Water Treatment Equipment for In-Center Hemodialysis

Introduction

During the provision of contemporary hemodialysis, the dialysate flows through the dialysate chamber of a dialyzer, which separates the toxic substances from the bloodstream into the dialysate via diffusion. The dialysate is composed of solute electrolytes in solvent water. In order to produce the final dialysate solution, a concentrated electrolyte solution is mixed with water. The ratio of concentrated electrolyte solution to water can vary based on many factors, including the type of concentration solution used and the proportioning system of the dialysis machine. In general, dialysate-proportioning systems mix around 1 part concentrate with 35–45 parts water. This final mixed dialysate is delivered to the dialyzer at a flow rate of 500–800 mL/min. Therefore, over a standard dialysis session, patients are exposed to vast quantities of water in the dialysate, that is, for a 4-hour session, anywhere from 120 to 192 L. To give you an idea of the magnitude of this volume, compare it with the estimated volume of total body water in a 70-kg man, which is around 42 L, with only 3.5 L of the total body water present in the plasma space. Therefore, dialysate water must be completely clear of potential contaminants to prevent injury to the patient during dialysis. Even contaminants found in dialysate water in small concentrations should be a cause for concern because their levels can reach toxic concentrations in the blood just by virtue of the vast quantity of water to which the blood is exposed. In addition, because of the absence of water contaminants in the blood, the diffusive pressure that can drive toxic solutes into the plasma space from the dialysate is high, and dialysis membranes do not offer selective protection to impede their entry from the dialysate into the bloodstream. Even contaminants in the dialysate that do not cross the dialysis membranes because of their size (e.g., bacteria) are found to be associated with systemic inflammation in dialysis patients. Systems that lack a separation of the blood from the treated water are available in some locations and demand an even higher level of water purity. For example, hemofiltration demands ultrapure or sterile water produced “on-line” for infusion into the bloodstream as replacement fluid. These mentioned considerations, among others, highlight the obligatory need for water purification methods that are effective and reliable in order to provide a safe dialysis therapy.

This chapter serves to highlight common contaminants found in municipal water sources that can be harmful to dialysis patients; it reviews the equipment used to prepare product water for use in the production of dialysate and covers some of the maintenance, monitoring, and design considerations for water treatment systems, as well as regulatory aspects that clinicians should be aware of when caring for dialysis patients. In addition, the reader is advised to consult guidelines published by the Association for Advancement of Medical Instrumentation (AAMI)/International Organization for Standardization (ISO), as well as their local governmental guidelines and regulations when considering water treatment systems for their dialysis patients. For the purposes of this chapter, we will focus on the most recently available ISO/AAMI guidelines, which are a product of the latest available knowledge and expertise. The content within may differ from local or governmental regulations, which tend to lag behind ISO/AAMI recommendations.

Water Contaminants

Water used for the production of dialysate fluid must meet a higher purity standard than what most municipal water supplies can provide. Because the production of dialysis water takes place in the dialysis facility or hospital, the responsibility for purification of water for dialysis rests on the dialysis provider. Common chemical contaminants in the water and their acceptable ranges are listed in Table 7.1 . Contaminants can be present naturally in the water source, can be added to the water for specific purposes, or can enter the water at the level of the dialysis facility. Each of these sources of potential contamination should be appreciated and monitored. Because the contaminants present in water can vary over time, dialysis providers are encouraged to establish a relationship with local water authorities so that they are apprised of any changes to the water supply contents. Examples of substances added to water by water authorities that can be toxic to dialysis patients include chlorine and chloramines for the control of microbiologic growth, fluoride for dental prophylaxis, and alum, which can be used as a flocculent to decrease water turbidity. Chloramines can also be naturally present in water. Occasionally, lime is added to acidic or ion-poor water to raise the pH and prevent damage to metal piping systems or lead leaching from older piping systems. Other trace elements, organic matter, agricultural products such as pesticides and fertilizers, and industrial products can also work their way into the water supply. Metals such as lead and copper can leach from plumbing systems. Microorganisms such as bacteria, fungi, protozoa, endotoxins, and other microbiologic fragments can also be present in water. Knowing the source of the water in your dialysis unit can help you anticipate which contaminants are more likely to be present. For example, water from surface sources such as rivers, lakes, and reservoirs is more likely to be ion poor (low conductivity) but is more prone to having organic surface contaminants present in it, such as particulates, pesticides, and others. Water that is derived from an aquifer or well tends to have more inorganic or ionic contaminants present (high conductivity), which the water accumulates as it percolates down through various sedimentary layers. Occasionally, city water authorities issue boil water advisories when water is excessively contaminated with microorganisms. Care should be taken to vigilantly monitor the water treatment system at this time. If the water treatment system is equipped with a reverse osmosis (RO) membrane, then dialysis can continue because this membrane will serve as a bacterial and endotoxin barrier. However, if a deionizing system is used, then ultrafilters or bacterial and endotoxin retentive filters downstream of the deionization (DI) system should be in place to prevent exposure to patients. Municipalities can often treat the water with higher concentrations of chlorine and chloramines during periods of infection, and care should be taken to make sure that the absorptive effect of the carbon tanks is not overwhelmed, thus causing spillover of chlorine and chloramines into the product water.

Exposure to these various contaminants in the water when in high concentrations can present a catastrophic nature, with multiple patients on dialysis being affected simultaneously. Sudden onset of illness in multiple patients, in particular, symptoms of hemolysis and intoxication, should prompt consideration of water or dialysate contamination. Episodes of hemolytic anemia involving multiple patients have been witnessed in dialysis units where carbon tanks were exhausted or overwhelmed, sometimes during times of system upgrades or increased water demands. Exhausted DI systems have been linked to copper and fluoride exposures. Exposures to disinfectants such as hydrogen peroxide and formaldehyde can result from incomplete or improper rinsing of water treatment systems after the disinfection procedure. Deleterious exposure to metals such as copper, lead, or brass in piping, fittings, valves, or aluminum in pumps used to transfer concentrates has occurred in case reports. Although case reports highlight drastic incidents of water contamination, it should also be recognized that exposure to contaminants in lower concentrations can manifest in atypical and subtle ways, which can be missed by health care providers. For example, chronic exposure to bacterial fragments like endotoxin may present with signs of systemic inflammation that are difficult to distinguish from other disease processes. Table 7.2 lists some of the common water contaminants and the symptoms that they can cause.

Water purification systems in dialysis units should take care to address each potential contaminant in the source water. Equipment used for water purification should be designed with knowledge of the contaminants that are present in the source water. Tables 7.3 and 7.4 contain some suggested steps to take when planning a new water system and considerations for water system design. Table 7.5 provides a list of considerations to take into account regarding the maintenance and monitoring of water treatment systems.

Equipment Used for Water Purification

Multimedia Filter ( Fig. 7.1 )

The purpose of the multimedia filter, also known as the sediment filter or depth bed filter, is to remove particulate matter from the source water. Plant debris, rocks, rust, silt, clay, and other debris are removed from water as it passes through the multimedia filter. Multimedia filters are usually composed of layers of gravel, sand, and anthracite. The water passes from larger gravel to finer sand and anthracite as it moves through the filter tank. Bigger particulate matter is trapped in the initial gravel layers, and smaller particulates are trapped in the finer layers of sand and anthracite. Most multimedia filters are capable of removing particulate matter down to 10 microns in diameter. In some ways, this mechanism of water filtration is similar to what happens in nature as groundwater filters down through the water table into an underground aquifer. The multimedia filter is usually the first filter that source water passes through. It is a necessary component of the water treatment system for the protection of the downstream water treatment components. For example, if particulate debris is not removed from the source water, it can end up fouling or damaging the RO membrane.

Monitoring the Multimedia Filter

• With use, the multimedia filter can become obstructed with debris. To prevent this, the filter is backwashed daily at a time when patients are not on dialysis.

• Pressure gauges should be present pre and post filter to measure the difference between the two. If the filter is occluded with debris, the pressure drop will increase. The pressure difference (delta) across the filter should be monitored regularly. Trends in pressure changes should be established and investigated if abnormal.

• Pressure delta of > 10 mm Hg above baseline across the multimedia filter indicates a problem.

• If automated systems are used for backwashing the filter, backwash timers should be monitored regularly to ensure that rinsing of the filter occurs during off-hours when patients are not on dialysis.

Water Softener/Brine Tank ( Fig. 7.2 )

Groundwater can accumulate calcium and magnesium as it percolates through deposits of limestone or chalk. Water that has large amounts of calcium and magnesium can leave behind hard residue or scale. One example of this left-behind residue is water spots on a glass shower door. The hardness of water refers to its overall content of polyvalent ions that cause this scale, the major contributors being divalent calcium and magnesium. The process of “softening” the water is often accomplished by the exchange of calcium and magnesium ions for sodium ions. Calcium and magnesium ions will be effectively removed through RO; however, the softening process is essential prior to water reaching the RO unit to protect membranes from damage due to scaling. Additionally, in the case of systems that use DI to form product water, the presence of large amounts of divalent ions can overwhelm the deionizer resin beds, leading the to release of other toxic ions into the treated water. Water softener units are typically composed of a tank that holds ion-exchange resin and is connected to a brine tank. As water moves through the softener tank, the resin releases sodium ions in exchange for higher-affinity divalent calcium and magnesium. The brine tank is used to hold a concentrated sodium chloride solution. Regeneration of the resin in the softener is accomplished by overcoming the affinity of the resin beads for polyvalent cations with a supersaturated sodium chloride solution from the brine tank. Recharging can be initiated on a timed basis or after a monitor, which measures a certain amount of water processed, indicates the need. Recharging should take place at a time when patients are not dialyzing to prevent accidental spillage of hypertonic sodium chloride solution into the water treatment system.

Monitoring the Water Softener

• Measurement of water hardness in grains per gallon (GPG) or parts per million (PPM) is used to monitor the efficiency of the water softener.

• Water hardness can be measured on site using colorimetric test strips. Personnel performing the testing should be able to distinguish between the colors on the test strips. There should be a well-labeled water sample port post water softener for the measurement of hardness.

• Source water hardness should be compared with treated water hardness periodically.

• After treatment by the water softener, the water should measure 1 GPG (17 PPM) or less.

• The hardness should be checked at the end of the day so that the capacity of the water softening system is fully appreciated.

• The brine tank should be checked daily to ensure that the solution is adequately supplied with sodium chloride (e.g., salt pellets are above the level of the water and are not forming a salt bridge).

• Pressure drop across the softener should be monitored regularly. A pressure change of > 10 mm Hg above baseline suggests a problem with the resin bed.

• Regeneration of the softener resin should take place during a time when patients are not on dialysis.

• Timers set for automatic softener regeneration should be monitored to ensure that regeneration does not occur during facility operating hours. There is potential for a high concentration of sodium to enter the water if regeneration occurs during dialysis treatments. Conductivity monitors on the RO system should sound an alarm if a high sodium concentration is detected; however, they should not be relied upon exclusively.