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Help Needed! Aluminium Sulphate Dosing

Postby Dingle » 19 Nov 2010 10:24

I have a quick question about aluminium sulphate. The bags that have been supplied say that the product is 17% purity. Is aluminium sulphate similar to chlorine in that you need to increase the dose to take into account the active product (e.g. with chlorine, if you are using 70% HTH you need to increase the dose by a factor of 1.4 to take into account the strength of the product). If I am assuming a typical dose of 100mg/l for treating the water - does this mean that I need to increase the amount of aluminium sulphate by a factor of 5.8? e.g to give a worked example: to treat 10,000 liters of water I need 10,000 liters x 100mg = 1 kg of aluminium sulphate (assuming 100% strength) or 5.8 kgs of aluminium sulphate (assuming 17% strength). Thanks for the help.

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Re: Help Needed! Aluminium Sulphate Dosing

Postby Caetano Dorea » 19 Nov 2010 11:15

If you want to calculate your alum dosage based on the amount of active ingredient, then yes you need to take into account the purity of it. However, you'd be making an assumption that the product has the purity stated by the manufacturer. The fact of the matter is that even if you do this you will then need to recalculate the actual amount (either weight or volume) of the product needed.
So, to avoid this calculation and recalculation you can actually disconsider the purity of the product. Make up your stock solution with known quantities (volumes are usually easier to measure in the field) of alum, determine your dose with jar-tests, and dose accordingly.
This is analogous with the modified Horrock's method of chlorination (note that the strength/purity of the product is not needed for dose determination).
Hope this helps.
Caetano

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Re: Help Needed! Aluminium Sulphate Dosing

Postby MathieuLC » 19 Nov 2010 11:40

Hi,

To add to Caetano's answer a really low-tech jar-test can be done with a series of identical buckets filled with the to be treated water. You inject different doses of your alum mix into the buckets stirr each bucket with the same timing and vigour. You then look at floc size and measure turbidity.

A low-tech jar-test is commercialised in Cambodia by DTW : http://www.irc.nl/page/16750 I do not know if they export.

All the best,

Mathieu

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Re: Help Needed! Aluminium Sulphate Dosing

Postby HappyPlumber » 19 Nov 2010 12:04

Hey Dingle, as Caetano and Mathieu say, a jar test is crucial to efficient alum dosing. However take care as it's still easy to screw up and overdose. Back in 2004 in Chad I ended up getting confused with this 17% issue and ended up overdosing in our treatment plant. It's easily done especially if you are trying to treat water with colloids. If you overdose you end up with a process called 'sweep coagulation' which is different to the classic 'charge neutralization' coagulation. Ideally you want to be dosing at the low end of the 'charge neutralization' curve. Happy dosing!!

Sweep Coagulation.jpg


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Re: Help Needed! Aluminium Sulphate Dosing

Postby Caetano Dorea » 19 Nov 2010 12:05

Yes, by "jar-test" I meant a field jar-test with beakers/buckets and whatever is available as a stirrer.
Interesting idea, but the link (within the IRC page) doesn't work.
Caetano

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Re: Help Needed! Aluminium Sulphate Dosing

Postby MathieuLC » 19 Nov 2010 12:14

Aie !

I've sent a message to my colleagues in Cambodia to have some up to date information on this equipment.

Cambodia has many private operators running small treatment plants so from a development perspective the simplified jar-test equipment was a good idea... I haven't followed that question closely in a while!

Sorry for the dud link!

Mathieu

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Re: Help Needed! Aluminium Sulphate Dosing

Postby Caetano Dorea » 19 Nov 2010 12:25

In practice, considering the relevant contexts to this forum, it is very difficult to know in which coagulation you are in. My view is a more pragmatic one, that it doesn't really matter so long as you are getting the final turbidity you want.
From our research, we have seen that in most cases considering bulk emergency water treatment the prevailing coagulation mechanism is in fact that of sweep coagulation. Perhaps just a technicality.
Caetano

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Re: Help Needed! Aluminium Sulphate Dosing

Postby HappyPlumber » 19 Nov 2010 12:45

Very true, but if you are operating in a range where both 'sweep coagulation' and 'charge neutralization coagulation' are acheving the same level of turbidity reduction, then it is better to take the lower dosage method - especially if you can reduce your consumption by 1/5th and you are trucking in alum from Cameroon!! Because I've screwed up in the past I like to do a jar test with more than the usual 4/5 samples (sometimes I'll do ten) just to make sure I'm using the lowest dose that does the job!

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Re: Help Needed! Aluminium Sulphate Dosing

Postby Caetano Dorea » 19 Nov 2010 13:02

Yes, lowest dose is best as it's more economical... no doubt about that!
Charge-neutralisation is measured usually with zeta-potential, so the actual verification that you are in that mechanism zone is not possible in the field. Where I was getting to is that the lowest dose you refer to may still be (and probably is) in the sweep zone, considering typical field practice and conditions.
Sweep coagulation may use more coagulant than charge-neutralisation, but it also has some advantages such as more robustness to withstand water quality variations (e.g. turbidity). Charge-neutralisation is much more sensitive with this respect.
Caetano

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