Use of Ferric sulphate as a coagulant for water treatment

A. 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 sulphate 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 ⇦[this on on Amazonaffil links]

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 sulphate will increase the TDS, as the Fe precipitates as hydroxide, leaving sulphate ion in solution. Soluble sulphate 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 ⇦this on eBay or Amazon [affil links] , 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.

A. Hi, Alex. I think the best way forward would be contact a major supplier of ferrous sulfate ⇦this on eBay or Amazon [affil links] 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,