Don't assume the tanker that delivered diesel this week to your corner station wasn't hauling e85 last week. Don't assume the pipelines from the refinery to the tank farm are dedicated to a certain blend or type of fuel. It is normal for a small amount of just about everything to be mixed in with motor fuels. That said, pump out all you can, add a splash of the new fuel, pump it out (lather rinse repeat) and you will have a fraction of a cup of the old fuel in the tank and pump diluted into (in your case) 30 gallons of diesel. I would dump a bottle of fuel treatment into the tank (just for giggles and extra lubrication_.