You probably understand the basics behind a dishwasher: water is jetted around with soap to scrub the dishes and after a bunch of scrubbing they are rinsed with clean water. There's an important design restriction that we need to keep in mind: you can only use so much water. Since someone could open the door at any moment you can't have a large pool of water waiting to rush out.
So a dishwasher has cycles and each cycle starts with water being let into the compartment. If the water is too cold or a high heat wash is selected electric coils heat the water. For this reason, and to conserve water and soap use, the same water is used over and over for the entire cycle.
The way this works is that a large pump sucks in water from the bottom of the dishwasher and sends it shooting through the spray arm(s). The spray knocks food off of the dishes which washes back to the bottom. This brings us to our first problem: what happens to all of the food chunks?
Water pumps just don't like chunks. At the bottom of the dishwasher there are a couple of filtration stages. One is a fine mesh, which would quickly be clogged beyond use if it was the only path for the water. The other filtration path has larger holes that lead to a special section of the pump assembly: the grinder.
The grinder (my name for it) is like a small blender blade or sink disposal. It is powered by the pump and finely chops up any food stuff it encounters. There's a small screen after the grinder which lets the ground up, small chunks through. Anything else stays there until it is ground up.
Hidden within all of this pump plumbing are a couple of valves. One valve lets the fresh water in at the beginning of the cycle. Some dishwashers might locate this opening along the back wall of the dishwasher. The other valve redirects the pump's output to the drain pipe and is engaged at the end of a cycle.
That's about it. There's some electronics (and or mechanical timers) to orchestra all of this activity, including opening soap or rinse flaps at appropriate cycles.
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