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water treatment slides

  1. 1. Water treatment Sudha Goel, Ph.D. Associate Professor (Environmental Engineering) Civil Engineering Department, IIT Kharagpur Reference: Masters GM [1998] Water treatment systems in Introduction to Environmental Science and 1 Engineering, Prentice Hall
  2. 2. Goal: safe and clean drinking water REQUIREMENTS Identify source water in terms of Quantity and quality Location Cost and sustainability Protect source water from contamination Watershed management plans Appropriate treatment of raw water (source water) Safe distribution of treated water Clean, safe drinking water at the tap 2
  3. 3. Conventional drinking water treatment Design or primary objectives are removal of (coliforms coliforms) Microbial pathogens (coliforms) – health concerns Particles (color and turbidity) – health and aesthetic concerns Total dissolved solids removal (hard waters) - health and aesthetic concerns Secondary objectives are removal of dissolved pollutants – health concerns (based on IS:10500) General: Odor, taste, pH Inorganic Cl, Mn, Hardness, Alkalinity, Fe, Cl, F, Ca, Mg, Cu, Mn, SO42-, NO3- Hg, Cd, Se, As, CN-, Pb, Zn, Cr(VI), Al, B, radioactive materials, Cd, Pb, residual free chlorine, TDS Organic Pesticides, Natural Organic Matter (NOM), Pesticides, Oils, PAHs, Anionic detergents, Phenols 3
  4. 4. Conventional drinking water treatment Groundwater (GW): In comparison to surface waters GW tends to have lower dissolved oxygen compared to surface waters Can have very little microbial contamination especially if GW is from a deep aquifer Much higher concentrations of inorganic compounds (or ions) sulfides), Anions: chloride, carbonates, sulfates (or sulfides), bromide, nitrates, fluorides, arsenite and arsenate Cations: Mn, …..(Hardness Cations: Ca, Mg, Fe, Mn, Al, As, …..(Hardness is the conc of Ca and Mg in GW) Surface waters (SW) High turbidity and microbial concentrations Dissolved oxygen concentrations vary depending on organic matter concentrations 4
  5. 5. Water intake or infiltration well preScreening or pre-sedimentation tank: Turbidity, TSS removal Coagulation and flocculation: Turbidity, colloid removal TURBID SURFACE WATER Settling tank: floc removal Filtration: Turbidity, TSS, floc removal Disinfection and storage: Pathogen removal 5
  6. 6. HARD GROUNDWATER Aeration Low DO levels, presence of other gases, precipitation of reduced minerals like Fe, As, Mn due to oxidation Softening Removal of calcium and magnesium hardness Filtration, with or without pre-chlorination Turbidity, TSS, colloid removal Chlorine to prevent biological growth on filter media Disinfection and storage: pathogens are destroyed; provides contact time for disinfection apart from water storage 6
  7. 7. Conventional drinking water treatment processes Aeration: necessary for GWs that are anoxic Oxidation of reduced forms of Fe(II) to Fe(III) and Mn(II) to Mn(IV) For As-contaminated water, it can result in substantial removal of As, too Types of aerators: cascade, fountain, tray, diffusers Screening: necessary for most surface waters, especially at intake points Removes large floating and suspended debris 7
  8. 8. Cascade aerators (Gangtok water treatment plant) Source: RN Sharma 8
  9. 9. PLAIN SEDIMENTATION TANK (with fountain type aerators; Gangtok water treatment plant) Source: RN Sharma 9
  10. 10. PLAIN SEDIMENTATION TANK (with fountain type aerators; Gangtok water treatment plant) Source: RN Sharma 10
  11. 11. Cascade aerators (Gangtok water treatment plant) Source: RN Sharma 11
  12. 12. Solids and suspensions Discrete particles Particles do not change size, shape and specific gravity over time Flocculating particles Size, shape and specific gravity of particles changes over time as they aggregate or coalesce Dilute suspensions If conc of particles in suspension is insufficient to displace water as the particles settle Concentrated suspensions If conc of particles in suspension is sufficient to displace water as the particles settle 12
  13. 13. 13
  14. 14. Particle sizes Stable particles that must be chemically and physically conditioned for removal Discrete particles can be removed by settling QMZ, 2000 14
  15. 15. Solids separation: Sedimentation and clarification Sedimentation Removal of discrete particles (>1 micron) that are heavy enough to settle by gravity alone Sedimentation or settling tanks for floc removal as well Detention times range from 1 to 10 hours 15
  16. 16. Conventional drinking water treatment processes: coagulation Coagulation and flocculation: turbidity and suspended solids (SS) removal Design objective is removal of colloidal particles (1 nm to 1 micron) Can remove bacteria, soil, sand and clay particles Concomitant removal of associated compounds or smaller particles like NOM, heavy metals, pesticides, etc. Stable particles in natural systems Particles in natural waters (generally in pH range of 6 to 8) are –vely charged Like charges repel each other and remain suspended in solution (stable particles and no aggregation is possible) A turbid solution! 16
  17. 17. Dilute solution in nature – low ionic strength Particles with negative surface charges After addition of coagulants to solution – high ionic strength Particles with negative surface charges 17
  18. 18. Conventional drinking water treatment processes: coagulation Coagulation mechanisms Charge neutralization: Addition of Al or Fe salts and organic polymers provides high concentrations of counter ions that neutralize negative surface charges of particles Reduces electrostatic repulsive interaction forces, and net interaction energy becomes attractive (mainly Van der Waal’s forces) Net attractive forces lead to aggregation, and settling of aggregates or floc formation Sweep floc formation: precipitation of salts at high concentration In settling, the precipitate ‘sweeps’ colloidal particles along with itself Interparticle bridging: polymers attach to more than one particle leading to aggregation and floc formation 18
  19. 19. Adsorption and interparticle bridging PRT 1985 19
  20. 20. Residual turbidity results Procedure for coagulation and flocculation in the laboratory flocculator. 0 mg/L 1 mg/L 2 mg/L 5 mg/L 10 mg/L 20 mg/L Samples of the coagulated and settled supernatant from the jar tests (after step 3) Narayan and Goel - 2011 20
  21. 21. Conventional drinking water treatment processes: flocculation Flocculation or mixing Rapid mixing: for mixing the coagulant Detention time is approx. 0.5 min Slow mixing: for floc formation Too fast will break floc; slow enough to maximize number of particle collisions Optimum speed has to be determined experimentally Practical examples: milk and tea as colloidal suspensions! 21
  22. 22. Clariflocculator 22
  23. 23. Clariflocculator Source: Internet(msu) 23
  24. 24. Conventional drinking water treatment processes: filtration Filtration: removal of flocculated particles of smaller size (those that cannot be removed by settling) • Rapid sand filters: higher throughput • Slow sand filters: lower throughput • Adsorption is another important mechanism for particle removal • Backwashing of filters is essential to regain head loss due to clogging • Generally with chlorinated water to disinfect filters 24
  25. 25. Slow sand filters 25
  26. 26. Rapid sand filters Peavy HS, Rowe DR and Tchobanoglous G (1985) Chapter 4, Environmental Engineering, McGraw Hill 26 International Ed., NY, US
  27. 27. Disinfection Destruction of vegetative pathogens Not sterilization which implies destruction of all life forms (microbes, spores, cysts, viruses, etc.) Autoclaving, membrane filtration Physical methods Membrane Filtration XRadiation: UV, X-rays, gamma rays Chemical methods (disinfectants) Chlorinated compounds chlorine, chloramines, chlorine dioxide Ozone (hydroxyl radical mechanism) Potassium permanganate 27
  28. 28. Oxidation potentials and disinfection power of disinfectants Disinfectant Oxidation potential (Volts) Fluorine -3.06 Hydroxyl free radical (OH-) -2.80 Oxygen (atomic) -2.42 Ozone (O3) -2.07 Hypobromous acid (HOBr) -1.59 Hypochlorous acid (HOCl) -1.49 Chlorine (Cl2) -1.36 Oxygen (molecular) -1.23 Bromine (Br2) -1.07 Chlorine dioxide (ClO2) -0.95 Monochloramine (NH2Cl) -0.75 Dichloramine (NHCl2) -0.74 28
  29. 29. Chlorine remains the most popular, why? Potent germicide High oxidation potential Residual in distribution system Chloramine can do the same but is a less powerful oxidant Taste and odor control Oxidation of NOM and removal of compounds causing taste and odor problems Biological growth control Growth of algae and bacteria in storage reservoirs and water supply systems Chemical control Iron and manganese removal Oxidation of SOCs 29
  30. 30. Problems with chlorine! Hazardous material Difficulty in transportation, handling and storage Pungent compound Disagreeable taste and odor Dermal and eye irritation Microbial resistance to chlorine More effective against bacteria rather than spores, cysts and viral particles Disinfection by-products (DBPs) formation Potential health hazard Carcinogenic, mutagenic, teratogenic Non-carcinogenic effects – little information or discussion in literature 30
  31. 31. Chlorine chemistry: reactions in water Addition of chlorine to water, results in the formation of hypochlorous HOCl] [HCl HCl]: [HOCl] and hydrochloric acids [HCl]: Cl2 + H2O → HOCl + HCl pK = 3.39 Depending on the pH, hypochlorous acid partly dissociates to hydrogen and hypochlorite ions: HOCl → H+ + OClpK = 7.57 The hypochlorite ion then most often degrades to a mixture of chloride and chlorate ions: 3 OCl- → 2 Cl- + ClO3- 31
  32. 32. Effect of pH and temperature on chlorine speciation • Temperature effect on equilibrium constants • Arrhenius’ effect of temperature on reaction kinetics • HOCl is a stronger disinfectant than OCl- 32
  33. 33. Example of inactivation assays or disinfection experiments dN = − kN dt N ln = − kt N0 − kt N = N 0e Harriette Chick’s law of disinfection (1908) Inactivation rate k is a f(time, cell conc, disinfectant conc, temperature, pH) TFC-8ed 33
  34. 34. Hardness Hardness: due to presence of cations like Ca and Mg Other cations like Fe, Mn, Sr, Al, etc. may be present Formation of soap curd (lack of frothing or foaming that is essential for bringing dirt particles into solution), increased soap requirement and subsequent difficulty in all cleaning activities On heating, scale formation or precipitation of these ions, CaCO3 and Mg(OH)2, leads to reduced efficiency of heating elements, and failure Synthetic detergents can reduce the problem but not eliminate it General level of acceptance is ≤ 150 mg/L Carbonate hardness Due to anions like carbonates and bicarbonates Also called temporary hardness, since it can be precipitated by boiling Non-carbonate hardness Amount of hardness in excess of carbonate hardness 34
  35. 35. Hardness 35
  36. 36. Hardness classification Description meq/L Hardness, meq/L Hardness, mg/L <1 <50 to 75 Moderately hard 1 to 3 50 or 75 – 150 Hard 3 to 6 150 - 300 >6 > 300 Soft Very hard 36
  37. 37. Alkalinity Alkalinity is the measure of a water’s ability to absorb hydrogen ions without significant pH change Buffering capacity of water 37
  38. 38. Softening Surface waters are generally softer than GWs For hardness levels < 200 mg/L as CaCO3, no softening is required Softening is often required for GW Especially when hardness is > 1000 mg/L Processes Lime-soda (gives crude levels of removal, cheap) Quick lime (CaO) or hydrated lime (Ca(OH)2) is added to water Carbonates of Ca precipitate out of solution Mg(OH)2 precipitates at pH >11, excess lime has to be added Can bring hardness down to 30-40 mg/L of CaCO3 Ion exchange (for finer applications, expensive, for <30 to 40 mg/L of CaCO3) Zeolites: can be natural or synthetic Ion exchange resins: cationic or anionic Na+ or H+ is exchanged for Ca 2+and Mg2+, does not contribute to hardness Regeneration required; much higher removal efficiencies can be achieved 38
  39. 39. Zeolites Wikipedia 2007 39
  40. 40. Water classes based on salinity CLASS SOURCE TDS, mg/L Fresh Rivers, lakes, GW <500 Slightly saline Ground, river, lake 500 - 1000 Estuaries 1000 - 2000 Inland and brackish mix 2000 - 10,000 Inland and coastal 10,000 - 30,000 Offshore seas and oceans 30,000 - 36,000 Mildly saline Moderately saline Severely saline Sea water TDS = A*C where A = conversion factor, 0.55 to 0.75 C = electrical conductivity, microS or micromhos TDS = total dissolved solids, mg/L 40
  41. 41. Demineralization or TDS removal Processes for removing TDS from water Membrane processes Electrodialysis (ED) and Electrodialysis reversal (EDR) Reverse Osmosis (RO) Distillation Freezing Distillation and RO account for 87% of the desalination capacity in the world 41
  42. 42. Demineralization Processes for removing TDS from water Membrane processes Electric current driven: electrodialysis or electrodialysis reversal Pressure driven: reverse osmosis, nanofiltration, ultrafiltration, microfiltration Distillation Multi-stage flash distillation (MSF) Multiple effect evaporation (or distillation) - MED Vapor compression (VC) Solar distillation Freezing 42
  43. 43. Membrane Processes Defined as processes in which a membrane is used to permeate high-quality water while rejecting passage of dissolved and suspended solids Used for demineralization (or desalination) and removal of dissolved and suspended particles Major applications in water treatment are NOM removal, and desalting (demineralization) Analytical instruments and methods Industrial applications: Medical applications include separation of various components of body fluids Purification processes QMZ (2000) Ch-18; Sincero (1996) Ch-9 43
  44. 44. Membrane Processes Treated water or effluent Qp, Cp Raw water or influent, Q0, C0 Concentrate or rejectate, Qr, Cr Mass balance around system or process: Flow: Q0 = Qp + Qr Mass of contaminant: Q0C0 = QpCp + QrCr 44