series-production (or multi-pass) assemblyReading time:
When the quality of the water produced by a plant is not suitable for its intended use (especially process or drinking water produced from seawater or the production of ultrapure water), all or part of the water produced by the first stage is pumped back to the second stage that can be equipped with a different membrane, e.g. figure 10 (simplest case). In order to avoid any confusion with the series-discharge configurations described earlier (see section series assembly – desalination membrane discharge), we have two-pass systems where the water produced by the first pass is treated again during the second pass (each pass can comprise several stages).
Comment: the discharge produced by the second pass will usually be better than the incoming water and can, therefore, be recycled upstream from the first stage.