The aim of this work is to improve a mathematical model to describe the volume-decreasing diafiltration process by including macrosolute rejection. Water consumption in this continuous process was analyzed and a significant advantage when comparing both the classical diafiltration process and multi-stage diafiltration processes was noticed. For whey protein, a relative volume was used continuously. A decreasing diafiltration process was estimated at a level of 1.36, while the corresponding values of fresh water relative volumes were reported as follows: 5.4 for the classical process, 2.9 for the two-stage process and 1.5 for the six-stage diafiltration process.