Update: The standpipe is 2" pvc. If the manifold really is glued, you can cut the pipe and reconnect it with a regular schedule 40 pvc coupling. A few tips:
1) If you do cut the pipe, measure the height of the manifold before cutting the pipe and make sure the height is exactly the same when reinstalling. The coupling will add about a 1/4", so you would have to cut out about that much to avoid the manifold being too high.
2) When cutting the pipe, put something between the pipe and grids to avoid cutting the grids.