1. A fresh water generation method comprising: feeding raw water or pretreated water
thereof as feed water by applying a pressure into a semipermeable membrane module using a
booster pump (8, 8a, 8b), thereby separating the feed water into a concentrate and a permeate
having a low concentration,
wherein a scale inhibitor having a reducing function is dosed intermittently or continuously upstream of the semipermeable membrane module, thereby inhibiting scale generation and maintaining an oxidation-reduction potential of at least either the feed water or the concentrate to a threshold value or lower
wherein at least either the feed water of the semipermeable membrane module or the concentrate has the oxidation-reduction potential of 100 mV to 500 mV or a halogen concentration of 0.2 mg/L or lower.
wherein a reductant is dosed after dosing the scale inhibitor.
wherein an oxidant is dosed during water intake or pretreatment, and the reductant is dosed at a location downstream therefrom and also upstream from the semipermeable membrane module.
2. The fresh water generation method as claimed in claim 1 wherein the raw water comprises, as a main ingredient thereof, any of seawater, brackish water, river water, groundwater, wastewater and treated water thereof.
3. The fresh water generation method as claimed in any one of claims 1 to 2, wherein the raw water comprises seawater or brackish water as the main ingredient thereof, and a ratio of an amount of the permeate to an amount of the raw water is from 30% to 90%.
4. The fresh water generation method as claimed in any one of claims 1 to 3, wherein the concentrate remaining after the permeate has been separated from the raw water has a total dissolved solids concentration of from 0.1 wt% to 10 wt%.
5. The fresh water generation method as claimed in any one of claims 1 to 4, wherein the scale inhibitor is a phosphorous acid-based organic compound or a phosphonic acid-based organic compound.

6. The fresh water generation method as claimed in claim 5, wherein the scale inhibitor is an organic acid which comprises at least one selected from the group consisting of aminotris(methylenephosphonic acid), diethylenetriamine penta(methylene phosphonic acid), hexaethylenediamine tetra(methylene phosphonic acid), ethylenediamine tetra(methylene phosphonic acid), 1-hydroxyethylene-1,1-diphosphonic acid and tetramethylenediamine tetra(methylene phosphonic acid) or salts thereof, and the scale inhibitor has a molecular weight of 200 g/mole to 10,000 g/mole.
7. The fresh water generation method as claimed in claim 5 or 6, wherein the scale inhibitor contains, as an auxiliary ingredient, at least one selected from the group consisting of polyphosphoric acid, phosphorous acid, phosphonic acid, phosphorus hydride, phosphine oxide, ascorbic acid, catechol, catechin, polyphenol, gallic acid, and derivatives thereof.
8. The fresh water generation method as claimed in any one of claims 1 to 7, wherein the raw water or the pretreated water has, at least temporarily, the oxidation-reduction potential of 350 mV or higher.
9. The fresh water generation method as claimed in any one of claims 1 to 8, wherein the raw water or the pretreated water contains transition metals in an amount of 0.001 mg/L or higher.
10. The fresh water generation method as claimed in claim 9, wherein the transition metals include at least one selected from the group consisting of Fe(II/III), Mn(II), Mn(III), Mn(IV), Cu(I/II), Co(II/III), Ni(II) and Cr(II/III/IV/VI).
11. The fresh water generation method as claimed in any one of claims 1 to 10, wherein the oxidation-reduction potential of at least either the feed water or the concentrate is controlled to 350 mV or lower by dosing the scale inhibitor.
12. The fresh water generation method as claimed in any one of claims 1 to 11, wherein a reductant is dosed before dosing the scale inhibitor.

13. The fresh water generation method as claimed in claim 11 or 12, wherein the oxidation-reduction potential of the feed water is controlled to a range of 100 mV to 350 mV by dosing the reductant.
14. The fresh water generation method as claimed in any one of claims 11 to 13, wherein the reductant comprises at least one selected from the group consisting of sodium bisulfite, sodium sulfite, sodium metabisulfite and sodium thiosulfate.
15. The fresh water generation method as claimed in any one of claims 1 to 14, wherein the concentrate remaining after the permeate has been separated from the raw water has the total dissolved solids concentration of from 6 wt% to 10 wt%.
16. The fresh water generation method as claimed in any one of claims 1 to 14, wherein the concentrate remaining after the permeate has been separated from the raw water has the total dissolved solids concentration of from 0.5 wt% to 1 wt%.
17. The fresh water generation method as claimed in any one of claims 1 to 16, wherein a semipermeable membrane of the semipermeable membrane module has polyamide as a main constituent thereof.
18. The fresh water generation method as claimed in claim 17, wherein the semipermeable membrane has been subjected to a chlorine treatment during a production of the membrane or before using the membrane for the fresh water generation method.
19. The fresh water generation method as claimed in any one of claims 1 to 18, wherein a concentrate obtained in another semipermeable membrane module to which a scale inhibitor having a reducing function has been dosed is mixed into the feed water, thereby dosing the scale inhibitor into the feed water.