If the water in the solution is allowed to evaporate, some soluble metals can precipitate as metal sulfates. Did you do a total volatile solids test? What type of sludge - industrial or domestic sewage? Proteins reacting? You should analyze the solids.

Hi Mohan,

Metal sulfate seems to be a very likely explanation, as I am also seeing higher dose of persulfate causing higher total solids content. I am planning on running some volatile solids test next week. The sludge is from domestic sewage. 

Thank you very much for your reply. I really appreciate your help.

Did you account the solids from sodium persulfate?? By running a blank without Sludge but with the same amount of persulfate??

Hello Arun,

Thanks a lot for your input. The persulfate I am adding to the sludge is supposed to be consumed by solids in the sludge. If I add persulfate in a blank (may be water), the consumption will not be same. I am not sure a blank would be comparable to sludge sample.

Thanks again.

Hello Arun,

Thanks a lot for your input. The persulfate I am adding to the sludge is supposed to be consumed by solids in the sludge. If I add persulfate in a blank (may be water), the consumption will not be same. I am not sure a blank would be comparable to sludge sample.

Thanks again.

consumed means converted - sulfate will stay - oxygen (per) will leave as oxygen, CO2 or stay as oxidation product in water. So mass contribution will be in between sulfate and persulfate.

how much the sample you used to determine the solid content, before and after treatment??