A water treatment system comprising a first reactor and a second reactor arranged to be placed in series in a body of water, the first and/or second reactor comprising at least one cell for housing biomedia. A mid-settling zone is provided between the first and second reactors for separating solids in the water. A method of treating water, the method comprising passing water to be treated through a first reactor and then a second reactor arranged in series in a body of water, the first and/or second reactors comprising at least one cell for housing biomedia, wherein the water is passed through a mid-settling zone between the first and second reactors before passing through the second reactor.