We found the 5 gallon slow sand filter design to be inferior to the 55 gallon design. The five gallon filters just don’t work as well. They are less efficient at removing biological contamination at best; and actually introduce contamination at worst.
Another study totally not related to ours found basically the same results (the need for 2 separate containers because 1 five gallon container was not enough to provide adequate filtering):
(see page 56 in the conclusion section)