A hollow fiber membrane element may be used in a pressure vessel similar to those used with spiral wound elements, optionally as a replacement for a spiral wound element. A connecting sleeve is used to connect the insides of adjacent elements to each other or to one or more fittings in communication with the outside of the pressure vessel. The element has hollow fiber membranes suspended between two potting heads spaced apart by a cross flow tube that is open at the outside faces of both potting heads. The potting heads are each adapted to slide into an end of a connecting sleeve. A module has one or more hollow fiber elements located inside of a pressure vessel. At least one port is provided in communication between the outsides of the one or more elements and the outside of the vessel. An aerator may be provided on the bottom of the vessel. In an outside-in flow process, feed water enters through the port and flows to the outside of one or more elements while permeate is removed through a fitting at one or both ends of the module. In an inside-out process, feed water enters through a fitting and permeate is removed through a port.