I read the post and thought
"an extinguisher using grey water? How environmentally friendly"
Grey water - comes from wash basins, dishwashers, washing machines etc. Not poisonous, but wouldn't drink it.
Anyway - we had them on the RJ
100s. Its a water extinguisher, as opposed to Halon, BCF, Foam etc. It would be used to damp down burning material in, for example, a waste bin or a passenger's bag, after a BCF had been used.
Although the fire is probably out due oxygen starvation, the water makes sure it doesn't re-ignite. It may still be hot.
Regards - Musang