lntegrated Environmental Technology Series

Sewage Treatment Plants: Economic Evaluation of Innovative Technologies for Energy Efficiency

Edited by

Katerina Stamatelatou and -;:>

Konstantinqs, P: Tsagarakis

•

Contents

About the Editors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii

Part 1 Innovative technologies and economics in sewage treatment plants - an overview . . . . . . . . . . . . . . . . . . . . . . . . 1

Chapter 1 Reducing the energy demands of wastewater treatment through energy recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 lntroduction .......................................... 3

1.1.1 Wastewater management ......................... 3 1.1.2 Energy demands for wastewater treatment . . . . . . . . . . . . 4

1.2 Energy Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2.1 Use of efficient mechanical parts and sensors ... . ..... 7 1.2.2 Anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.2.3 Fermentation ................................... 9 1.2.4 Microbial fuel cells .............................. 10 1.2.5 Energy recovery from sewage sludge ............... 10

1.3 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.4 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

vi Sewage Treatment Plants

Chapter 2 The principles of economic eva/uation and cost-benefit analysis implemented in sewage treatment plants .. . ... . 15 Maria Molinos-Senante, Nick Hanley, Francesc Hernandez-Sancho and Ram6n Sala-Garrido 2.1 lntroduction ......................................... 15 2.2 Cost Benefit Analysis Methodology . . . . . . . . . . . . . . . . . . . . . . . 16

2.2.1 Cost benefit analysis basis . . . . . . . . . . . . . . . . . . . . . . . 16 2.2.2 Interna! benefit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.2.3 External benefit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.3 Conclusions 2 .4 References

Chapter 3 lntroduction to energy management in wastewater

28 28

treatment plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Catarina Silva, Helena Alegre and Maria Joäo Rosa 3.1 Energy management of wastewater treatment plants

put into context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.2 Energy management systems: highlights

of the ISO 50001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.3 Energy management and infrastructure asset management ... 40 3.4 A Framework of Energy Performance lndicators and Indices

for WWTPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 3.4.1 Background . ... . ... .. .................... . .... 42 3.4.2 Energy performance indicators . . . . . . . . . . . . . . . . . . . . 43 3.4.3 Energy performance indices . . . . . . . . . . . . . . . . . . . . . . 49 3.4.4 Methodology for PAS application . . . . . . . . . . . . . . . . . . 51

3.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Chapter 4 Innovative energy efficient aerobic bioreactors for sewage treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Eoin Syron 4.1 lntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4.2 Aeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

4.2.1 Innovative process design and improvement . . . . . . . . . 58 4.3 lncreasing Oxygen Transfer from a Bubble . . . . . . . . . . . . . . . . . 59

4.3.1 Fine bubble diffusers and oxygen transferring technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

4.3.2 lncreasing contact time .......................... 61

Contents vii

4.4 Bubbleless Aeration-Membrane Aerated Biofilm Reactor . . . . . 61 4.4.1 Submerged membrane aerated biofilm reactors ....... 63 4.4.2 Passively membrane aerated biofilm reactors . . . . . . . . 65

4.5 Low Energy Ammonia Removal . . . . . . . . . . . . . . . . . . . . . . . . . 66 4.5.1 Ammonia removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 4.5.2 Shortcut nitrification ....... . .. . ................. 67 4.5.3 Anammox . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.6 Other Aerobic Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 4.6.1 Aerobic granules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.7 Conclusions .. .. . ... ................. .. ............. . 68 4.8 References ....... ..... .. . .......................... 68

Chapter 5 Integration of energy efficient processes in carbon and nutrient removal from sewage . . . . . . . . . . . . . . . . . . . . 71 Simos Malamis, Evina Katsou and Francesco Fatone 5.1 lntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 5.2 Regulatory Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 5.3 Energy Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 5.4 Conventional Biological Nutrient Removal Processes ........ 74

5.4.1 Description of alternative conventional BNR processes and configurations . . ................... 74

5.4.2 BNR processes implemented in Europe and Northern America ............................... . ...... 80

5.4.3 Energy requirements and cost of conventional BNR processes ......... .. .................. .. . 81

5.5 Innovative Bioprocesses in the Mainstream and Sidestream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

5.6 Nitrous Oxide Emissions in BNR . . . . . . . . . . . . . . . . . . . . . . . . . 89 5.7 Conclusion .......... .. .... . . . ............. . ......... 90 5.8 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 5.9 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

Chapter 6 The aerobic granulation as an alternative to conventional activated sludge process . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Anuska Mosquera-Corral, Angeles Val de/ Rio, Helena Moralejo-Garate, Alberto Sanchez, Ram6n Mendez and Jose Luis Campos 6.1 lntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 6.2 Basics of Aerobic Granulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

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6.2.1 Conditions for aerobic granular biomass formation . . . . . 97 6.2.2 Sequencing batch reactors ....................... 99 6.2.3 Factors affecting aerobic granule characteristics

and stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 6.2.4 Biological processes inside the aerobic granules ..... 103

6.3 Comparison with Activated Sludge Systems ............... 105 6.4 Full Scale Applications of the Aerobic Granular

Technologies ....................................... 108 6.5 Acknowledgements .................................. 110 6.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

Chapter 7 Anaerobic digestion of sewage wastewater and s/udge ...... . .... . .......................... . Katerina Stamatelatou 7.1 lntroduction ....................................... . 7.2 The Process ...................................... . 7.3 The Technology .................................... . 7.4 Anaerobic Digestion of Sewage Sludge ................. .

7.4.1 Sonication .................................. . 7.4.2 Microwave .................................. . 7.4.3 Thermal hydrolysis ........................... . 7.4.4 Autohydrolyis - Enzymatic hydrolysis ............. . 7.4.5 Other methods .............................. . 7.4.6 Economic analysis of the pretreatment methods .... .

7.5 Anaerobic Digestion of Sewage ....................... . 7.5.1 Pretreatment of sewage via anaerobic digestion .... . 7.5.2 Treatment of preconcentrated sewage via anaerobic

digestion ................................... . 7.6 Conclusions ....................................... . 7.7 References

Chapter 8

115

115 116 118 119 122 123 123 123 123 124 129 130

132 133 134

Resource recovery from sewage s/udge . . . . . . . . . . . . . . 139 M. G. Healy, R. Clarke, D. Peyton, E. Cummins, E. L. Moynihan, A. Martins, P. Beraud and 0 . Fenton

8.1 lntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 8.2 Defining Trends for Municipal Sludge Treatment . . . . . . . . . . . . 140

Contents ix

8.3 Sewage Sludge as a Resource . . . . . . . . . . . . . . . . . . . . . . . . . 141 8.3.1 Nutrient recovery from sewage sludge . . . . . . . . . . . . . 141 8.3.2 Volatile fatty acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 8.3.3 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 8.3.4 Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

8.4 Legislation Covering Disposal of Biodegradable Waste on Land . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

8.5 Existing and Emerging lssues Concerning the Re-Use of Biodegradable Waste on Land . . . . . . . . . . . . . . . . . . . . . . . . . 148 8.5.1 Societal issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 8.5.2 Nutrient and metal lasses . . . . . . . . . . . . . . . . . . . . . . . 148 8.5.3 Pathogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 8.5.4 Pharmaceuticals .............................. 150

8.6 Quantification of Costs and Benefits from Re-Use of Sewage Sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 8.6.1 Impact of nutrient recovery, energy/product

generation on energy and cost savings in a sewage treatment plant . . ..................... 154

8.7 Acknowledgements .................................. 155 8.8 References ........................................ 155

Chapter 9 Odour abatement technologies in WWTPs: energy and economic efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Jose M. Estrada, Raquel Lebrero, Guillermo Quijano, N. J. R. Bart Kraakman and Rauf Mufioz 9.1 lntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 9.2 Odour Abatement Technologies ........................ 165

9.2.1 Design and economical parameters . . . . . . . . . . . . . . . 168 9.3 Comparative Parametric Efficiency

Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 9.3.1 Energy consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 9.3.2 Energy efficiency parameter . . . . . . . . . . . . . . . . . . . . . 174 9.3.3 Sustainability efficiency parameter . . . . . . . . . . . . . . . . 177 9.3.4 Robustness efficiency parameter . . . . . . . . . . . . . . . . . 179 9.3.5 lnfluence of the H2S concentration . . . . . . . . . . . . . . . . 181 9.3.6 Exploring alternatives to increase technology

efficiency: LID ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 9.4 Conclusions ........................................ 184 9.5 References ........................................ 185

X Sewage Treatment Plants

Chapter 10 Instrumentation, monitoring and real-time control strategies for efficient sewage treatment plant operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 Sergio Beitran, Ion lrizar and Eduardo Ayesa 10.1 lntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 10.2 Instrumentation for Monitoring and Control Purposes ........ 190 10.3 Control of Aeration Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 10.4 Control of Chemical Addition ........................... 197 10.5 Control of the Interna!, External and Sludge Wastage

Flow-Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 10.5.1 Control ofthe nitrates internal flow-rate and the

carbon external addition . . . . . . . . . . . . . . . . . . . . . . . . 198 10.5.2 Control of the external flow-rate or sludge

recirculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 10.5.3 Control of the sludge wastage flow-rate ............ 200

10.6 Control of Anaerobic Processes . . . . . . . . . . . . . . . . . . . . . . . . 201 10.6.1 Technological barriers .......................... 202 10.6.2 Applications of control in anaerobic digestion . . . . . . . . 202

10.7 Plant-Wide Control . ................................. 205 10.8 Conclusions ........................................ 206 10.9 References ........................................ 207

Chapter 11 Microbial Fuel Cel/s for wastewater treatment . . . . . . . . . 213 V. B. Oliveira, L. R. C. Marcon, J. Vilas Boas, L. A. Daniel and A. M. F. R. Pinto

11.1 lntroduction ........................................ 213 11.2 Operating Principle of a MFC .......................... 214 11.3 Fundamentalsand Challenges ............... . ......... 215 11.4 Scale Up .......................................... 217 11.5 Operational Conditions .... . .......................... 220

11.5.1 EffectofpH .................................. 220 11.5.2 Effect of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 11.5.3 Organic load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 11.5.4 Feed rate and shear stress . . . . . . . . . . . . . . . . . . . . . . 225

11.6 Mod~lling Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 11.7 Economic Evaluation ................................. 228 11.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 11.9 Acknowledgements .................................. 231 11.10 References ........................................ 231

Contents xi

Part II Innovative technologies and economics in sewage treatment p/ants - case studies . . . . . . . . . . . . . . . . . . . . . 237

Chapter 12 Management optimisation and technologies application: a right approach to ba/ance energy saving needs and process goa/s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 P. Ragazzo, L. Falletti, N. Chiucchini and G. Serra 12.1 lntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 12.2 Energy Saving with Maintenance and Control Operations .... 240

12.2.1 Initial situation of plants . . . . . . . . . . . . . . . . . . . . . . . . . 240 12.2.2 Interventions on pumps and piping system .......... 242 12.2.3 Interventions on mixers and engines ....... . ...... 243 12.2.4 Interventions on air compression and distribution . . . . . 244 12.2.5 When energy and process efficiency do not agree .... 246

12.3 Energy Saving Choosing the Right Technology ...... . . . ... 247 12.4 Conclusions . . . . .... .. .. ... ......................... 248 12.5 References ............... . ........................ 249

Chapter 13 Energy factory: the Dutch approach on wastewater as a source of raw materials and energy . . . . . . . . . . . . . . 251 Ruud M. W Schemen, Rutger Dijsselhof, Ferdinand D. G. Kiestra, Ad. W A. de Man, Coert P. Petri, Jan Evert van Veldhoven, Erwin de Valk and Henry M. van Veldhuizen Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 13.1 Energy Factory ............. . ....................... 252

13.1.1 The concept ................................. 252 13.1.2 The history ............. .. ................... 253 13.1 .3 The present state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 13.1.4 Economic aspects .............. . .............. 255 13.1.5 The future (Wastewater management roadmap

towards 2030) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 13.2 Cases ........................................ . ... 257

13.2.1 LNG production at 's-hertogenbosch . . . . . . . . . . . . . . 257 13.2.2 Thermophilie digestion at STP Echten ............. 262 13.2.3 Delivering biogas from STP Olburgen to

potato industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263

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13.2.4 Centralised sludge treatment at STP Tilburg . . . . . . . . 264 13.2.5 Hydrolizing secondary sludge with TPH at

STP Venlo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 13.2.6 Digestion of external biomass at STP Apeldoorn ..... 266 13.2.7 Reclamation of energy and resources at STP

Amersfoort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 13.3 Conclusion(S) ...................................... 268 13.4 References ........................................ 268

Chapter 14 A new perspective on energy-efficiency and cost-effectiveness of sewage treatment plants . . . . . . . . 269 Helmut Rechberger and Aficja Sobaritka 14.1 lntroduction ........................................ 269 14.2 Methods and Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

14.2.1 Application of eSEA for the assessment of the N-removal performance of STPs . . . . . . . . . . . . . . 271

14.2.2 Data of Austrian STPs . . . . . . . . . . . . . . . . . . . . . . . . . . 273 14.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274

14.3.1 Assessment of the N-removal performance of STPs: eSEA vs N-removal rate . . . . . . . . . . . . . . . . . . . 274

14.3.2 Determination of the best practice STP: energy-efficiency and cost-effectiveness . . . . . . . . . . . 275

14.3.3 The influence of plant size . . . . . . . . . . . . . . . . . . . . . . 276 14.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 14.5 References .... . .. . ... . ...... .. ... . ................ 279

Chapter 15 Techno-economic assessment of sludge dewatering devices: A practical tool . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 Matteo Papa and Giorgio Bertanza 15.1 lntroduction ................................ . ....... 283 15.2 Description of the Methodology ................ . ........ 284

15.2.1 Operating procedure for test execution ............. 284 15.2.2 Data processing ... . ... . .. . ....... . ........... 288

15.3 Application to a Real Case Study ............ . ....... . .. 288 15.3.1 Technical issues . . ..... . .. . .. . .......... . ..... 289 15.3.2 Economic issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290

15.4 Conclusions ..... . ....... . .. .... .................... 292 15.5 References ........ . ............................... 293

Contents xili

Chapter 16 Short-cut enhanced nutrient removal from anaerobic supernatants: Pilot sca/e results and fu// sca/e deve/opment of the S.C.E.N.A. process . . . . . . . . . . . . . . . 295 Daniele Renzi, Stefano Longo, Nicola Frison, Simos Malamis, Evina Katsou and Francesco Fatone 16.1 lntroduction ........................................ 295

16.1.1 Removal or recovery? .......................... 296 16.2 Short-Cut Nitrogen Removal and

Via-Nitrite Enhanced Phosphorus Bioaccumulation: Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

16.3 Capital and Operating Cost of Anaerobic Sidestream Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298 16.3.1 Energy consumptions and costs of short-cut nitrogen

removal from anaerobic sidestream . . . . . . . . . . . . . . . 298 16.4 S.C.E.N.A. System .................................. 301

16.4.1 Pilot-scale results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 16.4.2 S.C.E.N.A. system integrated in conventional

treatment of sewage sludge . . . . . . . . . . . . . . . . . . . . . 302 16.5 Conclusions ........................................ 306 16.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307

Chapter 17 lnvestigation of the potential energy saving in a pilot-scale sequencing batch reactor . . . . . . . . . . . . . . . . . . . . . 311 Luca Luccarini, Dalila Pulcini, Davide Sottara and Alessandro Spagni 17.1 lntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311

17.1.1 Sequencing batch reactors . . . . . . . . . . . . . . . . . . . . . . 312 17.1.2 Automation of sequencing batch reactors . . . . . . . . . . . 313

17.2 Description of the Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . 314 17.2.1 Pilot plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 17.2.2 Process monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 17.2.3 EDSS architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

17.3 Results ............................................ 320 17.3.1 Nitrification time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 17.3.2 Dissolved oxygen consumption ................... 322 17.3.3 Cost analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323

17.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 17.5 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324 17.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

xlv Sewage Treatment Plants

Chapter 18 Economic impact of upgrading biogas from anaerobic digester of sewage sludge to biomethane for public transportation: Case study of Bekkelaget wastewater treatment plant in Oslo, Norway . . . . . . . . . . . . . . . . . . . . . 327 Rashid Abdi Elmi and G. Venkatesh 18.1 lntroduction ....................... . ................ 327 18.2 Wastewater Treatment and Sludge Handling at

Bekkelaget WWTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330 18.3 Biogas Handling at Bekkelaget WWTP . . . . . . . . . . . . . . . . . . . 331 18.4 The Economics of the Upgrading Facility . ................ 333 18.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338 18.6 References ........................ . ............... 339

Chapter 19 A wind PV hybrid system for power supply of a sewage treatment plant in a small town in Southern Brazil . . . . . . 341 Giuliano Daronco and Alexandre Beluco 19.1 lntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 19.2 The Sewage Treatment Plant Considered in this Study ...... 342 19.3 Components of the Energy System .................... . . 345 19.4 Simulations with Homer ............................... 348 19.5 Results and Discussion ............................... 349 19.6 Final Remarks ....................... . ...... . ....... 353 19.7 References ..... . ..................... .. ........... 354