**1. Introduction**

The process of haemodialysis is mirrored on the physiological principles of the kidney. Specifically the process of diffusion, where blood is made to flow in a counter current to the direction of dialysis fluid and ultrafiltration, where fluid is pressured across a semi permeable membrane using a convective force.

A large quantity of high purity dialysis water is required to safely perform a dialysis treatment. A single 4-hour dialysis treatment can require up to 150L of dialysis quality "ultra pure" water. Producing this high quality water is a multi step filtration process requiring several levels of processing before it is of a sufficient quality to be presented to the dialyser membrane and the patient's blood. Depending on the quality of the source water, the production of this 150L of dialysis quality water, can require the processing of up to 1000L of drinking quality water. This results in between 60 – 90% of source water being rejected to waste.

Ideally pure water used for dialysis should contain no contaminants eg. particles, trace elements, chemicals, organic matter, bacteria or bacterial fragments. Since this water is in direct contact with the blood stream any impurities in the dialysis water has the potential to pass to the patient. Published accepted standards of water quality do exist and specify the minimum concentrations of contaminants that are allowed, bearing in mind that absolute purity is impossible and is often limited by testing thresholds.

Several contamination incidents, some fatal have been described in the literature where patients have inadvertently been exposed to contaminants due to a breakdown in the water purification process. Historical examples include fatal haemolysis from chlorine exposure, bone disease and dementia from aluminium exposure and fatal bacterial contamination

resulting in liver failure [1-3]. There is also evidence that bacteria and bacterial fragments especially gram-negative lipopolysaccharide can induce an inflammatory state contributing to erythropoietin resistance, hypotension and a poor nutritional status [4].

Ultrapure water has more stringent microbiological criteria than standard dialysis water and has become the standard in most dialysis units. This is particularly relevant when considering the increasing use of on-line haemodiafiltration, which necessitates large volumes of ultrapure replacement fluid to be infused directly into the patient bloodstream, without the traditional barrier protection of the dialyser membrane. In addition the use of high flux dialyser mem‐ branes, theoretically may also allow bacterial fragments to cross into the blood compartment hence further necessitating the need for stringent water standards. Use of ultrapure water is associated with improvement in inflammatory and nutritional markers as well as anaemia and can be produced safely [4-6].

In the home setting much training, education and preparation is necessary prior to a patient safely performing dialysis at home and in turn this is also important with regards to water quality. Patient factors such as dexterity, visual acuity, hygiene, desire to be independent and ability to follow protocols are important. Importantly in the home setting, patients must correctly perform their own chlorine testing, equipment maintenance and WRO disinfection.

This chapter will discuss the various components of the water purification process required for haemodialysis both in the home and in dialysis units. We will also discuss the components of water quality testing and international standards. Finally, with the increasing constraints on water supply, there is a growing awareness of the need for, so called 'green dialysis units', where water conservation practices are utilised.
