Water treatment

Chapter 8 Water treatment


Water is a complex and incompletely understood chemical compound. The purest form of water is freshly formed rain as it leaves the cloud. It is also a very high-energy form of water. In the journey from rain cloud to final delivery tap, much of this energy is expended in acquiring various impurities as solute or suspension.


Improved dialysis technology has made high-purity water critical for dialysis fluid preparation. Fortunately the science of water purification has made parallel advances. Enhanced membranes for reverse osmosis, ultrafiltration (UF) devices to screen out endotoxins as well as bacteria, and improved monitoring systems are being applied in the renal community.




What impurities may be present in tap water?


Three categories of contaminating substances can cause patient injury or equipment malfunction: (1) chemical solutes, (2) bacteria or bacterial products, and (3) particulate matter. Of these, the chemical and bacterial contaminants may be directly harmful to patients receiving dialysis.



How do impurities get into water?


Falling rainwater passing through the air contacts carbon dioxide and sulfur dioxide, forming carbonic and sulfuric acid in weak solution. Upon hitting the ground, this water encounters limestone and other minerals to form calcium bicarbonate and sulfate, magnesium carbonate, and other salts. Calcium carbonate is the most prevalent impurity in tap water and accounts for most of its hardness.



What other inorganic chemicals may be present in tap water?


Sodium, chloride, iron, aluminum, nitrates, manganese, copper, zinc, iodide, and fluoride are common ionic constituents. The types of mineral present in the geographic area and the time the water is in contact with them determine the content.


Trace elements that may be present include arsenic, silver, strontium, selenium, chromium, lead, cadmium, cyanide, barium, tin, and others.



Are these chemicals harmful?


Nitrates and chloramines may cause methemoglobinemia, in which red cell hemoglobin cannot transport oxygen. Copper in excess may cause hemolysis. Manganese can be toxic, and iron may possibly be toxic. Fluoride is believed to aggravate uremic bone disease; it does accumulate in bone, and it is toxic to enzyme systems. Tin has been found in significantly higher quantity in tissues of dialyzed uremic patients than in nonuremic individuals. Aluminum accumulates and is related to dialysis dementia syndrome, as well as to a form of anemia and osteodystrophy. Zinc causes gastrointestinal upset and may produce anemia. In areas of radioactivity, the presence of strontium-90 (90Sr) may pose a danger.



Are there other materials in tap water?


Nonionic organic compounds, particularly nitrogenous matter such as proteins and polypeptides, phenols, indoles, and aldehydes, may be present. Solid particles of iron, sand, and silica are frequent. Suspended material, including mud, algae, plankton, bacteria, viruses, pyrogenic matter, and dissolved gases (ammonia, carbon dioxide, chlorine), are often present. The most common soluble organic compounds are chloramines from urban water treatment systems. The content of these materials as well as ionic impurities vary with the water source, season, and distribution system.


Some water supplies have identifiable amounts of pesticides or herbicides, such as chlordane, DDT, aldrin, lindane, 2,4,4’-Trichlorobiphenyl, and others.


Most water supplies contain various kinds of bacteria. Many are not detected by routine testing for coliform organisms. Such organisms include Flavobacterium, Achromobacter, Serratia, Pseudomonas, and several atypical mycobacteria.



If a water supply meets the requirements of the safe drinking water act and environmental protection agency standards, is it safe for dialysis?


No. Contaminants in the water used to make dialysis fluid may enter the patient’s bloodstream through the dialysis membrane. In addition, most dialysis procedures use bicarbonate as the buffer. Some delivery systems, in bicarbonate mode, will not function properly if the pH is outside the range of 6.5 to 7.8. The pH of untreated tap water may be extremely acidic or extremely alkaline. It is a requirement that water, dialysate, and equipment used for the hemodialysis treatment meet the quality standards found in the Association for the Advancement of Medical Instrumentation (AAMI) publication Dialysate for Hemodialysis (ANSI/AAMI RD52:2004).



Why is special water treatment necessary to make dialyzing fluid?


During hemodialysis the amount of water that contacts the patient’s blood is more than 25 times the amount taken in by drinking. A substance present in water to only one quarter of its upper limit of safety for drinking purposes may enter the body during hemodialysis in amounts 10 to 25 times that much. Ingested water is processed by the gastrointestinal tract before reaching the bloodstream. This selective membrane can alter the rate at which foreign substances are absorbed from ingested water. In a dialyzer system the dialyzer membrane cannot select ions to be absorbed or rejected, and they pass by diffusion. Substances that are harmless in drinking water may be toxic in dialysis water.



What methods are used to treat water for use in hemodialysis?




• Filtration


• Activated carbon filters (adsorption)


• Water softeners


• Reverse osmosis (RO)


• Deionization (DI)


• Ultraviolet light exposure



What is accomplished by filtration?


Suspended particles (mud, sand, rust, algae) are removed by mechanical filtration through a wound filament or membrane cartridge or by tanks containing granular material that can be backflushed. These effectively filter particles down to about 5 mm in size. Submicron filters are available that screen out particles as small as 0.2 mm. The smaller the number of the filter, the more efficient it is at filtering substances. The number of the filter represents the size of the particle that it is capable of filtering.



What types of filters are used?


The use of multimedia depth filters (i.e., sand filters) is a very economical and efficient way to remove suspended particles by filtering the water through sand. Cartridge filters remove particulate matter by filtering the water through a very stiff or rigid device. An ultrafilter is a very thin and delicate filter that removes much smaller solutes, such as endotoxins. Ultrafilters are highly effective for the removal of fine particles, bacteria, endotoxins, and other pyrogenic matter, as well as high molecular weight organic molecules.



What is the silt density index?


The silt density index (SDI) is an indicator of the colloid and suspended particulate matter present in tap water. The measuring device indicates the pressure drop over time as the tap water crosses a 0.45-mm membrane filter. The more suspended material present, the slower the water passes. An SDI of less than 5 is required of feed water for most RO systems.



What is the action of the carbon tank?


The carbon tank contains granular activated carbon, which removes chlorine and chloramines from the water by adsorption. Chlorine and chloramines are toxic to the blood and patient exposure to these organics can be extremely harmful. Carbon filters or tanks also remove organic matter and odor-producing materials by the same method. Adsorption is a physical process that does not require a chemical reaction and is simply the process in which liquids, gases, or suspended materials cling to a surface, such as the activated carbon. The carbon tank will not remove electrolytes, such as calcium or sodium.


Two carbon tanks are required and they should be side by side, with water passing directly from the first tank into the second tank. A sample port should be present after each of them to test for chlorine and chloramines. The water immediately leaving the first tank should be sampled for chlorine and chloramines with an approved testing device. If the sample shows that the water coming out of the first tank contains chlorine, a second sample should be obtained from the water leaving the second tank. If the sample from the second testing port is within limits, operations can continue for 72 hours until a replacement bed is installed. Testing of the water should continue to ensure there is not further chlorine breakthrough. Dialysis treatments should be suspended if the sample from the second tank contains chlorine. Prior to testing, the water system should be operating for a minimum of 15 minutes in the normal operating mode so that you have a sample that represents the current status of your water. You would not want to test a sample that has been exposed to the carbon bed for a prolonged period of time. The maximum allowable level for total chlorines is 0.5 mg/L (ppm) and the maximum allowable level for chloramine is 0.1 mg/L (ppm). Chlorine and chloramine testing should be done at the beginning of each treatment day, prior to the beginning of each patient shift, or every four hours, whichever is shorter. Proper documentation of results is critical. Empty bed contact time (EBCT) is the amount of time it takes for water to flow through the carbon tanks. The AAMI standard (RD62:2006) and End-Stage Renal Disease Conditions for Coverage require an EBCT equal to or exceeding 10 minutes (minimum of 5 minutes of exposure in each tank).


The first tank removes virtually all of the chlorine, and the second tank acts as a standby or backup in the event that the first tank did not effectively remove all of the chlorine and chloramines. These tanks are sometimes referred to as the working tank and the polishing tank. The tanks must be backwashed nightly to redistribute the carbon for more effective adsorption. The polishing tank has a low flow through it and little chlorine present, making it a good location for bacterial growth. Rotating the polishing tank and the working tank will help minimize the growth of organisms and extend the life of the tanks.


Commonly, there are two types of carbon tanks: (1) portable exchange and (2) “permanent” portable exchange. Portable exchange tanks are “changed out” on a cycle that is developed by the facility. The vendor then replaces them with “new charcoal”–filled tanks. Permanent tanks are equipped with a control unit that allows them to be backwashed at the facility’s discretion. At intervals the carbon is replaced by the vendor or the facility. If the carbon tanks are rebedded at the facility, care must be taken to follow the local waste management guidelines as well as the manufacturer’s recommendations for personal protective equipment. Backwashing does not regenerate the carbon beds. It actually “fluffs” the carbon particles so that channels are removed and the total bed is once again available to contact water passing through it.



What is the action of the water softener?


A water softener is a device located after the carbon tanks that exchanges ions in the water. Water hardness is caused primarily by calcium and magnesium ions. A water softener exchanges calcium and magnesium ions for sodium ions on a milliequivalent-for-milliequivalent basis. Other positively charged ions, such as aluminum, are also removed by the water softener. For each calcium ion removed, two sodium ions are added. The sodium is later removed by the RO system. Permanent softeners have a concentrated brine tank that holds the sodium chloride and controls for on-site regeneration of the softener.


If the feed water is very hard, a softener will remove most of the calcium and magnesium before further treatment. If a deionizer is used downstream, the softener will reduce the divalent ion load presented to the deionizer resin bed and prolong its life. If the subsequent treatment is RO, the removal of calcium and magnesium by the softener may result in higher-quality product water and longer RO membrane life. If a softener was not part of the water treatment system, calcium could potentially build up on the RO membranes and decrease their effectiveness.



What problems occur with the use of water softeners?


If the raw water is very hard, considerable sodium is substituted in the exchange for calcium and magnesium. Municipal water supplies often vary seasonably or even during a single day if multiple sources are used.


There are two types of softeners: portable exchange and permanent. Portable exchange units are provided ready for use by the vendor. Regeneration of the media resin is performed by the vendor at a central facility.


There are no online monitors that will indicate “hard” or “soft” water, but commercial test kits for total hardness are available and are quite reliable. The degree of hardness of both source water and product water should be determined each day. Total hardness is measured post softener and in grains per gallon (gpg) or parts per million (ppm). This will indicate the need for the regeneration cycle, as well as any softener malfunction.



What is reverse osmosis?


RO represents the ultimate in ultrafiltration and is the most effective method of treating water used in dialysis. The RO process removes most contaminants left in the water by the pre-RO treatment systems, including bacterial endotoxins and other contaminants. The RO process involves the movement of water under high pressure across a semipermeable membrane. The dissolved solutes or contaminants will form on the feed side of the membrane and the pure water will form on the product side of the membrane. The product water will be virtually free of dissolved solutes and microorganisms. It is expected that the water going through the membrane will have a rejection rate of at least 80%. The product or purified water will then be sent to a holding tank, where it will be stored before use.



How are organic compounds processed by the membrane?


Organic compounds have no net charge and are not electrically repelled but are physically screened by the membrane. Almost all particles of molecular weight greater than 200 Da are rejected. This includes bacteria, viruses, and pyrogens.



What types of membranes are used for reverse osmosis?


Membranes for RO use must be (1) freely permeable to water, (2) highly impermeable to solutes, and (3) able to tolerate very high operating pressures. Desirable characteristics include tolerance to a wide range of pH and temperature and resistance to attack by bacteria and by chemicals, such as chlorine.


Membranes in general use include (1) cellulose, (2) aromatic polyamide, (3) thin-film composites, and (4) high-flux, chlorine-resistant polysulfone.



• Cellulose acetate membranes have high water permeability but poor rejection of low molecular weight contaminants. Range of pH tolerance is limited; the membranes degrade at temperatures greater than 35°C (95°F) and are vulnerable to bacteria. They are relatively inexpensive.


• Polyamide membranes have wide pH tolerance and are more resistant to bacterial action and to hydrolysis than are cellulosic membranes. They are very susceptible to degradation by free chlorine.


• Thin-film composites are expensive. The supporting layer is usually a porous polysulfone. Fixed to this is a thin, dense, solute-rejecting surface film such as polyfurane cyanurate or a polyamide. Composite membranes have better water flux and better solute rejection than cellulose acetate. They are less subject to compaction and bacterial action.


• Chlorine-resistant polysulfone membranes have a very long service life. They tolerate a wide span of pH and temperatures. Water flux is high. They differ from other membranes in that if divalent ions are present in the feed water, rejection of monovalent ions is sharply reduced. Therefore it is essential that the feed water be softened or deionized before entering the RO unit.

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Jul 24, 2016 | Posted by in NURSING | Comments Off on Water treatment

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