My best guess is it's some combination of struvite, limestone and gypsum.
Struvite is magnesium ammonium phosphate in a 1:1:1 molar ratio. It forms under alkaline conditions (that would occur where you added aqua ammonia). It has a very low solubility at alkaline pH so that's why it precipitates out onto surfaces. Some WWT plants intentionally make struvite to reduce total nitrogen and total phosphorous in their effluent.
The limestone CaCO3 and gypsum CaSO4 are formed when the pH is increased and their solubility decreases. This is a similar mechanism as cold lime softening in the water treatment world.
Depending on the capabilities of your lab they may be able to determine its composition. But that may not be very helpful for eliminating it because all three precipitates are hard to stop in the conditions that you have there.
A quick test would be to put HCl on a chunk you break off. If it fizzes greatly there is limestone in there. I would guess you could do an orthophosphate test to see if it's struvite. Maybe if neither of those tests come back positive then it's gypsum. But an atomic absorption spectrophotometer (AAS) could tell you immediately what's in it. Or an inductively coupled plasma mass spectrometer (ICP-MS) would do the trick too. I'm in the industrial world so we have these instruments in our main labs for products so I have access to them. I doubt they are common in the POTW world.
12
u/Haunting-Walrus7199 2d ago edited 2d ago
My best guess is it's some combination of struvite, limestone and gypsum. Struvite is magnesium ammonium phosphate in a 1:1:1 molar ratio. It forms under alkaline conditions (that would occur where you added aqua ammonia). It has a very low solubility at alkaline pH so that's why it precipitates out onto surfaces. Some WWT plants intentionally make struvite to reduce total nitrogen and total phosphorous in their effluent.
The limestone CaCO3 and gypsum CaSO4 are formed when the pH is increased and their solubility decreases. This is a similar mechanism as cold lime softening in the water treatment world.
Depending on the capabilities of your lab they may be able to determine its composition. But that may not be very helpful for eliminating it because all three precipitates are hard to stop in the conditions that you have there.
A quick test would be to put HCl on a chunk you break off. If it fizzes greatly there is limestone in there. I would guess you could do an orthophosphate test to see if it's struvite. Maybe if neither of those tests come back positive then it's gypsum. But an atomic absorption spectrophotometer (AAS) could tell you immediately what's in it. Or an inductively coupled plasma mass spectrometer (ICP-MS) would do the trick too. I'm in the industrial world so we have these instruments in our main labs for products so I have access to them. I doubt they are common in the POTW world.