Water and its treatment (3) Prepared By Dr. Biswajit Saha

Softening of water The process whereby remove or reduce the hardness of water, irrespective of whether it is temporary or permanent is termed as softening of water External treatment: Internal treatment: External treatment: Carry out before its entry into the boiler. Lime-soda, Zeolite and ion-exchange process Internal treatment: Suitable chemicals are added to the boiler water either to precipitate or to convert scale in to compounds.

Internal Treatment Method 1. Colloidal Conditioning: -Using kerosine, tannin, starch etc -Get coated over the scale forming particles -Removed by Blow down Process 2. Phosphate Conditioning: - Small amount of phosphate ions are added to precipitate Ca ions. - Chosen depending on the pH conditions of boiler 3CaCl2 + 2Na3PO4 → Ca3(PO4)2 + 6NaCl Removed by Blow down Process 3. EDTA method

Na2 [Na4 (PO3)6]→ 2Na+ + [Na4P6O18]2- 4. Carbonate conditioning: Na2CO3 is added to precipitate Ca salts as CaCO3, (concentration of CO32- increases and becomes greater than Ksp) Removed by Blow down Process, Used in low pressure boilers CaSO4 + Na2CO3→ CaCO3 + Na2SO4 5. Calgon Conditioning: Scale forming salts are converted into soluble complex. E.g. Sodium Hexameta Phosphate (Na2PO3)6 is added…reacts with Ca and forms Calcium Hexameta Phosphate (Ca2PO3)6 Prevents Scale formation Na2 [Na4 (PO3)6]→ 2Na+ + [Na4P6O18]2- 2CaSO4 + [Na4P6O18]2- → [Ca2P6O18]2- + 2Na2SO4 soluble complex

External Treatment Method for Water softening Lime-soda process: Principle Chemically converted all the soluble hardness causing impurities into insoluble Precipitates which may be removed by settling and filtration. For this purpose, a suspension of calculated amount of lime, Ca(OH)2, and soda Na2CO3 are added. Ca(HCO3)2 + Ca(OH)2 = 2 CaCO3 + 2 H2O Mg (HCO3)2 + 2 Ca(OH)2 = 2 CaCO3 + 2 Mg(OH)2 + 2H2O (MgCl2 or MgSO4 )+ Ca(OH)2 = Mg (OH)2 + (CaCl2 or CaSO4 ) CaSO4 + Na2CO3 = 2NaSO4 + 2 CaCO3

Types of Lime-soda process: Cold Lime –soda process At the room temperature, the ppt formed are very fine, do not settle down. So, small amount of Coagulants like [K2SO4.Al2(SO4)3.24H2O], [NaAlO2] to make coarse ppt. They hydrolysis to Hydroxide and entraps the fine ppt like CaCO3, Mg(OH)2. NaAlO2 +2H2O= NaOH + Al(OH)3 Types of Lime-soda process: Cold Lime –soda process Hot lime –soda Process What are the differences between them??

Advantages of soda lime process: It increases the pH of the softened water thereby reducing corrosion of feed and distribution pipes. The process is economical. The microbial load particularly that of pathogenic organisms, is considerably reduced in the softened water. DISADVANTAGES: Inability to soften water to less than 15-20 ppm. Generation of large amount of sludge Requires well trained and skilled operators.

External Treatment Method: Water softening Method Zeolite (or Permutit) Process: In this process, the raw (hard) water is allowed to percolate through a bed of zeolite (Na2Z) packed in a vertical cylindrical tank. The zeolite bed retains the Ca2+ and Mg2+ ions from hard water by exchanging with Na+ ions thereby the out flowing water contains sodium salts. Zeolite is Hydrated sodium alumino Silicate Na2O. Al2O3 XSiO2 YH 2O (X= 2-10, Y= 2-6 ) Na2Z + (Ca2+/Mg2+)X = (Ca2+/Mg2+) Z + Na2X

Process: Higher exchange capacity and porous in Nature. Consist of Steel Tank Having Thick Layer of Zeolite When water pass through it hardness causing ions (Ca, Mg) are retained by Zeolite.

Chemical Reaction: Ca(HCO3) 2 + Na2 Ze → CaZe + 2 NaHCO3 CaSO4 + Na2Ze → CaZe + Na2SO4 MgCl 2 + Na2Ze → MgZe + 2NaCl Mg (NO3)2 + Na2Ze → MgZe + 2NaNO3 Exchange of Na ions continues until Na ions are exhausted Regeneration: CaZe + 2NaCl → Na2Ze + CaCl 2 MgZe + 2NaCl → Na2Ze + MgCl 2 CaCl 2 and MgCl 2 led to drain and Na2Ze can be reused.

Advantages Hardness can be completely removed Process can be made automatically Easy operation. No experts required Less time and sludge Small area requires Disadvantages Only Ca+ and Mg+ ions can be removed Large amount of Na ions present in treated water. Leaves other acids which causes corrosion Fe 2+ and Mn 2+ containing water can not be treated because Fe and Mn Zeolite can not be regenerated Water should be free from suspended impurities to prevent clogging on Zeolite beds.

B. Ion Exchange (or) Demineralization (or) Deionization Process Complex organic compound is called resins. Resins act as ion-exchangers and removed all cations and anions from hard water. Resins are cross link polymers Cation exchange resins: exchange cations present in hard water with H+. They possess acidic groups such as –COOH, -SO3H etc Anion exchange resins: exchange anions present in hard water with OH-. They possess basic groups such as OH-, NH2- etc

Advantage: Produces soft water Can treat highly acidic or alkaline water Regeneration of ion resins are possible Disadvantage: Cost of equipment is high Highly turbid water can not be treated Expensive chemicals are required.

Desalination Processes that remove some amount of salt and other minerals from Water. Brackish Water: contain 3.5 % salts & salty taste. Unfit for drinking Separation of salts from water by evaporation followed by condensation. Commonly used methods: Electrodialysis Reverse Osmosis

Semi permeable membranes are placed A. Electrodialysis: Method of separation of ions from salt water by passing electric current. Semi permeable membranes are placed Consist of three compartments containing 1. Sea Water 2. Pair of electrodes 3. Semi permeable membrane

As current applied Na+ ions moves towards cathode and Cl- moves towards anode As result brine concentration decreases in the middle compartment. Pure water is removed from the central compartment. Conc. Brines are replaced by fresh brine water. Much more effective separation Ion selective membranes are used

Reverse Osmosis (or) Super Filtration (or) Hyper Filtration Movement of solvent molecule from concentrated side to diluted side. If only pressure higher than that of osmotic pressure Membranes: Cellulose Acetate, Polymide, Polymethylmethaacrylate Thickness: 0.0005 to 0.0000002 µm Advantages: Removes both ionic and non ionic and colloidal matters, SiO2 Maintenance cost is low Membrane replacement can be done with in few minutes. Raisin