WATER IN DIALYSIS

Paul Lowe has been working in dialysis for eleven years. He is responsible for technical and user support to the staff of the dialysis unit in Beaumont Hospital. There are few aspects of dialysis with which he is not familiar. We are grateful to Paul for this most informative article on water for haemodialysis.

Dialysis is a procedure in which a patients blood is brought into contact, via a membrane with a solution, the dialysis fluid that removes certain products of metabolism without damaging the red cells or destroying other essential blood components. In dialysis two main constituents are removed. Toxins produced by the body are removed by diffusion and excessive water consumed or produced by metabolism is removed by ultrafiltration.

Dialysis fluid consists of two major components:

The first is a concentrated solution which contains the same electrolytes as normal extracellular fluid. It contains none of the toxic solutes which should be eliminated from the blood of the patient e.g. urea and creatinine.

The second component is water. This is added to the concentrate to produce the final solution at a dilution close to that of normal extracellular fluid.

The electrolyte composition of the dialysis fluid is tailored so as to correct the electrolyte disorders which develop in the absence of kidney function i.e. to make up the deficiencies in and prevent losses of electrolytes. Therefore a high content of minerals in the water used to make up the dialysis fluid not only upsets this careful balance but could also have negative effects on a patient’s well being.

A dialysis patient is exposed to 30 times the amount of water a person normally drinks e.g. average human consumption per year 500 - 1000 litres. A dialysis patient is exposed to 30.000 litres per year. It is obvious therefore that more rigid standards should be set for water which is used in the preparation of dialysis fluid.

What is in tap water?

Tap water can contain any or all of the following compounds:

1. Insoluble particulates
2. Soluble organic compounds
3. Soluble inorganic compounds
4. Heavy metals and trace elements
5. Bacteria and pyrogens

Below are listed some examples of each and the possible effects they can have on dialysis patients.

insoluble Particulates:- These are usually particulates of iron sand and silica. These can cause plugging of equipment.

Soluble Organic Compounds:- These are chioramines normally used as a bactericidal agent in urban water supplies. Water containing chloramines can cause haemolytic anaemia in dialysis patients.

Soluble inorganic Compounds:- Calcium and magnesium salts, nitrates, sulphates, fluorides and sodium are all examples of these compounds.

Water which has a high calcium and magnesium concentration is known as hard water and use of this type of water on a dialysis patient will result in the patient displaying symptoms of hard water syndrome which is characterised by nausea and vomiting during dialysis.

Presence of a high level of nitrates or other nitrogen compounds would indicate the presence of bacteria in the water. High levels or nitrates can lead to haemolytic anaemia in the dialysis patient. Sulphates are normal products excreted in the urine. However, for patients with no kidney function sulphate retention contributes to metabolic acidosis. If the dialysis fluid has an excess concentration of suiphates the patient presents a syndrome of nausea, vomiting and metabolic acidosis.

Heavy Metals and Trace Elements:- examples of these are iron, copper, zinc, lead. cadmium and aluminium. The main effect of iron is damage to water treatment equipment and dialysis fluid supply equipment. Iron salts can cross dialyser membranes into the patient’s blood stream and can result in excessive storage of iron in the liver.

Copper is present in water from natural sources and as a result of passage through copper piping. High levels of copper could result in haemolytic anaemia, gastrointestinal problems and metabolic acidosis. Zinc can lead to nausea, vomiting and fever. Lead can cause neurological damage to patients. Aluminium is generally added to urban water supplies in the form of aluminium sulphate. This acts as a flocculation agent in the water (it helps bind other compounds together in the water so that they are easier to treat). However, there is strong evidence to suggest that chronic aluminium loading can lead to bone disorders and dialysis dementia. Aluminium accumulates in the bone, brain and liver.

Bacteria and Pyrogens:- Ideally the bacteria count in dialysis fluid should not exceed 100 counts per ml. When the count exceeds 1000 per ml the patient may stiffer pyrogenic and septemic complications especially if the bacteria pass through a break in the membrane into the patients blood.

Water Treatment Systems

In designing a water treatment system, the system should ensure that all the possible contaminants are removed to the levels laid down by International Standards for dialysis water. A typical water treatment system would consist of the following:- Mechanical Filter:- This is usually a sandfilter which will remove relatively large particles from the water. Usually these filters will remove particles down to 20 microns in diameter.

Carbon Filter:- This filter will absorb low molecular weight organics. chlorine chloramines and some pyrogens from the water. However, if there is a high organic load on the incoming water, an organic trap would be a better option in this case.

Oxidising Filter:- This filter would be used if the water has a high iron or manganese content. They contain a filter medium which has the ability to oxidise and absorb dissolved iron and manganese.

Ion-Exchanger:- The most common Ion-Exchanger in use in water systems is a water softener. This will exchange any calcium and magnesium salts present in the water with sodium salts thus changing hard water to soft water (high in sodium ions).

Pre-membrane Filter:- This usually consists of cartridge filters which will remove particles down to 3 to 5 microns. This part of the water treatment system is often not researched or designed adequately. The type of filter used here will determine the quality of water produced and the longevity of service of the next part of the water treatment system - the reverse osmosis membrane.

Reverse osmosis Unit:- This is the heart of any water treatment system. Reverse osmosis, originally designed to purify or desalinate sea water, is a process where feed water flows over a membrane, while pressure forces pure water through the membrane. The impurities are rejected at the membrane surface and concentrated in the rejected water stream. The water which is forced through the membrane is purified and constitutes the clean water stream. Reverse osmosis membranes can reject up to 98% of dissolved solids present in the feed water e.g. if the aluminium level in the feed water is 100, the level in the purified water is 2. 99% of bacteria and pyrogens are rejected by reverse osmosis membranes. However, reverse osmosis membranes are sensitive and can be damaged easily by excessive hardness iron and chlorine in the feed water. Hence, the need for the correct pre4reatment is essential for the membrane to function properly.

To counteract bacteria contamination even more it is now usual to include an ultrafiltration unit in the water treatment system. The ultrafiltration unit (placed between the pre-filtration unit and reverse osmosis) will remove organics bacteria and pyrogens. This used in conjunction with the reverse osmosis unit will result in 99% of all bacteria and pyrogens being removed from the water. An even better solution would be to have a polishing reverse osmosis unit. This is where two reverse osmosis units work in series with the purified water from one unit being the feed water for the second unit. This, if functioning correcfly, will produce very high quality water.

Distribution Systems

Having produced water to the correct quality the water must still be distributed to the dialysis machines. Until recenfly the way to produce and distribute dialysis water was through a central reverse osmosis unit to a large treated water storage tank which was connected to the dialysis unit via a recirculating ring main. An U.V. sterilizer was connected to the ring main which will kill bacteria and a 0.2 micron filter was installed to collect any dead bacteria. However, the problems with this system are that the storage tank provides a good medium for growth of bacteria even though the water is constantly circulating. It has been shown that some bacteria are resistant to U.V. and endotoxins (dead bacteria fragments) can still pass through 0.2 micron filters. Because of these problems this distribution system must be chemically disinfected on a regular basis. In the past few years the design of distribution systems has changed. There is now a tendency to go for non-storage systems with the reverse osmosis unit feeding directly into the distribution loop. The elimination of the storage tank reduces the risk of contamination, however, if the reverse osmosis unit breaks down the dialysis has to stop. One way around this problem is to have a duplex reverse osmosis unit, so that if one unit breaks down the other unit can still provide water to the unit.

Sanitization of the Distribution Loop

To prevent the build up of bacteria and endotoxins in the distribution loop, it must be disinfected. Usually, the loop is disinfected using a chemical sanitizing agent. However, heat disinfection is now a realistic alternative.

For heat disinfection to work correctly a minimum temperature (85C) must be achieved throughout all of the loop. Heat disinfection of the distribution system must be seen as a process of keeping the contaminant levels to a minimum not as a way of reducing high levels already in the system. A proper heat disinfection process must be initiated and implemented in full in order to keep contaminant levels low.

Water Standards and Analysis

As with any quality system the system only works if the standards laid down are achieved and in some respects more importantly the methods of analysis to show the standards are being achieved are implemented correcfly. The standard for chemical contaminants for dialysis water adhered to by most dialysis units is the A.A.M.I. (Association for the Advancement of Medical Instrumentation). A properly designed and functioning water treatment plant will achieve these standards and monthly chemical analysis should be carried out to ensure standards are being achieved.

The standard for bactericidal contamination is laid down in the European Pharmacopoeia which state that the total colony forming units per ml should be less than 100 C.F.U/ml and endotoxins levels should be less than .251.U./ml.

The method of analysis of the bacteria count here is critical as the right plate medium must be used as well as incubation time - seven days at 200C. Analysis of the sample according to the way in which water for human consumption is analysed with an incubation period of only 48 will lead to incorrect results.