污泥处理处置工艺设计手册chap16

EPA 625/1-79-011 PROCESS DESIGN MANUAL FOR SLUDGE TREATMENT AND DISPOSAL U.S. ENVIRONMENTAL PROTECTION AGENCY Municipal Environmental Research Laboratory Office of Research and Development Center for Environmental Research Information Technology Transfer September 1979 Chater 16. Sidestreams from Solids Treatment Processes TABLE OF CONTENTS Page FOREWORD ABSTRACT LIST OF TABLES LIST OF FIGURES ACKNOWLEDGEMENTS CHAPTER 1. PURPOSE AND SCOPE 1- 1 1.1 Purpose 1- 1 1.2 Scope 1- 2 1.3 Process Classification 1- 2 1.4 References 1- 2 CHAPTER 2. GENERAL CONSIDERATIONS 2- 1 2.1 Introduction and Scope 2- 1 2.2 Legal and Regulatory Considerations 2- 1 2.2.1 Effect of Effluent Discharge Limitations on Wastewater Solids Management 2- 1 2.2.2 Restrictions on Wastewater Solids Treatment 2- 2 2.2.2.1 Air Emissions Limits 2- 2 2.2.2.2 Nuisances 2- 3 2.2.2.3 State and Local Requirements 2- 3 2.2.3 Laws and Regulations Governing Wastewater Solids Utilization and Disposal 2- 3 2.2.3.1 Federal Water Pollution Control Act 2- 3 2.2.3.2 Resource Conservation and Recovery Act 2- 4 2.2.3.3 Toxic Substances Control Act 2- 3 2.2.3.4 Marine Protection, Research and Sanctuaries Act 2- 5 2.2.3.5 Environmental Policy Acts 2- 5 2.2.3.6 State and Local Reuse and Disposal Requirements 2- 5 2.2.4 The Comprehensive Nature of Section 405 of the Clean Water Act 2- 5 2.3 Other Non-Technical Factors Affecting Wastewater Solids Management 2- 6 2.3.1 Availability of Construction Funds 2- 6 2.3.2 Special Funding Requirements 2- 7 TABLE OF CONTENTS (continued) Page 2.3.3 Time Span of Decisions 2- 7 2.3.4 Uncertainties 2- 8 2.3.5 The Design Team 2- 8 2.3.6 Public Involvement 2- 9 2.3.7 Social and Political Factors Affecting Waste Export 2- 10 2.4 References 2- 11 CHAPTER 3. DESIGN APPROACH 3- 1 3.1 Introduction 3- 1 3.2 Systems Approach 3- 1 3.3 The Logic of Process Selection 3- 2 3.3.1 Identification of Relevant Criteria 3- 2 3.3.2 Identification of System Options 3- 4 3.3.3 System Selection Procedure 3- 6 3.3.3.1 Base and Secondary Alternatives 3- 6 3.3.3.2 Choosing a Base Alternative: First Cut 3- 7 3.3.3.3 Choosing a Base Alternative: Second Cut 3- 10 3.3.3.4 Third Cut 3- 11 3.3.3.5 Subsequent Cuts 3- 12 3.3.4 Parallel Elements 3- 12 3.3.5 Process Selection at Eugene, Oregon 3- 13 3.4 The Quantitative Flow Diagram 3- 18 3.4.1 Example: QFD for a Chemically Assisted Primary Treatment Plant 3- 18 3.4.2 Example: QFD for Secondary Plant With Filtration 3- 24 3.5 Sizing of Equipment 3- 27 3.6 Contingency Planning 3- 29 3.6.1 Example of Contingency Planning for Breakdowns 3- 29 3.7 Other General Design Considerations 3- 34 3.7.1 Site Variations 3- 34 3.7.2 Energy Conservation 3- 35 3.7.3 Cost-Effective Analyses 3- 36 3.7.4 Checklists 3- 38 3.8 References 3- 39 CHAPTER 4. WASTEWATER SOLIDS PRODUCTION AND CHARACTERIZATION 4- 1 4.1 Introduction 4- 1 4.2 Primary Sludge 4- 1 4.2.1 Primary Sludge Production 4- 1 4.2.1.1 Basic Procedures for Estimating Primary Sludge Production 4- 1 4.2.1.2 Industrial Waste Effect 4- 2 4.2.1.3 Ground Garbage Effect 4- 3 4.2.1.4 Other Sludges and Sidestreams 4- 3 vi TABLE OF CONTENTS (continued) 4.2.1.5 Chemical Precipitation and Page Coagulation 4- 3 4.2.1.6 Peak Loads 4- 3 4.2.2 Concentration Properties 4- 6 4.2.3 Composition and Characteristics 4- 7 4.3 Biological Sludges 4- 9 4.3.1 General Characteristics 4- 9 4.3.2 Activated Sludge 4- 9 4.3.2.1 Processes Included 4- 9 4.3.2.2 Computing Activated Sludge Production - Dry Weight Basis 4- 9 4.3.2.3 Example: Determination of Biological Sludge Production 4- 19 4.3.2.4 Interaction of Yield Calculations and the Quantitative Flow Diagram (QFD) 4- 24 4.3.2.5 Concentration of Waste-Activated Sludge 4- 25 4.3.2.6 Other Properties of Activated Sludge 4- 27 4.3.3 Trickling Filters 4- 29 4.3.3.1 Computing Trickling Filter Sludge Production - Dry Weight Basis 4- 29 4.3.3.2 Concentration of Trickling Filter Sludge 4- 33 4.3.3.3 Properties - Trickling Filter Sludge 4- 34 4.3.4 Sludge from Rotating Biological Reactors 4- 34 4.3.5 Coupled Attached-Suspended Growth Sludges 4- 35 4.3.6 Denitrification Sludge 4- 36 4.4 Chemical Sludges 4- 36 4.4.1 Introduction 4- 36 4.4.2 Computing Chemical Sludge Production - Dry Weight Basis 4- 37 4.4.3 Properties of Chemical Sludges 4- 38 4.4.4 Handling Chemical Sludges 4- 38 4.4.4.1 Stabilization 4- 39 4.4.4.2 Chemical and By-Product Recovery 4- 39 4.5 Elemental Analysis of Various Sludges 4- 39 4.5.1 Controlling Trace Elements 4- 39 4.5.2 Site-Specific Analysis 4- 41 4.5.3 Cadmium 4- 42 4.5.4 Increased Concentration During Processing 4- 43 4.6 Trace Organic Compounds in Sludge 4- 44 4.7 Miscellaneous Wastewater Solids 4- 45 4.7.1 Screenings 4- 46 4.7.1.1 Quantity of Coarse Screenings 4- 46 4.7.1.2 Quantity of Fine Screenings 4- 48 4.7.1.3 Properties of Screenings 4- 48 4.7.1.4 Handling Screenings 4- 48 4.7.1.5 Screenings From Miscellaneous Locations 4- 49 4.7.2 Grit 4- 50 vii TABLE OF CONTENTS (continued) Page 4.7.2.1 Quantity of Grit 4- 51 4.7.2.2 Properties of Grit 4- 52 4.7.2.3 Handling Grit 4- 53 4.7.3 Scum 4- 45 4.7.3.1 Quantities of Scum 4- 55 4.7.3.2 Properties of Scum 4- 56 4.7.3.3 Handling Scum 4- 57 4.7.4 Septage 4- 59 4.7.4.1 Quantities of Septage 4- 59 4.7.4.2 Properties of Septage 4- 59 4.7.4.3 Treating Septage in Wastewater Treatment Plants 4- 60 4.7.5 Backwash 4- 61 4.7.6 Solids From Treatment of Combined Sewer Overflows 4- 62 4.8 References 4- 63 CHAPTER 5. THICKENING 5- 1 5.1 Introduction 5- 1 5.1.1 Definition 5- 1 5.1.2 Purpose 5- 1 5.1.3 Process Evaluation 5- 1 5.1.4 Types and Occurrence of Thickening Processes 5- 2 5.2 Sedimentation Basins 5- 2 5.2.1 Primary Sedimentation 5- 2 5.2.2 Secondary Sedimentation 5- 3 5.3 Gravity Thickeners - 5- 3 5.3.1 Introduction 5- 3 5.3.2 Theory 5- 3 5.3.3 System Design Considerations 5- 5 5.3.3.1 Minimum Surface Area Requirements 5- 6 5.3.3.2 Hydraulic Loading 5- 8 5.3.3.3 Drive Torque Requirements 5- 8 5.3.3.4 Total Tank Depth 5- 9 5.3.3.5 Floor Slope 5- 10 5.3.3.6 Other Considerations 5- 11 5.3.4 Design Example 5- 12 5.3.5 Cost 5- 15 5.3.5.1 Capital Cost 5- 15 5.3.5.2 Operating and Maintenance Cost 5- 15 5.4 Flotation Thickening 5- 16 5.4.1 Dissolved Air Flotation (DAF) 5 -18 5.4.1.1 Theory 5- 19 5.4.1.2 System Design Considerations 5- 19 5.4.2 Design Example 5- 33 5.4.3 Cost 5- 35 5.4.3.1 Capital Cost 5- 35 5.4.3.2 Operating and Maintenance Costs 5- 36 5.5 Centrifugal Thickening 5- 36 viii TABLE OF CONTENTS (continued) Page 5.5.1 Introduction 5- 36 5.5.2 Theory 5- 38 5.5.3 System Design Considerations 5- 39 5.5.3.1 Disc Nozzles 5- 39 5.5.3.2 Imperforate Basket 5- 45 5.5.3.3 Solid Bowl Decanter 5- 49 5.5.4 Case History 5- 53 5.5.5 Cost 5- 55 5.5.5.1 Capital Cost 5- 55 5.5.5.2 Operating and Maintenance Cost 5- 56 5.6 Miscellaneous Thickening Methods 5- 59 5.6.1 Elutriation Basin 5- 59 5.6.2 Secondary Anaerobic Digesters 5- 60 5.6.3 Facultative Sludge Lagoons 5- 60 5.6.4 Ultrafiltration 5 -60 5.7 References 5 -60 CHAPTER 6. STABILIZATION 6.1 Introduction 6.2 Anaerobic Digestion 6.2.1 Process Description 6- 6- 6- 6- 1 1 2 2 6.2.1.1 History and Current Status 6- 2 6.2.1.2 Applicability 6- 3 6.2.1.3 Advantages and Disadvantages 6- 4 6.2.1.4 Microbiology 6- 5 6.2.2 Process Variations 6- 7 6.2.2.1 Low-Rate Digestion 6- 7 6.2.2.2 High-Rate Digestion 6- 7 6.2.2.3 Anaerobic Contact Process 6- 15 6.2.2.4 Phase Separation 6- 16 6.2.3 Sizing of Anaerobic Digesters 6- 18 6.2.3.1 Loading Criteria 6- 18 6.2.3.2 Solids Retention Time 6- 18 6.2.3.3 Recommended Sizing Procedure 6- 20 6.2.4 Process Performance 6- 23 6.2.4.1 Solids Reduction 6- 26 6.2.4.2 Gas Production 6- 29 6.2.4.3 Supernatant Quality 6- 31 6.2.5 Operational Considerations 6- 34 6.2.5.1 pH 6- 34 6.2.5.2 Toxicity 6- 36 6.2.6 System Component Design 6- 42 6.2.6.1 Tank Design 6- 42 6.2.6.2 Heating 6- 46 6.2.6.3 Mixing 6- 52 6.2.6.4 Covers 6- 62 6.2.6.5 Piping 6- 66 6.2.6.6 Cleaning 6- 67 6.2.7 Energy Usage 6- 72 6.2.8 Costs 6- 74 i x TABLE OF CONTENTS (continued) 6.3 6.2.9 Design Example 6.2.9.1 Design Loadings 6.2.9.2 System Description 6.2.9.3 Component Sizing Aerobic Digestion 6.3.1 Process Description 6.3.1.1 History 6.3.1.2 Current Status 6.3.1.3 Applicability 6.3.1.4 Advantages and Disadvantages 6.3.1.5 Microbiology 6.3.2 Process Variations 6.3.2.1 Conventional Semi-Batch Operation 6.3.2.2 Conventional Continuous Operation 6.3.2.3 Auto-Heated Mode of Operation 6.3.3 Design Considerations 6.3.3.1 Temperature 6.3.3.2 Solids Reduction 6.3.3.3 Oxygen Requirements 6.3.3.4 Mixing 6.3.3.5 pH Reduction 6.3.3.6 Dewatering 6.3.4 Process Performance 6.3.4.1 Total Volatile Solids Reduction 6.3.4.2 Supernatant Quality 6.3.5 Design Example 6.3.6 Cost 6.3.6.1 Capital Cost 6.3.6.2 Operation and Maintenance Cost Page 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 6- 74 74 75 77 82 82 82 82 82 82 83 84 84 84 85 86 86 86 88 89 90 91 92 92 93 93 99 99 99 6.4 Lime Stabilization 6-100 6.4.1 Process Description 6-101 6.4.1.1 History 6-101 6.4.1.2 Current Status 6-102 6.4.1.3 Applicability 6-102 6.4.1.4 Theory of the Process 6-103 6.4.2 Design Criteria 6-103 6.4.2.1 pH and Contact Time 6-104 6.4.2.2 Lime Dosage 6-104 6.4.3 Process Performance 6-107 6.4.3.1 Odor Control 6-108 6.4.3.2 Pathogen Reduction 6-109 6.4.3.3 Dewatering and Settling Characteristics 6-110 6.4.3.4 Chemical Characteristics 6-110 6.4.4 Process Design 6-112 6.4.4.1 Design of Lime Handling Facilities 6-112 6.4.4.2 Mixing Tank Design 6-118 6.4.5 Costs and Energy Usage 6-121 6.4.5.1 Capital and Operating Costs 6-121 6.4.5.2 Energy Usage 6-122 TABLE OF CONTENTS (continued) Page 6.4.6 Design Example 6-124 6.4.6.1 Design Loading 6-124 6.4.6.2 System Description 6-124 6.4.6.3 Component Sizing 6-126 6.5 Chlorine Stabilization 6-127 6.5.1 Process Description 6-128 6.5.2 Uses, Advantages, and Disadvantages 6-131 6.5.3 Chlorine Requirements 6-132 6.5.4 Characteristics of Chlorine-Stabilized Materials 6-133 6.5.4.1 Stabilized Slude 6-133 6.5.4.2 Supernatant/Filtrate/Subnatant Quality 6-134 6.5.5 Costs 6-134 6.5.5.1 Operating Costs 6-135 6.5.5.2 Capital Costs 6-136 6.6 References 6-138 CHAPTER 7. DISINFECTION 7.1 Introduction 7.2 Pathogenic Organisms 7.2.1 Pathogen Sources 7.2.2 Pathogen Characteristics 7- 7- 7- 7- 7- 1 1 1 2 2 7.2.2.1 Viruses 7- 2 7.2.2.2 Bacteria 7- 3 7.2.2.3 Parasites 7- 4 7.2.2.4 Fungi 7- 6 7.2.3 Pathogen Occurrence in the United States 7- 6 7.3 Pathogen Survival During Sludge Stabilization Processes 7- 7 7.3.1 Pathogen Reduction During Digestion 7- 7 7.3.1.1 Viruses 7- 7 7.3.1.2 Bacteria 7- 8 7.3.1.3 Parasites 7- 9 7.3.2 Long Term Storage 7- 10 7.3.3 Chemical Disinfection 7- 10 7.3.3.1 Lime 7- 10 7.3.3.2 Chlorine 7- 10 7.3.3.3 Other Chemicals 7- 11 7.4 Pathogen Survival in the Soil 7- 11 7.4.1 Viruses 7- 11 7.4.2 Bacteria 7- 11 7.4.3 Parasites 7- 12 7.5 Potential Human Exposure to Pathogens 7- 12 7.6 Heat Disinfection Processes 7- 13 7.6.1 Sludge Pasteurization 7- 14 7.6.1.1 Process Description 7- 15 7.6.1.2 Current Status 7- 16 7.6.1.3 Design Criteria 7- 16 x i TABLE OF CONTENTS (continued) 7.6.1.4 Instrumentation and Operational Page Considerations 7- 17 7.6.1.5 Energy Impacts 7- 17 7.6.1.6 Cost Information 7- 17 7.6.1.7 Design Example 7- 20 7.6.2 Other Heat Processes 7- 24 7.6.2.1 Heat-Conditioning 7- 25 7.6.2.2 Heat-Drying 7- 25 7.6.2.3 High Temperature Processes 7- 25 7.6.2.4 Composting 7- 25 7.7 Pathogen Reduction With High-Energy Radiation 7- 26 7.7.1 Reduction of Pathogens in Sludge With Electron Irradiation 7- 26 7.7.1.1 Process Descritpion 7- 27 7.7.1.2 Status 7- 28 7.7.1.3 Design Considerations 7- 28 7.7.1.4 Instrumentation and Operational Considerations 7- 30 7.7.1.5 Energy Impacts 7- 30 7.7.1.6 Performance Data 7- 30 7.7.1.7 Production Production and Properties 7- 31 7.7.1.8 Cost Information 7- 31 7.7.2 Disinfection With Gammer Irradiation 7- 32 7.7.2.1 Process Description 7- 33 7.7.2.2 Current Status - Liquid Sludge 7- 33 7.7.2.3 Current Status - Dried or Composted Sludge 7- 34 7.7.2.4 Design Criteria 7- 35 7.7.2.5 Instrumentation and Operational Considerations 7- 35 7.7.2.6 Energy Impacts 7- 36 7.7.2.7 Performance Data 7- 37 7.7.2.8 Cost Information 7- 38 7.8 References 7- 44 CHAPTER 8. CONDITIONING 8- 1 8.1 Introduction 8- 1 8.2 Selecting a Conditioning Process 8- 1 8.3 Factors Affecting Wastewater Solids Conditioning 8- 1 8.3.1 General Wastewater Solids Properties 8- 1 8.3.1.1 Particle Size and Distribution 8- 3 8.3.1.2 Surface Charge and Degree of Hydration 8- 4 8.3.1.3 Particle Interaction 8- 4 8.3.2 Physical Factors 8- 4 8.3.2.1 Effect of Processing Prior to Conditioning 8- 5 8.3.2.2 Conditioner Application 8- 5 8.4 Inorganic Chemical Conditioning 8- 6 8.4.1 Introduction 8- 6 xii TABLE OF CONTENTS (continued) Page 8.4.2 Dosage Requirements 8- 6 8.4.3 Availability 8- 7 8.4.4 Storage, Preparation, and Application Equipment 8- 8 8.4.5 Design Example 8- 8 8.4.6 Cost 8- 9 8.4.6.1 Capital Cost 8- 9 8.4.6.2 Operation and Maintenance Cost 8- 10 8.5 Chemical Conditioning With Polyelectrolytes 8- 14 8.5.1 Introduction 8- 14 8.5.2 Background on Polyelectrolytes 8- 14 8.5.2.1 Composition and Physical Form 8- 14 8.5.2.2 Structure in Solution 8- 17 8.5.2.3 How Polyelectrolyte Conditioning Works 8- 17 8.5.3 Conditioning for Thickening 8- 18 8.5.3.1 Gravity Thickening 8- 18 8.5.3.2 Dissolved Air Flotation Thickening 8- 18 8.5.3.3 Centrifugal Thickening 8- 20 8.5.4 Conditioning for Dewatering 8- 20 8.5.4.1 Drying Beds 8- 21 8.5.4.2 Vacuum Filters 8- 21 8.5.4.3 Recessed Plate Pressure Filters 8- 22 8.5.4.4 Belt Filter Presses 8- 23 8.5.4.5 Centrifuges 8- 24 8.5.5 Storage, Preparation, and Application Equipment 8- 25 8.5.6 Case History 8- 25 8.5.7 Cost 8- 27 8.5.7.1 Capital Cost 8- 27 8.5.7.2 Operation and Maintenance Cost 8- 29 8.6 Non-Chemical Additions 8- 29 8.7 Thermal Conditioning 8- 31 8.7.1 Advantages and Disadvantages 8- 33 8.7.2 Process Sidestreams 8- 34 8.7.2.1 Gaseous Sidestreams 8- 34 8.7.2.2 Liquid Sidestreams 8- 35 8.7.3 Operations and Cost 8- 36 8.7.3.1 General Considerations 8- 36 8.7.3.2 USEPA Survey Results 8- 38 8.8 Elutriation 8- 39 8.9 Freeze-Thaw 8- 40 8.9.1 Indirect Mechanical Freezing 8- 40 8.9.2 Direct Mechanical Freezing 8- 41 8.9.3 Natural Freezing 8- 41 8.10 Mechanical Screening and G r inding 8- 41 8.11 Miscellaneous Processes 8- 42 8.11.1 Bacteria 8- 42 8.11.2 Electricity 8- 42 8.11.3 Solvent Extraction 8- 43 TABLE OF CONTENTS (continued) 8.11.4 Ultrasonic 8.12 References CHAPTER 9. DEWATERING Page 8- 8- 9- 43 43 1 9.1 Introduction 9- 1 9.1.1 Process Evaluation 9- 1 9.1.2 Methods of Dewatering 9- 3 9.2 Natural Sludge Dewatering Systems 9- 3 9.2.1 Drying Beds 9- 3 9.2.1.1 Basic Components and Operation 9- 4 9.2.1.2 Types of Drying Beds 9- 5 9.2.1.3 Process Design Criteria 9- 9 9.2.1.4 Costs 9- 12 9.2.2 Drying Lagoons 9- 14 9.2.2.1 Basic Concept 9- 15 9.2.2.2 Design Criteria 9- 15 9.2.2.3 Costs 9- 16 9.3 Centrifugal Dewatering Systems 9- 17 9.3.1 Introduction 9- 17 9.3.2 Imperforate Basket 9- 18 9.3.2.1 Principles of Operation 9- 18 9.3.2.2 Application 9- 19 9.3.2.3 Performance 9- 19 9.3.2.4 Case History 9- 19 9.3.3 Solid Bowl Decanters 9- 23 9.3.3.1 Application 9- 23 9.3.3.2 Performance 9- 24 9.3.3.3 Other Considerations 9- 24 9.4 Filtration Dewatering Systems 9- 25 9.4.1 Introduction 9- 25 9.4.2 Basic Theory 9- 26 9.4.3 Filter Aids 9- 26 9.4.4 Vacuum Filters 9- 27 9.4.4.1 Principles of Operation 9- 28 9.4.4.2 Application 9- 32 9.4.4.3 Performance 9- 33 9.4.4.4 Other Considerations 9- 33 9.4.4.5 Case History 9- 39 9.4.4.6 Costs 9- 41 9.4.5 Belt Filter Press 9- 43 9.4.5.1 Principles of Operation 9- 45 9.4.5.2 Application 9- 46 9.4.5.3 Performance 9- 46 9.4.5.4 Other Considerations 9- 47 9.4.5.5 Design Example 9- 49 9.4.5.6 Costs 9- 51 9.4.6 Recessed Plate Pressure Filters 9- 52 9.4.6.1 Principles of Operation 9- 52 9.4.6.2 Application 9- 55 9.4.6.3 Performance 9- 56 xiv TABLE OF CONTENTS (continued) Page 9.4.6.4 Other Considerations 9- 56 9.4.6.5 Case History 9- 59 9.4.6.6 Cost 9- 60 9.4.7 Screw and Roll Press 9- 63 9.4.7.1 Screw Press 9- 63 9.4.7.2 Twin-Roll Press 9- 66 9.4.8 Dual Cell Gravity (DCG) Filter 9- 67 9.4.9 Tube Filters 9- 68 9.4.9.1 Pressure Type 9- 68 9.4.9.2 Gravity Type 9- 68 9.5 Other Dewatering Systems 9- 69 9.5.1 Cyclones 9- 69 9.5.2 Screens 9- 70 9.5.3 Electro-Osmosis 9- 70 9.6 References 9- 70 CHAPTER 10. HEAT DRYING 10- 1 10.1 Introduction 10- 1 10.2 Heat-Drying Principles 10- 1 10.2.1 Drying Periods 10- 1 10.2.2 Humidity and Mass Transfer 10- 2 10.2.3 Temperature and Heat Transfer 10- 3 10.3 Energy Impacts 10- 5 10.3.1 Design Example 10- 6 10.3.2 Energy Cost of Heat-Dried Sludges Used for Fertilizers 10- 11 10.4 Environmental Impacts 10- 12 10.4.1 Air Pollution 10- 12 10.4.2 Safety 10- 13 10.4.3 Sidestream Production 10- 13 10.5 General Design Criteria 10- 13 10.5.1 Drying Capacity 10- 13 10.5.2 Storage Requirements 10- 14 10.5.3 Heat Source 10- 14 10.5.4 Air Flow 10- 14 10.5.5 Equipment Maintenance 10- 15 10.5.6 Special Considerations 10- 15 10.6 Conventional Heat Dryers 10- 15 10.6.1 Flash-Drying 10- 16 10.6.1.1 Process Description 10- 16 10.6.1.2 Case Study: Houston, Texas 10- 18 10.6.2 Rotary Dryers 10- 19 10.6.2.1 Direct Rotary Dryers 10- 19 10.6.2.2 Indirect Drying 10- 22 10.6.2.3 Direct-Indirect Rotary Dryers 10- 24 10.6.3 Incinerators 10- 25 10.6.4 Toroidal Dryer 10- 25 10.6.4.1 Process Description 10- 25 10.6.4.2 Current Status 10- 27 10.6.5 Spray-Drying 10- 27 xv TABLE OF CONTENTS (continued) Page 10.6.5.1 Process Description 10- 27 10.6.5.2 Current Status 10- 27 10.7 Other Heat-Drying Systems 10- 28 10.7.1 Solvent Extraction--BEST Process 10- 28 10.7.1.1 Process Description 10- 28 10.7.1.2 Current Status 10- 29 10.7.1.3 Operating Experien