Dispose pure HMPA in a separate bottle.
Waste solvent cannisters remain unclosed during collecting the waste. Therefore it is a risk to fill in highly toxic compounds. You may avoid this problem, if you add the HMPA-waste at last before closing the cannister. Ensure, that the quantity is small enough that there will be no reaction with the other waste.
HMPA is completely decomposed on heating in conc. hydrochloric acid for 5 hours. Per Mol HMPA (179 g) 750 ml of the acid are needed. Install an gas outlet pipe into the fume hood's flue and boil the mixture vigorously. Take care to use enough of the acid because otherwise the deactivation may not be complete.