Ferric Sulfate, Phosphorus precipitation & pH

[Subystud]

Hello -

The biosolids treatment process I work with uses chemistry to produce Class A biosolids. An acidification phase occurs in this process where the pH is lowered to 1.5 to 2.0 for 6 hrs. Ferric is added at the initial phase, the pH lowers from about 7.0 to 6.2. Sulfuric acid is then added to bring the pH to 1.5 to 2.0. At the end of the process, the pH is elevated to

Question: is this counter-productive for precipitating phosphorus? A colleague mentioned that binding phosphorus with Ferric and then lowering the pH re-solubilizes the phosphorus

Thanks -

Rob

 

[bimr]

Your colleague is correct. However, it is a complex question because of the numerous different processes that are used to remove phosphorus in wastewater treatment plants. The processes may be biological or inorganic, or a combination.

The phosphorus removal methods that are biological will have some of the phosphorus bound in the cellular structure of the treatment plant activated sludge microorganisms. The bound phosphorus is less likely to leach out of solution with a pH change.

The inorganic processes will be likely to leach phosphorus with a pH change.

What process are you working with? Most of the Class A processes elevate the pH instead of dropping the pH.

 

[Subystud]

Neutralizer process.

This would be inorganic precipitation. We utilize 12,500 gal well-mixed tanks with thickened sludge and produce ~ 4000 dry lbs Class A/batch. Ferric is the first chemical we add, then the process proceeds. This should change to adding Ferric at the end of the process.

Can you speak to how quickly inorganic precipitation occurs? I understand it be almost an instantaneous reaction.

Interesting point on the biological v chemical precipitation.

Thanks!

Rob

 

[bimr]

The website states that phosphorus is being precipitated with ferric. This precipitation should occur relatively quickly. I would expect the solids to settle out quickly as well. It seems like adding the ferric as the last step would make more sense.

The advertisements say that nitric acid is being used. You say that you are using sulfuric acid. What is the final pH?