Great piece!
Scrubbing and spraying with the hose will remove most loose debris.
If you have a concern for bacteria/parasites, I don't see a way to boil wood of that size and anything less than water over 150 degrees just will not kill most things inside wood.
If that is a concern, get a large 55 gallon plastic drum, fill it with water and add some bleach (e.g., 1 cup) and let it soak for a few hours. A heavy does of pool chlorine will work as well (and simplify treating it afterwards). With bleach, hose off thoroughly and then give several soakings in clean water. With chlorine, rinse and add extra de-chlorinator to the tank.
Many people don't do much with the wood (just scrub and rinse, so it's not always needed, but sterilization is a good step if you have important pets in the tank.