While there may be a reason for going overboard about sterilizing the wood, and there are many ways to do it, I have just cleaned it (scrubbed the dirt off) and soaked it until it sank. The larger pieces did not even fit in a garbage can, so I soaked one end for a week then turned it upside down to soak the other end for a few days. I still had to weight some of the pieces down in the tank.
Another way I soak wood it to throw it into my pond.
When the wood is that large, I just left it out on my driveway though a years' worth of sun and rain. (I was not ready to set up that tank for a while).
Then I weighted it down in the tank to sink.
Bleach is pretty safe, then do several water change/soakings with dechlor in the water.