Place ginger into a blender, cover with 1 1/2 cup water, and blend until ginger is thick and pasty. Strain and squeeze juice from ginger pulp into a 2-quart pitcher, squeezing ginger mixture as dry as possible.
Stir lemon juice and sugar into ginger juice until sugar has dissolved. Pour 7 cups water into mixture, stir to combine, and serve garnished with crushed mint leaves.
