I am replacing an old shower and after ripping out the concrete base and water damaged sub floor, there is a contraption into which the shower drain goes to first. The container has some water in it. There is an outlet pipe which leads to the final drain stack going to the to the septic. It appears to be similar in size to a windshield washer reservoir in a car with a cap for a clean out. My assumption is that the "fall" from the shower drain may not be enough and that this somehow compensates by creating the necessary pressure to the drain? I have never seen this before and an wondering if anyone else has.