A membrane module assembly for treating water, the membrane module assembly comprising at least one guide member for directing gas bubble flow along a vertically arranged membrane surface of a flat sheet membrane (25, 26), the guide member being placed adjacent the membrane surface in use, the guide member comprising: a bottom edge, a top edge, first and second side edges (54, 56) extending vertically from the bottom edge, at least one bar extending vertically from the bottom edge to the top edge, parallel to each of the first and second side edges (54, 56), each of the first side edge, the second side edge and the at least one bar defining a plurality of vertical channels for gas bubbles to flow along, the at least one bar lying flush against the membrane surface when in use, and each one of the plurality of vertical channels having an open bottom end through which the gas bubbles can enter the one of the plurality of vertical channels and an open top end through which the bubbles can exit the one of the plurality of vertical channels, so that, in use, the gas bubbles flowing upwardly along the one of the plurality of vertical channels are in contact with the membrane surface and guided along the membrane surface, the bottom edge having a diverting means adjacent the channel bottom end arranged to direct at least some of the incident gas bubbles in a first direction towards the channel bottom end, wherein, in use, the diverting means is also arranged to direct at least some of the incident gas bubbles in a second direction towards an adjacent membrane surface, wherein the diverting means has a first angled portion for directing the incident gas bubbles in the first direction and a second angled portion for directing the incident gas bubbles in the second direction, the first and second angled portions (63, 65) being in a substantially "V" shaped configuration; and wherein the membrane module assembly comprises first and second membrane modules having oppositely facing membrane surfaces, separated by the guide member and wherein the channels are adjacent both of the oppositely facing membrane surfaces and the guide member diverts the gas bubbles to the oppositely facing membrane surfaces or to an adjacent membrane surface. a bottom edge, a top edge, first and second side edges (54, 56) extending vertically from the bottom edge, at least one bar extending vertically from the bottom edge to the top edge, parallel to each of the first and second side edges (54, 56), each of the first side edge, the second side edge and the at least one bar defining a plurality of vertical channels for gas bubbles to flow along, the at least one bar lying flush against the membrane surface when in use, and each one of the plurality of vertical channels having an open bottom end through which the gas bubbles can enter the one of the plurality of vertical channels and an open top end through which the bubbles can exit the one of the plurality of vertical channels, so that, in use, the gas bubbles flowing upwardly along the one of the plurality of vertical channels are in contact with the membrane surface and guided along the membrane surface, the bottom edge having a diverting means adjacent the channel bottom end arranged to direct at least some of the incident gas bubbles in a first direction towards the channel bottom end, wherein, in use, the diverting means is also arranged to direct at least some of the incident gas bubbles in a second direction towards an adjacent membrane surface, wherein the diverting means has a first angled portion for directing the incident gas bubbles in the first direction and a second angled portion for directing the incident gas bubbles in the second direction, the first and second angled portions (63, 65) being in a substantially "V" shaped configuration; and wherein the membrane module assembly comprises first and second membrane modules having oppositely facing membrane surfaces, separated by the guide member and wherein the channels are adjacent both of the oppositely facing membrane surfaces and the guide member diverts the gas bubbles to the oppositely facing membrane surfaces or to an adjacent membrane surface.