I had a washing machine that drained into a barrel with a sump pump in the barrel a couple of months ago. I think this one is better than the barrel.
I think this was designed by an engineer probally took all weekend to get it together.
This is a washing machine drain line overflow tank because the drain does not flow fast enough to keep the standpipe from overflowing when the dishwasher is in use at the same time as the washing machine. Home constructed 1979 and had a 2 inch PVC standpipe visible for the washing machine.
This was on a FSBO today the seller tried to convenice the buyer my client that this was an exceptable practice on a concrete slab on grade foundation.
My only statement is; I was born at night but not last night