I think in general, IW treats used water, whether it came from your toilet, shower, kitchen etc... nowadays, with lots of chemical used in our daily life, mother nature is struggling to clean the water that we used.. thats when IW steps in ....
Water supply on the other hand is different...
you are being charge for the amount of clean water that u use....and when it becomes dirty again, IW will make sure that the water u used is safe to be release to the natural environment again...