A dehu pulls air from the living space. Living space air is statistically heavy in contamination.
A large portion of the "dust" in the air is dead skin cells. the dust that floats in the air and clings to the coils, condensing to water and running off is likely whats breaking down into ammonia.
Also, any chemicals sprayed or used in the home, the vapors would be pulled through the coils and collect in the pan.
If you were to treat the airspace with a serious decontamination device and air purifier (like the activetek 2000) and install a UV light over the coil and drain pan, then maybe you could use that water, but i would still test it first.
If it uses refrigerant (R410A R134A R22) then a leak would not cause the ammonia.