With fuel in the CWT and the (CWT) boost pumps running (this fuel is ALWAYS used first due to the fact that the pump pressure is 36psi as opposed to 12 psi for the wing tank pumps) the motive flow water scavenge pumps draw fluid from the bottom of the tank and pass it into the inlet side of its respective boost pump. As Swedish Steve points out these scavenge 'pumps' are small diameter pipes certainly less than 1/2 inch bore.
Each wing tank boost pump also has a fuel scavenge motive flow pump BUT this line (same or similar diameter!) has a float operated valve in the CWT AND a float operated valve in its respective wing tank. When the fuel is used up (down to about 900kgs) in the CWT the float valve will open and fuel will attempt to move into the wing tanks, BUT at this point because the wing tanks are full the float operated valve cannot open. When the fuel level in the wing tanks has dropped to a predetermined level the float valves open and the fuel is scavenged from the CWT into the wing tanks.