The Use of Carrier Water for Dilution of Aluminium Sulphate

[tipp79]

I'm currently involved in the upgrading of a coagulant dosing system and have seen conflicting guidelines regarding the use of carrier water for dilution of aluminium sulphate - some guidelines advise against diluting metal salts before mixing to prevent hydrolysis occurring too early & some guidelines indicate dilution is acceptable to a minimum conc of 3-5% as metal salt in the carrier water line. The current system is sized to give minimum line velocities, regardless of coagulant concentration.

What has other people's experience on the forum been with regard to the use of carrier water and aluminium sulphate

 

[SteveWag]

I have had problems with too much dilution water. %5 is as high as I would go. Also, make sure that final mixing with the water to be treated takes place at a location with high energy mixing. Too much dilution and/or low mixing energy will cause alum feed rates to sky rocket.

Steve Wagner

 

[bimr]

It is common to dose water chemicals at 5% solution strength.

Hydrolysis of alum is mentioned in the AWWA Water Treatment book although there is no source listed for the information.

However, hydrolysis of alum is probably not probable in a normal water treatment application. Aluminum sulfate poorly ionizes and you would need to raise the pH above 3.5 for hydrolysis to occur. That means that you would have to start out with a dilution water having a high pH for this to happen.

Liquid alum is actually 56% water, so water content is not really the problem with hdrolysis of alum, rather the problem is the mixing of the alum with a water having a high pH.

Here is a link for further infomation:

http://www.genchemcorp.com/markets/water/faq_03.shtml

"If the pH of the finished solution goes above 3.5, bring it back down by adding more alum or less water. This will prevent excessive hydrolysis in the stored product, which can reduce efficiency because the alum converts to aluminum hydroxide rather than remaining as an active form of aluminum. In some cases, a 1% to 5% solution can promote hydrolysis. Watch for increases in dilute coagulant solution pH, especially when using solutions made in day tanks."

 

[clonmel79]

Cheers for the advice. One more question though.

When the guidelines give a conc could someone clarify whether the % conc they give is as Al2O3 or Al2(SO4)3? If it's Al2O3 this doesn't allow much dilution of the 8% stock soln strength

 

[bimr]

% conc is as Al2(SO4)3.