Down the drain! There's nothing in your body that doesn't already go down the drain. The main chemical used during embalming is formaldehyde dissolved in methanol. Formaldehyde is naturally a gas, so just like CO2 bubbling out of soda, the formaldehyde eventually bubbles out of the methanol. Methanol is actually used during waste water treatment so that's also not a concern.

The only plumbing requirement in my area is that rooms for holding or preparing bodies need to have running water with a back-flow prevention valve.