Thank you Flight Detent for your detail explanation. This seems to make good sense.
However, if we are looking from a Boeing aircraft point of view, then this does not apply. Boeing aircrafts 737/757/767/777 all have center tank with center tank pumps that pump fuel directly to engines, but none is required to switch off the center tank pumps on take off. If contamination problem is so serious, I am sure Boeing would had think of it, isn't it?
Flight Detent, please don't take this the wrong way. I am in no way to dispute your explanation as I myself do not know the answer. What we want is a "healthy" discussion and hopefully can come up with the correct answer.
Anyone has any more input? Thanks.