Consumption of specific energy (total energy consumed to produce a unit volume of permeate) and scale inhibitors are key factors for efficient or inefficient operation of nanofiltration (NF) (or reverse osmosis, RO) systems. To minimize energy consumption, a suitable energy recovery device must be installed to recover waste energy. Even with optimum recovery of waste energy, still energy consumption exceeds 50% of the total operating costs. The best way to minimize the cost of scale inhibitors is to prevent scale (particularly sulfate) from forming. Scale cleanup (shut-down time and cost of inhibitors and dissolvers) and membranes replacement occupy the rest of the operating costs. As such, designers of NF systems must be aware of the hydraulic and scale envelops, that are the extreme boundaries of operational parameters over the lifetime of the systems. Such boundaries are determined by two factors. The first factor is the throughput ratio, which in turn, is governed by salinity, scale tendency, and temperature. The second factor is the overall system hydraulic, which in turn, is controlled by throttling losses, array losses, and pressure drops within membranes modules. However, the focus of this analysis is the first factor. This third article, in a four-part series, provides a modeling tool to predict: (1) the actual osmotic pressure difference between the membrane surface (Pi(m)) rather than the bulk of the feed stream (Pi(F)), and the permeate stream (Pi(p)) taking into account the actual rejection (R-a) factor; and (2) the saturation degree of calcium sulfate dihydrate (gypsum) and strontium sulfate (celestite) at the membrane surface, and on the concentrate side of the membrane.