A method of operating a filtration unit of a filtration system includes



feeding, during filtration, feed water containing suspended particulate material to an inside of each of a plurality of hollow fibres through a first inlet and a second inlet of each hollow fibre while simultaneously removing a filtrate from an outside of each of the hollow fibres through an outlet of a filtration elements. In addition, the method includes feeding,
during back-washing, back-wash water to the outside of the hollow fibres through the outlet of the filtration element. Further the method includes discharging, in a first back-wash cycle, back-wash water containing entrained particulate material from the inside of the hollow fibres from one end thereof. Still further, the method includes discharging, in a second back-wash cycle, back-wash water containing entrained particulate material from the inside of the hollow fibres from the other end thereof.