WATER TREATMENT

WATER TREATMENT

What is the basic nature of water?

Water is a complicated and incompletely understood chemical compound with a number of peculiar characteristics. Fresh formed rain is the purest form of water. It is also its highest energy form. Considerable energy is expended by acquisition of solute impurities in passing from rain cloud to the final water tap.

How do impurities get into water?

Falling rain water passing through the air contacts carbon dioxide and sulfur dioxide, forming carbonic acid and sulphuric acid in weak solution. Upon meeting with the earth , water encounters lime stone and other minerals to form calcium bicarbonate and sulphate. Calcium carbonate is the most prevalent impurity in tap water and accounts for most of its hardness.

What other minerals may be present in tap water?

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

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

Are these minerals harmful?  

Nitrates may cause methemoglobinemia – a situation in which red cell hemoglobin cannot transport oxygen. Copper in excess may cause hemolysis , pancreatitis or a type of hepatitis.

Manganese can be toxic and iron may possibly be so. Fluoride is believed by many to aggravate uremic bone disease but this is not proved. It does accumulate in bone and it is toxic to enzyme system.

Do the trace elements offered any hazard?

The effect of these is largely unknown. Tin has been found in significantly higher quantity in tissues of dialyzes uremic patients than in nonuremic individuals. Aluminum also accumulates  and has been tentatively related to the dialysis-dementia syndrome. Zinc causes gastrointestinal upset and may produce anemia,

Are there any other materials in tap water ?

Nonionic organic compounds particularly nitrogenous matter such as proteins an polypeptides , phenols, indoles and aldehydes may be present. Solid aprticles 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 resulting from urban water treatment systems. The content of these as well as of ionic impurities varies with the water sources , season and distribution system..

Why is special  treatment necessary for water used to make dialyzing fluid ?

During hemodialysis the amount of water that contacts the patient’s blood is 20 or more times the amount taken in by drinking. Water ingested by drinking is processed by gastrointestinal tract prior in reaching the bloodstream  This selective membrane can alter the rate at which foreign substances are absorbed from ingested  water. In a dialyzer system the water in dialysate  is separated from blood by a non living artificial membrane. The dialyzer membrane can not select ions to be absorbed or reject  and they pass diffusion. Thus ,substances hat might be harmless in drinking water may  be toxic in dialysis water.

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

·         Filtration

·         Softening

·         Adsorption by activated charcoal

·         Deionization

·         Revers Osmosis ( RO )

                               

What is accomplished by filtration ?

Suspended particles ( mud, sand, rust, algae ) are removed by mechanical filtration through a wound filament or membrane cartridge. These effectively filter particles down to about 5 µin size. Filtration does not remove dissolved particles or ions from the water.

                
                                                     

How is activated charcoal used in water treatment ?

Activated charcoal acts by adsorption. It does not remove electrolytes such as  sodium or calcium . It has a unique ability to adsorb chloramines , odor-producing materials and lower molecular weight ( 60 to 300 daltons )material s as well higher molecular weight organic matter.

               

What does softening of water achieve?

 

Hardness of water is produced primarily by calcium and magnesium ions. Commercial softening units replace these diavalent ions with sodium ions from an exchange resin bed on a milliequivalent-for-milliequivalent basis. For each calcium ion removed ,two sodium ions are added .

If the water is very hard, the use of a softener will remove most of the calcium and magnesium ion prior to further treatment. . If a deionizer is used , the softener will reduce the divalent ion load presented to the deionizer resin bed and prolong its life. If the subsequent treatment is by RO the removal of calcium and magnesium by the  softener may result in higher quality water and longer RO membrane life. 

water softener
                                                                      

What problems occur with use of water softener ?  

If raw water is very hard ,considerable sodium is substituted by the exchange for calcium and magnesium. Municipal water supplies often vary  seasonably or even during a single day if multiple sources are used. Monitoring of softener function is done by checking the total ‘Hardness “ of outflow water and is difficult on a continuous basis.  

What is deionization ?

 

Ion exchange resins  somewhat like those of a water softener but most sophisticated, are used to remove both cations and anions from the processed  water.

                                  
                                                        
                                                                  Deionization                           

How does a deionizing  unit work ?

Essential are a cation bed and an anion bed . Each bed is composed of grains of ion exchange resin about the size of fine sand, called zeolite. Zeolites are large organic chemical molecules that have active side chain groups attached to them.. The side chain of the zeolite  in the carbon bed is a carbonyl ( -COOH ) group . In the anion resin bed there are attached substitute

Ammonium group ( NH3 and OH ). In passage through the cation bed calcium ,magnesium sodium and other cations in the water combine with the carboxyl radicals with the release of hydrogen ions. In the anion bed anions – chloride , fluoride , sulphate, nitrate, bicarbonate and others  are exchanged for the hydroxyl  ion (-OH) . The release hydroxide ion   combine with hydrogen ions released at the cation bed to form water chemically.

CATIONS                   ANIONS

Na                                            Cl

K                                             F

Ca                                            So4

Mg                                           NO3

CATION RESIN BED

H                                             Cl

H                                             F

H                                             SO4

H                                             NO3

ANION BED RESIN

   
H                             OH

                      

                                   H2O

Above  Schematic diagram of  de ionizing apparatus

What is the difference between two bed and single bed deionizer system ?

The two bed system has the cation removing and anion removing resins in separate tanks; water flows through the cation bed and then to the anion bed . The single bed or mixed bed system system has the two resin mixed together. The single bed system because of the mixing produces a much higher quality ( pure ) water. It is also more costly. 

What are problems in deionizer-use ?

1.         One problem is obtaining the maximum flow rate needed ( standard units  for center use have a maximum of  about 300 gallons per hour )

2.         The ionic of raw water presents a problem -  a finite quantity of ions can be removed  by a bed of given size, hence a lesser  volume of  water with a high content can be processed than if the content is low.. The life of the deionizer bed depends on water composition and / or volume .  

3.         Beds tend to exhaust suddenly, it is necessary  to  have parallel or twin installation  so that  a switchover  can be made while the exhaust of tanks are replaced.

4.         Sometimes during regeneration of the resin tanks channel form in the bed. Such a tank may exhaust in an abnormally short time. Usually the anion and cation resin s exhaust at different .times. Depending on which exhausts first , their may be an excess of H or  of OH ion  in the product water.

What is reverse osmosis ? (RO)

This is a process that utilizes semipermeable membrane and pressure to remove both organic matter and electrolytes from water . The membrane repel ions, Organic particles are screened out while water passes through the micropores of the membrane surface by ultrafiltration. These pores are approximate 200 nm in diameter.

                                 

                                                         Reverse osmosis                                                         

        

How does the membrane “repel“ electrolyte ?

This results from electrochemical tension at the water membrane interface, and increases with high valence salts. Diavalent ions such as calcium, magnesium are repelled at a greater distance from the membrane surface than  are monovalent ions ( Sodium ). The rejection rate for divalent ions is greater than 95 % and for monovalent ions 90 to 95 % . Thus up to 10 % of the ions in the feed water may remains in the final product ( purified ) water,

How are organic compounds processed by the membrane ?

Organic compounds have no net charge and are not electrically repelled but are physically screened , depending on molecular size & shape, by the mambrane. Almost all particles of molecular weight greater than 200 daltons are rejected. This includes bacteria, viruses, and known pyrogens.

OVERHEAD     ---------->      IRON REMOVER       --------->  WATER SOFTNER ------->

                                                                           

 <--------  DE                       <--------- CHAR COAL  <---------- CHLORINATION <-------

              CHLORINATION                   TREATMENT                     ( 5 ppm Chroline )                    

 

  ---------------->   DEMINERALIZATION  ------------>  DISTILLATION

Chlorination : 0.5 % Sodium Hypochloride solution  2 drops / 250 ml