Water plays an important, life-sustaining role for dialysis patients. And as such, the water used for this critical function must be of special quality.
When hemodialysis started to blossom over 20 years ago, many dialysis centers used water right from the tap. We now know that certain contaminants in water supplies can cause severe complications in dialysis patients. Sophisticated water purification systems are currently used to purify water to a level determined to be safe for the patients. And the critical piece of equipment in these water purification systems is the reverse osmosis (RO) machine.
THE NUTS AND BOLTS
Simply stated, RO is a membrane process, which removes both organic molecules and salt ions from a solution (typically water). The membrane sieves organic molecules and repels salt ions while passing pure water through the micropores in its surface.
The driving force behind RO is pressure, which is typically supplied by a centrifugal pump. This pressure is needed to overcome the inherent osmotic pressure of the solution and to supply enough energy to force water through membrane pores which are only about 5 angstroms in diameter (by comparison, a human hair has a diameter of about 500,000 angstroms).
The basic components of an RO system are the prefilter, a pump, and sepralators (spiral-wound membrane elements). The sepralators are placed in stainless steel or PVC housings, which are then manifolded together.
RO systems operate in a crossflow mode whereby a single stream is fed into the RO and flows across the membrane surface. Two streams exit--the permeate and the concentrate. The permeate stream contains the water which passes through the membrane and is purified. The concentrate stream contains the water, salt ions, and organic molecules that do not pass through the membrane; the concentrate is typically plumbed to drain.
The advantage of operating in the crossflow mode is that it minimizes plugging of the very small membrane pores. However, as a result of crossflow operation, only a percentage of the feed is collected as permeate. This ratio of the permeate to the feed is called the 'recovery' of the RO. Though it would seem that one would want to have as high a recovery as possible, there is a trade off--the higher the recovery goes, the poorer the permeate quality becomes. In practice, the majority of RO systems producing less than 15,000 gallons of permeate per day operate at 33% or 50% recovery.
QUALITY, RELIABILITY, ECONOMY
A typical water purification system for dialysis is shown in Figure 1. The raw city water is fed through a water softener, which removes calcium and magnesium ions. Softeners also remove small amounts of iron; however, if a significant amount of iron is present, additional treatment should be specifically geared toward its removal. If high levels of suspended solids exist, a backwashable sand filter should be installed upstream of the softener.
After the softener, activated carbon tanks are employed (many times, in series) to remove chloramine, chlorine, and trace organics. It is important to keep the chloramine or chlorine in the system up to this point in order to minimize the chance of bacterial growth.
Next comes the RO machine, which typically removes 90% to 95+ % of the dissolved salts. RO also removes bacteria and pyrogens as well as 99+ % of organic molecules over 200 daltons in molecular weight. In some cases where the raw water has a very high level of dissolved solids, a second RO machine in series or an ion exchange unit after the RO may be used to lower the dissolved solids level to an acceptable number.
One component missing from the water system depicted in Figure 1 is storage. Most new dialysis centers, however, are staying away from storage tanks, preferring instead the 'direct feed' type systems whereby the RO permeate feeds directly into a loop, which serves the dialyzers. This minimizes stagnant areas where bacteria can establish. In many cases, the unused water is recirculated back to the inlet of the RO. This lowers the feedwater dissolved solids level and thus gives an even higher quality permeate.
What level of water quality is necessary for dialysis? The Association for the Advancement of Medical Instrumentation (AAMI) has set forth water standards as guidelines for dialysis centers to follow. These standards list maximum levels for ions found in water as well as for heavy metals and bacteria (see Table 1). Adoption of these standards had been voluntary in the past, but now, dialysis centers are required to meet these standards in order to be reimbursed by the government. In fact, many physicians, nurses, and dialysis technicians feel that these standards are not strict enough and require water for their facilities that is much purer than the AAMI standards.
The trend is definitely toward higher purity for dialysis water. So, the question becomes what is the most reliable and economical way to produce water which meets or exceeds the AAMI standards? In a very small number of U.S. communities, the natural water supply meets AAMI standards. However, this amounts to less than 5% of the cases, making water treatment necessary for over 95% of dialysis centers.
RO vs. DI
When choosing which types of water treatment equipment to use, some decisions are easier than others. For instance, water softeners and activated carbon tanks have few alternatives, none of which are simple, inexpensive, and reliable. Choosing the RO machine, on the other hand, is a much more complex task. First of all, some centers choose to go with an alternative to RO known as ion exchange or deionization (DI). Some centers originally used DI, but many of them have since installed RO upstream of or in lieu of the DI.
Basically, DI removes ions from water via electrochemical attraction to a charged resin. The resin is contained in a fiberglass or steel tank. The DI tank has a given capacity for ions, and when it reaches its capacity, the tank must be taken off-line and regenerated with concentrated acid and caustic. The DI can be regenerated either on-site or off-site. Most centers have regeneration done off-site so that they do not have to keep the concentrated chemicals on-site, handle them, and dispose of them.
Comparing RO to DI shows advantages for each. As previously stated, RO will remove bacteria and pyrogen; DI does not remove pyrogen and, in some cases, can actually bring about increases in the bacteria and pyrogen levels. For this reason, when DI is employed, a membrane filter or an ultrafiltration (UF) machine is used downstream to capture bacteria.
DI will typically have a lower capital cost than RO, but the operating costs of DI are far greater than for RO. The DI operating costs are mainly for the acid and caustic, and if the DI tanks are regenerated off-site, a charge for labor and freight to and from the local regeneration facility is also levied. These costs can easily add up to $2-3 per thousand gallons of water, about twice as much as the cost for RO.
The main operating cost for RO is the electricity used to run the high-pressure pump. Other costs include changing the membrane elements approximately every 3 years and cleaning the RO every 4-8 weeks. These costs typically add up to $1-1.50 per thousand gallons of water, about 50% of the cost of DI.
RO is usually chosen over DI by dialysis centers because of the operating cost difference plus the fact that RO is such a good bacteria and pyrogen filter. As previously mentioned, if the feedwater has a high dissolved solids level or a high concentration of one specific contaminant, a second RO in series or a DI unit operating on the RO permeate is needed. The operating costs of DI fed with RO permeate are far less than DI fed with untreated water.
The major choice when purchasing an RO unit is which type of membrane to use. The two primary RO membrane types are cellulose acetate (CA) and polyamide (PA), each of which has advantages and disadvantages. CA membrane is tolerant to chlorine disinfection treatments and is relatively inexpensive. PA membrane is not tolerant to chlorine and it costs significantly more than CA. PA membrane also generally has a higher salt rejection than CA, so if the permeate quality is borderline with CA, PA may work better.
Most dialysis centers use activated carbon to remove chlorine from the city water, so there is no need to worry about chlorine destroying the PA membrane. However, many centers disinfect their water loop with chlorine, and this can cause problems with PA systems. In addition, other disinfectants such as formaldehyde are being phased out due to exposure problems. One disinfectant which has shown promise for compatibility with PA membrane is peracetic acid; CA can be sanitized with chlorine, formaldehyde, and peracetic acid. It is also much easier to disinfect an RO containing CA membrane than one using PA membrane.
Since neither CA nor PA membrane has proven itself superior in all situations, one needs to look at each individual application and decide which membrane is best.
Nearly all dialysis centers use water purification equipment to purify water for dialysis. The centerpiece of these water purification systems has become reverse osmosis. In conjunction with sound pretreatment, RO has proven time after time that it is the safest, most reliable, and most economical method of purifying water for dialysis.
Maximum Allowable Level (mg/L)
Total Hardness 21.0
Calcium as CaC03 5.0
Magnesium as CaC03 16.0
Chlorine (free) 0.5
Chloramine (combined) 0.1
Nitrate (N) 2.0
TYPICAL RO TERMS
Feed - Water entering an RO machine.
Permeate – Portion of the feed that passes through the membrane and is collected for use. In water purification, this is the product.
Concentrate (reject) – Portion of the feed which does not pass through the membrane, which exists as a separate stream containing concentrated impurities and is usually discharged to drain.
Rejection – Percentage of dissolved material, which does not pass through the membrane.
Recovery – Ratio of the permeate rate to the feed rate.
Osmotic Pressure – Head equivalent difference which arises when a dilute solution and a concentrated solution are separated by a semipermeable membrane. Approximately equal to 1 psi per 100 mg/L total dissolved solids for water.