2hardness.pdf for getting better knowledge of hardnee
- 1. 23-09-2024 1 Water Treatment Dr Bharti Arora 1 2
- 2. 23-09-2024 2 •Statement or supporting text Demineralization/ Deionization/ Ion Exchange Process Ion-Exchange resins are insoluble, cross linked long chain organic polymers with a micro porous structure, and the “functional Groups” attached to the chains are responsible for the ion-exchanging properties. Anion Exchange Resins Cation Exchange Resins • Contain Basic groups Usually co-polymer of styrene & divinyl benzene containing amino, substituted/ quaternary ammonium groups as an internal parts of the resin matrix. • Are capable of exchanging their OH- ions with anions in water. • Contain Acidic groups (-COOH, -SO3H) • Usually co-polymer of styrene & divinyl benzene, which on sulphonation or carboxylation can exchange their H+ ions with cations in water. Ion Exchange Resins 3 4
- 4. 23-09-2024 4 In ion-exchange process, hard water is allowed to pass through cation exchange resins, which remove Ca+2 and Mg+2 ions and exchange equivalent amount of H+ ions. Anions exchange resins remove bicarbonates, chlorides and sulphates from water exchange equivalent amount of OH- ions. Thus by passing hard water through cation hardness is observed by the following reactions. Cation Exchange Resins 2RH + Ca+2 → R2Ca + 2H+ 2RH + Mg+2 → R2Mg + 2H+ (RH = cation exchange resin) Anion Exchange Resins ROH + Cl- → RCl- + OH- 2ROH + SO4 -2 → R2SO4 + 2OH- 2ROH + CO- 3 2 → R2CO3 + 2OH- (ROH = anion exchange resin) The Ion Exchange Resin is in the granular form and performs as adsorptive exchange of ions, but it requires regeneration when adsorptive capacity is consumed. 7 8
- 5. 23-09-2024 5 H+ And OH- ions, thus released in water from respective cation and anion exchange columns, get combined to produce water molecules. H+ + OH- → H2O The water coming out from the exchanger is ion free i.e., free from anions and cations. Regeneration: When cation exchanger losses capacity of producing H+ ions and exchanger losses capacity of producing OH- ions, they are said to be exhausted. The cation exchanger is regenerated by passing it through dilute sulphuric acid. R2Ca+ 2H+ → 2RH + Ca+2 (washing) The exhausted anion exchanger is regenerated by passing a dilute solution of NaOH R2SO4+ 2 OH - → 2ROH- + SO42- (washing) Merits of Ion-exchange process: • The process can be used to soften highly acidic or alkaline water. • It produces water of very low hardness (2 ppm) • So it is very good for treating water for use in high-pressure boilers. Demerits of Ion-exchange process: The equipment is costly and more expensive chemicals are needed. If water contains turbidity, the output of the process is reduced. 9 10
- 6. 23-09-2024 6 The process of removing dissolved salts (NaCl) from the water, is known as desalination. Desalination The water containing high concentration of dissolved salts (1000-3500ppm) with a salty taste is called brackish water. Ion Exchange Membrane allows ions to permeate by DC electric current applied to saline water. The sodium ions start moving towards the cathode while the chloride ions start moving towards the anode. As a result ,the concentration of brine decreases in the central compartment; while it increases in the two side compartments. No regeneration is required and allows continuous use for an extended period of time. Electrodialysis cell 11 12
- 7. 23-09-2024 7 It has large no. of alternate cation & anion permeable membranes. An electric field is applied perpendicularly to the direction of water flow. The fixed positive charges in the membrane repel sodium ions and allow chorine ions to pass through. Similarly the other membrane allow sodium ions to pass through. Then we get pure water in one compartment. Electrodialysis Cell ELECTRODIALYSIS cell is most compact unit The cost of installation of plant and its operation is economical. If electricity is easily available ,it is best suited Advantages Disadvantages Non-charged, higher molecular weight, and less mobile ionic species will not typically be significantly removed. Electro dialysis becomes less economical when extremely low salt concentrations in the product are required. 13 14
- 8. 23-09-2024 8 Reverse Osmosis In normal osmosis: O Water molecules move from a region of low concentration to a region of high concentration through a partially permeable membrane. In reverse osmosis (high to low concentration): O The reversal of osmosis to purify water. The flow of water can be reversed with an opposing pressure that exceeds osmotic pressure. With Reverse Osmosis, water is forced out of a concentrated solution, leaving the solute (impurities) behind.. Membranes: nylon cellulose acetate, polymethacrylate, polyamide polymers Process of Reverse Osmosis A reverse osmosis membrane will remove impurities and particles larger than .001 microns. 15 16
- 9. 23-09-2024 9 How Reverse Osmosis Works A semi permeable membrane, like the membrane of a cell wall is selective about what it allows to pass through, and what it prevents from passing. These membranes in general pass water very easily because of its small molecular size; but also prevent many other contaminants from passing by trapping them. Water will typically be present on both sides of the membrane, with each side having a different concentration of dissolved minerals. Since the water with the less concentrated solution seeks to dilute the more concentrated solution, water will pass through the membrane from the lower concentration side to the greater concentration side. Eventually, osmotic pressure will counter the diffusion process exactly, and an equilibrium will form. pH 7.2 - 7.8 Total Alkalinity : 80 - 120 ppm as CaCO3 Calcium Hardness 250+ ppm as CaCO3 Total Dissolved Solids less than 1500 ppm The recommended parameters and factors relating to water balance are :- 17 18
- 10. 23-09-2024 10 Zeolite Process Zeolite are Naturally occurring hydrated alumino silicate minerals (Na2O.Al2O3.x SiO2.y H2O) Where x=2-10, y=2-6 For softening of water by zeolite process, hard water is percolated at a specified rate through a bed of zeolite, kept in a cylinder. The hardness causing ions like Ca+2, Mg+2 are retained by the zeolite as CaZe and MgZe; while the outgoing water contains sodium salts. 19 20
- 11. 23-09-2024 11 Sodium Zeolite to Calcium & Magnesium Zeolite Exhausted Zeolite + Conc. Brine solution (10% NaCl solution) CaZe + 2 NaCl → Na2Ze + CaCl2 MgZe + 2 NaCl → Na2Ze +MgCl2 Regeneration of Zeolite Advantages: 1.It removes the hardness completely and water of about 10 ppm hardness is produced. 2.The equipment used is compact, occupying a small space. 3.No impurities are precipitated, so there is no danger of sludge formation in the treated water at a later stage. 4.The process automatically adjusts itself for variation in hardness of incoming water. 5.It is quite clean 6.It requires less time for softening. 7.It requires less skill for maintenance as well as operation. Disadvantages: 1.The treated water contains more sodium salts. 2.This method causes caustic embrittlement. 3.High turbidity water cannot be treated efficiently by this method. Limitations: 1. If the supply of water is turbid, the suspended matter must be removed, before the water is admitted to the zeolite bed. Otherwise the turbidity will clog the pores of zeolite bed, thereby making it inactive. 2. If water contains large quantities of coloured ions such as Mn+2 and Fe+2 they must be removed first, because these ions This cannot be easily regenerated. produce magnesium and Ferrous zeolites. 3. Mineral acids, if present in water, destroy the zeolite bed and therefore, they must be neutralized with soda, before admitting the water to the zeolite softening plant. 21 22