what kind of floor do you have in the shed? would it be possible to trench from the riser drain to a low spot outside the shed? Then a pipe could be run through the trench, and the low spot back filled with whatever is used locally for a septic leach field. I used this approach with an old cellar that was host to a seasonal stream.
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Dance like you have never been hurt, work like no one is watching,love like you don't need the money.