A conductivity control system was used to control the draw solution at a constant concentration of 1.0 M. When the NaCl
concentration in draw solution reservoir was less than 1.0 M,
the probe would give a signal to the control device, and then a
peristaltic pump was started to transfer the concentrated
draw solution of 5 M NaCl to the reservoir until the draw solution
recovered its initial concentration of 1.0 M.