Re: Does water treatment with alum salts

The treatment of water with alum salts is effective at removing colloidal 
material. Most colloids have a negative surface charge. Aluminium ions 
adsorb strongly onto these surfaces, and help the colloidal material to 
coagulate and settle out.

Aluminium ions are very insoluble in water at normal pH values. Unless the 
water is very acidic (pH 4 or less), the aluminium ions from the alum will 
slowly form an aluminium hydroxide precipitate, which will also settle out. 
I do not think there is a problem with residual aluminium, provided the 
water is left to settle for some time, and filtered.

It is fairly easy to test water for the presence of dissolved aluminium. We 
have such an experiment in the first year of our environmental chemistry 
laboratory. We have had difficulty in finding waters that even give a 
detectible result. I boiled some water containing lemon juice in an 
aluminium billy. It gave a measurable result a day later. But when we did 
the same experiment the following year, the residue of that water had a 
visible aluminium hydroxide cloud, and gave a zero test result.

If ever there were a problem with significant amounts of dissolved 
aluminium in water, the very acidic water involved could easily be 
neutralized with a small amount of lime or magnesia (this would make the 
water 'hard' as a side effect, but not make it toxic or unpalatable). Any 
aluminium in solution would be quite effectively removed.

It would be worth checking with the local water supply authorities. They 
sometimes do use alum, and they certainly monitor the water supply 
regularly for dissolved aluminium, and would have safe and effective 
remedies to remove it if it were found.