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Article Effect of temperature, pH and DO concentration on filterabil...

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https://www.researchgate.net/post/What_do_you_think_about_Aeration_and_pH_increases

Yes, it does appear that your solution is strongly buffered! The pK of the H2CO3 and HCO3 (-1) system is 9, so you are bumping up near the edge of the good buffering range of that system near pH 8. You must have a very high concentration of carbonates in your tank to buffer all that sulfuric acid! There are a lot of chemical players in your tank that infuence pH; but perhaps one strategy to lower pH would be to precipitate some of the carbonates--unfortunately I don't know what the identity of a good/suitable precipitate would be (? maybe CaCl?) ; and perhaps there would be other problems associated with such a precipitate in your tank. I would investigate what others in the tanning industry do to more reliable stabilize tanning waste solution pH!

1. Vigorous sulfate reduction at or before the aeration tank may have contributed to the high pH above 7.7, because one of the products from sulfate reduction is HCO3-, and it may buffer pH to higher values of 7.5~8.0. Stripping CO2 can also be a cause of increasing pH if CO2 is supersaturated in the water. You can also do a test about it. Pour the inflow of aeration tank in a beaker and place an aerator (a small and cheap aerator for a mini aquarium (fishbowl) can be got in an aquarium shop). Monitor the pH. If the pH increases, stripping CO2 can be the main cause. If not, sulfate reduction and aeration of hydrogen sulfide should be minimized. If organic matter content cannot be reduced, bactericide may be applied to reduce activity of sulfate-reducing bacteria, if it is OK to reduce sulfate reduction. Besides, using ferric chloride can oxidize and precipitate hydrogen sulfide.

2. The blackish particles and odor may be caused by iron sulfides and hydrogen sulfide, respectively.

3. If you put sulfuric acid in the tank, it may also be reactant of sulfate reduction, so changing to hydrochloric acid should be considered regarding discharge criteria of chloride.

Adding sulfate just provides an electron acceptor for the production of more H2S and creates an alkalinity increase.

Check the equilibrium pH of the water with air in a filtered sample. By just aeration with outside air at 415 ppmv of CO2, the equilibrium pH will allow calculation of the carbonate alkalinity which is probably increasing by sulfate removal. ( CO2Calc software from USGS is free and works well)

For decreasing the alkalinity and equilibrium pH, using HCl or HNO3 as acids. Using nitricThe addition acid would also provide the oxygen to the interior of the flock where the ORP is lower and H2S is formed. Addition of nitrate will suppress H2S formation.

I don't know what form of Fe you adding but sulfate could be an issue.

Thanks everybody for your input!!!

I have followed most of the hints I received here on this thread.

After about a couple days or so I managed to get the pH down to reasonable levels.

I believe the culprit of this was our dosage of ferric chloride. I have since read some papers about how the ferric chloride (or lack thereof) influences on phosphate buffering and on the overall pH level. It turns out the presence of the element iron is a lot more crucial to the WWTP balance than I had anticipated.

We doubled the ferric chloride dosage and after 4-5 hours the pH of the tank came down to 7.3-7.4. Now we are closely monitoring ferric chloride levels in the tank to ensure we do not go through all this nightmare again. It's been over 2 weeks since we did this and so far the WWTP has had no more hiccups

Viktor--the ferric chloride was an unexpected culprit by me--normally it is just a minor constituent, isn't it, due to limited Fe solubility in oxygenated water? Does the Fe combine with carbonates to form low-solubility precipitates?