Replacing the impeller for the second time in two days. I use the macerator to flush out the holding tank for winter lay-up. There was a originally a paddle switch that broke while loading things in the cabinet. I replaced the broken paddle switch with a push / pull switch. That switch would randomly turn the macerator on. The old impeller ran dry for I don't know how long, and was ground to dust. I didn't realize the switch was bad. Before going out for the weekend, I tested all systems and found that the macerator sounded wrong. I replaced the bad old impeller.
Shurflo 3200 Series
Impeller Kit 94-571-00
Impeller Housing Kit 94-570-00
After a long weekend out on the lake, sitting quietly back at the dock, I hear a slight humming. I discovered that the macerator pump was running. The new impeller was ground to dust. I replaced the faulty switch and yet another new impeller. All is well. The first time I was replacing the impeller, the faulty switch could have come on and the macerator would have ripped up my fingers!
Danger  I know some boaters don't have the macerator and Y-valve. I am glad to have it on board. Here's one example why:
There is no Y-valve between the toilet and the tank. All black water goes to the holding tank. For inland cruising, the Y-valve is set for pumpout and the handle is removed.
 | A macerator is used to grind waste from the toilet into a slurry so that it can be pumped into a holding tank or other sewage system. |