AWWA 1942-2016 Water Treatment Grades 3 & 4.pdf

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1、 #MBOL1BHFGrades 3 the precipitation of magnesium hydroxide requires a pH of 10.6. In both cases, the necessary pH is achieved by adding the proper amount of lime.RecarbonationRecarbonation is the reintroduction of carbon dioxide into the water either dur-ing or after limesoda ash softening. When ha

2、rd water is treated by conventional lime softening, the water becomes supersaturated with calcium carbonate and may have a pH of 10.4 or higher. This very fine, suspended calcium carbonate can deposit on filter media, cementing together the individual media grains (encrus-tation) and depositing a sc

3、ale in the transmission and distribution system piping (postprecipitation). To prevent these problems, carbon dioxide is bubbled into the water, lowering the pH and removing calcium carbonate as follows:CaCO3+ CO2(g) + H2O Ca(HCO3)2calciumbicarbonatecalciumcarbonate(in suspension)watercarbondioxide(

4、gas)(1-11)This type of recarbonation is usually performed after the coagulated and floc-culated waters are settled but before they are filtered, thereby preventing the sus-pended CaCO3from being carried out of the sedimentation basin and cementing the filter media.When the excess- lime technique is

5、used to remove magnesium, a considerable amount of lime remains in the water. The result is a water that is undesirably caus-tic and high in pH. Carbon dioxide introduced into the water reacts as follows:Ca(OH)2+ CO2(g) CaCO3+ H2Oexcesslimecalciumcarbonate(precipitate)watercarbondioxide(gas)(1-12)Th

6、is form of recarbonation is performed after coagulation and flocculation but before final settling. Carbon dioxide reacts with the excess lime, removing the cause of the caustic, high- pH condition and, incidentally, removing the calcium that added to the hardness. The product, calcium carbonate, is

7、 removed by the filtration process.It is important to select the correct carbon dioxide dosage. If too much CO2is added, the following can happen:Ca(OH)2+ 2CO2(g) Ca(HCO3)2excesslimecalciumbicarbonatecarbondioxide (gas)(1-13)recarbonationThe reintroduction of carbon dioxide into the water, either du

8、ring or after limesoda ash softening.8 WSO Water Treatment, Grades 3 after softening, these anions are associated with the sodium cations released by the resin. Hence, the softened water contains sodium bicar-bonate (NaCHO3), sodium sulfate (Na2SO4), and sodium chloride (NaCl). These compounds do no

9、t cause hardness and are present in such small concentrations that they do not cause tastes. Unlike limesoda ash softening, ion exchange soft-ening operates the same for both carbonate and noncarbonate hardness. Both types are removed by the same exchange reactions.After most of the sodium ions are

10、removed from the exchange resin in the softening process, the resin must be regenerated in order to restore its soften-ing capacity. That is, the exchange process must be reversed, with the hardness cations of calcium and magnesium being forced out of the resin and replaced by cations of sodium. Thi

11、s reverse exchange is achieved by passing a strong brine solution (a concentrated solution of common table salt) through the resin bed. The two ion exchange regeneration reactions are shown below:CaCl2+ Na2XCaX + 2NaCl(1-22)MgCl2+ Na2XMgX + 2NaCl(1-23)When sodium is taken back into the exchange resi

12、n, the resin is again ready to be used for softening. The calcium and magnesium, released during regeneration, are carried to disposal by the spent brine solution.Properly maintained and operated, cation exchange removes all hardness. Wa-ter of zero hardness is corrosive, so the final step in ion ex

13、change softening is to mix a portion of the unsoftened water with the softened effluent to provide water that is still relatively soft, but that contains enough hardness to be noncorrosive (stable).Scaling and Corrosion ControlScaling and corrosion are closely related problems in water treatment. Th

14、ey may be thought of as being at opposite ends of a hypothetical stability scale, as shown in Figure 1-1.The objective of scale and corrosion control is to stabilize the water, thus pre-venting both scale formation and corrosion. The stable range is relatively narrow, requiring careful monitoring du

15、ring treatment in order to avoid under- or over-shooting the stable range.regenerationThe process of reversing the ion exchange softening reaction of ion exchange materials. Hardness ions are removed from the used materials and replaced with nontroublesome ions, thus restoring the exchange capacity

16、of the resin for further softening.Figure 1-1 Hypothetical stability scaleCorrosivewaterStablewaterScale-formingwater10 WSO Water Treatment, Grades 3 however, the coating is removed, partly by the scrubbing action of moving water and partly by combination with oxygen (O2) normally dissolved in the w

17、ater:2H2O2H2+ O2(1-28)Basic Microbiology and Chemistry 11The metal surface is exposed again and corrosion continues. Failure to protect the metal surface or remove corrosion- causing elements will result in destruction of pipes or equipment.Once the reaction in Equation 1-26 has occurred, subsequent

18、 reactions de-pend on the chemical characteristics of the water. If water is low in pH, is low in alkalinity, and contains dissolved oxygen, then the ferrous ion reacts with water to form ferrous hydroxide, Fe(OH)2:Fe(OH)2+ 2H2Fe+2+ 2H2O(1-29)The insoluble ferrous hydroxide immediately reacts with C

19、O2present in low- alkalinity water to form soluble ferrous bicarbonate, Fe(HCO3)2:Fe(HCO3)2+ 2H2OFe(OH)2+ 2H2CO3(1-30)Since ferrous bicarbonate is soluble, it detaches from the metal surface and is mixed throughout the water. The dissolved oxygen in the water then reacts with the ferrous bicarbonate

20、 to form insoluble ferric hydroxide, Fe(OH)3:4Fe(HCO3)2+ 10H2O + O2Fe(OH)3+ 8H2CO3(1-31)Because this reaction occurs throughout water, ferric hydroxide does not form a protective coating on the metal surface. The ferric hydroxide appears as sus-pended particles that cause red water.If water begins w

21、ith a higher pH and alkalinity (where CO2is not present), then the corrosion reaction can be controlled. The ferrous ion shown in Equation 1-26 combines with the hydroxyl alkalinity that is present naturally or induced by lime treatment, forming an insoluble film of ferrous hydroxide on the metal su

22、rface:Fe(OH)2Fe+2+ 2(OH)(1-32)If dissolved oxygen is present in the water, it will react with the ferrous hydroxide to form an insoluble ferric hydroxide coating:4Fe(OH)2+ 2H2O + O24Fe(OH)3(1-33)Both ferrous and ferric hydroxide are somewhat porous, and although their coatings retard corrosion, they

23、 cannot fully protect the pipe. However, the same high- pH and high- alkalinity conditions that cause the rust coating to form also favor the formation of a calcium carbonate coating. Together these coatings pro-tect the pipe from further corrosion.Chemical Methods for Scale and Corrosion ControlTab

24、le 1-1 shows common methods used to control scale and corrosion. The fol-lowing paragraphs contain brief discussions of each method. Lime, soda ash, and caustic soda are typically used to raise pH and alkalinity. Carbon dioxide and Table 1-1 Scale and corrosion control methodsMethodFor Control of:Sc

25、ale CorrosionpH and alkalinity adjustment with lime X XChelation XSequestering XControlled calcium carbonate scaling XOther protective chemical coatings XSoftening X12 WSO Water Treatment, Grades 3 sodium pyrophosphate, Na4P2O7; and a group known as bimetallic glassy phosphates. After being fed into

26、 the water, polyphos-phates form a phosphate film on interior metal surfaces, protecting them from corrosion. Polyphosphates are also effective as sequestering agents for preventing calcium carbonate scale and for stabilizing dissolved iron and manganese. Dosages of 510 mg/L are recommended when tre

27、atment is initiated. After 12 months worth of protective film is established, the dosages are reduced and maintained at approximately 1 mg/L.Sodium silicate (Na2Si4O9), or water- glass, can also be used to control corro-sion in water systems. Sodium silicate combines with calcium to form a hard, den

28、se calcium silicate film (CaSiO3). Dosages vary widely depending on water quality.SofteningA major problem caused by water hardness is scale formation. Hard waters form calcium carbonate scales in pipelines and boilers. These scales reduce pipeline capacities, lower boiler heat transfer efficiencies

29、, and cause heat exchange tube failures because excessive heating is required to overcome the insulating effects of scale. The results are higher pumping and maintenance costs, longer repair times, shortened equipment life, and higher fuel and power costs.There are four major water characteristics t

30、hat control hardness. The ten-dency to form scale is greater when the hardness, alkalinity, pH, or temperature is increased. Figures 1-3, 1-4, and 1-5 demonstrate how these characteristics are Figure 1-3 Total alkalinity versus total calcium hardness at 77F (25C)Note: Data were computed for one wate

31、r sample and are not generally applicable.pH = 7.0200000,10 Total calcium hardness, mg/LTotalalkalinity, mg/LHardnessprecipitatesto formscaleEquilibriumlineHardnessremainsin solution Basic Microbiology and Chemistry 15interrelated. The graphs in the figures were determined for one water sample; they

32、 should not be applied in general, but they do illustrate the interaction between the indicated variables.The 10 chemical reactions involved in water softening were presented earlier in the discussion of limesoda ash softening. Refer to that material to review hardness removal using lime and soda as

33、h.Study Questions1. Which disinfectant would work best against Cryptosporidium?a. Ozoneb. Dichloraminec. Hypochlorite iond. Hypochlorous acid2. The pH of a water sample is greater than the pHsso the Langlier index?a. Indicates that the water is corrosive.b. Is negative.c. Indicates the water is scal

34、e forming.d. Indicates the water is stable.3. When water high in calcium bicarbonate is softened by ion exchange _.a. All cations and anions are removed.b. Regeneration is usually accomplished by an acid.c. Lime is used at a pH of 9.5d. Sodium bicarbonate remains in the product water.pH = 7.0pH = 7.

35、5pH = 8.0pH = 8.3200002,10 Total calcium hardness, mg/LTotalalkalinity, mg/LpH = 7.021232 Temperature, F1,2500Solubility of totalcalcium hardness, mg/LFigure 1-4 Effect of total alkalinity and pH on the amount of total calcium hardness that can be kept in solution Note: Data were calculated for one

36、water sample and are not generally applicable.Figure 1-5 Effect of temperature on the amount of calcium that will stay in solution Note: Data were calculated for one water sample and are not generally applicable.16 WSO Water Treatment, Grades 3 & 44. Lime softening can effectively disinfect water be

37、cause:a. Particles containing microorganisms are settled out in the process.b. The pH level is higher than 10.5.c. The pH level is below 10.5.d. Microorganisms are sensitive to calcium level.5. A noncarbonate hardness compound isa. CaCO3b. MgCO3c. Ca(HCO3)2d. MgSO46. What term refers to the reintrod

38、uction of carbon dioxide into water either during or after limesoda ash softening?a. Regenerationb. Recarbonationc. Replenishmentd. Recycling7. _ is the oxidation of unprotected metal surfaces.a. Corrosionb. Saturationc. Softeningd. Chelation8. What is the cause of carbonate hardness?9. Which two io

39、ns are most commonly associated with water hardness?10. For noncarbonate hardness to be removed, what must be added to re-move the noncarbonated calcium compounds?17Chapter 2Advanced Math for Operators sPipeline and Hydraulic CalculationsHeadHead is one of the most important measurements in hydrauli

40、cs. It is used to cal-culate the hydraulic forces acting in a pipeline and to determine a pumps capacity to overcome or pump against these forces.Head is a measurement of the energy possessed by the water at any particular location in the water system. In hydraulics, energy (and therefore head) is e

41、x-pressed in units of foot- pounds per pound:= headft-lblbThese somewhat cumbersome units of measure cancel out so that head can be expressed in feet:= = ft headft-lblbWhen head is expressed in feet, as it normally is, the measurement can al-ways be considered to represent the height of water above

42、some reference ele-vation. The height of water in feet can also be expressed as pressure in pounds per square inch gauge (psig) by dividing the feet- of-water height by 2.31.Types of HeadThere are three types of head, as discussed in detail in the following sections: Pressure head Elevation head Vel

43、ocity headPressure Head Pressure head is a measurement of the amount of energy in wa-ter due to water pressure. As shown in Figure 2-1, it is the height above the pipeline to which water will rise in a piezometer. The water pressure is easily measured by a head(1) A measure of the energy possessed b

44、y water at a given location in the water system, expressed in feet. (2) A measure of the pressure or force exerted by water, expressed in feet.pounds per square inch gauge (psig)Pressure measured by a gauge and expressed in terms of pounds per square inch.pressure headA measurement of the amount of

45、energy in water due to water pressure.18 WSO Water Treatment, Grades 3 & 4pressure gauge, and the resulting pressure reading in pounds per square inch gauge may then be converted to pressure head in feet.Pressure head describes the vertical distance from the point of pressure mea-surement to the hyd

46、raulic grade line (HGL). So, if you were interested in locat-ing the HGL for a particular pipeline, you would take gauge pressure readings at several critical points along the pipeline, convert those pressure readings to feet of pressure head, and then plot the HGL.The normal range of pressure heads

47、 in water transmission and distribution systems can vary from as little as 50 ft (about 20 psig) to over 1,000 ft (about 450 psig). At certain locations within the treatment plant, pressure heads can be very small, perhaps 2 ft (about 1 psig) or less.Elevation Head Elevation head is a measurement of

48、 the amount of energy that water possesses because of its elevation. It is measured as the height in feet from some horizontal reference line or benchmark elevation (such as sea level) to the point of interest in the water system. For example, a reservoir located 500 ft above sea level is said to ha

49、ve an elevation head of 500 ft relative to sea level. The concept is illustrated in Figure 2-2. Elevation head is quite useful in design but has little day- to-day operating significance.Velocity Head Velocity head is a measurement of the amount of energy in water due to its velocity, or motion. The greater the velocity, the greater the en-ergy and, therefore, the greater the velocity head. Anything in mo