The drain to our kitchen sink has a half moon opening in the pipe which limits drainage when I run water in the sink. What is the purpose of this?
The “half moon” pipe is a baffle tee which helps make sure the water from the garbage disposer goes down the drain and not back up the other sink. I have never heard of this plugging up. If your disposal is running properly it should grind up the debris before it ever reaches the drain pipe. Hopefully you are not dumping fibrous or starchy vegetables like lettuce, potatoes, rice or celery down the drain as these will plug the drain even with a disposal.
I have had a number of clients contact me with some rather interesting stories about what has happened to their garbage disposal. One would think that the ole garbage disposal just will grind away without fault. Like the Cookie Monster, always eating everything thrown its way. And indeed, a garbage disposal is quite a persistently effective kitchen appliance. You don’t really see it at work, but you sure can hear it and it seems just about nothing can escape its wrath as it just goes right through just about anything. But here is the reality. Yes, while these machines can operate with great effectiveness, they simply do not obliterate everything object. They reduce them to small parts, but what remains sometimes can contribute to blockage. And over time, with the accumulation of many food particles and other objects that were intended or unintended for grinding, there can be a slow, but steady decline in the effectiveness of the drain.
If you don’t have a garbage disposal, you can install a regular san tee which does not have a baffle. But my advice is invest in a garbage disposal and learn how to use it properly and us it often.