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> Third, no safety margin is needed here because this is not a safety problem.

Sewage overflow is certainly a safety problem. Just health safety though, not flight safety.

1.7L per passenger is a large volume of excretions, but flight operations may require multiple flights before emptying the tank, especially if there's a problem with the emptying equipment at some airport. Would a failure to empty the tank prevent a plane from taking off? Probably only if the remaining capacity is likely to be insufficient.