Two things interesting here. The first of course is that line where they basically say "we had them send us patients ("Normals") on any pretext" ("Physicians in the other departments were requested to refer patients to our department for the slightest indications so that it might be possible to collect a series of ") which brings up significant ethical questions.
The second was the number. 1.5L seems pretty small. When I'm in left-lateral I can feel when the water starts to run down into the ascending colon, and that's at over 2L. I guess if they did the study in supine it would flow into the ascending before fully filling the rectosigmoid, descending and transverse, but it does seem a bit low if you're talking "capacity." It's also possible that I'm just a large colon.
Might explain why a lot of medical bags/buckets are 1500 ml?
Read here more information what is bonus group membership.