The invention provides a blood treatment device comprising: a dialyser； an inlet pump assembly； an outlet pump assembly； and a control system. The inlet pump assembly configured to deliver a first volume of dialysate from a dialysate source to the dialyser in an inlet pump cycle having a dialysate delivery stroke. The outlet pump assembly configured to remove a second volume of dialysate from the dialyser and deliver the dialysate away from the dialyser in an outlet pump cycle having a dialysate removal stroke. The control system is configured to operate the inlet pump assembly in the inlet pump cycle, and configured to operate the outlet pump assembly in the outlet pump cycle. For each inlet pump cycle there is a corresponding outlet pump cycle, and each inlet pump assembly dialysate delivery stroke has a commencement time t1 and a termination time h, and each outlet pump assembly dialysate removal stroke has a commencement time t3 and a termination time t4. The blood treatment device is operable such that either: each dialysate removal stroke commencement time t3 is after the respective corresponding dialysate delivery stroke commencement time ti and before the respective corresponding dialysate delivery stroke termination time t2； or each dialysate delivery stroke commencement time t1 is after the respective corresponding dialysate removal stroke commencement time t3 and before the respective corresponding dialysate removal stroke termination time t4.
