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Use of Ferric Sulfate as a coagulant for water treatment

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We are dealing in the Processing of Sports Socks. We are at the moment in a phase of erection/commissioning of Effluent treatment plant to treat the Waste Water of our Dye-House. I have some technical questions.

1- What is the main difference between Ferric Sulfate and Ferrous Sulfate to be used as a Coagulant for the plant.

2- Is there possibility that Ferric sulfate after working as a coagulant will further increase the TDS of the treated water?

3- How Ferric Sulfate will chemically interfere the process of the effluent treatment to be act as a coagulant?

Ijaz Hussain
manager processing - Faisalabad, Punjab, Pakistan


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You haven't given enough information on the effluent composition, treatment processes and efficacy, and effluent limits. In any event, some jar testing‹ should be done. 1) Ferric sulfate is superior as a coagulant, especially for co-precipitating other metals as hydroxides. "Not only do the Fe(III) cations as the positively charged basic salt ions neutralize the negative charge on finely dispersed precipitates, they also (in slight excess) act as flocculating agents. In consequence, readily sedimentable and easily filterable materials are formed". L. Hartinger, p. 194 in "Handbook of Effluent Treatment & Recycling for the Metal Finishing Industry" [link is to product info at Amazon]

However, if your treated effluent contains residual excess oxidizer or Cr(VI), Fe(II) may be advantageous as a reducing agent. Also, Fe(III) compounds are more expensive than Fe(II), and their solutions are less easily pumped due to lower solubilities. It may be preferable to use Fe(II) and then convert it to Fe(III) by air agitation or via reaction with excess oxidizer as mentioned or with hydrogen peroxide.

2) Yes, either Fe sulfate will increase the TDS, as the Fe precipitates as hydroxide, leaving sulfate ion in solution. Soluble sulfate can be lowered to ~2000 ppm with lime, but this may be too high for effluent limits, e.g., for reuse in irrigation. You can add iron using ferric chloride, but chloride is also difficult to remove (except by energy intensive processes such as reverse osmosis). You can also add iron ions without the undesirable anions by passage through scrap iron or an electrocoagulation unit with mild steel plates.

3) Add the coagulant after chemical treatment (and before final pH adjust and polymer addition); it should not interfere with the treatment. It should improve flocculation and settling.

Ken Vlach
- Goleta, California  

Ken received a special
"Contributor of the Year" award
from finishing.com for his numerous
helpful and well researched responses


February 3, 2010

I am looking for Reference Water Treatment Plants using Ferrous Sulfate as coagulant we could contact ?

Alex Moubarak
Tampa, Florida


February , 2010

Hi, Alex. I think the best way forward would be contact a major supplier of ferrous sulfate and ask them for the names of a few of their water treatment customers. Alternately a literature search for technical articles on the subject should get you there. Good luck.

Regards,

Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

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