Hi Katie, it's true that early on in the process (on day 1 or 2) the rise may be caused by a type of bacterium that gives off carbon dioxide, rather than representing true yeast activity. However, once your starter is rising predictably and has a pleasant aroma, then the discard should be safe to use. I would give it another day or two to be sure that your starter continues to rise predictably after each feeding and to smell good. Even though you haven't been discarding, the starter will weed out the unhelpful bacteria and the discard should be fine to bake with. 

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