Use of a fluid processing system for determining if the level of chloramine in purified water exceeds a safe threshold or increases and for either delivering filtered water to a medical treatment device adapted to receive the filtered water if the level of chloramine does not exceed said safe threshold or for halting or diverting the flow of filtered water if the level of chloramine exceeds said safe threshold, wherein the system is adapted for providing purified water and comprises: a fluid circuit including a replaceable first filter module configured to remove chloramine from primary water and a resistivity testing element positioned downstream of the first filter module such that filtered water from the first filter module is tested thereby to detect for a threshold level of chloramine remaining in the filtered water; the resistivity testing element being adapted to indicate resistivity above 2.5 megohm- cm; the first filter module, or an optional deionization filter upstream or downstream of the first filter module, being adapted to filter water so as to achieve a resistivity of the water filtered thereby and received by the resistivity testing element, to a level higher than 2.5 megohm-cm; a controller connected to the resistivity testing element, configured to control a flow of water in the fluid circuit, wherein the control of the flow of water comprises halting or diverting the flow of water, and to output an alarm signal; the resistivity testing element being connected to apply to the controller a resistivity signal indicating a resistivity of the filtered water received thereby, and the controller being configured to control the flow of filtered water responsively to the resistivity signal such that said controller, in response to a resistivity below a predefined level of at least 2.5 megohm-cm indicating that the chloramine level exceeds said safe threshold, or a rate of change of a resistivity above a predefined rate indicating an increasing chloramine level, controls the flow of water and outputs a signal indicating a water quality error; the controller further being configured to determine a time for replacement of said first filter module and to output an alarm signal indicating that said first filter module should be replaced, the time for replacement being determined based on a measured total volume and/or a total time since a last replacement of the first filter module and taking into account a predicted primary water quality, to ensure that only a fraction less than 100% of a chloramine removal capacity of the first filter module has been consumed by said time for replacement. a fluid circuit including a replaceable first filter module configured to remove chloramine from primary water and a resistivity testing element positioned downstream of the first filter module such that filtered water from the first filter module is tested thereby to detect for a threshold level of chloramine remaining in the filtered water; the resistivity testing element being adapted to indicate resistivity above 2.5 megohm- cm; the first filter module, or an optional deionization filter upstream or downstream of the first filter module, being adapted to filter water so as to achieve a resistivity of the water filtered thereby and received by the resistivity testing element, to a level higher than 2.5 megohm-cm; a controller connected to the resistivity testing element, configured to control a flow of water in the fluid circuit, wherein the control of the flow of water comprises halting or diverting the flow of water, and to output an alarm signal; the resistivity testing element being connected to apply to the controller a resistivity signal indicating a resistivity of the filtered water received thereby, and the controller being configured to control the flow of filtered water responsively to the resistivity signal such that said controller, in response to a resistivity below a predefined level of at least 2.5 megohm-cm indicating that the chloramine level exceeds said safe threshold, or a rate of change of a resistivity above a predefined rate indicating an increasing chloramine level, controls the flow of water and outputs a signal indicating a water quality error; the controller further being configured to determine a time for replacement of said first filter module and to output an alarm signal indicating that said first filter module should be replaced, the time for replacement being determined based on a measured total volume and/or a total time since a last replacement of the first filter module and taking into account a predicted primary water quality, to ensure that only a fraction less than 100% of a chloramine removal capacity of the first filter module has been consumed by said time for replacement.