While (to my knowledge) this has no official explanation, many of us asume those systems are between the floors.
Most ship maps assume a height 3 m per floor
1 but a ceiling height of about 2.4 m. This would leave about 60 cm of interfloor space, and many of us asume all plumbing (and most recycling, water storage, etc) is inside those spaces in most ships.
Note 1: at 14 m3 per ton, with 3 m heigh a dton is about 4.6 m2, while a 1.5 m square is 2.25 m2, so making about 2 such squares/dton.