Once you get the gas out, you (offically) need to dump it at a hazardous materials collection area ( check with your local gov. for disposal areas where you dump oil, gas, paint, etc.....) (unoffically) all you need to do is completely drain the tank, rense it good, cut it into small enough peices to fit in a trash bag and send it out in the garbage. Once it is "cut into peices it "becomes peices of plastic" and not a fuel tank any more. Same goes with metal tanks and hauling scrap metal to the recyclers. If you drain/rense and cut up a metal tank the scrap yard will take it as scrap, but in one peice they will/can not accept it.
Zig