Enforcing use of biodegradable laundry soap

[David C.]

My wife and I are closing on a house with a guest home we will be using as a house hack and preparing to list one unit for rental. The unit we will be renting out only has a gray water drain in the laundry room. For this to be legal (and non-lethal to our plants) a biodegradable laundry soap must be used.

My question is how do we enforce this in our rental agreement? Has anyone done this before?