1. Send pictures to poolforum AT gmail DOT com -- I'll post them, and you'll get much better answers that way. Pay particular attention to the pump AND pump label, as well as the motor label. We'll also need to see the piping coming out of the ground, and returning.
2. There are benefits to both sand and DE. But, if you get a sand filter, make sure it's at least a 30" filter, for a 40,000 gallon pool.
3. We can give info on the manifolds, once we have pictures of the filter and filter label
Good luck!
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