Heating and settling your waste diesel fuel are good ideas. You would want to be careful how and where you heat a barrel of diesel fuel, because the whole thing could blow up in you. And, make sure the lid is off so the pressure in the drum does not build up. I would put the drum in the sun on a sheet of ply wood and place a clear plastic 55 gallon "contractor" bag over the barrel and let the sun heat it. The temp should not exceed 150F. The water should separate out in a day or two of cycling the drum temperature.