My understanding of clarifying (floccing) liquids is that we add a mineral with an electric charge, which attracts suspended particles (like bacteria) which I assume have the opposite charge. The particles clump together, and then the rate at which it sinks increases due to geometric reasons (the Stokes' drag on a small sphere increases proportional to its radius, but its weight increases as the cube of its radius).

My question is why a drinking-water treatment plant would use alum (aluminum sulfate) while people making alcoholic beverages use bentonite (aluminum phyllosilicate).

  • $\begingroup$ I think this is surprisingly broad. Not for the punctual question itself but for the many simplification you took, for the mechanisms of action you don't know, for the fact that flocculation removes many and different thing in the two cases and it can be done with other methods as well. Moreover the properties of water and other drinks are different in themselves and by regulation limits. $\endgroup$
    – Alchimista
    Nov 1 '21 at 7:49

Waste water, if not too polluted, is treated in wastewater treatment plants by adding aluminum salts like aluminum sulfate $\ce{Al2(SO4)3}$ in order to remove bacteria. But this solution is acidic, because aluminum sulfate produces aluminum ions that are partly hydrolyzed according to the equation $$\ce{Al^{3+} + H2O -> [Al(OH)]^{2+} + H+}$$ This acidity (plus the sulfate ions) does not disturb the wastewater treatment plant. But it would disturb drinkers if this aluminum sulfate would be added to their beverages.

Bentonite is a special porous clay that adsorbs impurities in its pores. So if you add a sort of mud made of bentonite plus water on top of a barrel of beer, it will not be dissolved. It will not bring anything to the beer. It will slowly fall down the barrel, absorbing bacteria and unwanted stuffs during its way. After one week, it has fallen down the barrel and the beverage can be recovered and drunk safely. Its quality has improved.

  • $\begingroup$ Okay, so the alum dissolves into ions, and the bentonite just breaks up into tiny solid particles. Does the mechanism by which they clump particles together differ, though? If I google "bentonite wine", I see many links talking about its "negative electrostatic charge" which is the same explanation I see for alum (though I've seen them described as having both positive and negative charges in different references). $\endgroup$
    – AshleyZ
    Nov 1 '21 at 4:53
  • $\begingroup$ I edited the question to specify that I was talking about drinking water, not wastewater. Alum is commonly used to clarify drinking water. $\endgroup$
    – AshleyZ
    Nov 1 '21 at 5:05

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