Ferric Chloride vs Ferric Sulfate 1

[adesbarats]

Hi Folks,

I would like to get some feedback on the following:

We are trying to remove phosphorous from aquaculture effluent prior to re-entry to a nearby river. We chose ferric chloride and a high molecular weight anionic polymer based on successful jar tests.

Since then, another chemical supplier has entered the scene and is telling our client that ferric sulfate is a better option for three reasons a) it is higher molecular weight and hence the floc will display better settling rate in our clarifier tank b) ferric sulfate will produce less discoloration upon re-entry to the stream and c) it will not corrode the stainless steel belt filter (he has the company convinced their belt filter will fall apart within the next year or so).

My counter argument has been that ferric sulfate and ferric chloride both function in the same way - ie, they dissociate in water in the presence of alkalinity to produce ferric hydroxide and it is the ferric hydroxide component which then reacts with the phosphorous to produce a precipitated floc. So although ferric sulfate does have a higher specific weight versus ferric chloride it is only because of the sulfate portion which plays no role in the settling process. Am I correct with this statement?

Regarding stainless steel corrosion, I did submit that ferric sulfate is less aggressive versus ferric chloride but we are dosing at such low levels (60 ppm ferric) with 30 sec retention inside a PVC flocculator prior to the SS belt filter that corrosion is not a concern. For example, we have dosed at 1000 ppm ferric chloride directly into a SS flocculator w/o any issues providing we used an injection quill.

Regarding discoloration, in jar tests the ferric sulfate did produce noticeably less discoloration versus the chloride so I will give him that. Does anyone else have experience with ferric sulfate vs ferric chloride with respect to water clarity post treatment?

Regarding my first two points, are my statements correct? The ferric sulfate will be significantly more expensive for my client so at the end of the day, I want to be sure they are getting the best choice both performance and operating cost wise.

Thanks..

 

[moltenmetal]

If the water is hot (> 40 C), chloride is potentially a big concern. Not hot? Much less of a problem. You're growing rather than cooking the fish, so water will probably be cold- and there's probably at least some chloride in the water already at the 10s of ppm level.

Both work the same way as you correctly note, by the formation of Fe(O)OH flocs. The counterion molecular weight shouldn't matter.

 

[adesbarats]

Thanks Molten. And yes, you are correct - we're not cooking the fish! System temps in this application would max out at approx 14C so yeah, fe-chloride at this low a dose should be ok.

The fe-sulfate does seem to produce a clearer effluent which I find interesting and would like to understand. If both of these metal salts produce the same molecule, namely FeOH, then why does sulfate produce a clearer effluent while fe-chloride produces an effluent with a slight brown tinge?

 

[moltenmetal]

Unless the effluent is acidic, I don't understand your observation. Neither the Cl- nor the SO4^-2 counterion should matter to the colour of the effluent if the Fe+3 completely precipitates as the Fe(O)OH. If there isn't enough alkalinity around to do that, or there's some other ligand present which keeps Fe+3 soluble at neutral pH, you'll have some unprecipitated Fe+3 and an orange colour in the effluent using either starting material.

Be aware that you might be regulated on the colour of your discharge, so this might matter. If so, paying extra for the sulphate might be cheaper than spending the time to figure out what's really going on.

Sand filter the water and see if the colour persists. If it does, analyze the filtrate for iron. That might help you to figure out what's happening.