Water demineralisation by NF/MD integrated processes Original Research Article

The production of demineralised water from drinking water by nanofiltration/membrane distillation system (NF/MD) has been investigated. The precipitation of CaCO3 on the membrane surface and a rapid flux decline was observed when tap water was used directly as a feed. The problem of scaling was eliminated by the acidification of the feed to pH = 4 and stable module efficiency was observed. The performance of the MD module was significantly improved when tap water was subjected to softening in NF process. The precipitation of the silica compounds during the operation of MD process with the NF permeate was found. These compounds caused clogging of the capillary membrane inlets, resulting in a gradual decline of the module efficiency. The application of filtration net assembled at the module inlet prevented blocking of the capillary membrane inlets. The MD process can be run without a flux decline, associated with the scaling, when the pH of NF permeate was adjusted at a level of 4. A partial wettability of the membrane resulting in a flux decline by 30% was observed after 1100 h of process duration. The quality of obtained distillate was stable and practically independent of the feed concentration. The produced distillate has the electrical conductivity in the range of 1.5–2.5 μS cm−1 and contained 0.5 ppm of inorganic carbon and below 1 ppm TOC.