The "open channel" disc tube module type DT can operate effectively and economically at increased turbidity and silt density index levels for reverse osmosis (RO). This module is applied reliably on seawater which has been pretreated only with normal mechanical filters such as a sand filter and a cartridge filter, usually making dosing of disinfectants as well as acid and anti-sealant pretreatment unnecessary. More than 1,800 plants with a permeate production in the range of 2 to 300 m(3)/d have been installed since 1985 and are operating successfully to secure the water consumption on cruise liners and merchant ships as well as in the navy and on installations offshore. The chemical pretreatment, necessary in conventional RO practice in the field, is avoided using this module type. Therefore, on-shore applications are gaining importance because there is no negative impact on the environment that usually is expected because of longterm accumulation effects of the additives from conventional RO plants that are going back with the concentrate to the sea or the raw water source. Due to the low pretreatment requirements of the module, it can be operated for a wide range of desalination and purification applications. Design details of the DT module and results from practical field work in seawater desalination and treatment of brackish water are discussed.