The outer shell of the mechanical filter is also fitted the same way. You can also see the outlet pipe it's about an inch short below the top of the 100mm pipe. This will enable to take the screw top off without the water pouring in - I have not glued it yet as I want to test it out first. The mechanical filter (A netted bag housing a soft sponge/s) fits between the inner pipe and the outer pipe. The outer pipe has four 20mm holes drilled down the bottom, just above the retaining flange.