Never flown the A320 and not sure if its the same principal but the 747 has ejector pumps which suck accumulated water (and fuel) at the low point in the main tanks. The water is mixed with fuel from the Jet Nozzle and sprayed at the Boost Pump inlet there by removing water constantly, the engines are capable of running with these small amounts.
Perhaps the same principal is used while the A/C is still on the ground in the case of the A320 as stated.