Water Treatment: Fundamentals and Practical Implications of Bubble Formation

Transcription

1 Water Treatment: Fundamentals and Practical Implications of Bubble Formation Paolo Scardina Thesis submitted to the Faculty of the Virginia Polytechnic Institute and State University In partial fulfillment of the requirements for the degree of Masters of Science In Environmental Engineering Marc Edwards, Chair Bill Knocke John Little February 11, 2000 Blacksburg, Virginia Keywords: Bubble, Nucleation, Air Binding, Filtration, Water Treatment, Coagulation Copyright 2000, Paolo Scardina

2 Water Treatment: Fundamentals and Practical Implications of Bubble Formation Paolo Scardina Advisor: Dr. Marc Edwards Abstract Water utilities can experience problems from bubble formation during conventional treatment, including impaired particle settling, filter air binding, and measurement as false turbidity in filter effluent. Coagulation processes can cause supersaturation and bubble formation by converting bicarbonate alkalinity to carbon dioxide by acidification. A model was developed to predict the extent of bubble formation during coagulation which proved accurate, using an apparatus designed to physically measure the actual volume of bubble formation. Alum acted similar to hydrochloric acid for initializing bubble formation, and higher initial alkalinity, lower final solution ph, and increased mixing rate tended to increase bubble formation. Lastly, the protocol outlined in Standard Methods for predicting the degree of supersaturation was examined, and when compared to this work, the Standard Methods approach produces an error up to 16% for conditions found in water treatment. Air entrainment and ozonation are the key causes of dissolved gas supersaturation and eventual bubble formation in water treatment plants. Total dissolved gas probes (TDGP) are now available to directly measure supersaturation and have many advantages compared to conventional techniques. Bubble formation during coagulation-flocculation hindered particle sedimentation, producing settled turbidities double that of solutions without dissolved gases. In a filtration study, run time to one half of initial flow was decreased by 54% when the source water was increased from 0.1 to 0.2 atm supersaturation. Indeed, even at 0.05 atm supersaturation, run length was only 21 hours in solutions without added particulate matter. A case study confirmed that bubble formation can interfere with coagulation and filtration processes at conventional treatment plants.

3 ACKNOWLEDGEMENTS I would like to thank my advisor, Marc Edwards, whose unyielding patience and guidance kept this project alive. I would also like to thank all members in the research group, especially Steve Kvech whose help with the project is appreciated. iii

4 TABLE OF CONTENTS LIST OF TABLES...vi LIST OF FIGURES... vii AUTHOR S PREFACE...ix CHAPTER I. THE FUNDAMENTALS OF BUBBLE FORMATION IN WATER TREATMENT...1 Introduction...1 Fundamentals of Bubble Formation...2 Model Confirmation...7 Practical Extensions...14 Conclusions...17 Appendix I. Model Equations...18 Appendix II. Equations for Standard Methods Approach...20 Appendix III. References...21 II. THE PRACTICAL IMPLICATIONS OF BUBBLE FORMATION IN WATER TREATMENT...23 Introduction...23 Bubble Formation and Dissolved Gas Supersaturation...24 Measuring Dissolved Gas(es) Supersaturation...27 Previous Work...28 Materials and Methods...29 Results and Discussion...31 Water Treatment Plant Case Study...40 Conclusions...42 Acknowledgements...43 Notation...43 iv

5 Appendix. Operational Parameters of Utility A...43 References...44 AUTHOR S VITA...46 v

6 LIST OF TABLES Table 1-1: Comparison of Standard Methods and the Bubble Apparatus : Sources of Dissolved Gas Supersaturation : Gas Diffusion Values in Different Media : Previous Work on Bubble Formation in Treatment Plants : Interfacial Area for Experimental Apparatus versus Full Scale Treatment...35 vi

7 LIST OF FIGURES Figure 1-1: Heterogeneous Nucleation : Model Conceptualization : Model Illustrative Example : Bubble Formation Potential as a Function of Initial Alkalinity and ph : Bubble Formation Potential as a Function of Temperature : Bubble Formation Potential as a Function of Pressure : Schematic of Bubble Volume Measuring Apparatus : Bubble Volume Kinetics : Mixing Intensity Effects on Bubble Kinetics : Model Verification with Extreme Conditions : Model Verification for 2 Different Acids and ph s : Bubble Volume Estimation with Standard Methods Approach : Predictions of this Work Considering CO 2 Compared to Standard Methods Assumptions : Dissolve Gas Supersaturation from Source Water Air Entrainment : The Effects of Ozonation on the Dissolved Gas Saturation : Pilot Filter Schematic : Extent of Dissolved Nitrogen Concentration during Coagulation Experiment : Turbidity Measurements during Flocculation : Bubble Attached to Floc Particles : Comparison of Floc Formation at 2.5 mg/l Fe : Final Settled Turbidity for Coagulation Experiments : Relationship between Bubble Formation Potential and Settled Turbidity in the Amirtharajah Study : Air Bubble Formation in Media during Filtration of Supersaturated Waters : Effluent Filter Flow at Constant Head : Time for 50% Reduction of Effluent Flow per Supersaturation : Total Dissolved Gas Profile through Utility A...41 vii

8 2-14: Air Bubble Release during Filter Backwashing at Utility A A1: Schematic of Utility A...43 viii

9 AUTHOR S PREFACE Both chapters of this thesis are a separate manuscript and formatted to the specifications of the journal to which it will be submitted. The first chapter, entitled The Fundamentals of Bubble Formation in Water Treatment, was submitted to Journal of ASCE. The second chapter, The Practical Implications of Bubble Formation in Water Treatment, was submitted to Journal American Water Works Association. Both chapters are aimed at better understanding bubble formation in water treatment plants. The first chapter describes the fundamental science of bubble formation at water utilities, and develops a model that can predict the volume of bubbles formed under various conditions. The second chapter describes how waters can become supersaturated, and for the first time illustrates consequences of bubble formation in coagulation and filtration processes. ix