you are going to have to lift the wort at some point. you can gravity drain into buckets and then lift and pour back in but I can't think of a way that you could gravity drain into a holding tank and then gravity drain back into the kettle. so either runoff into 2 or more holding vessels or be prepared to lift ~50-60 lbs of hot liquid high enough to pour or drain into the kettle or to lift the kettle with that much hot wort into it. not a big deal, I do this. My 'system' is a cooler mash tun that sits on a table. I transfer hot liqour from my kettle to the mash tun first with a long handled sauce pan then, when the level is low enough to feel safe about it I lift the kettle and pour the rest of the liqour into the mash tun. When mash is done I drain into the kettle on the ground and then lift the kettle onto the burner. Or, for 10 gallon batches I run off into two or more fermenter buckets and then pour them into the kettle after adding the sparge water to the mash.