It looks like the side branch of the tee goes down under where the water level would be. This serves as a trap as well as an oil separator type. Any oil that gets in the basin will float on top of the water and the water flows out the tee as it rises. The bottom of the pipe going horizontal is the flow line of the drain. The pipe coming up could be plugged with sludge and or cement. Looks like a cap on the end of the tee. Can you get that off and run a snake or water hose down the lower pipe to check if it is plugged?