Flocculation
Colour Removal
The invisible Strengthener
Colour removal is a completely different process than particle removal. Particles are distincly different from the solution they are in, they are "not in solution". Consequently they scatter (bounce back) light and are picked up by a turbidity meter. Colour is caused by molecules which are in solution and are not picked up as turbidity but they absorb light of different wavelength and are picked up by a colorimeter. The main molecules responsible for colour in water are humic and fulvic substances.
Now we are all used to molecules as very, very tiny, such as the water molecule (0.278 nm). However, the molecules that make up colour are much, much larger than that. Fulvic acid molecules have a molecular weight of about 700 to 15,000, while humic acid molecules have a molecular weight of 15,000 to about 70,000, though humic acid molecules with a molecular weight of 10,000,000 have been reported. An example of a molecular structure of fulvic acid is given below:
What I am trying to get across to you is that compared to our polycation these molecules are very large. They are negativly charged and will therfore readily combine with a positivly charged polycation. They will be destabilised by free iron or aluminium ions and can become intangled in the larger polycation chains/networks.
Because humic and fulvic are molecules they are held together by very strong bonds (covalent bonds) which are very difficult to break. We would assume, therefore, that a flocc which is full of humic acids is much stronger than a flocc without humic acids. Fulvic acids would have the same effect but to a lesser extend. This is a assumption which seems to hold true in practice and we can put this down as another rule.
RULE 5: Highly coloured waters form a strong flocc
There is something else we need to consider though. During low rainfall the size of the colour molecules is, on average, lower than during high rainfall conditions. Larger molecules means a stronger flocc and less need for poly-electrolytes.