epc_fa2011_lecture_6 - Introduction to Water Treatment...

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Unformatted text preview: Introduction to Water Treatment Processes -- Purpose of Water treatment; a) Protect users from pathogens and other "unhealthy" substances b) Insure a certain quality before industrial or commercial use (degree of water quality depends on Intended Use) The Source The Users the Users (Water treatment) Receiving waterbody (Wastewater treatment) -- Source characteristics (river, reservoir, groundwater, snowmelt) -- Receiving waterbody characteristics (river, reservoir, estuary, ocean) 1 Water Supply Sources -- Public water supplies are consist of lakes and reservoirs, rivers and groundwater -- Large municipalities/utilities prefer lakes/reservoirs for their main water supply sources, because a) Seasonal variation in supply is far less than rivers b) More economical than the groundwater c) Quantity of the available source is generally larger than the groundwater d) Easier to manage/protect/control the water quality due to their enclosures e) Source Isolation 2 Main Objectives of Water Treatment -- With given source characteristics (water or wastewater) and required water quality objectives, objectives of water treatment are to design, build, and operate a cost-effective treatment plant. -- However, the science time to time lags behind practice, and as a result, proven treatment plant designs are normally used with very conservative safety factors built into the design, i.e., to ensure its 'lifesupporting' infrastructure grid capacity. 3 Treatment Process Categories (Unit processes) -- Unit processes (or Unit operations) used in environmental engineering can be classified as physical, chemical, or biological treatments according to their functional principles. -- (In a strict sense,) an Unit process = a chemical or biological treatment an Unit operation = a physical treatment However, the terms unit processes and unit operations are frequently used interchangeably. 4 Common Treatment Processes in Water Treatment Physical process Separation using physical properties Unit Operation Chemical process Chemical reactions Biological process Micro-organisms enhance chemical reactions Unit Process Unit Process •screening •flocculation •settling •sedimentation •gas transfer •coagulation •softening •oxidation •precipitation •disinfection •activated sludge •trickling filters •anaerobic digestion 5 Municipal Water Treatment -- Traditional basic layout of municipal water treatment processes 6 Common types of treatment plant -- 1 -- Depending on type of the source water to be treated: Surface water source (lake/reservoir/river) a) Rapid Sand Filtration plants (RSF) b) Lime-Soda Softening plants (LSS) Groundwater source a) Gas stripping (to remove supersaturated CO2 from groundwater) and Chlorination plants b) Softening plants (lime-soda or ion exchange) 7 Common types of treatment plant – 2 Rapid Sand Filtration (RSF) plants Disinfection (Chlorine solution) Coagulant Mixing Traveling Screen Flocculation Settling Granular Filtration (Rapid Sand Filter) Clear Well Distribution System (High‐pressure Pumps) Raw water Sludge Bar Rack Filter Press Filtrate -- Alum, Ferric salts, or other coagulants 8 Common types of treatment plant – 3 Lime-Soda Softening (LSS) plants Slaked Lime Ca(OH)2 Traveling Screen Disinfection (Chlorine solution) Soda Ash Na2CO3 Mixing Granular Filtration Mixing Flocculation Settling Recarbonation (Rapid Sand Filter) Raw water Sludge Bar Rack CO2 Centrifuge Centrate Distribution System (High‐pressure Pumps) Cake -- Ca and Mg ions are precipitated as CaCO3 and Mg(OH)2 -- Recarbonation (CO2) lowers pH and stabilize water Clear Well 9 Common types of treatment plant – 4 Groundwater Treatment plants CO2 and H2S Cascade Step Aerator Disinfection (Chlorine solution) Clear Well or Ground Storage Distribution System (High‐pressure Pumps) Well Piezometric Water Level -- Gas stripping to remove supersaturated CO2 from groundwater 10 Classification of Municipal Water Uses -- 1 -- Municipal water uses can be classified into following five categories: 1) 2) 3) 4) 5) Domestic Use Industrial Use Commercial Use Public Use Loss (Unaccounted for) such as leakage -- Factors affecting average daily use are expressed as gallons per capita per day (gpcd) or liters per capita/day (lpcd). 11 Classification of Municipal Water Uses -- 2 1) Domestic Use -- For households, hotels, sanitary and lawn-sprinkling purposes. -- Average daily use, on whole year basis, is approximately 80 gpcd or 300 liters per capita/day 12 Classification of Municipal Water Uses -- 2 2) Industrial Use -- Averaging 40 gpcd nationally (4 to 8 L/m3 - day) -- General purposes -100 -200 gpd/1000 ft2 of floor space -- Process water - highly variable depending on the type of industry using the water. -- Many industries have on-site water treatment facilities and do not use municipal water supply for their manufacturing processes. In such case, industry water facilities are not subjected to comply with standard/strict design requirements/considerations applicable to a water treatment plant. 13 Classification of Municipal Water Uses -- 3 3) Commercial Use -- Averaging 15 gpcd nationally -- General use is 244 to 367 gpd/100 ft2 (10 to 15 L/m2-day) of floor space -- Mostly used in air conditioning and cooling. 4) Public Use -- Averaging about 16 gpcd (60 liters per capita per day) for street and sewer flushing, hydrant testing, fire fighting, park watering, schools and public swimming pools, etc. -- This is the portion of water which is normally not charged. 14 Classification of Municipal Water Uses -- 4 5) Loss (Unaccounted for) such as leakage -- Averaging about 10 gpcd. In the sense that it is not assigned to a specific user, and due to leakage from failing infrastructure, defect in meterage and unauthorized connections. -- Leakage in water pipes - 1,500 to 15,000 gpd/mile, averaged 5,000 gpd/mile 15 Factors Affecting Water Use -- Size of city, industry and commerce -- Characteristics of the population - by economic levels - high-value districts consume more (>>80 gpcd) - low-value districts consume less (~25 gpcd) -- Metering - shown to reduce overall consumption by 50% if imposed appropriately! Incentive for water conservation employed by many municipalities. -- Climate: during the cold weather season, consumption increases to prevent freezing of distribution systems. -- Quality: poor water quality typically results in less consumption. -- Pressure: high pressure systems result in greater use, also increases losses from leaks. 16 Variations in Water Use -- Water consumption varies during the day (lower at night), from day to day during the week and so on. -- Peak consumption usually occurs between 7-9 AM and 5-7 PM. -- Flow measurements at the pump station are important in evaluating variations in demand. -- If there is no data, one should use the maximum rates for design. The maximum daily consumption is likely to be 180 percent of the annual average and may reach 200% depending on seasonal variation. 17 Water Treatment Targets -- 1 Organic Contaminants a) Chlorinated Hydrocarbons (CHC) Organochlorine pesticides such as lindane and DDT, industrial chemicals such as polychlorinated biphenyls (PCB), and chlorine waste products such as dioxins. Resistant to biological degradation by phenols, tannic and lignin acid, persistent in the environment and most likely to bioaccumulate in the food chain. Carcinogenic, controlled by established MCL b) Volatile Organic Chemicals (VOCs) VOC's are a group of commonly used chemicals that evaporate or volatilize when exposed to air. Since they dissolve many other substances, VOC's are widely used as cleaning and liquefying agents in fuels, degreasers, solvents, polishes, cosmetics, drugs, and dry cleaning solutions such as benzene, carbon tetrachloride and trichloroethylene, vinyl chloride, etc. Carcinogenic, controlled by established MCL 18 Water Treatment Targets -- 2 Inorganic Contaminants -- Inorganic contaminants that contain ions and affect water quality including changes in pH of the water and toxicity caused by metals. -- Inorganic contaminants include both suspended and dissolved materials. -- Dissolved inorganics that have health effects include aluminum (Al), arsenic (As), barium (Ba), cadmium (Cd), chromium (Cr), fluoride (F-), lead (Pb), mercury (Hg), nitrate (NO3-), selenium (Se), and silver (Ag). 19 Water Treatment Targets -- 3 Hardness -- Cause scaling and soap consuming capacity of water. -- Caused by Fe2+, Mn2+, Sr2+ and Al3+. Usually represented by sum of Ca2+ and Mg2+ ions, and as mg/L of CaCO3. Ca2+ + HCO3(Heat) CaCO3(s) + CO2 + H2O (Heat) Mg(OH)2(s) + 2CO2 Mg2+ + 2HCO3-- Water can be classified as; 20 Quick Recap -- 1 1) Objectives of water treatment are to design, build, and operate a cost-effective treatment plant. 2) Unit processes (Chemical & Biological) vs. Unit operations (Physical) 3) Municipal water treatment processes are primarily in form of either Rapid Sand Filtration (RSF) or Lime-Soda Softening (LSS) plants in the East coast. 4) Daily use are expressed as gallons per capita per day (gpcd) 5) Hardness of the water is an important water treatment target 21 Coagulation -- 1 -- Coagulation is the first step (or first unit process) in typical treatment process in water treatment plant (WTP). -- Purpose of coagulation is to chemically aggregate and subsequently remove by settling those waste material in suspended or colloidal form that would not settle without treatment, or to increase the settling velocity of already settleable solids. -- Coagulation" is done by treating water or wastewater with alum, ferric salts, or other coagulants to destroy the stability of colloids. Chemical and Electrical processes 22 Coagulation – 2 Colloidal Stability & Van der Waals Force -- Colloidal particle sizes range between 1x10-9 and 1x10-6 meter (=1x10-3 to 1 µm), generally will not settle out of solution. -- Colloidal suspensions that do not agglomerate naturally are called stable. (= stable electrostatic repulsion) -- If colloids can be brought close together sufficiently, they will conjoin to form larger particles due to the Van der Waals forces (VAN-der-VALS; i.e., attracting force in a *very* close range). Gecko!! 23 Coagulation – 3 Zeta Potential & Van der Waals Force -- Zeta potential is a measure both of the charge on a colloidal particle and of the distance into the solution to which the effect of the charge extends. -- Lowering Zeta potential is the key to the destabilization of colloidal particles, i.e., a successful coagulation!! 24 Coagulation – 4 Coagulants - I -- Key properties of a coagulant include; - Trivalent cations: Al3+, Fe3+ - Non-toxic - Insoluble in the neutral pH range (precipitate and no ion residual) -- Most common coagulants are; - Alum (Aluminum Sulfate) -- Al2(SO4) - Ferric salts - Ferric sulfate (Fe2(SO4)3) and Ferric chloride (FeCl3) - Ferrous Sulfate (Copperas) -- FeSO4 25 Coagulation – 5 Coagulants - II Alum (Aluminum Sulfate) -- Al2(SO4) -- Most commonly-used coagulant in water treatment -- Will form flocs in form of Aluminum hydroxide, Al(OH)3 (s) and consequently pH will be lowered -- Lime (=Calcium hydroxide, Ca(OH)2) or soda ash (=Sodium carbonate, Na2CO3) are then added for acid neutralization 26 Coagulation – 6 Coagulants - III Ferric salts - Ferric sulfate (Fe2(SO4)3) and Ferric chloride (FeCl3) -- Predominantly used in wastewater treatment -- Will form flocs in form of Ferric hydroxide, Fe(OH)3 (s) -- Effective in removing color and odor components -- More difficult to handle due to its corrosiveness 27 Coagulation – 6 Coagulants - IV Ferrous Sulfate (Copperas) -- FeSO4 -- Used in water treatment -- In forms of green granular or crystalline or hydrated -- Will form flocs in form of Aluminum hydroxide, Al(OH)3 (s) and consequently pH will be lowered -- Lime (=Calcium hydroxide, Ca(OH)2) is generally added to raise pH -- Much cheaper than alum coagulation, but dosing operation with two chemicals is more difficult 28 Determination of Appropriate Coagulant Dosage -- Jar Test is used to experimentally determine the optimal dose of coagulant, the optimal pH for coagulation and concentration of coagulant aid. -- Even a very-well maintained water treatment plant performs jar tests in daily basis since quality and characteristic of the raw source water are slightly different seasonally and after any hydrologic event such as a big rainfall. 29 Flocculation -- 1 -- Following the coagulation, flocculation is to physically enhance the formation of LARGER clumps or flocs of the destabilized colloids. -- Aided by mild agitation for a period of 20 to 30 minutes, to allow time for maximum floc formation and growth. The agitation should be gentle, in order not to break up flocs already formed. Flocculators with mechanical paddle-type agitators 37th St. WTP City of Norfolk 30 Flocculation -- 2 -- Flow velocities through the flocculator tank should be greater than 3 to 5 ft/min to prevent sedimentation, but should not be so rapid as to cause turbulence (that will break flocs apart!). 31 Flocculation -- 3 -- New plants have rapid mixing (=dispersion of coagulant) flocculation and sedimentation in a single unit. This rapid mixing unit is typically called 'secondary settling tank' or 'secondary clarifier.' 32 Flocculation – 4 Secondary Settling Tank/Clarifier HRSD VIP BNR (Biological Nutrient Removal) Wastewater treatment plant, City of Norfolk 33 Rapid Mix Tank/Flocculator Basin Design -- 1 1) Calculate Volume with V = Q*θ, where θ=θH=V/Q 2) If you are designing “n” tanks of the same dimension, divide the Volume with “n” to find a design Volume of a tank 3) With a design Volume, find the DIMENSION of a tank, Area x Height - Rectangular tank = Width x Length x Height - Circular tank = Surface Area x Height 4) With the surface area of the tank, calculate Equivalent Tank Diameter, T or TE to do a "design sanity check" ⎛ 4 ⋅ Area ⎞ TE = ⎜ ⎟ ⎝π ⎠ 0. 5 34 Rapid Mix Tank/Flocculator Basin Design -- 2 5) Calc. Required input power, P (in Watts) using G= W μ = P μV P = G 2⋅ μ ⋅ V (you may have to interpolate the dynamic viscosity value for the water temperature used in your design) 6) Do "design sanity check" and find the right impeller diameter for your design 7) Calc. Rotational speed for impeller, n (rps) with available diameters values, Di and Required input power, P, and Power number (=impeller constant) Np 3 5 P = NP ⋅ n ⋅ Di ⋅ρ ⎛ ⎞1 P ⎜ ⎟3 n =⎜ NP ⋅ Di 5 ⋅ ρ ⎟ ⎝ ⎠ 35 Rapid Mix Tank/Flocculator Basin Design -- 3 8) Finally, most importantly, neatly summarize all our your calculated design parameters for the rapid mixer/flocculator. - Number of Tanks - Power Input, P (kW) - Tank dimensions: Area = Width & Length Height and T; H; B - Type of impeller -- Radial or Axial - Diameter of impeller, Di - Rotational speed of impeller, n (rpm) 36 Rapid Mix Tank/Flocculator Basin Design Design Sanity Check – Would It Fit/Do Harm?? Radial-flow impellers Axial-flow impellers ** Tangential velocity (=blade tip speed) should not exceed 2.7 m/sec – else flocs will start breaking up! Blade tip speed = n (rps) * π * Di (m) T(=TE) = Equivalent tank diameter 37 (calculated with area) Water Softening -- Softening is the process removing the hardness (divalent metallic Ca2+ and Mg2+ cation concentration) from water. -- Minimize the scale forming tendency and mineral deposit problems. -- Reduce corrosion [in water distribution system, plumbing, etc.] caused by hard water. 38 Hardness -- 1 -- Total hardness = Ca2+ and Mg2+ -- Figuratively speaking, the hardness of water is a measure of soap’s water-consuming power. For example, the "harder" the water, the “greater amount of soap” is required to produce a lather or foam. -- Because, Ca2++ (Soap) Ca(Soap)2 (s) combination, soap cannot interact with the dirt on clothing. 39 Hardness -- 2 -- Level of the hardness in water can be classified as: -- Water treatment process removes hardness if it is greater than 150 mg/L CaCO3 -- Desired range of the hardness in treated water is 75 to 120 mg/L CaCO3 (Moderately hard). 40 Problems Associated with Hard Water -- 1 -- Objectable taste (above 200 mg/L). -- [hard magnesium silicate] Scale formation in water distribution system and plumbing, resulting in low flow conditions due to the reduced cross-sectional area. 41 Problems Associated with Hard Water -- 2 -- Increased fuel consumption due to poor efficiency of heat exchangers, i.e., heating coils in a boiler are all scaled up (both interior and exterior of the coil pipes). -- To remove such scales, you have to disassemble the boiler and repeat expensive acid bathes (for large-scale boiler units), or throw old water heater/boiler away and buy new one (for smaller/home units). 42 Problems Associated with Hard Water -- 3 -- Blocks shower heads and taps!!! (i.e., low flow problems) -- Itchy dry skin and hair. -- Scales up washing machines, dishwashers, kettles and other appliances. -- Increased soap/detergent requirement. Decreased fabric life. -- Some public water treatment plants add hardness if it is below 25 mg/L CaCO3 for improving taste of the water. 43 Source of Hardness -- 1 -- Hardness is often found in groundwater due to the dissolution of limestone (= CaCO3/MgCO3) caused by rain water that enters the top soil and percolates to the aquifer. -- At top soil level, zone of bacterial reduction of OM (produces) CO2 -- At the subsoil level, CO2 dissolved into water to form Carbonic acid (=H2CO3) CaCO3 + H2O H2CO3 Carbonic acid further on producing Ca(HCO3)2 (=Calcium bicarbonate) -- Limestone is after all CaCO3 and MgCO3 that are the main source of hardness CaCO3 + H2CO3 MgCO3 + H2CO3 Ca(HCO3)2 Mg(HCO3)2 44 Source of Hardness -- 2 -- Following ions are causes of hardness (listed from the top in the order of usual abundance) * Limited solubility at pH of natural waters 45 Types of Hardness -- 1 1) With respect to the metallic ions (cations) -- Ca2+ or Mg2+ (Magnesium Hardness + Calcium Hardness) -- Used to determine Lime and Soda ash dosages for softening process - Lime: CaO or Ca(OH)2 - Soda ash: Na2CO3 -- Lime and Soda ash are added to precipitate the calcium as CaCO3 and magnesium as Mg(OH)2 46 Types of Hardness -- 2 2) With respect to Anions associated with metallic ions a) Carbonate hardness (= temporary hardness, CH) - caused by bicarbonate, HCO3- may be partially removed as a precipitate by boiling - can be precipitated with Lime (=CaO or Ca(OH)2) Ca(HCO 3 ) 2 + Ca(OH) 2 → 2CaCO 3 ↓ + 2H 2 O Ca 2 + + 2 HCO 3 - + Ca(OH) 2 → 2CaCO 3 ↓ + 2 H 2 O 47 Types of Hardness -- 3 2) With respect to Anions associated with metallic ions b) Non-Carbonate Hardness (= permanent hardness, NCH) - caused by sulfates, sodium, chlorides, nitrates, etc. - cannot be removed by boiling - can be precipitated with Soda ash (= Na2CO3) CaSO 4 + Na 2 CO 3 → CaCO 3 ↓ + Na 2 SO 4 (soluble) 48 Types of Hardness -- 4 -- TH (Total hardness) = CH + NCH where TH = Ca + Mg (mg/L as CaCO3) CH = HCO3- (mg/L as CaCO3) NCH = TH - CH -- If CH ≥ TH, CH = TH Example) If TH = 100 and CH = 145 mg/L as CaCO3 (from HCO3 conc.), CH = 145 ≥ 100 = TH then CH becomes 100 mg/L as CaCO3 instead of 145 mg/L as CaCO3. 49 Types of Hardness -- Example 1) Since TH = Ca + Mg (mg/L as CaCO3) mg 50 = 242 as CaCO 3 40/2 L mg mg 50 Mg : 30.4 × = 126.7 as CaCO 3 L 24/2 L Ca : 96 mg L × ∴ TH = 242 + 126.7 = 368.7 mg/L as CaCO 3 2) CH (mg/L as CaCO3) with HCO3 CH : HCO 3 = 318 mg 50 mg × = 260.7 as CaCO 3 L 61 L 3) NCH = TH - CH = 368.7 - 260.7 = 108 mg/L as CaCO3 50 Lime/Soda Softening Reaction -- 1 -- Unit Process Components 1) Lime: Regulate the pH (the pH of water is raised) 2) Lime: Precipitate CH caused by Ca (Calcium) find HCO3 3) Lime: Precipitate CH caused by Mg (Magnesium) 4) Lime: Put excess lime to drive the reaction “flowchart” find CO2 find Mg2+ use the 5) Soda ash: Remove NCH caused by Mg (Magnesium) find Mg2+ 51 Lime/Soda Softening Reaction -- 2 52 Lime/Soda Dosage Determination -- 1 1) If conc. are given anything other than 'mg/L as CaCO3,' convert to it first mg/L as CaCO3 = (mg/L as species) x Molarity, M (mole/L) = Normality, N (eq/L) = Milliequivalent, meq. (meq/L) = ⎛ MW ⎞ CaCO 3 ⎜ 50 CaCO 3 ⎟ ⎜n ⎟ = ⎜ MW EW Species ⎟ Species ⎜ ⎟ ⎝n ⎠ mg L × 1g mole × g (MW) 1000 mg mg 1g eq mole × × × L g (MW) 1000 mg mole mg 1g eq 1000 meq mole × × × × L g (MW) 1000 mg mole 1 eq 53 Lime/Soda Dosage Determination -- 2 2) Find Total Hardness, TH = (Ca + Mg) mg/L as CaCO3 3) Find Carbonate Hardness, CH = HCO3- mg/L as CaCO3 4) If CH ≥ TH, CH = TH 5) Find "initial" Non-Carbonate Hardness, NCHi NCHi = (TH - CH) mg/L as CaCO3 6) Start calculating Lime dosage a) Add Lime of equal amount to CO2 b) Add Lime of equal amount to HCO3c) Add Lime for Mg using following 'flowchart' (instead of using one given in the course reference book) 54 Lime/Soda Dosage Determination -- 3 7) Calculate Soda dosage Mg Mg Mg < 40 mg/L Mg ≥ 40 mg/L Add (Mg - 40) mg/L = Δ Mg a) Find "final" Non-Carbonate Hardness, NCHf NCHf = Desired final hardness - 40 Just Add 20 mg/L!! (as excess) ('40' represents the target Mg conc. for preventing hard magnesium silicate scale) If ΔMg < 20 mg/L, Add 20 mg/L, again If 20 ≤ Δ Mg ≤ 40, Add Δ Mg mg/L , again If Δ Mg > 40 mg/L, Add 40 mg/L , again b) find 'Removed' Non-Carbonate Hardness, NCHR NCHR = NCHi - NCHf = Soda dosage (If NCHR ≤ 0, no Soda is added) 8) All done -- make it sure to summarize total Lime and Soda dosages clearly. 55 Lime/Soda Dosage Determination Chart Mg Mg Mg < 40 mg/L Mg ≥ 40 mg/L Add (Mg - 40) mg/L = Δ Mg Just Add 20 mg/L!! (as excess) If ΔMg < 20 mg/L, Add 20 mg/L, again If 20 ≤ Δ Mg ≤ 40, Add Δ Mg mg/L , again If Δ Mg > 40 mg/L, Add 40 mg/L , again 1 ...
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This note was uploaded on 10/19/2011 for the course CEE 350 taught by Professor Jaewanyoon during the Fall '10 term at Old Dominion.

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