The "adsorption-amphoteric" model is used to investigate the role of the main phenomena determining the mechanism of the membrane charge formation in nanofiltration membranes. The prevailing mechanisms considered are: acid/base dissociation of the hydrophilic groups of the membrane, counter-ion site-binding on dissociated hydrophilic sites and competitive adsorption of ions on the hydrophobic groups of the membrane. The total membrane charge is given by the contribution of a proper charge, which is screened by the dissolved ions giving rise to site-binding phenomena, and by the contribution of an adsorption charge located on the hydrophobic sites. Aqueous solutions containing single symmetric (1:1, 2:2) as well as non-symmetric (1:2, 2:1) electrolytes are studied, by a simulation of the performances of a polyamide Desal-type membrane, taken as a reference, as a function of pH and salt concentration. The role of counter-ion site-binding on the total membrane charge is investigated in comparison with competitive adsorption. A sensitivity analysis is performed in which the overall effect of competitive adsorption is studied. With symmetric salts, the membrane charge is determined by the competitive adsorption on hydrophobic sites. With non-symmetric salts, on the contrary, the behaviour of the total volume membrane charge as a function of salt concentration is given by the relative effects of counter-ion site-binding vs. competitive adsorption.