Water treatment. Sudha Goel, Ph.D. Assistant Professor (Environmental Engineering) Department of Civil Engineering, IIT Kharagpur

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1 Water treatment Sudha Goel, Ph.D. Assistant Professor (Environmental Engineering) Department of Civil Engineering, IIT Kharagpur Reference: Masters GM [1998] Water treatment systems in Introduction to Environmental Science and Engineering, Prentice Hall 1

2 Conventional drinking water treatment Design or primary objectives are removal of Microbial pathogens (coliforms coliforms) health concerns Particles (color and turbidity) health and aesthetic concerns Total dissolved solids removal (hard waters) - health and aesthetic concerns 2

3 Conventional drinking water treatment Groundwater (GW): In comparison to surface waters 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) Anions: chloride, carbonates, sulfates (sulfides), bromide, nitrates, fluorides, arsenite and arsenate Cations: Ca, Mg, Fe, Mn,..(Hardness is the conc of all multivalent cations mainly Ca and Mg in GW) Surface waters (SW) High turbidity and microbial concentrations Dissolved oxygen concentrations vary depending on organic matter concentrations 3

4 Water intake or infiltration well Screening 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 4

5 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 5

6 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 6

7 Cascade aerators (Gangtok water treatment plant) Source: RN Sharma 7

8 PLAIN SEDIMENTATION TANK (with fountain type aerators; Gangtok water treatment plant) Source: RN Sharma 8

9 Cascade aerators (Gangtok water treatment plant) Source: RN Sharma 9

10 Particle sizes Stable particles that must be chemically and physically conditioned for removal Discrete particles can be removed by settling QMZ,

11 Conventional drinking water treatment processes: coagulation Coagulation and flocculation: turbidity and 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! 11

12 Clariflocculator 12

13 Circular clariflocculator Source: Internet(msu) 13

14 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 rapid sand filters is essential to regain head loss due to clogging Generally done with chlorinated water to disinfect filters 14

15 Disinfection Destruction or removal of vegetative pathogens Not sterilization which implies destruction of all life forms (microbes, spores, cysts, viruses, etc.) Autoclaving, membrane filtration Physical methods Membrane Filtration Radiation: UV, X-rays, gamma rays Chemical methods (disinfectants) Chlorinated compounds chlorine, chloramines, chlorine dioxide Ozone (hydroxyl radical mechanism) Potassium permanganate 15

16 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 16

17 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 17

18 Chlorine chemistry: reactions in water Addition of chlorine to water, results in the formation of hypochlorous [HOCl HOCl] and hydrochloric acids [HCl [ HCl]: Cl 2 + H 2 O HOCl + HCl pk = 3.39 Depending on the ph, hypochlorous acid partly dissociates to hydrogen and hypochlorite ions: HOCl H + + OCl - pk = 7.57 The hypochlorite ion then most often degrades to a mixture of chloride and chlorate ions: 3 OCl - 2 Cl - + ClO

19 Effect of ph and temperature on chlorine speciation Arrhenius effect HOCl is a stronger disinfectant than OCl - 19

20 Example of inactivation assays or disinfection experiments dn = kn dt N ln = kt N 0 kt N = N 0e Harriette Chick s law of disinfection (1908) TFC-8ed Inactivation rate k is a f(time, cell conc, disinfectant conc, temperature, ph) 20

21 Hardness Hardness: due to presence of multivalent cations like Ca, Mg (mainly), and Fe, Mn, Sr, Al, etc. 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, CaCO 3 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; IS standard 300 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 21

22 Hardness 22

23 Hardness classification Description Hardness, meq/l Hardness, mg/l Soft < 1 <50 to 75 Moderately hard 1 to 3 50 or Hard 3 to Very hard > 6 >

24 Softening Surface waters are generally softer than GWs For hardness levels < 200 mg/l as CaCO 3, no softening is required Softening is often required for GW Especially when hardness is > 300 mg/l (IS 10500) 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 mg/l of CaCO 3 Ion exchange (for finer applications, expensive, for <30 to 40 mg/l of CaCO 3 ) Zeolites: can be natural or synthetic Ion exchange resins: cationic or anionic Na + or H + is exchanged for Ca 2+ and Mg 2+, does not contribute to hardness Regeneration required; much higher removal efficiencies can be achieved 24

25 Zeolites Wikipedia

26 Water classes based on salinity CLASS Fresh Slightly saline Mildly saline Moderately saline Severely saline Sea water SOURCE TDS, mg/l Rivers, lakes, GW <500 Ground, river, lake Estuaries Inland and brackish mix ,000 Inland and coastal 10,000-30,000 Offshore seas and oceans 30,000-36,000 TDS = A*C where A = conversion factor, 0.55 to 0.75 C = electrical conductivity, micros or micromhos TDS = total dissolved solids, mg/l 26

27 Demineralization Processes for removing TDS from water Ion exchange Membrane processes Electric current driven: electrodialysis or electrodialysis reversal Pressure driven: reverse osmosis (RO), nanofiltration, ultrafiltration, microfiltration Distillation Multi-stage flash distillation (MSF) Multiple effect evaporation (or distillation) - MED Vapor compression (VC) Solar distillation Freezing Distillation and RO account for 87% of the desalination capacity in the world 27

28 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 28

29 Membrane Processes Raw water or influent, Q0, C0 Treated water or effluent Qp, Cp Concentrate or rejectate, Qr, Cr Mass balance around system or process: Flow: Q0 = Qp + Qr Mass of contaminant: Q0C0 = QpCp + QrCr 29

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