**3. Components of the water system**

Improving water quality to a standard required for safe haemodialysis utilises a step wise, water filtration process. Each component is specially designed to remove certain contaminants and is arranged in a manner so as to protect and increase the efficiency of the downstream components (Figure 1). For example, carbon filters efficiently remove chlorine, which is not removed by water reverse osmosis (WRO) and can damage WRO membranes, thereby reducing their efficiency.

Figure courtesy of Fresenius Medical Care © Australia Pty Ltd. A typical dialysis water treatment setup consists of a reservoir tank, particle filters (multimedia, micron, bag filters), a water softener and 2 carbon tanks in series.

**Figure 1.** Schematic representation of a typical dialysis water treatment circuit

In centre dialysis units supplying multiple dialysis machines, which are used up to thrice a day, require large amounts of dialysis water and hence more involved filtration systems. However, even though home haemodialysis setups supply only one machine, they also are used in different feedwater settings, which may require minor changes in the type and size of filters (Figure 2).

(A) The Cartridge unit is an easily changed, optional unit sometimes used to "pre treat" the source water. (B) micron particle filters, usually either 5u and 1u size, (C) activated carbon tanks

**Figure 2.** Schematic of home water treatment setup

#### **3.1. Micron filters**

Figure courtesy of Fresenius Medical Care © Australia Pty Ltd. A typical dialysis water treatment setup consists of a

In centre dialysis units supplying multiple dialysis machines, which are used up to thrice a day, require large amounts of dialysis water and hence more involved filtration systems. However, even though home haemodialysis setups supply only one machine, they also are used in different feedwater settings, which may require minor changes in the type and size of

(A) The Cartridge unit is an easily changed, optional unit sometimes used to "pre treat" the source water. (B) micron

reservoir tank, particle filters (multimedia, micron, bag filters), a water softener and 2 carbon tanks in series.

**Figure 1.** Schematic representation of a typical dialysis water treatment circuit

particle filters, usually either 5u and 1u size, (C) activated carbon tanks

**Figure 2.** Schematic of home water treatment setup

filters (Figure 2).

220 Updates in Hemodialysis

Water contains particulate matter, which may include sand, clay, silt, or colloidal matter. This particle load can be quantified by calculating the silt density index, which measures the time taken for a 0.45µm filter to experience a reduction in flow. These filters function to exclude particles on the basis of size and prevent fouling of the RO membrane (Figure 3). They are located both pre and post carbon filters. In addition the post carbon filter also traps any carbon flecks that may pass out of the carbon tanks.

These micron filters are of a woven bag structure. Water passes through the filters in series, the 1st a 25u and the sec‐ ond 10u. They are changed every 4 weeks. Note the pressure gauges. The pressure differential between the 1st filter (inlet) and the gauge on the softener (outlet, not Figured here) is closely monitored. The pressure differential slowly rises as the filter integrity drops.

**Figure 3.** Micron particle filters (bag filters)

For **home dialysis,** typically, a 10-inch 1µm filter would be used but larger capacity e.g. 20 inch may be necessary in some areas with significant particle load such as areas with high clay concentrations. The International Organization for Standardization (ISO) recommends changing these according to the manufacturers specifications or using a change in pressure test, across the filter to check its integrity [9]. Common practice is to change these on schedule every two to four weeks.

#### **3.2. Softener and multimedia filter**

Dissolved salts within in the water contribute to the hardness of water, which can cause 'hard deposits' and can foul the downstream WRO filter. Softeners function to reduce this hardness by removing the salts, commonly calcium and magnesium, by passing water through a resin, exchanging them for sodium (Figure 4). The ISO guidelines recommend a softener be used where feed water has hardness in excess of 10 GPG (grains/gallon) [9]. The softener is 'regen‐ erated' by backwashing it with a high salt solution from the brine tank, effectively stripping the resin of bound calcium and magnesium salts. This can be set to a required time or volume schedule. Typically the softener is replaced every 3 – 5 yrs. Multimedia filters are composed of multiple layers of media, typically coal, sand and garnet, which are of differing sizes. As water passes through the filter, particles up to 10µm, suspended in the water are removed. Some units do not use multi-media filter, opting rather for the micron filters, which are cheaper and easier to replace.

(A) <sup>2</sup> carbon filters, <sup>1</sup> softener tank and <sup>a</sup> brine tank in series. (B) Mechanism of softener function,

illustrating ion exchange

#### Figure: 1: Softeners and mechanism of action. **Figure 4.** Softeners and mechanism of action.

Large amounts of particulate matter and salts have the potential to affect the efficiency of the carbon and WRO filters. In home dialysis installations, our local experience has been that multimedia filters and softeners do not affect the water quality standards. We do not routinely use these filters in our home water setup. Our RO membranes are routinely changed approximately every 5 years and the absence of the multimedia filter and softeners has not led to early failure of the WRO. Once again this will be dependent on the quality of the feed water. a. **Activated carbon:** Activated carbon is a highly porous carbon material that is created by adding heat and steam to carbon containing products such as Large amounts of particulate matter and salts have the potential to affect the efficiency of the carbon and WRO filters. In **home dialysis** installations, our local experience has been that multimedia filters and softeners do not affect the water quality standards. We do not routinely use these filters in our home water setup. Our RO membranes are routinely changed approx‐ imately every 5 years and the absence of the multimedia filter and softeners has not led to early failure of the WRO. Once again this will be dependent on the quality of the feed water.

With regular use the carbon becomes spent with no further binding available along its large surface area.

'fluffing the carbons'. This is usually performed weekly and the tanks are changed every 12 months.

manufacturer and that regenerated carbon not be used(9).

coal and wood. The carbon is then acid washed to clean and increase its porosity. The primary purpose of the carbon filter is to remove chlorine and its related compounds, including chloramines and halogenated organic material, like trihalomethanes (Figure 5).

Chloramines are derived from chlorine and ammonium and is added to drinking water, as a disinfectant as it is more stable, does not dissipate as rapidly as free chlorine and reduces the formation of chlorinated organic compounds such as trihalomethanes(10) (see section on water testing). It is important to note that the RO filter does not remove chlorine. Activated carbon is the best method of removing chloramines and is achieved by physical adsorption and chemical catalysis to its parent compounds. The adsorptive capacity of the carbon filter is determined by the contact time of the carbon with water, known as empty bed contact time (EBCT) and the Iodine number of the carbon. The iodine number of a carbon is calculated by its ability to absorb iodine per gram of carbon. The EBCT is directly proportional to the volume of carbon in the carbon tank and inversely proportional to the flow and can be

The tanks are connected in series with the first tank providing most of the chlorine removal; hence it is usually referred to as the 'worker' tank and the second called the 'polisher'. With regular use, the absorptive sites on the carbon particles become exhausted and their efficiency declines. Conditions within the carbon tanks, especially the polisher, are ideal for bacterial proliferation in that it is chlorine deplete and contains organic matter. To increase the efficiency and prolong its lifespan, the carbons are backwashed ie

Both in the home and in-centre setting, patients and staff, respectively, test the dialysis water daily prior to treatment. If chlorine is detected, dialysis is not initiated and an immediate carbon change is effected. Our center, also tests for trihalomethanes (THMs) (*see section on water testing*), in both the home and in centre setting. The carbons effectively remove THM's. In our experience, elevated THM levels often precede a chlorine breakthrough and are also a marker of the organic load in the water. The ISO guidelines recommend an EBCT of at least 10 minutes and a Iodine number > 900. It also recommends that carbon be acid washed by the

exactly calculated using a formula. The larger the carbon tanks, the higher the EBCT provided the flow rate is constant.

#### **3.3. Activated carbon**

**3.2. Softener and multimedia filter**

222 Updates in Hemodialysis

and easier to replace.

Figure: 1: Softeners and mechanism of action.

**Figure 4.** Softeners and mechanism of action.

a. **Activated carbon:**

manufacturer and that regenerated carbon not be used(9).

the quality of the feed water.

Dissolved salts within in the water contribute to the hardness of water, which can cause 'hard deposits' and can foul the downstream WRO filter. Softeners function to reduce this hardness by removing the salts, commonly calcium and magnesium, by passing water through a resin, exchanging them for sodium (Figure 4). The ISO guidelines recommend a softener be used where feed water has hardness in excess of 10 GPG (grains/gallon) [9]. The softener is 'regen‐ erated' by backwashing it with a high salt solution from the brine tank, effectively stripping the resin of bound calcium and magnesium salts. This can be set to a required time or volume schedule. Typically the softener is replaced every 3 – 5 yrs. Multimedia filters are composed of multiple layers of media, typically coal, sand and garnet, which are of differing sizes. As water passes through the filter, particles up to 10µm, suspended in the water are removed. Some units do not use multi-media filter, opting rather for the micron filters, which are cheaper

(A) <sup>2</sup> carbon filters, <sup>1</sup> softener tank and <sup>a</sup> brine tank in series. (B) Mechanism of softener function,

Large amounts of particulate matter and salts have the potential to affect the efficiency of the carbon and WRO filters. In **home dialysis** installations, our local experience has been that multimedia filters and softeners do not affect the water quality standards. We do not routinely use these filters in our home water setup. Our RO membranes are routinely changed approx‐ imately every 5 years and the absence of the multimedia filter and softeners has not led to early failure of the WRO. Once again this will be dependent on the quality of the feed water.

With regular use the carbon becomes spent with no further binding available along its large surface area.

'fluffing the carbons'. This is usually performed weekly and the tanks are changed every 12 months.

Large amounts of particulate matter and salts have the potential to affect the efficiency of the carbon and WRO filters. In home dialysis installations, our local experience has been that multimedia filters and softeners do not affect the water quality standards. We do not routinely use these filters in our home water setup. Our RO membranes are routinely changed approximately every 5 years and the absence of the multimedia filter and softeners has not led to early failure of the WRO. Once again this will be dependent on

Activated carbon is a highly porous carbon material that is created by adding heat and steam to carbon containing products such as coal and wood. The carbon is then acid washed to clean and increase its porosity. The primary purpose of the carbon filter is to remove chlorine and its related compounds, including chloramines and halogenated organic material, like trihalomethanes (Figure 5).

Chloramines are derived from chlorine and ammonium and is added to drinking water, as a disinfectant as it is more stable, does not dissipate as rapidly as free chlorine and reduces the formation of chlorinated organic compounds such as trihalomethanes(10) (see section on water testing). It is important to note that the RO filter does not remove chlorine. Activated carbon is the best method of removing chloramines and is achieved by physical adsorption and chemical catalysis to its parent compounds. The adsorptive capacity of the carbon filter is determined by the contact time of the carbon with water, known as empty bed contact time (EBCT) and the Iodine number of the carbon. The iodine number of a carbon is calculated by its ability to absorb iodine per gram of carbon. The EBCT is directly proportional to the volume of carbon in the carbon tank and inversely proportional to the flow and can be

The tanks are connected in series with the first tank providing most of the chlorine removal; hence it is usually referred to as the 'worker' tank and the second called the 'polisher'. With regular use, the absorptive sites on the carbon particles become exhausted and their efficiency declines. Conditions within the carbon tanks, especially the polisher, are ideal for bacterial proliferation in that it is chlorine deplete and contains organic matter. To increase the efficiency and prolong its lifespan, the carbons are backwashed ie

Both in the home and in-centre setting, patients and staff, respectively, test the dialysis water daily prior to treatment. If chlorine is detected, dialysis is not initiated and an immediate carbon change is effected. Our center, also tests for trihalomethanes (THMs) (*see section on water testing*), in both the home and in centre setting. The carbons effectively remove THM's. In our experience, elevated THM levels often precede a chlorine breakthrough and are also a marker of the organic load in the water. The ISO guidelines recommend an EBCT of at least 10 minutes and a Iodine number > 900. It also recommends that carbon be acid washed by the

exactly calculated using a formula. The larger the carbon tanks, the higher the EBCT provided the flow rate is constant.

illustrating ion exchange

Activated carbon is a highly porous carbon material that is created by adding heat and steam to carbon containing products such as coal and wood. The carbon is then acid washed to clean and increase its porosity. The primary purpose of the carbon filter is to remove chlorine and its related compounds, including chloramines and halogenated organic material, like trihalo‐ methanes (Figure 5). With regular use the carbon becomes spent with no further binding available along its large surface area.

Chloramines are derived from chlorine and ammonium and is added to drinking water, as a disinfectant as it is more stable, does not dissipate as rapidly as free chlorine and reduces the formation of chlorinated organic compounds such as trihalomethanes [10] (see section on water testing). It is important to note that the RO filter does not remove chlorine. Activated carbon is the best method of removing chloramines and is achieved by physical adsorption and chemical catalysis to its parent compounds. The adsorptive capacity of the carbon filter is determined by the contact time of the carbon with water, known as empty bed contact time (EBCT) and the Iodine number of the carbon. The iodine number of a carbon is calculated by its ability to absorb iodine per gram of carbon. The EBCT is directly proportional to the volume of carbon in the carbon tank and inversely proportional to the flow and can be exactly calculated using a formula. The larger the carbon tanks, the higher the EBCT provided the flow rate is constant.

The tanks are connected in series with the first tank providing most of the chlorine removal; hence it is usually referred to as the 'worker' tank and the second called the 'polisher'. With regular use, the absorptive sites on the carbon particles become exhausted and their efficiency declines. Conditions within the carbon tanks, especially the polisher, are ideal for bacterial proliferation in that it is chlorine deplete and contains organic matter. To increase the efficiency and prolong its lifespan, the carbons are backwashed ie 'fluffing the carbons'. This is usually performed weekly and the tanks are changed every 12 months.

Both in the home and in-centre setting, patients and staff, respectively, test the dialysis water daily prior to treatment. If chlorine is detected, dialysis is not initiated and an immediate carbon change is effected. Our center, also tests for trihalomethanes (THMs) (*see section on water testing*), in both the home and in centre setting. The carbons effectively remove THM's. In our experience, elevated THM levels often precede a chlorine breakthrough and are also a marker of the organic load in the water. The ISO guidelines recommend an EBCT of at least 10 minutes and a Iodine number > 900. It also recommends that carbon be acid washed by the manufacturer and that regenerated carbon not be used [9].

#### **3.4. UV**

The ultra violet (UV) light emitters function to deactivate microorganisms. Exposure to UV results in damage to the nucleic acids of the cell. The ISO recommends that a minimum radiant energy dose should be 16 milliwatt-s/cm2 and that unit replacement and maintenance should occur annually [11-13].

Adapted from Culp, G.L., and R.L. Culp. 1974. New Concepts in Water Purification. Van Nostrand Reinhold Co., New York.

**Figure 5.** Activated carbon particle

#### **3.5. De-ioniser**

Deionisers work on the principle of ion exchange to remove organic or inorganic ions from the water. Typically a mixed bed, anion and cation exchange resin would be used. These have largely been replaced by the use of reverse osmosis technology.

#### **3.6. Reverse osmosis**

Water reverse osmosis (WRO) units operate by pumping water, at pressure, across a semi permeable membrane, using a cross flow, membrane filtration system (Figure 6). Here a single stream of water is presented to the membrane, at which point it can either pass across the membrane as pure permeate or be "rejected" by the membrane and flow to waste. The WRO will remove metals (e.g. manganese, iron and fluoride), as well as organic molecules (e.g. bacteria). Effective and efficient operation of the WRO is proportional to the quality of feedwater; hence making the pre-treatment process obligatory to maximise the longevity of the membrane.

There are various WROs available on the market, differing in membrane type (e.g. cellulose, synthetic, composite) and membrane configuration. Typically, a polyamide, thin film compo‐ site in a spiral configuration is used in haemodialysis.. Water pH ideally should be between 5 and 8.5. A higher pH will cause the carbon filters chloramine absorption to be less effective and also reduce the efficiency of the RO membrane.

**Figure 6.** Used RO membrane from a home dialysis WRO machine

**3.5. De-ioniser**

224 Updates in Hemodialysis

**Figure 5.** Activated carbon particle

York.

**3.6. Reverse osmosis**

the membrane.

Deionisers work on the principle of ion exchange to remove organic or inorganic ions from the water. Typically a mixed bed, anion and cation exchange resin would be used. These have

Adapted from Culp, G.L., and R.L. Culp. 1974. New Concepts in Water Purification. Van Nostrand Reinhold Co., New

Water reverse osmosis (WRO) units operate by pumping water, at pressure, across a semi permeable membrane, using a cross flow, membrane filtration system (Figure 6). Here a single stream of water is presented to the membrane, at which point it can either pass across the membrane as pure permeate or be "rejected" by the membrane and flow to waste. The WRO will remove metals (e.g. manganese, iron and fluoride), as well as organic molecules (e.g. bacteria). Effective and efficient operation of the WRO is proportional to the quality of feedwater; hence making the pre-treatment process obligatory to maximise the longevity of

There are various WROs available on the market, differing in membrane type (e.g. cellulose, synthetic, composite) and membrane configuration. Typically, a polyamide, thin film compo‐ site in a spiral configuration is used in haemodialysis.. Water pH ideally should be between 5 and 8.5. A higher pH will cause the carbon filters chloramine absorption to be less effective

largely been replaced by the use of reverse osmosis technology.

and also reduce the efficiency of the RO membrane.

The WRO unit has an internal conductivity sensor and uses this to monitor the efficiency of the WRO membrane by measuring the conductivity both pre-and post-filtration and then calculating a percentage efficiency. Post membrane water usually has a conductivity of between 2 – 10 µSm/cm. The machine has programmable alarm limits, which can be adjusted by the technicians. Although our unit routinely sets the initial alarm at 50µSm/cm, our technicians usually intervene if the post RO water conductivity exceeds 20µSm/cm. The machine is programmed to shut down if this exceeds 150µSm/cm. Measurements of RO conductivity efficiency is only a guide and not an absolute measure of suitability for dialysis, which can only be ascertained by performing a detailed water analysis.

In the home setting, portable WROs are disinfected, by the user, either using heat, weekly or chemically, twice a week. Chemical disinfection is performed using agents, such as Dialox® solution (peracetic acid 0.35%, hydrogen peroxide 6.6%). Some units use a weekly or even fortnightly disinfection schedule. The use of chemicals reduces the longevity of the RO membrane. Newer WRO's use only heat to disinfect by heating water up to 90°C. This has the advantage of not needing to store and transport chemicals and also prevents the rare but real danger of mistakenly using the dialysis machine bleach in the WRO. Using heat disinfection WRO's also offers the unique advantage of integrated disinfection. Here the heated solution is not limited to the WRO but extends simultaneously to the HD machine, thus disinfecting the piping in between and resulting in fewer breaches in water quality. In the in-centre setting, the loop delivery piping connecting the WRO and the dialysis machines, is heat disinfected daily, using water heated to 85°C with the WRO membranes being disinfected weekly.

As the WRO is used over time, several different processes start to affect its efficiency. These 3 processes include:


Hence, WROs do need to be serviced regularly. This involves calibrating the conductivity sensors, checking the pump flows, descaling, checking for leaks and sterilising the machine (see Figure 8). In the home setting, the machines first undergo a high pH, sodium hydroxide, flush followed by a low pH acidic solution (e.g. a citrate based solution.) Specific practices will be dependent on whether or not softeners have been used, the quality of the feed water and manufacturer specific guidelines for servicing. The WRO membranes are usually replaced every 3 – 5 years.

The ISO recommends daily monitoring of the WRO unit's instrumentation panel. This usually includes, a constant readout of the product water conductivity and percentage efficiency [9]. The purpose is to monitor and log trends and confirm that the machine is operating within the manufacturer's specifications. It also recommends repeating a laboratory water analysis when significant seasonal changes in water quality are suspected or if rejection rates change by more than 10%.

**Figure 7.** Portable WRO with casing removed

#### **3.7. Ultra filters or endotoxin retentive filters**

**Figure: 8 Example of an ultrafilter showing dialysate flow**

change by more than 10%.

**Figure: 7 Portable WRO with casing removed**

the loop delivery piping connecting the WRO and the dialysis machines, is heat disinfected daily, using water heated to 85°C with the WRO membranes being disinfected weekly.

As the WRO is used over time, several different processes start to affect its efficiency. These 3

Hence, WROs do need to be serviced regularly. This involves calibrating the conductivity sensors, checking the pump flows, descaling, checking for leaks and sterilising the machine (see Figure 8). In the home setting, the machines first undergo a high pH, sodium hydroxide, flush followed by a low pH acidic solution (e.g. a citrate based solution.) Specific practices will be dependent on whether or not softeners have been used, the quality of the feed water and manufacturer specific guidelines for servicing. The WRO membranes are usually replaced

The ISO recommends daily monitoring of the WRO unit's instrumentation panel. This usually includes, a constant readout of the product water conductivity and percentage efficiency [9]. The purpose is to monitor and log trends and confirm that the machine is operating within the manufacturer's specifications. It also recommends repeating a laboratory water analysis when significant seasonal changes in water quality are suspected or if rejection rates change by more

**•** fouling – the entrapment of particles in the membrane,

**•** scaling – deposition of eg calcium salts and

processes include:

226 Updates in Hemodialysis

every 3 – 5 years.

than 10%.

**Figure 7.** Portable WRO with casing removed

**•** membrane degradation.

Ultra filters, also known as endotoxin retentive filters, are cartridge type filters that are installed onto the dialysis machine (Figure 9). They are composed of a polysulfone material used to achieve "ultra pure" water by removing bacteria and endotoxin. This is by a process of adsorption and exclusion by particle size. Flow through the filter cartridge can be either in a "dead end" or "cross flow" configuration. In 'cross flow' mode, water flows parallel to the membrane surface with impurities "washed away" in the reject stream. In 'dead end' mode, water flows perpendicular to the membrane surface. **g. Ultra filters or endotoxin retentive filters**  Ultra filters, also known as endotoxin retentive filters, are cartridge type filters that are installed onto the dialysis machine (Figure 9). They are composed of a polysulfone material used to achieve "ultra pure" water by removing bacteria and endotoxin. This is by a process of adsorption and exclusion by particle size. Flow through the filter cartridge can be either in a "dead end" or "cross flow" configuration. In 'cross flow' mode, water flows parallel to the membrane surface with impurities "washed away" in the reject stream. In 'dead end' mode, water flows perpendicular to the membrane surface.

*Figure courtesy of Fresenius Medical Care © Australia Pty Ltd.* **Figure 8.** Example of an ultrafilter showing dialysate flow

These filters are usually operated in dead end mode as a cost saving measure. Any "fouling" of the membrane is limited by regular flushing to a drain valve. In addition, these filters are included in the dialysis machine disinfection cycle further limiting bacterial contamination. Prior to each treatment, the dialysis machine conducts a These filters are usually operated in dead end mode as a cost saving measure. Any "fouling" of the membrane is limited by regular flushing to a drain valve. In addition, these filters are included in the dialysis machine disinfection cycle further limiting bacterial contamination. Prior to each treatment, the dialysis machine conducts a pressure integrity check (**Δ**P) to ensure the membrane is functioning adequately.

Patients on haemodiafiltration (HDF) require a high quality "substitution fluid' to be infused directly in to the circulation. Hence, a second ultrafilter is used, which greatly minimises the possibility of contamination.

### **3.8. Plumbing**

Only qualified plumbers registered with the local water board and with prior experience in water systems are used to plumb new installations. The main components are the filters and the piping in between. Plumbing installations including those for haemodialysis must comply with the *Plumbing and drainage standard* of that country.

#### *3.8.1. Backflow device and stop valve*

This prevents treated water, containing disinfectants, from back flowing into municipal water supply. The device cannot be tested and so are changed routinely every two years according to the manufacturer's specification.

#### *3.8.2. Piping, couplings, micro fittings and sealants*

The 2009 ISO guidelines recommend that piping should not contribute any chemicals eg copper, lead, zinc or chemicals. [9]. Common practice is the use of PVC (polyvinyl chloride) piping as it is non corrodible, is able to withstand high temperatures achieved during disin‐ fection and has a smooth inner surface to prevent biofilm.

Much controversy surrounds the potential leaching of plasticizer compounds from the dialysis tubing, including the dialyser membrane and their effects on health. Two compounds in particular, Bisphenol A and phthalate diesters are known to act as 'oestrogenic disrupting chemicals' which have been shown in rodent models, to cause liver, pancreatic, thyroid and developmental abnormalities [28, 29]. Their role in human disease remains unclear. Copper piping is used only for reject water and not for the piping that supplies the WRO and the dialysis machine, as copper can leach from the piping and result in copper toxicity to the patient [14]. Brass fittings may be minimally used for certain fixtures due to the risk of leaks and blowout with plastic couplings. Particular care needs to be taken to prevent use of and contamination from adhesives, epoxy resins or bonding cements.

#### **3.9. Water disposal and saving – Green dialysis**

There are two grades of wastewater created from the water filtration process. The first is reject water from the WRO, which has not come into contact with the patient and second, post dialysis effluent, which is produced during the actual dialysis process. Safe disposal of dialysis effluent water poses two issues. **Firstly**, because dialysis fluid is in contact with blood, it is a biological waste product and theoretically may contain bacterial or viral particles from the patient. There is however no evidence that this poses a definite infective risk. In one study dialysis wastewater was analysed and compared to municipal, industry, Food and Agriculture Organization of the United Nations (FAO) and World Health Organization (WHO) standards for wastewater use for agricultural applications. Apart from an expected higher conductivity, the dialysis wastewater did not exceed FAO standards for biochemical oxygen demand or bacteria [15]. **Secondly**, dialysis wastewater in the home setting is diverted to sewerage, which poses a practical dilemma for patients in the rural setting, where septic tanks are used. With the large volumes of dialysis wastewater created per treatment, this could quickly overflow these tanks and hence there is a need to be able to accommodate this volume. **Thirdly**, addition of chemicals used in the disinfection process would damage the bio flora essential in the normal function of the septic system used by some patients. For these situations, the current practice is to construct an independent water disposal pit for chemically tainted water during the disinfection process whilst all other effluent water is diverted to sewerage.

There is a high percentage of reject water (approximately 65 – 90%) created by the WRO. This water, created from drinking standard feed water, has already passed through, in most cases, particle *filters,* carbon filters, softeners or multimedia filters. Although "reject" by dialysis purity standards, it is "pre patient" and is still drinkable in some cases [16]. Although not traditional standard practice, experts in the field, especially in water shortage areas in Australia, advocate the use of water conserving practice. This entails storing and using the 'reject' water for "grey water' purposes (e.g. gardening, cleaning etc.) [16, 17]. As a rough estimate, a dialysis unit in a tertiary hospital, servicing approximately 20 dialysis beds, can reject almost 3,000,000 litres of water annually! Secondly, reject water (RW) can be recycled back into the dialysis filtration system [18]. Here the RW is pumped into a separate storage tank, which is then re-presented to the filtration system for reuse and reprocessing. This can result in up to an 80% water saving [18]. Thirdly, water saving WRO's that internally reuse RW can also result in water saving. For example, there would typically be a 3 way valve that would redirect reject water flow away from waste and back into the supply pipe. This valve would operate in short eg. 20-second cycles, alternating between waste and the supply pipe. The recirculation rate can be varied on the machines. Commonly home dialysis and in-centre WRO units use a recirculation rate of between 10% and 65%.

However recirculating reject water or using the water saving feature, will increase the feed water concentration of contaminants, which will reduce the efficiency of the WRO membrane and likely reduce its lifespan. The average water conductivity in an urban area is usually <500mSm/cm and it is sensible in such areas to trial a higher water recirculation percentage eg 30%-50%. Areas where feed water quality is poor with high conductivity > 1000mSm/cm should probably not use the water saving feature to better protect the RO membrane. Ulti‐ mately, the cost and safety concerns involved in balancing water saving against membrane longevity and water quality need to be monitored. Without trialing the water saving feature, it is impossible to know where this balance lies.
