Water demineralisation by NF/MD integrated processes

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.

The application of membrane distillation for the production of demineralised water from tap water requires pretreatment of feed water, in particular, the removal of bicarbonate ions. Heating of the feed in the MD process caused decomposition of HCO– ions present in water and forma3 tion of a deposit layer (mainly containing CaCO3) on the membrane surface. This phenomenon can be eliminated by acidification of the feed to pH = 4, which was associated with the consumption of considerable amounts of acids. Preliminary softening of water in the nanofiltration process resulted in the reduction of acid consumption. Taking into consideration the fact that bicarbonate ions remaining in the NF permeate can still cause membrane fouling, the NF permeate (MD feed) was acidified by HCl to pH = 4. During the concentration of tap water (also softened by NF process) the silicon compounds precipitated in the feed, and they were deposited at the inlets of the capillary membranes. This resulted in a rapid decline of the MD module efficiency. The application of a protecting net at the module inlet allowed preventing this phenomenon. Additionally, the performance of the MD module indicates that the use of the net also limits formation of the deposit on the membrane surface in a significant degree, which in turn reduces the rate of membrane wettability. The membrane wettability occurring in the MD process is one of the major reasons for the process efficiency reduction, particularly during the initial period of operation (200–300 h). The wettability of a fraction of the membrane pores caused a decline of the module efficiency by 30% after 1100 h of the process duration. Drying of membranes allows restoring the initial module efficiency for a short period only. The quality of the distillate obtained was stable and practically independent of the feed concentration. The distillate produced had 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. The volatile organic compounds, the vapour of which (similarly to water vapour) diffuse from the feed to the distillate, therefore, they cannot be completely removed in the MD process. The hydrodynamic conditions in the MD module have a significant influence on the module efficiency. Thus, minimum values of the flow rate both of the distillate and the feed stream should be determined for each module design. The driving force of MD is the partial vapour pressure difference and the dependence of the vapour pressure on temperature is represented by an exponential function, therefore, a large permeate flux can be achieved for high feed temperature (over 353 K).