The duck leg Confit, brings added richness to the bisque, but the soup is also delicious without it. Remove meat from bones, discarding bones, and shred meat. Cut reserved duck skin into thin strips and in a small non-stick skillet cook over moderate heat until fat is rendered and skin is crisp and golden brown.
Transfer cracklings with a slotted spoon to paper towels to drain and season with salt.
Thinly slice shallots and chop celery ribs. In an 8-quart heavy saucepan cook vegetables in butter with salt and pepper to taste over moderately high heat, stirring, until golden, about 6 minutes. Add water and boil, uncovered, until celeriac is very tender, about 15 minutes. Stir in lemon juice and, if necessary, more water to reach desired consistency. Season soup with salt and pepper and keep warm.