A sorbent based monitoring system for measuring the solute concentration of at least one component of a fluid, comprising: a sorbent regeneration system for regeneration of the fluid, wherein the fluid is a dialysate for hemodialysis or hemodiafiltration and wherein the sorbent regeneration system has a sorbent cartridge having a plurality of material layers (102,103,104) and wherein the sorbent cartridge in use receives spent dialysate containing at least one species from a dialysate outlet port of a dialyzer and removes at least a portion of a species from the dialysate to regenerate the dialysate for recirculation of at least a portion of the regenerated dialysate to a dialysate inlet port of the dialyzer and wherein the dialysate is conveyed through the sorbent cartridge and contacts at least one sensor after having contacted at least one material layer, and wherein the system comprises a plurality of sensors (201,202) that measure one or more solute concentrations and wherein the plurality of material layers comprise an urease-containing material layer and one or more layers (103,104,105) selected from, alumina, zirconium phosphate, magnesium phosphate, zirconium oxide, and activated carbon and wherein the system further comprises a controller; and characterized in that the controller determines a solute concentration of urea in the dialysate entering the urease-containing material layer by obtaining a first measurement from the dialysate before contacting the urease containing layer and a second measurement from the dialysate after having contacted the urease containing layer and wherein the first and second measurements are conductivity measurements or pH measurements. a sorbent regeneration system for regeneration of the fluid, wherein the fluid is a dialysate for hemodialysis or hemodiafiltration and wherein the sorbent regeneration system has a sorbent cartridge having a plurality of material layers (102,103,104) and wherein the sorbent cartridge in use receives spent dialysate containing at least one species from a dialysate outlet port of a dialyzer and removes at least a portion of a species from the dialysate to regenerate the dialysate for recirculation of at least a portion of the regenerated dialysate to a dialysate inlet port of the dialyzer and wherein the dialysate is conveyed through the sorbent cartridge and contacts at least one sensor after having contacted at least one material layer, and wherein the system comprises a plurality of sensors (201,202) that measure one or more solute concentrations and wherein the plurality of material layers comprise an urease-containing material layer and one or more layers (103,104,105) selected from, alumina, zirconium phosphate, magnesium phosphate, zirconium oxide, and activated carbon and wherein the system further comprises a controller; and characterized in that the controller determines a solute concentration of urea in the dialysate entering the urease-containing material layer by obtaining a first measurement from the dialysate before contacting the urease containing layer and a second measurement from the dialysate after having contacted the urease containing layer and wherein the first and second measurements are conductivity measurements or pH measurements.