Coagulation and Flocculation: Color Removal

Transcription

1 Coagulation and Flocculation: Color Removal Submitted to: Dr. Hashsham Research Complex Engineering Department of Civil and Environmental Engineering Michigan State University East Lansing, MI Authors Dipa Dey Amanda Herzog Vidya Srinivasan ENE 806 March 14, 2007 i

2 Acknowledgement First of all, we would like to thank Professor Syed A. Hashsham who instructed this course and gave us useful advices for our project. Professor Hashsham has given us plenty of guidance, which was greatly helpful for the successful completion of this study. Finally we are thankful to Joseph Nguyen for his help, valuable information and advices during this study. May 2, 2007 Dipa Dey Amanda Herzog Vidya Srinivasan ii

3 CONTENTS Caption Title page Acknowledgement Contents List of Figures and Tables Page No. i ii iii iv Abstract 1 Introduction 2 Objective 4 Material and methods 4 Results and discussion 5 Conclusions 10 References 11 Appendix Appendix-2 14 Experimental setup 14 Appendix-3 15 Appendix-4 17 Coagulation and Flocculation: Color Removal: Lab Protocol 17 Procedure to determine Alkalinity 19 iii

4 LIST OF FIGURES AND TABLES Caption Page No. Figure 1: Concentration vs Absorbance for Humic Acid (254 nm wavelength) 5 Figure 2: Effect of Alum Dose on Supernatant Turbidity, ph and Remaining Humic Acid 6 Figure3: Effect of Initial ph on Humic Acid and Turbidity Removal 7 Figure 4: Effect of Initial ph on Humic acid and Turbidity Removal (Spiked Sample) 8 Figure5: Effect of Initial ph on Humic acid and turbidity Removal (Non-Spiked Sample) 9 Figure A: Removal of TOC by alum coagulation 12 Figure B: Alum Coagulation diagram and its relation to Zeta Potential 12 Figure C: Color removal domains on Alum coagulation 13 Conventional Jar Test Setup 14 Formation of Flocs 14 Settled Particles 14 Table 1: Trial Run 15 Table2: Run1 15 Table 3: Run 3 16 Table 4: Run 3 and Run 4 16 Table 5: Different Jars with Different Alum Dosages (Spiked) 17 Table 6: Different Jars with Different Alum Dosages (Non Spiked) 17 iv

5 ABSTRACT Water is expected to be free from pathogenic organisms, undesirable taste and odor. In addition it should be clear and free from minerals before supplied as a drinking water. Jar test experiments on coagulation and flocculation were conducted to remove color from water caused by humic substances. These tests help to obtain an optimum alum dosage required for color removal. Water sample were collected from Red Cedar River and spiked with humic acid to a concentration of 35mg/L. Alum varying from mg/l was used as a coagulating agent and ph ranged from was studied to obtain the optimum conditions in terms of removal of humic acid, turbidity and settling of flocs. Results were obtained after doing four runs and compared to similar published studies. An approximate 70% removal of color was achieved with an alum dosage of 140 mg/l at ph 6. When reducing cost was considered the optimal conditions are an alum dosage of 100 mg/l at ph 6.5 which results with a 60% removal. 1

6 INTRODUCTION An ideal waterworks management ensures that the water supplied for public distribution is free from pathogenic organisms, undesirable taste and odor, clear, palatable, of reasonable temperature, neither corrosive nor scale forming and free from minerals which could produce undesirable physiological effects. Potable water does not mean pure water but water that is free from harmful impurities, harmful salts and harmful bacteria. In fact water may contain certain salts and minerals in limited amounts as they impart good taste and assist in digestion. Therefore, the water should be tested before it is distributed. The aim of laboratory test is to ensure that potable water conforming to standards is supplied to consumers. In the present study, Coagulation and Flocculation experiments or Jar Tests were conducted to remove color from water due to humic substances using alum as the coagulating agent. These tests help in determining the optimum alum dosage and ph required for color removal. Chemicals used for coagulation in water treatment should be cost-effective for coagulating impurities and to help remove any traces of toxic or other undesirable residues in water. The jar test is a common laboratory procedure used to determine the optimum operating conditions for water or wastewater treatment. This method allows adjustments in ph, variations in coagulant or polymer dose, alternating mixing speeds, or testing of different coagulant or polymer types, on a small scale in order to predict the functioning of a large scale treatment operation. A jar test simulates the coagulation and flocculation processes that encourage the removal of suspended colloids and organic matter which can lead to turbidity, odor and taste problems. Testing is done for the reasons such as, correct chemical dosage and ph adjustments, sludge characteristics, turbidity removal, color removal, THM (trihalomethanes) removal, and control chemical costs. In water and wastewater treatment operations, the processes of coagulation and flocculation are employed to separate suspended solids from water. Although the terms coagulation and flocculation are often used interchangeably, or the single term "flocculation" is used to describe both; they are, in fact, two distinct processes. Coagulation and flocculation processes are used to improve removal of solids in both drinking water and wastewater treatment, through destabilization and aggregation of suspended material. Floc formation and electric charges are the basis of coagulation. When a coagulant is added to the water and mixed thoroughly, a thick gelatinous precipitate is formed which is insoluble in water. 2

7 This precipitate is called as floc. As the floc settles down, it coalesces with the colloidal particles. Also, the floc is positively charged and the colloidal particles are negatively charged. The flocs thus attract the colloidal particles and hence make them settle down. Flocculation is the action of coagulant or polymers to form bridges between the flocs and bind the particles into large agglomerates or clumps. Bridging occurs when segments of the polymer chain adsorb on different particles and help particles aggregate. An anionic flocculants reacts with a positively charged suspension, thus adsorbing on the particle and hence causing destabilization either by bridging or charge neutralization. In this process it is essential that the flocculating agent be added by slow and gentle mixing to allow for contact between the small flocs and to agglomerate them into larger particles. The newly formed agglomerated particles are quite fragile and can be broken apart by shear forces during mixing. Care must also be taken to avoid overdosing the polymer as doing so causes settling/clarification problems. Anionic polymers are lighter than water, hence, increasing the dosage will increase the tendency of the floc to float and not settle. Once suspended particles are flocculated into larger particles, they can usually be removed from the liquid by sedimentation, provided that a sufficient density difference exists between the suspended matter and the liquid. Such particles can also be removed or separated by media filtration, straining or floatation. When a filtering process is used, the addition of a flocculants may not be required since the particles formed by the coagulation reaction may be of sufficient size to allow removal. The flocculation reaction not only increases the size of the floc particles, but also affects the physical nature of the floc, making these particles less gelatinous and thereby easier to dewater. 3

8 OBJECTIVE The objective of the jar test experiments is to determine the optimal conditions for removal of color due to humic substances using aluminum sulfate octadecahydrate as the coagulant agent. MATERIAL AND METHODS Surface water was obtained from the Red Cedar River, East Lansing, which had a concentration of humic acid of approximately 25 mg/l and an alkalinity of 219 mg/l as CaCO 3. An additional 10 mg/l of humic acid was spiked in the Red Cedar River samples to adjust the humic acid to approximately 35 mg/l or a 70% absorbance. The coagulant a was prepared as a stock solution of 10 mg/ml aluminum sulfate octadecahydrate (Al 2 (SO 4 ) 3 18H 2 O). The jar test was executed with the conventional apparatus with twelve 2-liter beakers (Appendix-2). Each run was mixed for 160 rpm for 1 min, 70 rpm for 15 min, 30 rpm for 35 min and settled for 2 hours. The absorbance was measured by the Spectronic Genesys 2 spectrophotometer at 254 nm using 1 cm quartz cells. A standard curve was generated using tap water and a humic acid range of 0-40 mg/l to evaluate the relationship of concentration vs. absorbance (Fig 1.). The turbidity was measured using the Turbidimeter 2100A. 4

9 RESULTS AND DISCUSSION A total of 4 jar test experiments were conducted by varying parameters such as ph and alum dosage. The main objectives of the experiments were to determine the optimum alum dosage and ph for efficient color and turbidity removal. Changes in ph, turbidity and color were noted for each run. To enhance the absorbance to 70% which was determined using the standard curve (Fig 1), the Red Cedar River sample was spiked with humic acid to a concentration of 35mg/L. The first run determined the optimum alum dosage required for color removal, by varying the alum dosage and fixing the initial ph of the sample. Alum dosages varying from mg/l were chosen. The effects of the remaining absorbance and turbidity, including final ph were compared in figure 2. The percentage of humic acid remaining in the sample decreased with an increase in alum dosage. The highest alum dosage of 220 mg/l resulted in about 76% removal of humic acid (Fig 2). These results were compared with results from previous studies (Appendix-1, Fig A) and were in significant correlation. Turbidity showed a gradual decrease with increase in alum dosage which for alum dosage of 220 mg/l resulted in approximately 86% removal. 40 Humic Acid Concentration, mg/l Absorbance 5

10 Figure 1: Concentration vs Absorbance for Humic Acid (254 nm wavelength) A substantial decrease in humic acid content was observed beyond an alum dosage of 30 mg/l (Fig 2). Alum dosages higher than 100 mg/l did not result in a significant decrease in turbidity or absorbance, which was in accordance with a previous study carried out with different alum dosages and color causing components (Appendix-1, Fig A) Measured ph and Turbidity (NTU) % Absorbance ph Remaining Turbidity, NTU % Absorbance Alum Dosage, mg/l Figure 2: Effect of Alum Dose on Supernatant Turbidity, ph and Remaining Humic Acid The alkalinity of the river water sample was measured to be 219 mg/l as CaCO3. If the water is poorly buffered, any addition of alum results in a drop in ph. In our study, addition of 220 mg/l of alum resulted in a drop of approximately 1 ph unit. 6

11 Initial ph Measured ph or Turbidity (NTU) Final ph % Absorbance Remaining Turbidity, NTU % Absorbance Initial Adjusted ph Figure3: Effect of Initial ph on Humic Acid and Turbidity Removal The second run determined the optimum ph for color removal by varying the initial ph of the sample and fixing the alum dosage. Minor changes in turbidity and absorbance removal were observed for alum dosages higher than 140 mg/l, therefore, 140mg/L was selected as the optimum alum dosage. A gradual decrease in color and turbidity removal was observed between ph 4 and 6. Beyond a ph of 6, there was minimal change in turbidity and absorbance. A dip in the final ph curve (between 6 and 6.5) shows that there exists an optimum ph for color removal. At a ph of 6, the color removal was about 72%. Any increase in ph beyond 6.5, did not result in significant color removal. From Fig B (Appendix-1), it is evident that our study followed the trend as depicted in the charge neutralization zone and did not show any significant color removal in the sweep coagulation zone. Figure C (Appendix-1) shows that about 40% removal for 100cu was obtained for a ph of around 6. 7

12 The third and fourth runs were with spiked and non spiked samples respectively. Spiked and non spiked samples were used to compare the effects of the optimum alum dosage and ph on the addition of humic acid and the natural occurring humic substances. The main objective of run four and five was to further optimize the experiment, where the alum dosage is reduced and the initial ph is adjusted closer to neutrality. This would assist in reducing the cost of alum required and the time in adjusting the ph to 7 after treatment. The alum dosage was reduced to 100 mg/l and the initial ph values used ranged from 5 to 7.5 for each jar respectively. Such a set up allowed us to verify if effective color and turbidity removal can be obtained by using a lower alum dosage and a ph closer to 7. There was a steep decrease observed in turbidity and color removal in Fig 4, for ph values ranging from 5 to 6, while the change is much more gradual after a ph of 6. Therefore ph 6 is the optimum for an alum dosage of 100mg/L. Measured ph or Turbidity (NTU) Alum Dose = 100 mg/l Initial ph Final ph % Absorbance Remaining Turbidity, NTU % Absorbance Initial adjusted ph 0.00 Figure 4: Effect of Initial ph on Humic acid and Turbidity Removal (Spiked Sample) 8

13 10 Alum Dose = 100 mg/l Measured ph or Turbidity (NTU) Initial ph Final ph % Absorbance Remaining Turbidity, NTU % Absorbance Initial Adjusted ph Figure5: Effect of Initial ph on Humic acid and turbidity Removal (Non-Spiked Sample) The non spiked samples reacted similarly to the spiked samples (Fig 5) resulting in a ph of 5.7 to be an optimum ph for effective color and turbidity removal. An alum dosage of 100 mg/l at a ph between 5.5 and 6 is the optimal conditions for effective color and turbidity removal. 9

14 CONCLUSIONS This experiment yielded the following conclusions: At alum dosage of 140 mg/l the color is removed by 70%; concentrations higher than 140mg/L had little significance in color removal after this point. A maximum of 70% of color removal was achieved at an initial ph of 6. Approximately 40% in color reductions occurred below or above ph 6. When cost is considered the optimal conditions for color removal is an alum dosage of 100 mg/l at ph 6. 10

15 REFERENCES 1. American Water Works Association. Water Quality and Treatment: A Handbook of Community Water Supplies, 4 th Ed McGraw-Hill 2. Syed Hashsham. Coagulation and Flocculation: Color Removal, Lab Report flocculati.html

16 APPENDIX-1 Figure A: Removal of TOC by alum coagulation 12

17 Figure B: Alum Coagulation diagram and its relation to Zeta Potential Figure C: Color removal domains on Alum coagulation 13

18 APPENDIX-2 Experimental setup Conventional Jar Test Setup Formation of Flocs Settled Particles 14

19 APPENDIX-3 Table 1: Trial Run Trial Run Raw Water Raw Water Spiked w/ Humic Acid ph Turbidity (NTU) Temperature ( o C) Alkalinity Color Initial fixed ph N/A 7.5 After Settling Spiked Sample (+ alum dosage) ph Temperature Turbidity Absorbance Table2: Run1 Run 1 Raw Water ph 8.06 Turbidity 3.1 Color Alkalinity 17.8 Temp 23.3 After Setteling the samples Alum Doses Turbidity ph Absorbance Alkalinity(ml) Background=

20 Table 3: Run 3 Run 2 Alum Dosage: 140 mg/l Initial ph Initial ph Final ph Turbidity Absorbance Table 4: Run 3 and Run 4 Run 3 and 4 alum dosage = 100 mg/l WITHSPIKED Initial adjusted ph Turbidity ph Absorbance WITHOUTSPIKED Initial adjusted ph Turbidity ph Absorbance

21 APPENDIX-4 Coagulation and Flocculation: Color Removal: Lab Protocol Standard Curve: 1. Using the spectrophotometer find the absorbance of tap water (to calculate background) 2. Spike 1L of tap water with 1 mg of Humic acid (salt) and then use the spectrophotometer to find the absorbance. 3. Continue spiking tap water for concentration 5, 10, 15, etc (in increments of 5) until the absorbance reaches a reading of approximately 95%. 4. Minus the background (step 1); plot Humic acid concentration (mg/l) vs Absorbance Run 1: (spiked) 1. Measure the initial ph, temperature, turbidity and alkalinity (refer to the procedure at the end of the handout) of the river water sample (Red Cedar) 2. Spike river water sample with Humic acid to an absorbance to 70%; Note: concentration of Humic acid to be added can be determined from the standard curve plot. 3. Measure ph and adjust to Make up Alum Dosages with distilled water: Table 5: Different Jars with Different Alum Dosages (Spiked) Error! Not a valid link. 5. Add the alum dosage to 1L of river water for each jar. 6. Mix at 100+ RPM for 1 min 7. Mix at 70 RPM for 15 min 8. Mix at 30 RPM for 35min 9. Let settle for 30 min. 10. After settling measure supernatant ph, turbidity, and absorbance (degree of color removal). 11. Plot the effect of alum dose on supernatant turbidity, ph, and remaining humic acid. Run 2: (spiked) 1. From run 1 chose an appropriate alum dosage. 2. Adjust ph from (increments of 0.5) which is the initial adjusted ph 3. Add the chosen alum dosage 17

22 4. Mix at 100+ RPM 1 min 5. Mix at 70 RPM for 15 min 6. Mix at 30 RPM for 35 min 7. Let settle for 30 min 8. After settling measure supernatant ph, turbidity and absorbance (degree of color removal). 9. Plot the effect of initial ph on Humic acid and turbidity removal Run 3: (not spiked) 1. Measure the initial ph, temperature, and turbidity of the river water sample (Red Cedar) 2. Adjust ph to Make up Alum Dosages with distilled water: Table 6: Different Jars with Different Alum Dosages (Non Spiked) Error! Not a valid link. 4. Add the alum dosage to 1L of river water for each jar. 5. Mix at 100+ RPM for 1 min 6. Mix at 70 RPM for 15 min 7. Mix at 30 RPM for 35min 8. Let settle for 30 min. 9. After settling measure supernatant ph, turbidity, and absorbance. 10. Plot the effect of alum dose on supernatant (of the river water sample only) turbidity, ph, and remaining humic acid. 11. Compare plot from run 1 with run 3 Run 4: (not spiked) 1. Measure the initial ph, temperature, and turbidity of the river water sample (Red Cedar) 2. Use the appropriate alum dosage used in run Adjust ph from 4-8 (increments of 0.5) which is the initial adjusted ph 4. Add the chosen alum dosage 5. Mix at 100+ RPM 1 min 6. Mix at 70 RPM for 15 min 7. Mix at 30 RPM for 35 min 18

23 8. Let settle for 30 min to 1 hour 9. After settling measure supernatant ph, turbidity and absorbance (degree of color removal). 10. Plot the effect of initial ph on Humic acid and turbidity removal 11. Compare run 2 with run 4. Procedure to determine Alkalinity 1. Use ph 7.0 buffer and adjust the ph meter to 7.0. Use ph 4.0 buffer and adjust the ph meter to Titrate 100 ml of sample with 0.02 N H2SO4 to obtain ph 4.5 by vigorously stirring towards the end of the titration step. This is done in order to break the surface and to obtain rapid equilibrium between CO2 in solution and CO2 in the atmosphere. 3. Total Alkalinity is measured as CaCO3 in mg/l= (ml of titrant) X 10 To prepare 0.02N H2SO4, Use the following formula: Nf*Vf=Ni*Vi Where: Nf: Final normality desired Vf: Final total volume Ni: Initial normality Vi: Initial volume Note: The addition of coagulants to waste water causes chemical reactions where repulsive forces surrounding the colloids are neutralized, thereby resulting in floc formation. These particles can be removed by filtration or sedimentation. Soon after settling, the appearance of each jar is observed. If no jar appears clear, increase alum dosage in the next set of experiments. Deficient dosage causes the jar to look cloudy with insignificant floc formation/settling. An excess amount of alum results in dense floc formation and poor settling. The jar with the optimum alum dosage 19

24 will have settled floc at the bottom and a relatively clear supernatant. The clearest sample is determined by measuring it s turbidity. If turbidity values are not satisfactory, the experiment needs to be repeated with different alum dosages. 20