Polypropylene is pretty inert stuff. What you asked for is a 1 to 100 ratio of bleach to water. In most cases, household bleach is pretty inocent stuff in the sense that it is not very concentrated to begin with, i.e. it is already in a light concentration when you buy it in the bottle at the grocery store (if it was highly concentrated there would be a lot more household related accidents). So, the fact that you intend to let it down even more means that it will be well under a 1% concentration -hardly worth worrying about in PP pipe. Good luck.