In a 4 litre soup pot over medium heat, saute leeks, celery and potatoes in a little water for 5 to 7 min. Add more water as needed to prevent sticking. Add the 6 cups of water, salt & pepper. Bring to a boil, lower heat and simmer potatoes are very soft, about 15 min. Add cooked rice and heat through. Puree soup 2 to 3 cups at a time in a blender or food processor until very smooth. Return to soup pot and rewarm if needed.