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How can I estimate the quantity of flocculants needed for my surface water treatment station?

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This is a question that I've been asked in the field so I wanted to share it with everyone on KnowledgePoint

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You need to do a jar test, the easiest method to determine the required quantity of product to add to the raw water.

In the south of Chad, we had to set up quickly a water treatment station to supply potable water to some thousands of refugees running away from Central African Republic. The only available source of water in sufficient quantity was the large brown river passing in the outskirt of the village. The high turbidity of the raw water (NTU>50) forced us to set up a flocculation/sedimentation procedure before chlorination and distribution by water trucking.

Flocculation and Sedimentation

You add chemical products, coagulants, in the water, combining suspended matters into flocs, heavier matter that you eliminate through sedimentation. It’s very useful when the turbidity of the raw water is high and to accelerate and improve the sedimentation.

We used the most known coagulant, the Aluminium Sulphate Al2(SO4)3 efficient with a pH between 6 and 7.5. Another coagulant, the Ferric Sulphate Fe(SO4)3 has a wider pH range, between 5 and 9.

Jar test

Firstly, you prepare a 5% (50g/L) stock solution of coagulant using clean water in a 20L jerry can. Then, you pour raw water into four 10L buckets. Using a syringe, you add in each bucket respectively, 4 mL, 10 mL, 20 mL and 30 mL of stock solution. You stir vigorously for 30 seconds, then gently during 5 minutes. Leave it for 1 hour and observe the coagulation/flocculation. The smallest dose giving you the best results would be the recommended one. If the dose is too small, the flocs do not form, or are too small, or do not settle. If none of the tests is convincing, repeat the operation using different stock solution doses.

For example if the 10 mL dose of stock solution shows good results in 10 L of raw water, and if you use a 1 m3 tank for your flocculation/sedimentation, you’ll add 1L of your stock solution (convert all the figures in liter = Volume of stock solution added to the bucket x Volume of your tank / Volume of the bucket = 0.01 x 1000 / 10).

Flocculation and sedimentation in the water tank

The flocculation is improved through rapid mixing followed by slow and regular stirring. The time needed for proper flocculation/sedimentation can vary from 1 hour up to half a day. We spent one morning doing jar tests and running few tests with the 1 m3 onion tank to determine the required time for a proper flocculation/sedimentation.

The coagulant can be introduced into the tank through the suction pipe or during the filling of the tank. Then, you must transfer the clarified water into another tank to proceed with the disinfection. The sedimentation tank must be cleaned regularly and the floc disposed properly.

General recommendations for your treatment station

Remember the following point when you install and run a surface water treatment station:

  • Record: record all the ...
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Asked:
2017-09-07 14:50:11 -0500
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Last updated:
Feb 22