UNIT IV : WATER TECHNOLOGY

Water : Water is A clear, colorless, odorless, and tasteless liquid, H₂O essential for most plant and animal life and the most widely used of all solvents . Whose Freezing point 0°C (32°F); boiling point 100°C (212°F); specific gravity (4°C) 1.0000; weight per gallon (15°C) 8.338 pounds (3.782 kilograms).


Sources of Water : 


(A) Surface Waters : Surface waters generally contains suspended matter , disease producing bacterias .


        1.Rain water : It is obtained as a result of evaporation from the surface water .


       2.River water : Rivers are fed by rain and spring waters . Water from these sources flow over the surface of land .


       3.Lake water : A large inland body of fresh water or salt water. It has a more constant chemical composition .


      4.Sea water : It is the most impure form of natural water . Rivers carry the impurities to the sea .


(B) Underground waters : A part of the rain water which reaches the surface of the earth percolates in to the earth . Spring and well water contains more dissolved salts . Thus water from these sources contains more hardness .




Effects of water on rocks and minerals :


When water flows over/percolates through the ground rocks or solids, It gets contaminated due to the following physical and chemical changes :


1.Dissolution : Mineral constituents of rocks like sodium chloride , gypsum (CaSO4.2H₂O) , etc readily dissolve in water and collect in it .


2.Hydration : Some materials like anhydrite(CaSO4) , olivine (Mg₂SiO₄) , etc . readily undergo hydration , leading to the formation of products of increased volume . It leads to the dis-integration of such mineral breaking rocks . Thus :


- CaSO₄ + 2H₂O ----> CaSO₄.2H₂O (Gypsum of 33% more volume)


- Mg₂SiO₄ + xH₂O ----> Mg₂SiO₄.xH₂O (Serpentine)


3.Action of dissolved oxygen : Oxidation and hydration reactions are brought by dissolved oxygen . 


eg:
- 2Fe₃O_{4 (oxd)(+1/2O2)--->3Fe2O3 (Hyd)(2H2O} )---> 3Fe₂O₃.2H₂O (Limonite) 
- 2FeS₂+2H₂O+7O₂ (oxd&hyd)----> 2Feso₄+2H₂SO₄ 


4.Action of dissolved carbon dioxide :


(i) Converts insoluble carbonates of Ca,Mg and Fe into soluble bicorbonates . eg:


- CaCO₃+CO₂+H₂O ---> Ca(HCO₃)₂ (soluble)
- MgCO₃+CO₂+H₂O ---> Mg(HCO₃)₂(soluble)


(ii) Converts insoluble rock-forming silicates and alumino silicates of Na , K , Ca and Fe into soluble carbonates , Bicarbonates and silica. 


eg: 
K₂O.Al₂O₃.6SiO₂+CO₂+2H₂O ---> Al₂O₃.2SiO₂.2h₂+K₂CO₃+4SiO₂

Dissolved salts , fine clay and silica finally are collected in water .


Electro-dialysis : It is based on the principle that the ions present in the saline water migrate towards their respective electrodes through ion selective membranes . Under the influence of applied e.m.f 


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The unit consists of a chamber , two electrodes , The cathode and anode. The chamber is divided into three compartments with help of thin rigid ion-selective membranes which are permeable to either cation or anion . The anode is placed near anion selective membrane . While the cathode placed near cation selective membrane . The anion selective membrane is containing of positively charged functional groups such as R₄N⁺ and is permeable to anions only . The cation selective membrane consists of negatively charged fucntional groups . Such as RSO₃^-  and is permeable to cations only . Under the influence of applied e.m.f across the electrodes the cations move towards cathode through the membrane and anions move towards anode through the membrane . The net result is depletion of ions in the central compartment while it increases in the cathodic and anodic compartments . Desalinated water is periodically drawn from the central compartment while conc brakish water is replaced with fresh sample .


Advantages :