I get itchy thinking about putting drained fuel back into the tank. When we defueled a transport jet prior to heavy maintenance, particularly those with foreign registry, it went into a fueling truck. It could be reused only after our lab checked it out for contamination and biological growth. You might be surprised to see what can grow in a fuel tank. Sumping a large jet presents problems. Many tanks, many sump ports. Seen and participated in "on the ramp" disposal in early days. Later, bowsers to collect sumped fuel were made available. Lot of water collected from condensation at high altitudes where fuel got very cold and you then descended into warm, moist air.
Even worse, back in big piston days, checking oil screens resulted in pails of thick, combustion contaminated oil. Remember, we had lots of lead in 108/135 and 115/145 fuel. I remember seeing more than a few buckets poured into the storm sewers at the then KIDL airport. These went out to Jamaic a Bay. Admittedly, it was a very long walk across the ramp to the oil pit.
Good Old days were sometimes bad.