Hi everyone!
I am doing a filtration lab tests with flocculation. It used to work very well, but from one day to another it stopped. I didn't change anything, as far as I know. The water looks very transparent, yellow-brownish. It seems that the Fe solutes perfectly in the water and does not react/attach with the organics. When everything worked, the water was more turbid, because of all the emerging flocs.
Has anyone experienced the same? What is the reason that the Fe ions don't catch the organics?
Process parameters:
Feed water with 10 mg/l Humic Acid
Coagulant: Fe3Cl at 10 mg/l (Fe) (inline coagulation)
Coagulation Aid: Polymer at 0,1 mg/l
pH: 6,8-7
What I have done:
- Jar tests (it worked well with these concentrations). I even used the exact solutions and feed water from the setup, so they cannot be wrong/too old/poorly mixed/rotten.
- Cleaned the whole setup including pipes.
- Checked dosing pumps (they pump the right amount)
- Increased polymer concentration which usually makes better flocs even at poor conditions
Could you have a sudden increase in sodium ion. Did you measure conductivity of the water before so you can compare?
No, I did not. I take tap water to dissolve the humics, so there might be changes.
As far as I know, Na+ is a very bad flocculant, so you wouldn't even call it like this. However, would the water look perfectly clear in the presence of too much Na?
Sodium ion compete with your coagulant for the anionic groups on the humic molecules. As the ion is monovalent sodium doesn't coagulate anything.
Thanks for your answers! I added a static mixer after the Fe dosing and now I have flocs again. Don't know if the water quality changed again, or the mixer is crucial (although it has worked before without).
Why was there no formation of FeOH, if the Na ions caught the humics?
You should expect to have almost all Fe3+ precipationg as Fe(OH)3 at that pH if it doesn't bind to organics.
I found out the problem: There were minor residuals of acid from chemical cleaning in the system. The pH was not influenced, but obviously flocculation. I assume the humic acid is very sensitive to acid, because I never had this effect with other waters.
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