Measure 6 cups filtered water, dividing it into 4 cups and 2 cups. Remove all rhubarb leaves, weigh the stalks, measure the raw honey and salt, and juice the lemons.
Put 4 cups water and the rhubarb in a medium stainless-steel pot with a fitted lid. Bring to a boil, lower the heat, cover, and simmer for 10 minutes without letting the liquid boil over.
Remove from the heat. Add the remaining 2 cups cool water and the sea salt. Cool to room temperature; refrigerate the pot to speed cooling when needed.
Once fully cooled, stir in 1 cup raw honey until no sticky ribbons remain.
Transfer the mixture to the Ninja BN801 pitcher. Secure the lid and blend on medium-high for about 30 seconds, until evenly pink and smooth with no rhubarb pieces remaining.
For a slightly smoother drink, press through a medium-mesh stainless-steel sieve with a whisk. Skip this step to retain all blended rhubarb.
Stir in the fresh lemon juice.
Chill for 2 to 3 hours or overnight. Stir and serve. Add ice only in individual glasses at serving time so melting ice does not dilute the measured formula.