The operation of pre-industrial membrane distillation (MD) units is of great importance in order to implement this new technology. The systematic development from module to plant design for MD is conducted in an industrial research-cooperation. The consequent up-scaling from a lab plant at Hamburg University of Technology (TUHH) for MD module evaluation to a pre-industrial plant is presented. Permeate gap MD (PGMD) and Direct Contact MD (DCMD) spiral wound modules with different channel lengths have been used in this study to evaluate the most suitable module configuration and design. It has been decided to go along with DCMD modules with a relatively short channel length. A pilot plant for on land testing has been built and operated for 9 months before constructing the pre-industrial unit. In the pre-industrial plant waste heat was used and a heat recovery and recycle concept was implemented and tested successfully. It has been proven that long-term operation with real seawater is possible without significant performance decline. Besides operational data also MD plant design specifics will be discussed. The importance of ambient pressure conditions for the MD modules was determined. In addition, the possibility of piping system deaeration is discussed.