Hello Umiar,

How did the calcium get there and what is the concentration?

Regards, Paul. 

Why can't you get a chelating agent specific for calcium?

Why not dissolve away the phosphorus and leave the calcium behind? Tie up the calcium with sulfate to make it insoluble.

Hi Umair, 

Did you try using different chelation agents as the ones in the attached file. How much Ca & phosphate load you have? what is the pH? low pH extracts phsophorous without affecting Ca much. 

Regards,

Hi Umair, 

The best way to remove phosphate from the sludge is use natural coagulants. This will remove phosphate and leave the calcium as such. Again it depends on the concentration of the phosphate and calcium. 

Thank you so much for all of you taking keen interest towards my inquiry. But I think we need more discussion of figure out the problem. According to my little knowledge I try to discuss all the answers and I hope you all will again suggest me more ways to combat this phenomena.

Dear  Dr.Paul,  Concentration of Ca is about 13 mg/g, P is about 43 mg/g of sludge, while it has high Al content that is 77 mg/g of sludge.

Dear Dr. Imam and Dr.Aasem Zeino, I have already used EDTA for specifically binding Ca but the results are not promising. The pH of original sludge is 6.4, low pH also dissolve P which is not desirable. If we can specifically dissolve Ca then free P will bind with Al which is already present in it and can be leached out by Alkaline leaching step. Still the problem is that we donot want to lose our Phosphorus with Ca dissolution.

Dear Mark Krause, I have already used alkaline solution (1.25 M NaOH) for dissolving the P, but still acidic dissolving P remain and then we use acidic leaching for Ca or appatite bound phosphorus to recover but still Ca remain inside which interfere with our final product. Usually people used ash and before using alkaline leaching, a pretreatment with acid performed in order to make Al-P which could be dissolve in alkaline. The problem remain the same some Ca remain in it and interfere the final outcome.

Dear Dr. Richard Thilagaraj, Al and Fe are already present in it so the sludge is already in form of Precipitants.

EGTA is much more Ca specific than EDTA