Water in Dialysis
Dialysis is a life-saving and life-sustaining therapy from which millions of patients have benefited. The effectiveness of (hemo)dialysis is, in large part, due to our ability to provide ultra-clean water to the patient. More than any other compound, water is crucial in providing safe and effective hemodialysis. It’s no surprise, then, that the water purification system in each hemodialysis unit is relied upon heavily to provide high-quality dialysis to our patients.
Water and its iatrogenic conversion to dialysate happens “behind the scenes”. Most hemodialysis units have a water-treatment system out-of-sight (and mind) of the patients and staff. And while being “out-of-sight” can lead to it being “out-of-mind”, every kidney doctor knows that the water room is the most integral part of the dialysis unit. Without proper water treatment, dialysis could not function. So let’s open the door that leads into the water room and uncover how dialysate is created from ordinary municipal water.
Depending on your water source, the water you start with can have a variety of ions and molecules that can be toxic (and/or deadly) to your patient. Water sources are generally of 3 types: municipal, ground, and surface. Municipal water is filled with chlorine/chloramine to disinfect and provide safe potable water. Ground water commonly has many ions and heavy metals (aluminum, magnesium, copper, etc.) and surface water has a lot of organic compounds (from farm run-offs, animal waste, etc.).
In order for water (unsafe for the patient) to become dialysate (safe), you need to do a few things. Shown below is a schematic of how water is converted into dialysate using carbon tanks, reverse osmosis machines, and deionizers.
Carbon Tank
The first step in the conversion is to expose water to the carbon tank. Carbon tanks(usually 2 tanks in a series circuit) remove chlorine and chloramine from the water. These compounds are usually added to water as a disinfectant to prevent bacterial growth and help in creating potable water. However, when exposed to the patient’s blood, chlorine (for example, in the form of sodium hypochlorite) and chloramines diffuse into the patient and cause hemolysis and neurologic abnormalities. In addition, chlorine can damage the next piece of equipment, the reverse osmosis (RO) machine. So carbon filters exist to protect both the patient and our equipment.
Kidney doctors must calculate the EBCT (empty-bed contact time) using a simple mathematical formula. The contact time must be at least 10 minutes in order for chloramine (in the water) to bind to carbon filters in the tank. Usually, the first carbon tank should remove almost 100% of the chloramine. To check this, doctors must check the [chloramine] every 4 hours in the water the exits the first carbon tank. The allowable concentration of chloramine is <= 0.1 parts per million.
A second carbon tank is also found — connected in series with the first tank. The second tank is considered the backup tank and used only if the first tank fails.
Reverse Osmosis (RO) Tank
The RO tank is the “workhorse” of the entire water treatment system. That’s because the RO tank must move water through a permeable membrane in a direction that is against the solute concentration gradient. In osmosis, water moves from an area of low solute concentration to an area of higher solute concentration. With reverse osmosis, water must move from an area of high solute concentration to an area of low solute concentration.
Accomplishing this requires a lot of energy in the form of a highly pressurized tank. Generally, the RO tank is under 260 PSI!! (most car tires are inflated to 40 PSI → 4 tires with a total of 160 PSI carrying a 2–3 ton vehicle).
Through reverse osmosis, permeate (the water the exits the RO tank) is free of ions, organic compounds (e.g., endotoxin), bacteria and viruses.
Deionizer (DI) Tank
The deionizer tank is an optional component of the water treatment system. Many hemodialysis units do not use a DI tank because all of the solutes, ions, bacteria/virions are removed by the RO tank. DI tanks exist in case there is a failure in the RO system.
RO systems operate under extremely high pressure and high volumes. Both of these increase the likelihood of a mechanical failure within the RO tank. If this happens, the water must pass through a DI tank in order to have ions exchanged for H+ and OH-. The DI tank substitutes these 2 ions for cations and anions found in the water. Remember that the carbon filter only removes chlorine/chloramine — it does not bind cations or anions.
The DI tank has 2 flaws that relegate it to “back up status” instead of as a primary component of the water treatment system. First, the DI tank cannot sterilize water. Unlike an RO tank which can remove bacteria, virus particles and endotoxins, the DI tank can only exchange cations/anions for H+ and OH-. A separate detoxification system (ultrafilter) must be attached to the DI tank to remove these substances. Second, the DI tank operates at much lower volumes than the RO tank. An RO tank can generate double-to-triple-digit volumes of permeate; a DI tank generates much less permeate.
So why have a DI tank? All hemodialysis centers have DI tanks “waiting to be used” in case of a water emergency. In cases of natural disasters, where the supply of water is severely limited, DI tanks are used to de-ionize the available water so that hemodialysis can be performed.
There you have it. A quick guide on the water treatment system and how water becomes dialysate. Check out our 10-Minute Rounds video on the same subject:
