Which of the following fixtures should NOT be piped to a grey water holding tank?

The fixture that should not be piped to a grey water holding tank is the water closet. This is because water closets, which are toilets, primarily handle black water—waste that contains human waste and toilet paper. Black water is considered contaminated and requires different treatment compared to grey water, which is relatively clean and comes from sources like bathtubs, showers, and laundry machines.

Grey water can be reused for various purposes, but mixing it with black water can lead to contamination issues and complications during treatment processes. Therefore, water closets should be directly connected to a sewage system rather than to grey water holding tanks.

In contrast, fixtures like bathtubs, laundry machines, and showerheads produce grey water, making them suitable for piping to grey water systems intended for reuse in irrigation or other non-potable applications.