In normal operation, a sand filter for pools has water come in at the top, and the water is collected by the laterals at the bottom.

In backwash mode, water is injected through the laterals, and is collected at the top.

Plumbing is used to select for all the correct pathways.

Laterals are slitted pipes in this example, lending themselves to 3D printing:

The last mechanism that is important is the selector valve for switching between different functions:

The best available flow diagram is this:

So this needs to be opensourced and detailed.

Multiselector Valve

Hint: Perfect thing to 3D print

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