A separation unit for separating oil from oily water having a first oil to water ratio, said separation unit comprising: a housing defining an inner chamber and having a first end-wall constituting a bottom end wall and an opposite second end-wall constituting a top end-wall, one or more side-walls extending between said first end-wall and said second end-wall, said inner chamber being at least partly delimited by said first end-wall, said second end-wall, and said one or more side-walls, a first set of separation walls extending from said first end-wall towards said second end-wall and defining a first set of fluid passages below said second end-wall and above each of said separation walls of said first set, a second set of separation walls extending from said second end-wall towards said first end-wall and defining a second set of fluid passages above said first end-wall and below each of said separation walls of said second set, said second set of separation walls being interlayered between said first set of separation walls, an inlet defined in one of said side-walls and at said first end-wall or alternatively defined in said first end-wall, a first outlet defined in one of said side-walls and at said first end-wall or alternatively defined in said first end-wall, a further set of outlets defined in said second end-wall above each of said separation walls of said first set of separation walls, and a fluid path (22, 32, 40) extending from said inlet to said first outlet within said inner chamber passing said first set of fluid passages and said second set of fluid passages and having a gradually increasing flow restriction from said inlet to said first outlet so as to separate fractions of said fluid from said fluid path to said second set of outlets having higher oil to water ratios than said first oil to water ratio and so as to expel fluid from said first outlet having a lower oil to water ratio than said first oil to water ratio. a housing defining an inner chamber and having a first end-wall constituting a bottom end wall and an opposite second end-wall constituting a top end-wall, one or more side-walls extending between said first end-wall and said second end-wall, said inner chamber being at least partly delimited by said first end-wall, said second end-wall, and said one or more side-walls, a first set of separation walls extending from said first end-wall towards said second end-wall and defining a first set of fluid passages below said second end-wall and above each of said separation walls of said first set, a second set of separation walls extending from said second end-wall towards said first end-wall and defining a second set of fluid passages above said first end-wall and below each of said separation walls of said second set, said second set of separation walls being interlayered between said first set of separation walls, an inlet defined in one of said side-walls and at said first end-wall or alternatively defined in said first end-wall, a first outlet defined in one of said side-walls and at said first end-wall or alternatively defined in said first end-wall, a further set of outlets defined in said second end-wall above each of said separation walls of said first set of separation walls, and a fluid path (22, 32, 40) extending from said inlet to said first outlet within said inner chamber passing said first set of fluid passages and said second set of fluid passages and having a gradually increasing flow restriction from said inlet to said first outlet so as to separate fractions of said fluid from said fluid path to said second set of outlets having higher oil to water ratios than said first oil to water ratio and so as to expel fluid from said first outlet having a lower oil to water ratio than said first oil to water ratio.