter bed will usually be
grade 1 AFM® with a 50mm to maximum 100mm layer of activated carbon. It is very important not to use any more than
100mm of activated carbon, to prevent the carbon from becoming a biofilter. A small amount of activated carbon works
well as a catalyst, but any more than 100mm could start to cause issues resulting from biofouling of the carbon
The following reactions will take place on the surface of the activated carbon. In the first stage the hypochlorous will oxidise
the surface of the carbon for form very active CO* sites. By this mechanism, activate carbon will remove some of the
hypochlorous from the water.
Stage 1.
HOCl + C* → CO* + H + Cl -
The chlorine will also react with chemicals in the water such as ammonium to form inorganic chloramines such as
monochloramine, and organic matter to form organic chloramines.
Monochloramine HOCl + NH 3 ↔ NH 2 Cl + H 2 O
In addition to monochloramine, other inorganic chloramines including;
- dichloramine and trichloramine, but this is a function of pH and water chemistry.
- Organic chloramines, formed by reaction with protein and amino acids.
The mechanism by which chloramines are catalytically oxidised by activated carbon in the presence of chlorine are as
follows;
Stage 2a
NH 2 Cl + H 2 O + C* → NH 3 + CO* + H + + Cl –
Stage 2b
2NH 2 Cl + CO* → N 2(g) + C* + H 2 O + 2H + + 2Cl –
The end products will be nitrogen gas, hydrochloric acid and water as well as carbon