This is an old thread, but still open.
The decision to select a water softener or nanofiltration depends on a number of factors including raw water quality, flow rate, treatment plant skill set, and the desired finished water quality.
For the application of 12.5 cubic meter/ hr, the water softener may be the best option. A nanofilter will be much more expensive and require more operator attention.
In the US, the acceptable standard for TDS is 500 ppm (mg/L), a standard established by the USEPA under the authority of the Safe Drinking Water Act. This is a secondary standard, that is, it’s a standard that is based on aesthetics (taste) rather than health impacts. It is not enforced, and many public water systems deliver water that has a TDS concentration that is higher than 500 mg/L.
The WHO (World Health Organization) also considers the TDS standard as an aesthetic standard and has published the following guidance for TDS: excellent, less than 300 mg/L; good, between 300 and 600 mg/L; fair, between 600 and 900 mg/L; poor, between 900 and 1200 mg/L; and unacceptable, greater than 1200 mg/L.
Many people in the world drink water every day that has a TDS level higher than 1000 mg/L. The water doesn’t taste good, but the TDS by itself isn’t necessarily unhealthy. However, high TDS levels are frequently accompanied by concentrations of other substances that are harmful to health. The most common are usually the heavy metals (arsenic, chromium, cadmium, mercury, lead, and a few others).
Nanofiltration membranes have a nominal pore size of approximately 0.001 microns and an MWCO of 1,000 to 100,000 daltons. Pushing water through these smaller membrane pores requires a higher operation pressure than either MF or UF. Operating pressures are usually near 600 kPa (90psi) and can be as high as 1,000 kPa (150psi). These systems can remove virtually all cysts, bacteria, viruses, and humic materials.
Because NF membranes also remove alkalinity, the product water can be corrosive, and measures, such as blending raw water and product water or adding alkalinity, may be needed to reduce corrosivity. NF also removes hardness from water, which accounts for NF membranes sometimes being called “softening membranes.” Hard water treated by NF will need pretreatment to avoid precipitation of hardness ions on the membrane. However, more energy is required for NF than MF or UF.
Typically, NF membranes used for softening applications remove more than 95 percent of total hardness.
AWWA
AMTA Org
Finally, agree with the previous posters that "salt free" water softeners or electronic water treatment gadgets are scams.
