SOLUTION PROCESSING METHOD AND SOLUTION PROCESSING APPARATUS

An object of the present invention is to provide a solution treatment method which is to be applied to a solution containing at least an electrolyte in an amount of 1000 mg/L or more and containing a low-molecular-weight nonelectrolyte molecule having a molecular weight of 70 or less, and by which the electrolyte and water are recovered while the low-molecular-weight nonelectrolyte molecule is removed, with combination of a plurality of types of membranes. The present invention provides a solution treatment method comprising treating a raw liquid with a separation membrane 1 and a separation membrane 2 which each separate a feed liquid into a permeate liquid and a concentrated liquid, and separating the raw liquid into a purified liquid containing water and an electrolyte and a discharge liquid containing a low-molecular-weight nonelectrolyte molecule, whereinthe raw liquid is a solution containing at least an electrolyte in an amount of 1000 mg/L or more and containing a low-molecular-weight nonelectrolyte molecule having a molecular weight of 70 or less,a permeate liquid from the separation membrane 1 is fed to the separation membrane 2 and subjected to separation treatment,the separation membrane 1 exhibits, when a solution 1 with pH 7 in which 10000 mg/L of sodium chloride (NaCl) and 250 mg/L of urea are mixed is fed thereto at 36°C and a pressure of 1.2 Mpa, an NaCl removal rate of 90% or more, the NaCl removal rate being defined by a reduction rate of a component concentration in a permeated solution relative to a component concentration in a feed liquid,the separation membrane 2 exhibits, when a solution 2 with pH 7 in which 1000 mg/L of sodium chloride (NaCl) and 700 mg/L of urea are mixed is fed thereto at 36°C and a pressure of 1.8 Mpa, a urea removal rate of 85% or more, the urea removal rate being defined by a reduction rate of a component concentration in a permeated solution relative to on a component concentration in a feed liquid,a difference between a urea removal rate of the separation membrane 2 under the evaluation conditions of the separation membrane 2 specified above and a urea removal rate of the separation membrane 1 under the evaluation conditions of the separation membrane 1 specified above is 40% point or more, andthe solution treatment method satisfies any one of the following requirements (i) and (ii):(i) a part of a permeate liquid from the separation membrane 2 is mixed with the raw liquid and then fed to the separation memb