What is the downside of using carbon dioxide to reduce the pH of alkaline waste water from an anodizing plant?

Question

We are considering switching from acid to carbon dioxide since we're rejecting too much sulfate according to the environmental regulations. The only information we are getting about this new technology is from commercial people, but we are interested to know practical and actual information about the disadvantages of using carbon dioxide in the pH reduction process.
If you have used this method then please describe the advantages and disadvantages of adopting carbon dioxide in pH reduction process.

Guido Bönig-Martius · Accepted Answer

First of all I think the sulfate load, which is too high in your case, has its origin not mainly in the neutralisation process of alkaline waste waters. The origin is mainly the anodizing bath itself. Sometimes you have to dump a part of this bath to lower the Aluminium content and this discharge has a huge load of sulfate (aliminium sulfate and sulfuric acid). This load can be lowered with using e.g. an acid Retardation plant. This plant can recover a big part of the free sulfuric acid. Of course the sulfate as Aluminium sulfate will remain in the discharged water. Now the neutralization process with carbon dioxide:
Advantages:
no salting of the waste water, which is ecologically advantageous; no handling of aggressive concentrated acids; lowering of sulfate (or Chloride when using hydrochloric acid) content in the waste water
Disadvantages:
handling of liquid gas, new neutralization plant is necessary; higher Investment costs (carbon dioxide tank, gassing system)
Operating costs: They depend on the costs for acid and carbon dioxide. My experience is that the operating costs of carbon dioxide neutralization are lower than neutralization with hydrochloric acid but higher than neutralization with sulfuric acid.

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