Presentation on theme: "Coagulation and Flocculation at Water Treatment Plants"— Presentation transcript:
1 Coagulation and Flocculation at Water Treatment Plants WQT 131Water Works Operation IIIWater TreatmentChapter 4 Coagulation and FlocculationLecture 3
2 Week 3 ObjectivesReading assignment:AWWA Water Treatment, Principles and Practices of Water Supply Operation, Third Edition, American Waterworks Association, ISBNChapter 4 Coagulation and FlocculationUnderstand conventional treatment to remove turbidityUnderstand turbidity and its primary constituentsExplain coagulation chemistryUnderstand mechanism of function involved in coagulant aggregationExplain flocculation chemistryUnderstand mechanism of function involved in flocculent aggregationUnderstand the role of pH, alkalinity, turbidity, temperature on coagulation and flocculation application
3 Key WordsCoagulation: adding and rapid mixing of chemicals to remove particles from water. (flash mixing)Flocculation: adding and slow mixing of chemicals and particles to create flocs that settle out of water.Turbidity: suspended, dissolved, and colloidal particles in pretreated water that need to be removed to optimize treatment efficiency.Suspended Solids: particles held in suspension by the natural action of flowing waters.Colloidal Solids: fine silt that does not settle out of water but remain in suspension.Dissolved Solids: organic or inorganic molecules that are dissolved into the aqueous phase.
4 Which of the following is the main purpose of the coagulation/flocculation process? a. to remove turbidityb. to soften the waterc. to add oxygend. to disinfect.
5 The most important raw water constituent for a surface water plant is: a. temperatureb. hardnessc. turbidityd. pH
6 WQT 131 Water Works Operation III Water Treatment Chapter 4 Coagulation and Flocculation at Water Treatment PlantsWQT 131Water Works Operation IIIWater TreatmentChapter 4“Ironically, it is easier to clean up dirty water than to make clean water cleaner. The reason is because particles must collide before they can stick together to make larger flocs. More particles means more collisions.”
8 Conventional Treatment Conventional Treatment – common treatment steps used to remove turbidity from the initial source water.1. Coagulation2. Flocculation3. Sedimentation4. FiltrationRapid MixingSlow MixingSettlingCleaning
9 Turbidity 1. Suspended Solids 2. Colloidal Solids (~0.1 to 1 mm) Turbidity – particles (sand, silt, clay, bacteria, viruses) in the initial source water that need to be removed to improve treatment.1. Suspended Solids2. Colloidal Solids (~0.1 to 1 mm)3. Dissolved Solids (<0.02 mm)213
10 The turbidity of a water treatment plant effluent cannot be above? 5 ntu1 ntu.5 ntu.3 ntu
11 TurbidityTurbidity/Colloids– negative charged particles particles (sand, silt, clay, organic matter) in the initial source water that need to be removed to improve treatment.
12 Coagulation Coagulants tend to be positively charged. Due to their positive charge, they are attracted to the negative particles in the waterThe combination of positive and negative charge results in a neutral , or lack, of chargeVan der Waal's forces refer to the tendency of particles in nature to attract each other weakly if they have no charge.
13 Settling ForcesZeta Potential refers to the electrostatic potential generated by the accumulation of ions at the surface of the colloidal particle. It can help you understand and control colloidal suspensionsvan der Waals Force- van der Waals forces are weak attractive forces that hold non-polar molecules together. They tends to pull molecules together and forms flocs.Zeta potential analyzervan der Waals in action
14 Stability behavior of the colloid Settling ForcesZeta Potential -is the electrical potential that exists at the "shear plane" of a particle, which is some small distance from its surface. Keeps particles apart and in suspensionZeta Potential [mV]Stability behavior of the colloidfrom 0 to ±5,Rapid coagulation or flocculation
15 Water Treatment Coagulants Particles in water are negative; coagulants usually positively charged.1. Alum- aluminum sulfate2. Ferric chloride or ferrous sulfate3. Polymers
16 What determines the optimum and most cost-effective amount of a coagulant to use?: Beyond that dose, it takes a very large increase in the amount of chemical to produce a small increase in turbidity removalBelow that dose the coagulant results in poor settlingThe treatment plant budgetDivide the number of gallons of water in the coagulation tank by the nephelometric turbidity unit reading to determine the dosage in mg/L.
17 Which is NOT a common method for determining optimum coagulant effectiveness?: Jar testZeta potential detectorStreaming current detectorColorimetric method
18 Water Treatment Coagulant Alum Alum- (aluminum sulfate)- particles suspended in natural, untreated water normally carry a negative electrical charge. These particles are attracted to the positive charges created by aluminum hydroxides. Dosage is generally around 25 mg/L.1. Trivalent Al+3 charge attracts neg – particles2. Forms flocs of aluminum hydroxide (AlOH3).3. Impacted by mixing, alkalinity, turbidity and temp.4. Ideal pH range
19 Alum CHEMISTRYAlum- (aluminum sulfate)- made by dissolving aluminum hydroxide (bauxite or clay) in sulfuric acid2Al(OH)3 + 3H2SO4 + 10H2O → Al2(SO4)3·16H2OWhen ALUM is dissolved in alkaline water, it undergoes hydrolysis (reacts with water) to produce a high surface area gelatinous precipitate of aluminum hydroxide, Al(OH)3 (gibbsite)(Al(OH)3 sticks the negatives.When ALUM is reacted with water it hydrolyzes to form aluminum hydroxide and dilute sulfuric acid (lowers pH) Need alkalinity adjustment
21 Alum MSDS Safety and Handling Alum- (aluminum sulfate)-Health Rating: 2 - Moderate Flammability Rating: 0 - None Reactivity Rating: 1 - Slight Contact Rating: 2 - Moderate Lab Protective Equip: GOGGLES; LAB COAT; VENT HOOD;PROPER GLOVES Storage Color Code: Green (General Storage)Inhalation: Causes irritation to the respiratory tract.Ingestion: Causes nausea, vomiting and diarrhea. There have been two cases of fatal human poisonings from ingestion of 30 grams of alum. Skin Contact: Causes redness, itching, and pain. Eye Contact: Causes irritation, redness, and pain.
22 When alum is added to water, a floc is formed from the combination of alum and a. alkalinityb. acidc. chlorined. lime
23 The precipitate formed by coagulation with alum is aluminum ________. BicarbonateCarbonateHydroxideSulfate
24 Adding Alum to water will cause the pH of the water to increase. TrueFalse
25 Alum comes in dry grade as a minimum of 17 Alum comes in dry grade as a minimum of 17.5% pure product, in liquid form it is 49% pure or 8.23% by weight Al2O3?TrueFalse
26 The coagulation process will most likely improve when: The hardness of the influent increasesThe temperature of the influent decreasesThe temperature of the influent increasesThe alkalinity of the influent decreases
27 Water Treatment Coagulant Iron Salt Iron salt- (Iron chloride or sulfate)- particles suspended in natural, untreated water normally carry a negative electrical charge. These particles are attracted to the positive charges created by Fe(III) salts.1. Trivalent Fe+3 charge attracts neg – particles2. Work over a larger pH range than alum3. Lower costs than alum4. Better removal of natural organics5. Corrosive6. Special handling necessary7. Leave a residue of Fe in water (taste, stains)
28 Which one of the following chemicals would you most likely use as a coagulant?: Cationic polymerSulfuric acidHydrochloric acidSodium hydroxide
29 A coagulant aid is a chemical added during coagulation to improve coagulation; to build stronger, more settleable floc; to overcome the effects of temperature drops; to reduce the amount of coagulant needed, and/or to reduce the amount of sludge produced. Which of the following is not a type of coagulant aid:Activated silicaGreen sandPolyelectrolytes or polymersWeighting agents
30 A microfloc is a colloid that has reacted with a chemical coagulant? TrueFalse
31 In determining the proper dosage of alum, the most useful test is the _______ test: a. marbleb. jarc. carbonated. pH
32 Alum added to turbid water containing alkalinity forms _________, which increase in size and settle out.a. floc particlesb. coagulantsc. coagulant aidsd. aluminum sulfate
33 Overcoming problems of cold-water floc can be corrected by operating the process at the best pH for that water temperature, increasing the coagulant dosage, or:Adding weighting agentsPerforming the jar testIncreasing the number and strength of floc particlesIncreasing the detention time for floc formation
34 Which of the following conditions most affect coagulation performance? a. velocity, chlorine dosage, detention time, and air temperatureb. velocity, water temperature, detention time and coagulant dosagec. water temperature, detention time, air temperature, and chlorine dosaged. detention time, velocity, air temperature, and chlorine dosage
35 With the coming of winter, the water temperature drops With the coming of winter, the water temperature drops. A likely operational problem at a filtration plant with coagulation is:Floc carryover from the sedimentation systemHigh chlorine residualHigh alkalinityOdor
36 Water Treatment Coagulant Aids Activated silica (sodium silicate)- helps improve coagulation, decreases volume of coagulant necessary. Typically is sodium silicate.1. secondary coagulant2. reduces primary coagulants needed3. Sodium silicate are alkaline4. widens pH range for coagulation5. used at 7-11% of alum6. Heavier denser floc that settles faster7. Can be formed on site8. Corrosion inhibitor (forms a surface coating)
37 The three most commonly used coagulants in water treatment are: Aluminum hydroxide, lime and sodium hydroxideAluminum sulfate, ferric chloride, and ferrous sulfateLime, sodium hydroxide, and chlorineSoda, lime and chlorine
38 Water Treatment Coagulant Aids Bentonite (clay)- helps improve coagulation, decreases volume of coagulant necessary.1. high in color, low turbidity, low mineral contentwatermg/L dosage3. Heavier denser floc that settles faster
39 Which of the following would most likely improve the coagulation/flocculation process? a. increase in raw water hardnessb. decrease in water temperaturec. increase in water temperatured. decrease in raw water alkalinity
40 Water Treatment Coagulant Aids Polyelectrolytes- are water-soluble organic polymers that are used as both primary coagulants and coagulant aids. Act as "bridges" between the already formed particles :• Anionic—ionize in solution to form negative sites along the polymer molecule.• Cationic—ionize to form positive sites.• Non-ionic—very slight ionization.effectiveness: particles type, turbidity present, and the turbulence (mixing) available during coagulation.
41 Which one of the following chemicals would be most suitable as a filter aid? a. alumb. soda ashc. sodium hydroxided. anionic polymer
42 A high molecular weight substance that is formed by either a natural or synthetic process. Can have either positive or negative charge.PolymerProteinCarbohydrateEnzymeDeoxyribonucleic acid
43 Water Treatment Coagulant/pH Alkalinity- Alkalinity is a measure of the buffering capacity of water. These buffering materials are primarily the bases bicarbonate (HCO3-), and carbonate (CO32-), and occasionally hydroxide (OH-), borates, silicates, phosphates, ammonium, sulfides, and organic ligands.Chemicals applied to raise alkalinity• Lime—CaOH2 accompanies alum or iron salt• Sodium bicarbonate- NaHCO3- raise alkalinitySoda Ash—Na2CO3 -raise alkalinity• Caustic Soda—NaOH -raise alkalinity
44 Water Treatment WHO Coagulants NameAdvantagesDisadvantagesAluminumSulfate(Alum)Al2(SO4)3.18H2OEasy to handle and apply; most commonly used; produces less sludge than lime; most effective between pH 6.5 and 7.5Adds dissolved solids (salts) to wa-ter; effective over a limited pH range.SodiumAluminateNa2Al2O4Effective in hard waters; small dos-ages usually neededOften used with alum; high cost; ineffective in soft watersPolyaluminum Chloride (PAC)Al13(OH)20(SO4)2.Cl15In some applications, floc formed is more dense and faster settling than alumNot commonly used; little full scale data compared to other aluminum derivativesFerric SulfateFe2(SO4)3Effective between pH 4–6 and 8.8–9.2Adds dissolved solids (salts) to wa-ter; usually need to add alkalinityFerric ChlorideFeCl3.6H2OEffective between pH 4 and 11Adds dissolved solids (salts) to wa-ter; consumes twice as much alka-linity as alumFerrous(Copperas)FeSO4.7H2ONot as pH sensitive as limeLimeCa(OH)2Commonly used; very effective; may not add salts to effluentVery pH dependent; produces large quantities of sludge; overdose can result in poor effluent quality
45 Optimum flocculation requires: a. violent agitationb. gentle agitationc. high pHd. low pH
46 Agglomeration of colloidal and finely divided suspended matter after coagulation by gentle mixing is called what?FlocculationSedimentationPolymer accretionBallasting
47 Which laboratory test is concerned with indicator changes at pH 8 Which laboratory test is concerned with indicator changes at pH 8.3 and about pH 4.5?a. total hardnessb. pHc. alkalinityd. total chlorine residual
48 _________ is the measure of how much acid can be added to a liquid, without causing a great change in pH.AlkalinityHardnesspHAcidity
49 When operating a surface water plant, which laboratory tests are most significant for establishing dosages for coagulation?a. pH and alkalinityb. sulfatesc. calcium and magnesiumd. total hardness
50 A test that is commonly performed to monitor the treatment process is : pHAlkalinityTurbidityAll of the above
51 Tonight's Lecture Objectives: To understand coagulation processes has been met? Strongly AgreeAgreeNeutralDisagreeStrongly Disagree
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