Assuring water quality for hemodialysis

INTRODUCTION — Water used for hemodialysis should comply with established quality standards to ensure patient safety. Compliance with these standards requires two well-functioning components: a water treatment system that decontaminates water up to the quality standards and a distribution system that delivers the treated water to its points of use without recontamination. The design and maintenance of such water treatment and distribution systems will be reviewed here.

The waterborne contaminants to which hemodialysis patients may be exposed, the clinical manifestations of toxicity, and the safe levels of those contaminants are presented separately. (See "Contaminants in water used for hemodialysis".)

DESIGN OF A WATER TREATMENT SYSTEM — The design and operation of a water treatment system should be based upon standards published by the International Organization for Standardization (ISO) and the American National Standards Institute (ANSI), in collaboration with the Association for the Advancement of Medical Instrumentation (AAMI), with a focus on ensuring water quality and facilitating disinfection and maintenance [1]. There may also be additional mandatory local regulations that apply. The planning should take into account the capability of operating with the available water supply under worst possible conditions and working with a range of variation in the quality of that water. Highly treated water is aggressive and will leach metals and chemicals from materials with which it comes into contact. Thus, surfaces of all components of the water storage and distribution system that will come in contact with treated water must be fabricated with inert materials; no brass, aluminum, or galvanized metal parts can be used.

In-center hemodialysis — The water treatment room for in-center hemodialysis should be in a dedicated, secure, and access-controlled area. It should be fitted with a source of water, drains, and electrical power capable of supporting the operation of the water treatment system. Any newly built dialysis facilities should have a drinking water supply source that is separately directed to the dialysis unit and independent from other water supply sources on site. If an existing water supply is used, there are potential risks from chemicals used either to minimize legionellae growth or to periodically clean the system [2,3].

The water treatment room should have enough space so that equipment can be safely accessed and serviced. If the room is not on the ground floor, the load-bearing capacity of the floor should be professionally assessed to ensure that it is sufficient. A tough waterproof coating should be applied to the floor so that any water and/or chemical spills or leaks may be contained.

Only qualified plumbers with prior experience in dialysis water systems should be hired to install new systems or modify existing systems. In accordance with local plumbing codes, all water treatment systems should be isolated from the drinking water supply by a backflow prevention device, such as a Pressure Principle Backflow Assembly, a vacuum breaker, or a reduced pressure zone valve. The drainage system should have the capability to support maximum flow from all components of the water treatment system. It should have a device to prevent backflow of water from the main sewer system into the dialysis water treatment system in case of a blockage in the sewer. Drain systems can sometimes become malodorous for which bleach can be effective.

Each component of the water treatment system should be labeled with the following:

●Name and contact information of the component manufacturer

●The location of the manufacturer's recommendations for correct use, either in the form of a web address (electronic resource) or a physical handbook

●Signs and flow diagrams indicating the direction of flow and the location and operational setting of on and off valves

In addition, a logbook should be maintained for recording the readings of pressure, flow rate, and water quality, and the logbook should be made available for review. The logbook may be kept in the water treatment room or maintained electronically. For electronic logbooks, readings may be recorded automatically.

The following stepwise process may be used to design the basic elements of a system or to evaluate the overall suitability of systems proposed by vendors:

●Step 1 – Obtain an analysis of the water entering the dialysis facility with respect to the chemical contaminants. This analysis should be supplemented by information from the municipal water supplier. The water supplier should be asked to provide information about seasonal variations in water quality. As examples, aluminum and monochloramine levels tend to vary seasonally. Thus, the system design and evaluation should be based upon handling the worst possible case (eg, highest levels of aluminum and monochloramine).

●Step 2 – Ensure that the quality of the product water complies with standards of water for hemodialysis and related therapies published by the ISO or the equivalent National Standards Organization (eg, the ANSI) [4]. The water used for the preparation of dialysis fluid can also be used for dialyzer reprocessing or for on-line convective therapies (eg, hemodiafiltration). Thus, where dialyzers are routinely reprocessed, more rigid requirements for microbiologic contaminants should be set. For convective therapies, water quality should be as specified by the manufacturer of the dialysis machines.

●Step 3 – Determine the required reduction in concentration for each contaminant by dividing the feed water concentration by the maximum concentration as per the appropriate standards (table 1).

●Step 4 – Compare the values determined in step 3 with what can be achieved by various treatment methods (table 2).

●Step 5 – Identify possible purification cascades that will provide the desired reduction for all contaminants.

●Step 6 – Review the treatment cascades to eliminate redundancy. As an example, a possible treatment cascade might include both reverse osmosis and deionization as a means of reducing the levels of ionic contaminants. However, in most cases, one of the two processes would be sufficient and the other, usually deionization, could be eliminated for being redundant.

●Step 7 – Choose a water distribution system.

Planning and design consideration relating to water treatment should also include but not necessarily be limited to:

●Contingency planning – A risk assessment and contingency plan should be developed that describes how to deal with events that completely prevent dialysis from being performed. Unexpected events, such as natural disasters leading to failure of the facility's water supply or electrical service or a water main break, can occur at any time. Contingency planning should also address how to deal with unplanned changes in municipal water quality. These can result from additional disinfection following a water main break or with the introduction of nonstandard chemicals.

●Calculation of product water capacity for sanitization – If heat sanitization is planned for the system, the distribution loop is sanitized along with the links from the distribution loop to the dialysis machines. The demand for water during such sanitization may be higher than required by the dialysis machines during operation.

●Calculation of product water capacity during the winter months – The output of a reverse osmosis system is rated at a specified incoming water temperature. This temperature may not be achieved during winter months, resulting in a decrease in system efficiency. Thus, for reverse osmosis to function optimally, preheating the feed water may be required. Alternatively, installation of a higher-capacity system that can accommodate the fall in the efficiency of reverse osmosis during winter months may be needed.

●Expected future growth in the number of patients to be treated.

●Plan for sanitization of the system – Heat sanitization of the distribution loop is preferred over chemical sanitization as it is less labor intensive and can be performed without disruption to dialysis schedules. However, if chemical sanitization is used, sufficient time must be allowed for rinsing of the chemicals from the system prior to a dialysis shift. This rinsing requirement generally limits chemical sanitization to once per week, whereas heat sanitization can be performed daily, if needed.

●Placement of connectors for potable water outlets within the dialysis area – Whenever possible, outlets for potable water should be avoided in dialysis areas. However, if they are present in dialysis areas, such outlets should be clearly marked and, wherever possible, fitted with a connector that is incompatible with those used to connect the dialysis machine to the dialysis water distribution system. This additional measure is to prevent use of potable water for hemodialysis without undergoing appropriate treatment. All staff working within the dialysis area should be familiar with the different types of connections that may be present in the area.

Home hemodialysis — In principle, the treatment of water for home hemodialysis is comparable with that for in-center hemodialysis. (See 'In-center hemodialysis' above.)

Water treatment systems for home hemodialysis should include carbon filters or beds with built-in redundancy, heat disinfection, a reverse osmosis or deionization unit, and a point-of-use ultrafilter. If needed, a water softener should also form part of the treatment process. In locations with appreciable seasonal variations in feed water temperature, a tempering valve might be needed to provide a consistent feed water temperature by blending hot and cold water. The hemodialysis water treatment system should be isolated from the municipal water supply by installation of a backflow prevention device, in accordance with local plumbing regulations.

One important difference between home and in-center hemodialysis is that the water supply for home hemodialysis may come from a small community water supply or from a well. When the water is from a well, its quality needs to be examined closely. This is because well water is subject to seasonal changes and contamination from agricultural waste, chemicals used for hydrocarbon extraction, septic tanks, and underground fuel storage tanks. Thus, well water may be subject to higher rates of microbiologic and chemical quality failure [5,6].

Low-volume hemodialysis systems that are used for home hemodialysis (eg, NxStage System One, Tablo Hemodialysis System) do not require a separate system of water purification, because water treatment is built into the system. (See "Short daily home hemodialysis: The low dialysate volume approach".)

Hemodialysis in other settings — Hemodialysis machines used at the hospital bedside, such as in an intensive care unit, require a mobile water treatment system capable of producing dialysis water that meets the same quality requirements as that for outpatient hemodialysis.

The equipment used generally includes a softener, carbon filtration, reverse osmosis system, and point-of-use ultrafilter. If the feed water has a low degree of hardness, then the softener may be omitted to reduce the weight and space requirements of the system. However, omission of a softener may result in increased fouling of the reverse osmosis membrane by calcium and magnesium salts. (See 'Softeners' below.)

Where possible, the water treatment system should connect to a potable water supply. In systems where the water treatment system is not connected to a potable water supply, there is potential for presence of chemicals used to prevent Legionella, such as chlorine dioxide and silver-stabilized hydrogen peroxide. Testing for presence of these chemicals should be performed prior to dialysis. A backflow prevention device compliant with local plumbing codes should be installed at the point of connection to the feed water supply.

Compared with fixed systems, such as those used for in-center facilities, mobile units are more susceptible to bacterial proliferation due to their intermittent use. Thus, certain design and maintenance aspects that could be beneficial are:

●A direct-feed system without a storage tank. The lack of a storage tank helps minimize bacterial proliferation (figure 1).

●A design that allows for easy sanitization, preferably using hot water.

●A point-of-use ultrafilter.

●A plan for operation of the equipment for at least 15 minutes per day, whether a treatment is performed or not. When use is intermittent, even in the absence of a storage tank, there is the potential for bacterial growth because of stagnant water in the system. Daily operation minimizes such bacterial proliferation.

In addition to structural specifications, it is important that electrical noise and current leakage be minimized, particularly if the equipment is used in an intensive care setting. The hemodialysis water treatment systems used in such locations should comply with applicable electrical safety standards regarding electrostatic discharge, electromagnetic compatibility, and any other electrical safety standards.

ELEMENTS OF A WATER TREATMENT SYSTEM — A water treatment system sources water from a potable water supply and decontaminates it as per the quality requirements set for hemodialysis. Ideally, the water treatment system is directly connected to its source of water supply. (See 'In-center hemodialysis' above.)

Typically, a water treatment system consists of three distinct parts: a pretreatment section; a primary and, if needed, a secondary treatment section; and the distribution system (figure 2). The functions of these different sections are discussed below.

Pretreatment — The pretreatment section is intended to condition the incoming water for optimal operation of the primary treatment process. Pretreatment generally includes:

●A booster pump to provide adequate pressure for optimal operation of the water treatment system. Pressure or flow switches of the booster pump help regulate flow when the water pressure falls below predetermined levels.

●A depth filter (ie, a bed filter, sediment filter, or multimedia filter) that can remove coarse particulate matter.

●A softener to reduce levels of calcium and magnesium in locations where the incoming supply brings in hard water. (See 'Softeners' below.)

●Activated carbon beds or filters to remove chlorine and monochloramine. (See 'Activated carbon beds or filters' below.)

Other components of the pretreatment section can include a dedicated water heater and a tempering valve to provide water year-round at a suitable temperature. This is essential for the optimal operation of reverse osmosis, chemical injection, and activated carbon beds or filters. (See 'Activated carbon beds or filters' below and 'Reverse osmosis' below and 'Chemical injection' below.)

Primary treatment — The primary treatment process should be capable of removing a wide range of harmful contaminants, both chemical and microbial, to which a patient might otherwise be exposed. In the United States, reverse osmosis is used as the primary treatment process in 98 percent of the water treatment systems [7]. Reverse osmosis has low operating costs, essentially unlimited capacity for contaminant removal, and the ability to effectively remove a wide range of inorganic and organic contaminants. (See 'Reverse osmosis' below.)

Important exceptions to the contaminants removed by reverse osmosis are chlorine and monochloramine, which degrade the polyamide membranes used in reverse osmosis, thereby compromising their ability to remove other contaminants [8]. Thus, the pretreatment section should include a means for chlorine and monochloramine removal, such as carbon filtration. (See 'Activated carbon beds or filters' below.)

Secondary treatment — The removal of high concentration ionic contaminants by reverse osmosis is, at times, insufficient. In such situations, deionization is helpful as a secondary treatment step following reverse osmosis. (See 'Deionization' below.)

Although mixed-bed deionizers effectively remove ionic contaminants, they do not remove uncharged contaminants, bacteria, or endotoxins. In addition, the resins of the deionizers provide a fertile environment for bacterial proliferation and often worsen the microbiologic quality of water passing through them. Thus, a bacterial control procedure is necessary after deionization to ensure that the product water meets the appropriate microbiologic quality. Consequently, bacteria- and endotoxin-retentive filters are installed following the deionizers to prevent contamination of the water distribution system.

As an alternative to deionization, the primary and secondary treatment sections can be combined in the form of a double-module or two-stage reverse osmosis system. In that configuration, two reverse osmosis modules operate in series, with the permeate from the first module serving as the feed water for the second module. A double-module reverse osmosis system offers enhanced removal of both ionic and uncharged contaminants and provides a better barrier against microbiologic contaminants compared with a single-stage reverse osmosis system, alone or in combination with deionization.

Distribution system — The distribution system must be capable of delivering the treated water to its points of use without recontamination. (See 'Disinfection' below.)

There are two basic designs for distribution systems: direct feed, which does not include a water storage tank, and indirect feed, which incorporates a water storage tank (figure 1). Both systems offer distinct advantages and disadvantages:

●In an indirect-feed system, the extra capacity provided by the storage tank allows for use of a water treatment system that produces water that is less than required at the time of peak demand. Additionally, in case of a water system failure, the water in the storage tank provides a reserve that permits an orderly discontinuation of dialysis. By comparison, in case of water system failure in a direct-feed system, dialysis ends abruptly due to the sudden interruption of water supply to the dialysis machine.

●The pressure in an indirect-feed system can be increased, as desired, by means of a booster pump. By comparison, the pressure in a direct-feed system is limited to the outlet pressure of the last purification device. Thus, the indirect-feed system might be the only suitable option if the patient care area is located one or more floors above the water treatment system.

●Bacterial control is easier in a direct-feed system since it is not associated with a storage tank containing semi-stagnant water.

PRINCIPAL COMPONENTS AND MONITORING OF WATER TREATMENT SYSTEMS — Every hemodialysis unit should have regularly updated written policies and procedures for the safe operation of the water treatment plant covering:

●Access to the room housing the water treatment system.

●Appropriate labeling to identify each component of the water treatment system and its operation. Identifying information should be directly affixed to each component by the manufacturer (International Organization for Standardization [ISO] standards indicate that such markings should include name and address of manufacturer, model and serial number, identification of flow direction through the device, and warnings). Operational information specific to each component should be affixed by the facility (figure 3).

●Procedures for operation and maintenance of all components of the water treatment and distribution system.

●Procedures for the collection of water samples, the frequency of collection, and the type of testing to be performed.

●Policies related to recording and trend analysis of results.

●Guidelines for the actions to be taken when out-of-range test results are obtained.

●Policies and audit related to third parties who provide services relating to the water treatment system.

●While the medical director has the ultimate responsibility, nursing and technical staff do share some responsibility for the safe operation of the water pretreatment plant as well. The entire team of the medical, nursing, and technical staff should hold regular meetings to review the safe operation of the water treatment system and participate in audits related to water quality.

Any component of a water treatment system can fail without warning. Thus, regular monitoring of all components is necessary. Without regular monitoring, any such failure places patients at risk. The principal components of the water treatment and distribution system along with routine monitoring practices are discussed here.

Ultrafilters, particle filters, multimedia filters, and depth filters — Pre- and post-filter pressure gauges should be fitted on all filters and the pressure difference between the two gauges should be monitored. To counter the loss in performance, filters should be backwashed every day when there are no patients connected to the hemodialysis machines. Automated systems should have timers monitored to ensure that backwash occurs outside of patient treatment hours.

Pressure differences should be trended to aid investigation of any abnormal values. With use, all types of filters become progressively obstructed with debris, resulting in an increase in pressure difference and/or a decrease in filtrate flow rate. A decrease in pressure difference without a corresponding decrease in flow rate can indicate a breach of filter integrity.

Softeners

●Component design and purpose – Softeners are a form of deionizer in which calcium and magnesium ions are exchanged for sodium ions. Softeners are used in locations where the water supply is "hard" due to high levels of calcium and magnesium. Although reverse osmosis sufficiently clears calcium and magnesium ions, the use of a water softener before the reverse osmosis unit protects the reverse osmosis membranes from fouling by calcium and magnesium salts. The fouling of the reverse osmosis membrane can lead to a reduction in its permeate flow rate and life expectancy.

Softeners can be placed either before or after the carbon beds. Placing the softener before the carbon beds can help impede microbial growth and decrease the bacterial bioburden to the reverse osmosis unit. Placing the softener after the carbon beds can protect the softener resin from degradation by chlorine or monochloramine in the incoming water. Either of these orientations are acceptable depending upon the chlorine and monochloramine content of the source water.

Softeners have a finite capacity and must be regularly regenerated. Regeneration is generally undertaken on an automatic cycle by exposing the resin to a strong brine solution. The volume of water that can be softened before the softener will need regeneration can be calculated by:

Safety during regeneration should be ensured by inclusion of an automatic lockout device to prevent patient exposure to water (from the dialysis water line) containing high quantities of calcium, magnesium, and sodium [9]. This is most readily achieved by using a bypass valve, which is activated during the regeneration cycle. Alternatively, timers can be set for automatic softener regeneration, which can then be monitored to ensure that regeneration does not occur during patient treatment hours. Additionally, conductivity monitors on the reverse osmosis system should sound an alarm if a high concentration of sodium is detected. However, alarms should not be relied upon exclusively.

●Monitoring – Softeners should be monitored to ensure that they continue to effectively remove calcium and magnesium ions. They are monitored by measuring the water hardness in grains per gallon (GPG) or parts per million (PPM).

Colorimetric test strips can also be used to measure water hardness on site. Colors on the test strips should be distinguishable by personnel using the test strips. For sampling, there should be a clearly labeled port following the softener (figure 2). Softened water should have a hardness of 1 GPG (17 mg/L CaCO₃) or lower. Values above 1 GPG should prompt an evaluation of the softener for loss of resin, fouling of the resin by iron, an inadequate regeneration cycle, or inadequate capacity due to high water throughput.

At the end of the day, hardness should be checked to determine the capacity of the softener over its full operating cycle. In addition, the brine tank should be checked daily to ensure adequate quantities of sodium chloride. It should be verified that salt pellets are above the water level and there are no salt bridges. The pressure difference should be monitored across the softener daily (table 3). A change in pressure difference >10 mmHg, in the absence of a change in flow rate, can indicate a problem with the resin bed.

Activated carbon beds or filters

●Component design and purpose – Activated carbon beds, filters, or blocks are the principal method of removing chlorine and monochloramine, substances that can degrade reverse osmosis membranes [8], and among dialysis patients, cause hemolysis and resistance to erythropoiesis-stimulating agents (ESAs) (table 4 and table 5) [10-12]. Carbon filtration also provides nonspecific removal of other substances, such as disinfection byproducts and organic contaminants.

Traditionally, granular-activated carbon (GAC) has been used in carbon beds. An alternative is catalytic-activated carbon, which removes monochloramine more effectively than GAC [13].

Efficacy of removal of chlorine and monochloramine is indicated by either the iodine number, which is a measure of the carbon's capacity to adsorb low-molecular-weight substances, or by the empty bed contact time (EBCT), which is a measure of how much contact occurs between the carbon and water as it flows through the carbon bed. For removal of monochloramine, the carbon bed should have an iodine number of at least 900 and an EBCT of at least 10 minutes [1]. EBCT can be calculated from:

where V is the volume of carbon (ft³) and Q is the water flow rate (gallons/min). Alternatively:

where V is the volume of carbon (m³) and Q is the water flow rate (L/min).

Although carbon can remove a wide range of organic contaminants by adsorption, removal of monochloramine takes place via an oxidation-reduction reaction at the active sites on the carbon surface. Over time, access to these active sites is diminished by adsorption of soluble organic molecules and by entrapment of suspended material present in the water. This leads to monochloramine having insufficient contact time with active sites and is known as breakthrough. Patients should be protected from exposure to monochloramine arising from breakthrough by placing two carbon beds in series and monitoring the monochloramine concentration after the first bed. When breakthrough is detected in setups with exchangeable carbon beds, the first bed should be discarded, the second bed should be moved into the first position, and a new bed should be installed in the second position. When breakthrough is detected in setups with permanent carbon beds, it is often simpler to replace the carbon in both beds.

Inadequate removal of monochloramine can occur when the pH of the water is high or when water contains high levels of organic material or additives (such as orthophosphate added for lead and copper control). In those circumstances, other strategies for its removal might be needed. One approach that has been used successfully is the injection of sodium bisulfite prior to the reverse osmosis system. Other approaches include installing an anion exchange resin before the carbon beds for removal of organic matter and other contaminants and injection of a mineral acid before the carbon beds to reduce the pH of water, if it is alkaline. (See 'Chemical injection' below.)

Assays may test positive and falsely indicate inadequate removal of monochloramine when N-chloramines are present in the water. N-chloramines are large molecules formed by the chlorination of amine groups that are present on a wide variety of organic molecules in water [14,15]. N-chloramines are effectively removed by reverse osmosis. False positive tests can be distinguished from true presence of monochloramine by checking its level in the reverse osmosis permeate. A negative test in the reverse osmosis permeate in the presence of a positive test in the water leaving the carbon beds suggests that the substance detected is N-chloramine rather than monochloramine.

●Monitoring – Carbon beds, filters, or blocks are monitored to detect exhaustion of their capacity for removal of monochloramine and total chlorine. The risk of patient injury is high if the carbon fails due to breakthrough (table 4 and table 5). Thus, rigorous monitoring, recording, and review of the performance of the carbon is essential. (See "Contaminants in water used for hemodialysis".)

The maximum allowable level for free chlorine is 0.5 PPM and for monochloramine 0.1 PPM. There is no direct way to measure the monochloramine level. It is determined indirectly by calculating the difference between the levels of total chlorine and free chlorine.

Total chlorine measurements can be made by using the N,N-diethyl-p-phenylenediamine (DPD) assay, by using a "dip and read" test strip (with a DPD or Thio-Michler's ketone [TMK]), or by an on-line monitor. Free chlorine may also be measured using "dip and read" test strips that use syringaldazine or tetramethylbenzidine. Many dialysis facilities only check total chlorine with the upper limit set to 0.1 PPM.

For appropriate use of "dip and read" test strips, the personnel performing the tests should be screened for color blindness to ensure that they can read the color change accurately. (Alternatively, hand-held meters capable of reading the test strips are available.) If commercially produced indicators are used, then the selected indicator should be appropriate for the application. Test strips can be sensitive to heat and humidity, and therefore, strict adherence to manufacturer's instructions for storage and use is essential.

The testing should be performed on a sample collected from the port located after the first carbon tank filter or block. If the level in that sample is not acceptable, then a sample collected from the port located after the second carbon tank should be tested. If that sample is acceptable, then dialysis can be continued with plans to replace the failing first carbon tank. Testing may be performed more frequently until the first tank filter or block has been replaced. If the sample collected after the second carbon tank filter or block does not meet the allowable limit, then dialysis cannot continue until the tanks are replaced. Testing should be performed at the beginning of the day after allowing the system to run for 15 minutes and then at least every three to four hours while patients are on dialysis. Allowing the system to run for 15 minutes ensures that water that has dwelt in the carbon overnight is fully expelled and that the tested sample reflects active flow of new water through the tanks.

EBCT of the carbon must be at least 10 minutes. Changes in water flow rate, such as might occur with the addition of dialysis stations to a unit or an increase in dialysis fluid flow rate, or a change in carbon volume should prompt a recalculation of the EBCT. In addition, the pressure difference across each carbon tank filter or block should be monitored. An increase in the pressure difference of >10 mmHg suggests fouling with particulate matter or bacteria. A decrease in the pressure difference of >10 mmHg suggests channel formation. Back-flushing of carbon tanks can help resolve these problems, although it will not regenerate exhausted carbon. Back-flushing should occur when patients are not on dialysis. Automated backflushing should be affixed with appropriately functional timers.

Reverse osmosis

●Component design and purpose – Reverse osmosis removes dissolved substances, such as metal ions, salts, and inorganic and organic chemicals, and microbial elements, including bacteria, endotoxins, and viruses. The effectiveness of reverse osmosis depends upon characteristics of the contaminant to be removed. Ionic contaminants are more effectively removed compared with neutral solutes and polyvalent ions are more effectively removed compared with monovalent ions. The removal of organic contaminants is largely based on size; organic contaminants with molecular weight >200 Da are effectively filtered out.

Reverse osmosis reduces contaminant levels by using high pressure (200 to 250 pounds per square inch) to force a portion of the feedwater across a semipermeable membrane. The remaining feed water that does not go across the membrane carries the concentrated contaminants to the drain (figure 4). Reverse osmosis may be operated in a single- or double-module (two-stage) configuration. With a single module, the feed water enters as a single stream and exits either as reject water containing concentrated contaminants or as permeate (product water) (figure 4).

Although there is potential for up to 75 percent of the feed water to be recovered as product water, recovery rates of 35 to 50 percent are more common. Recovery rates are highly dependent upon the concentration of contained substances capable of fouling the membranes and the temperature of the feed water. In areas with deficient feed water supply, a part of the reject water can be added back to the feed water to allow recovery of a greater quantity of product water.

Overall water recovery can also be increased by using a two-stage reverse osmosis system. A two-stage system operates two reverse osmosis modules in series with the permeate from the first module serving as the feed water for the second module. While traditional single-pass reverse osmosis systems are typically less than 75 percent efficient, two-stage reverse osmosis can achieve an efficiency of 90 percent or more. Two-stage systems also provide redundancy, which can help maintain operations if one stage of the reverse osmosis unit fails.

To aid optimal recovery, a tempering valve is often included in the pretreatment section so that optimal feed-water temperature can be maintained, particularly in colder climates. Failure to maintain an optimal temperature can lead to an inability of the reverse osmosis system to keep up with the demand for treated water. (See 'Pretreatment' above.)

When the product water flow rate or percent recovery of product water changes by some predetermined amount (eg, 10 percent with temperature, pressure, and feed water flow rate held constant), the membrane modules should be cleaned using the manufacturer's recommended method. A 10 percent increase in the pressure difference between the feed-side and product-side of the membrane should also prompt cleaning of the membranes. If cleaning fails to restore performance, then the membrane modules might need to be replaced.

The life of reverse osmosis membranes can be prolonged by pretreatment of the feed water. As examples, softening the feed water will help minimize fouling with calcium and magnesium salts and removal of chlorine by the carbon beds will protect membranes from being degraded. (See 'Pretreatment' above.)

Typically, a single-module reverse osmosis system will meet the microbiologic and chemical standards for hemodialysis. However, if the feed water quality varies over time, then a double-module or two-stage reverse osmosis system may be preferred. (See 'Secondary treatment' above.)

The most common reverse osmosis membrane for hemodialysis applications is made of polyamide. Such membranes, unlike those manufactured from cellulose acetate, are not susceptible to biological degradation or hydrolysis but are sensitive to chlorine and monochloramine exposure and to peracetic acid (disinfectant) at a concentration of >1 percent of peracetic acid [8]. Degradation of the membrane compromises its ability to remove contaminants from the water. Thus, chlorine and monochloramine should be removed by carbon beds installed before the reverse osmosis unit, although other methods might be required in certain circumstances. (See 'Activated carbon beds or filters' above.)

●Monitoring – Reverse osmosis systems are monitored to:

•Ensure continued removal of contaminants at the desired level

•Ensure production of sufficient product water

•Determine when the membranes should be cleaned or replaced

To facilitate such monitoring, samples are generally taken from multiple sampling points (figure 4). Measurements of conductivity and flow rate are used for calculation of the solute rejection (a measure of the removal of ionic contaminants) and the percent recovery of product water (a measure of membrane fouling).

Percent recovery can be calculated from:

The conductivity of product water should be monitored on a continuous basis during operation of the reverse osmosis system. Modern reverse osmosis systems monitor and display the percent rejection in real-time during operation. The conductivity is displayed as total dissolved solids. The percent rejection of a reverse osmosis system indicates the ability of the system to remove solutes. The percent rejection is calculated from:

There is no set value that is desirable for the percent rejection. Instead, the dialysis facility should use the percent rejection figures to monitor changes in performance of the reverse osmosis system over time. A decline in the percent rejection can indicate fouling of the reverse osmosis membranes. Alarm set points can be calibrated to alert the user when the percent rejection or conductivity of the product water falls short of desired.

The percent rejection or conductivity also depends upon whether a particular system mixes reject water back into the feed-water stream. If the measurement is made before the point at which the recycled water enters the feed-water stream, then the calculated solute rejection provides a measure of the overall performance of the reverse osmosis system. If the measurement is made after the point at which the recycled water enters the feed-water stream, then the calculated solute rejection provides a measure of the performance of the reverse osmosis membranes, and the value may be slightly lower.

Deionization

●Component design and purpose – Deionization is a process by which ion-exchange resins are used to remove ionic contaminants from water. Hydrogen ions are exchanged for cations (cationic resins) and hydroxyl ions are exchanged for anions (anionic resins). Mixed-bed deionizers contain both cationic and anionic resins; the exchanged hydrogen and hydroxyl ions combine to form water.

If the available feed water contains a high concentration of one or more ionic contaminants and reverse osmosis alone is insufficient, then deionization can be used as an additional secondary purification step. Although mixed-bed deionizers effectively remove ionic contaminants, they do not remove uncharged contaminants, bacteria, or endotoxins. In fact, the resins provide a good environment for bacterial proliferation and often worsen the microbiologic quality of water passing through them, thereby increasing the requirement for bacterial control. Thus, when deionizers are installed as a secondary purification step, they should be followed by a bacteria- and endotoxin-retentive filter to prevent contamination of the purified water distribution system.

●Monitoring – Deionizers should be monitored to ensure product water quality and to identify when new resin beds might be needed. The undetected failure of a deionizer can be extremely hazardous to patients. Thus, the resistivity (a measure of the concentration of ions in the water) of the product water must be monitored on a continuous basis with a temperature-compensated meter. Treated water should have a resistivity of 1 megaohm·cm (1 micro Siemens (S)/cm or 0.1 mS/m) or more. Water with resistivity less than 1 megaohm·cm is not suitable for hemodialysis. For reference, pure water (water with no dissolved solute) has a resistivity of around 18 megaohm·cm. Resistivity should be recorded at least twice daily and with regular review of the trends (table 3). The meter should be coupled to an alarm system that is audible in the patient treatment area and that diverts product water to drain if the resistivity falls below 1 megaohm·cm. Deionizer systems also need surveillance for microbiologic contaminants. Pre- and post-deionizer pressures should be monitored. A change in the pre- to post-deionizer pressure difference of >10 mmHg above baseline can indicate deionizer fouling with particulate matter or resin breakdown.

Disinfection — The distribution system must be capable of delivering the treated water to its points of use without recontamination. The risk of recontamination with chemical contaminants is low provided that the primary and secondary treatment processes are well maintained and the surfaces of all piping, valves, storage tanks, and pumps in the distribution system that may come in contact with water are constructed from inert materials.

The following measures can help limit microbiologic contamination and proliferation:

●Structural modifications – This risk can be minimized with careful design and maintenance of the distribution system. A continuous loop design is more beneficial (figure 1); dead-ends or multiple branches should be avoided. All surfaces within the distribution loop should be as smooth as possible. Chamfered joints are preferred over butt joints. The machine and drain outlet connections should be as short and simple as possible. The water flow velocity should be regularly evaluated, and the distribution loop should be visually inspected to ensure that there have been no unauthorized or inappropriate repairs or alterations to the system. Materials, such as polypropylene, cross-linked polyethylene, and polytetrafluoroethylene that can tolerate disinfection with either hot water or ozone are preferred. If hot water is used for system disinfection, then the distribution loop should be appropriately insulated to ensure that all parts of the loop are able to reach a sufficiently high temperature for the efficacy of disinfection.

Storage tanks should be designed to minimize the potential for bacterial growth. Design features that should be incorporated include: a tight-fitting lid and conical or bowl-shaped base; drain from its lowest point; venting through a 0.45 micrometer hydrophobic air filter; avoid the incorporation of a sight tube; and, in cases where an overflow pipe is incorporated, it should be fitted with means to prevent contamination arising from backflow. There should be a means of adequately disinfecting and rinsing the interior surfaces of the tank. Bladder-type or pressurized surge tanks should not be used.

The inclusion of a bacteria- and endotoxin-retentive filter at the point of use, such as at the dialysis machine, can ensure dialysis fluid of very high microbiologic quality (ultrapure dialysis fluid). However, such a point-of-use filter should not be used as an alternative to careful design and maintenance of the entire water treatment and distribution system. The details regarding production, quality assurance, and benefits of ultrapure dialysis fluid are discussed at length elsewhere. (See "Ultrapure dialysis fluid".)

●Disinfection practices – The risk of microbial recontamination in the distribution loop is high. This is because the water treatment system works to remove disinfectant chemicals, such as chlorine and monochloramine, that are added to drinking water for suppression of bacterial growth. The removal of these disinfectants provides a good environment for biofilm formation and microbiologic contamination of the treated water. Once established, the biofilm is difficult to remove. The continuous dispersal of cells from a biofilm enables it to spread and colonize new surfaces. Biofilm can be minimized by the use of appropriate disinfection practices.

Difficult-to-control levels of bacteria and endotoxin suggest the presence of biofilm. Disinfection should be performed on a predetermined schedule, ie, proactively, to prevent bacterial proliferation, rather than retroactively to reduce levels of bacteria once proliferation has occurred [16]. The frequency of disinfection depends upon the design of the system and is established by trend analysis and the action levels, which are typically set at 50 percent of the permitted maximum level. At a minimum, disinfection should be performed quarterly, even if the action levels have not been breached. A more frequent disinfection schedule using lower concentrations of disinfectant is preferable to less frequent disinfection with a higher concentration of disinfectant [17].

Disinfection can be performed either using chemicals or using hot or ozonated water. Chemical disinfection is performed with sodium hypochlorite (bleach) or with a mixture of peracetic acid and hydrogen peroxide. These agents are compatible with the PVC tubing that is commonly used in the manufacture of distribution loops. Disinfection with these agents is more effective if the pipes are first treated with a descaling agent. Disinfection with chemical agents can be difficult to implement on a frequent basis because of the lengthy period of rinsing required to remove residual disinfectant from the distribution system. Thus, ozonated or hot water is preferred over chemical disinfectants. Disinfection with hot or ozonated water can be performed daily or weekly provided that the tubing material is compatible with them. These methods leave either no chemical residues (hot water) or residues with a short half-life (ozone).

For removal of biofilm that is resistant to disinfection, cleaning the system with 2 to 3 percent citric acid before disinfection may help. However, it should be ensured that the materials of the system are compatible with citric acid [18,19]. In some cases, complete or partial replacement of the distribution system might be the only way to remove biofilm.

Ultraviolet (UV) light can be used as an optional auxiliary means of controlling the bacterial content of water in the distribution loop. UV light inactivates microbial pathogens in water but has no effect on any biofilm that has formed. UV lights used for the inactivation of pathogens emit light at a wavelength of 254 nm. They are monitored with a radiant energy meter that measures on-line energy intensity and triggers an alarm if the intensity is not sufficient. If the UV light has a calibrated radiant energy meter filtered to restrict its output to the disinfection spectrum, the minimum dose of radiant energy should be at least 16 mW·s/cm². If the irradiator is not fitted with a calibrated radiant energy meter, the dose of radiant energy provided by the lamp should be at least 30 mW·s/cm². The device used should be appropriate for the anticipated flow rate of water passing through it. The UV light should be housed within an opaque structure. UV light intensity checks should be recorded daily and reviewed routinely (table 3). This requirement is intended to prevent development of microbial resistance. The UV light source should be followed by an endotoxin-retentive filter to remove pyrogens.

●Alteration of water flow velocity – Maintaining a minimum flow velocity in the distribution loop has been suggested as a means of minimizing bacterial proliferation and biofilm formation, although the effectiveness of this strategy is unclear, particularly in systems that do not maintain flow through the distribution loop when the dialysis facility is not operating. In the United States, Centers for Medicare and Medicaid Services (CMS) recommends that when the system is operating under peak demand, the flow velocity in the distal part of the distribution loop in indirect feed systems should be at least 3 ft/s. For direct feed systems, the flow velocity should be at least 1.5 ft/s. Flow velocity is calculated using the formula

V = Q/A, where V is the flow velocity, Q is the flow rate, and A is the cross-sectional area of the pipe.

Measurements of flow rate and the flow velocity calculated from it should be reviewed routinely with trend analysis. In particular, the flow velocity should be recalculated following any change in peak water demand, such as when additional dialysis stations are connected or when there is a change in the dialysis fluid flow rate.

Chemical injection — Chemical injection systems can be used to adjust feed water properties to ensure optimal operation of other treatment processes. Examples include the addition of sodium bisulfite to remove monochloramine and the addition of mineral acid to optimize the pH of water. For alteration of pH, a mineral acid should be used to adjust pH rather than organic acid. Organic acids can serve as a nutrient for bacteria and enhance their proliferation. Injection systems should be designed to tightly control addition of the chemical to vary with certain parameters of water (eg, pH).

FREQUENCY OF MONITORING — The frequency of monitoring of water quality may vary from one system to another based upon data collected during the initial validation process that follows the installation of a water treatment system. The validation process is carried out over two to four weeks during which time all aspects of system performance are monitored and samples are collected for chemical and microbial analysis of the product water.

After the initial validation process, monitoring for chemical contaminants depends upon the quality of the water entering the treatment system, and for microbiologic contaminants and endotoxin levels, on the frequency of disinfection (table 3).

Levels of microbial contaminants are monitored to verify the adequacy of the disinfection schedule. However, such monitoring should take place at least monthly (table 3). During times of suspected contamination, the frequency of monitoring should be increased to at least weekly. Each dialysis station in the loop should be sampled at least once annually with at least two dialysis machines sampled every month. Monthly disinfection of the loop should include disinfection of the machine supply lines that are not routinely disinfected during the machine disinfection cycle. International Organization for Standardization (ISO) 23500-5 provides guidance on this aspect of maintaining water quality [1].

SAMPLING METHODS

Sampling for chemical contaminants — Chemical contaminants should be checked in water sampled from various sampling points (figure 2). The system should have been in continuous operation for at least 20 to 30 minutes before recording any readings or collecting any samples. Waiting for such time ensures that measurements are representative of conditions during routine operation. All sampling points should provide direct access to the water stream being sampled. Prior to sampling, water should be allowed to flow through the sampling point for at least 30 seconds before sampling.

Sampling for microbiologic contaminants — Samples for microbial testing are drawn from sample ports in the dialysis water distribution system. The exterior of the sample ports (not the lumen) may be disinfected by wiping with 70 percent ethanol or isopropanol (isopropyl alcohol). Bleach or other disinfectant solutions should not be used. The sample should not be collected until all the alcohol has evaporated. The sample port should be opened, and the dialysis water sample should be collected in a "clean catch" manner and placed into a sterile, endotoxin-free container. The "clean catch" technique entails opening the sample port and allowing the water to run (waste) before collection for at least 60 seconds, unless the sampling port manufacturer's instructions specifies another duration. A sample volume of approximately 50 mL is usually necessary. Samples should be collected just before a scheduled disinfection and 24 hours or more after disinfection. Detailed methods for sampling water for dialysis are available in ISO 23500-5 [1].

Generally, a sample from the return line of the distribution system is sufficient to demonstrate compliance with the water quality standard. However, a more comprehensive sampling approach is required for dialysis units in the United States in order to comply with CMS regulations; for example, sampling from the first station in the loop, the last station in the loop, reverse osmosis product water, and from storage tanks is required. Additional samples may be needed during times of contamination.

Samples should be analyzed as soon as possible after collection. If samples cannot be analyzed within four hours of collection or if they are sent to an off-site laboratory, then they should either be refrigerated or packed on ice in an insulated container that is suitable for shipping. The samples should not be frozen. For samples analyzed off site, the laboratory's instructions for shipping must be followed to ensure that analysis is performed within 24 hours of the sample collection. The storage and handling of samples for endotoxin analysis is dependent upon the measurement technique. Thus, individual laboratories' instructions for sample handling and storage must be followed for endotoxin analysis.

Water used for hemodialysis generally has very low levels of bacteria, and it, therefore, requires specific, validated, and sensitive analytical methods for sample plating and sample culture. Common clinical laboratory methods that are used for bloodborne bacteria, commercial water test kits, and dip samplers should not be used due to their lack of sensitivity and specificity for bacteria in hemodialysis water.

The following culture method options are considered suitable for sample plating and culture:

●Membrane filtration – This entails filtration of the sample through a membrane filter that has a pore diameter of 0.45 micrometer or less. Membrane filtration should be used when the sample needs to be concentrated for detection of low levels of contamination (usually <1 colony-forming unit [CFU]/mL). The volume to be filtered is determined by the suspected level of contamination and should be between 10 mL and 1000 mL. Membrane filtration is the most sensitive method for culture. However, it is also the most resource-intensive (labor and supplies) of the available methods for culture.

●Spread plate assay – The spread plate assay can be used where the membrane filter technique is not readily available. A sample (0.1 to 0.3 mL) is applied on the incubation medium in a Petri dish (typically 90 mm in diameter). The sample must be spread uniformly with a plate spreader or hockey stick, rather than a calibrated loop. Using a 0.2 mL sample, the detection limit of the spread plate assay is 5 CFU/mL.

●Pour plate technique – The pour plate technique is a second alternative to the membrane filter technique. The sample (typically 1 mL) is placed in a Petri dish and 15 to 20 mL of molten medium is added. The sample and medium are mixed carefully by gentle rotation and then allowed to set. The time between addition of the sample and addition of the molten medium should not exceed 15 minutes. The plate is then inverted and incubated. The detection limit for bacteria using this technique is 1 CFU/mL in a 1 mL sample.

The recovery of bacterial colonies and the types of bacteria recovered are influenced by the choice of culture medium, incubation period, and incubation temperature:

●Culture medium – A low-nutrient agar, such as Tryptone Glucose Extract Agar or Reasoner's Agar 2A should be used [1]. Blood agars should not be used due to their richness in nutrients that are incompatible with the bacteria that grow in the nutrient-poor environment of purified water.

●Incubation temperature – Samples should be incubated at 17 to 23°C [20,21].

●Incubation period – Samples should be incubated for 168 hours. A long incubation time allows the growth of species that are better adapted for growth at a higher temperature and/or on richer media.

Within the United States, the CMS Conditions for Coverage (CfC) specify a maximum allowable level for microbial contaminants in dialysis water of 200 CFU/mL. The CfC also specify that the microbial level be determined using Trypticase Soy Agar (TSA) incubated at 35°C for 48 hours as described in the 2004 edition of the American National Standard for Dialysate for Hemodialysis (ANSI/AAMI RD52:2004). Since 2004, the ANSI/AAMI standards have been updated and specify more stringent standards for microbiologic contaminant levels. CMS CfC do not recognize these more recent and more stringent requirements. Extending the culture time up to seven days and using an incubation temperature of 23 to 28°C increases the recovery of bacteria compared with incubation for 48 hours at 35°C [20-22].

Testing for endotoxins — The standard test for determining endotoxin concentrations is the Limulus amebocyte lysate (LAL) assay, which utilizes elements derived from the Atlantic horseshoe crab. Several different methodologies are in use. Concern over the harvesting of horseshoe crabs for LAL assays has resulted in the development of alternative endotoxin testing methods [23-26], which are not commonly used in the hemodialysis setting.

For storage of endotoxin test samples, it is important to use containers specified by the testing laboratory or those included with the assay kit. Endotoxin testing should be performed by fully trained personnel.

Dialysis centers should keep records of the microbiologic testing results and schedule testing for all loop ports and machines to ensure that samples are collected appropriately. Results of cultures and endotoxin testing should be subjected to trend analysis to help identify any tendency for bacterial contamination.

If the colony count or endotoxin reaches the action level (>50 CFU/mL or >0.1 endotoxin unit/mL, respectively), then the dialysis unit should have a plan for addressing the contamination and for subsequent testing to ensure that the plan was effective. Corrective action for positive culture counts or high endotoxin levels can include:

●Disinfection and cleaning of the reverse osmosis membranes

●Disinfection of the loop

●Installation of an endotoxin-retentive filter

●Confirming that endotoxin-retentive filters are not failing or contaminated

●Confirming that no dead spots exist in the loop plumbing

●Confirming that the water supply line connecting the machine to the loop is properly disinfected during loop disinfection

●More frequent water testing

●Increasing the number of sites tested

●Increasing the frequency of disinfection

●Ensuring that proper disinfection procedures are followed

OPERATIONAL RESPONSIBILITY — The treatment system needed to produce water consistent with set standards will vary from facility to facility depending upon the nature of the source water supply and the applications for which the water is intended. Specialist vendors are generally responsible for the detailed design and installation of a water treatment system. If the entire system is not obtained from a single vendor, then the user becomes responsible for ensuring that separate parts of the system are compatible and that the overall water treatment system is adequate.

Once a dialysis water treatment system is installed and operating properly, the responsibility for ensuring continued performance passes from the vendor to the dialysis facility. To meet that responsibility, all dialysis units should have a facility-specific quality management program for the water treatment and distribution systems [1]. The quality management program should be executed by appropriately trained personnel under the leadership of the medical director of the facility, and should include the following:

●Maintenance practices

●A structured monitoring program, including specification of acceptable ranges for monitored metrics

●Detailed written description of actions to be taken for deviations from normal operations with clearly delineated lines of responsibility

The monitoring program should be capable of ensuring that the system is performing optimally, procedures are being followed, water quality is assessed, and the appropriate standards are met. Results should be charted to allow trend analysis of equipment performance and of the overall water quality. Appropriate action limits should be established, together with the action to be taken, when abnormal test results are obtained.

All dialysis facilities should establish a robust system of communication with their water supplier. Facilities should request that they be notified in advance of any major changes in the supplier's water treatment practices, repair work to the municipal water distribution system supplying the dialysis facility, and changes in municipal water quality, such as might occur with seasonal algae blooms. Water utilities need to know where patients on home hemodialysis are located.

Within the United States, the Centers for Medicare and Medicaid Services (CMS) state that "the Medical Director is responsible for safety and quality of the water used for dialysis treatments" [27]. Adherence to this condition requires that a medical director has a fundamental understanding of the water treatment for hemodialysis, participates in the design and/or upkeep of the water treatment and distribution system, be involved in the implementation of standard operating protocols for safe operation of the system, and participate in the dialysis facility's quality improvement program. Dialysis organizations should prioritize the quality of their water treatment systems and promptly respond to medical directors' requests for equipment upgrades or modifications to improve the safety and effectiveness of a water treatment system.

WATER QUALITY IN AN EMERGENCY SETTING — Dialysis providers can be faced with the need to provide treatments following a natural disaster or other disruption to drinking water and wastewater services. All dialysis facilities should have an emergency plan to permit the continuation or initiation of hemodialysis treatments and have a kidney emergency management team available to implement the plan. A medical technician or technologist familiar with water treatment should be part of that team.

If an emergency dialysis facility is being set up where there is no existing support structure, there should be close collaboration with the water utility to ensure that a stable water supply is available and, if not, to bulk ship water to the location of the facility. Tankers normally used for the transport of foodstuffs, such as milk, can be used following sanitization of the tanker and associated hoses. Additional pumping equipment may be required at the point of use to ensure that the appropriate water pressure is maintained. Dialysis fluid flow rates can be reduced temporarily to ensure that there is sufficient water available for the treatment of all patients.

An emergency water treatment system should undergo expedited performance validation over 24 to 48 hours, with the first water samples sent for testing as soon as possible within this timeframe. Special attention needs to be paid to the levels of chlorine and chloramine in the feed water since this can vary substantially, and the size of the carbon beds should be appropriate to ensure that the required levels in the product water are met. Following the initial 48 hours of operation, the system should be sanitized, and additional daily water sampling undertaken for seven days to assess microbial contamination and develop data for a trend analysis of the system's performance. While a two-week validation period is typical under normal circumstances, with an expedited emergency validation, the system may begin operation after 48 hours, at which time at least one set of results for chemical and microbial contamination levels should be available. If microbial contaminant levels are not available at 48 hours, endotoxin levels can be used to assess for microbial contamination. To minimize risk, bacteria- and endotoxin-retentive filters should be fitted to individual dialysis machines.

RESILIENCE AND WATER CONSERVATION — Dialysis technologies require large volumes of water, and many areas face significant water shortages due to population growth and climate change.

Water may be conserved during the production of dialysate by careful configuration and operation of the water treatment system (eg, using a dual-pass reverse osmosis system and/or salvaging or reusing rejected water for alternate applications such as toilet flushing, plant watering, and laundry) [28-30]. Water also may be conserved by recovering and using spent dialysate (after additional filtration) for purposes such as irrigation [31,32].

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Dialysis".)

SUMMARY AND RECOMMENDATIONS

●General principles – Water used for hemodialysis should comply with established quality standards to ensure patient safety. Compliance with these standards requires that a water treatment system decontaminates water up to the quality standards and a distribution system delivers the treated water to its points of use without recontamination (figure 2 and figure 1). (See 'Introduction' above.)

●Design of a water treatment system – The design and operation of a water treatment system should be based upon standards published by the International Organization for Standardization (ISO) and the American National Standards Institute (ANSI), in collaboration with the Association for the Advancement of Medical Instrumentation (AAMI), with a focus on ensuring water quality and facilitating disinfection and maintenance. The design of the system is dependent upon whether it is set up for in-center hemodialysis, home hemodialysis, or hospital bedside hemodialysis. (See 'Design of a water treatment system' above.)

●Elements of a water treatment system – A water treatment system consists of a pretreatment section; a primary and, if needed, a secondary treatment section; and the distribution system. (See 'Elements of a water treatment system' above.)

●Principal components – The principal components of a water treatment system include (see 'Principal components and monitoring of water treatment systems' above):

•Various types of filters to remove particulate matter

•Softeners to remove calcium and magnesium from hard water

•Activated carbon beds to remove free chlorine and monochloramine

•Reverse osmosis to remove various metals and organic chemicals

•Deionizers to remove ionic contaminants

•Structural modifications and disinfection practices to maintain the distribution system

•Chemical injection for additional treatment, if needed

●Monitoring and water sampling – The components of the water treatment system need to be monitored regularly to ensure optimal performance (table 3). Monitoring of the water treatment and distribution system requires sampling of the treated water using methods that are specific to either chemical or microbial contaminants. (See 'Frequency of monitoring' above and 'Sampling methods' above.)

●Operational responsibility – Some specialist vendors design and install all components of the water treatment system and ensure its proper operation. In other cases, individual components of the system can be obtained from different vendors, and dialysis facilities are responsible for ensuring proper operation. Once the system is operating properly, the responsibility for ensuring continued optimal performance is the responsibility of the dialysis facility. (See 'Operational responsibility' above.)