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HYDRAULIC MODELING AND OPTIMIZATION OF WASTE STABILIZATION POND DESIGN FOR DEVELOPING NATIONS

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ABSTRACT 
 
Wastewater stabilization ponds (WSPs) have been identified and are used extensively to 
provide wastewater treatment throughout the world. It is often preferred to the conventional treatment systems due to its higher performance in terms of pathogen removal, its low maintenance and operational cost. A review of the literature revealed that there has been limited understanding on the fact that the hydraulics of waste stabilization ponds is critical to their optimization. The research in this area has been relatively limited and there is an inadequate understanding of the flow behavior that exists within these systems. This work therefore focuses on the hydraulic study of a laboratory-scale model WSP, operated under a controlled environment using computational fluid dynamics (CFD) modelling and an identified optimization tools for WSP.A field scale prototype pond was designed for wastewater treatment using a typical residential institution as a case study. This was reduced to a laboratory-scale model using dimensional analysis. The laboratory-scale model was constructed and experiments were run on them using the wastewater taken from the university wastewater treatment facility. 
The study utilized Computational Fluid Dynamics (CFD) coupled with an optimization 
program to efficiently optimize the selection of the best WSP configuration that satisfy 
specific minimum cost objective without jeopardizing the treatment efficiency. This was 
done to assess realistically the hydraulic performance and treatment efficiency of scaled 
WSP under the effect of varying ponds configuration, number of baffles and length to 
width ratio. Six different configurations including the optimized designs were tested in the 
laboratory to determine the effect of baffles and pond configurations on the effluent 
characteristics. The verification of the CFD model was based on faecal coliform 
inactivation and other pollutant removal that was obtained from the experimental data. 
 The results of faecal coliform concentration at the outlets showed that the conventional 
70% pond-width baffles is not always the best pond configuration as previously reported 
in the literature. Several other designs generated by the optimization tool shows that both 
shorter and longer baffles ranging between 49% and 83% for both single and multi-
objective optimizations could improve the hydraulic efficiency of the ponds with different 
variation in depths and pond sizes. The inclusion of odd and even longitudinal baffle 
arrangement which has not been previously reported shows that this configuration could 
improve the hydraulic performance of WSP. A sensitivity analysis was performed on the 
model parameters to determine the influence of first order constant (k) and temperature 
(T) on the design configurations. The results obtained from the optimization algorithm 
considering all the parameters showed that changing the two parameters had effect on the 
effluent faecal coliform and the entire pond configurations.  
This work has verified its use to the extent that it can be realistically applied for the 
efficient assessment of alternative baffle, inlet and outlet configurations, thereby, 
addressing a major knowledge gap in waste stabilization pond design. The significance of 
CFD model results is that water and wastewater design engineers and regulators can use 
CFD to reasonably assess the hydraulic performance in order to reduce significantly faecal 
coliform concentrations and other wastewater pollutants to achieve the required level of 
pathogen reduction for either restricted or unrestricted crop irrigation. 
TABLE OF CONTENTS 
         Title page ..................i 
         Declaration.........................ii           
         Certification .....iii 
         Dedication ...iv 
         Acknowledgements..... .....................v 
         Table of Contents ..............viii 
         List of Plates....................xv 
         List of Figures ............xvi 
         List of Tables ....xxiv 
         Abbreviations and symbols................xxvii 
         Abstract... ................xxxii 

Chapter 1:   
Introduction.........1 

1.1     Background to the study.......1 
1.2     Problem statement.........................5     
1.3     Aim of the research........6 
1.4     Objectives........6 
1.5     Scope of study...............6 
1.6     Justification of study.....7 
1.7     Limitation of the work........................7 
 
Chapter 2: Literature review.....8 

     2.1    The pressure on water demand ..8 
     2.2    Wastewater treatment systems in use.......................9 
     2.3    Waste stabilization ponds....11
2.3.1    Treatment units in Waste Stabilization. Ponds................12   
        2.3.2    Anaerobic ponds...13 
             2.3.2 .1 Design approach for anaerobic pond..15      
       2.3.3    Facultative ponds........................17 
             2.3.3.1 Design criteria for facultative pond.................17  
             2.3.3.2 Surface BOD loading in facultative ponds...............19 
        2.3.4 Model approaches for faecal coliform prediction in facultative pond.........20 
             2.3.4.1 Continuous stirred reactor (CSTR) model approach..21 
                2.3.4.2 Dispersed flow (DF) model approach................23 
        2.3.5 Maturation Pond..24 
    2.4 Waste Stabilization Ponds in Some Selected Institutions in Nigeria........26 
            2.4.1   Waste stabilization pond in University of Nssuka, Nigeria........29 
            2.4.2   Waste stabilization pond in Obafemi Awolowo University,  
                           Ile-Ife, Nigeria...30 
               2.4.3   Waste stabilization pond in Ahmadu Bello University, Zaria,  
                           Nigeria......32 
   2.5    Residence time-models in waste stabilization ponds ................35 
                 2.5.1 Plug flow pattern...35 
                 2.5.2 Completely mixed flow pattern.....................37 
                 2.5.3 Dispersed hydraulic flow regime.....................39 
   2.6 Wind effect and thermo-stratification on hydraulic flow regime.......42 
  2.7 Tracer experiment.43 
  2.8 Effects of baffles on the performance of waste stabilization .............44 
  2.9 Computational Fluid Dynamics Approach to Waste Stabilization Ponds...............48 
  2.10 Laboratory scale ponds........................56 
  2.11 Optimization of waste stabilization pond design...............59  
  2.12 Summary of literature review........................61 
   
Chapter 3: 
Methodology................62
 
  3.1   Description of the study area........................62
3.2   Collection of data on Water demand.......................65 
  3.3   Estimation of wastewater generated .......................66 
  3.4   Study of existing wastewater treatment system..................66 
  3.5   Analysis of wastewater samples..70 
  3.6   Design of the laboratory-scale plant layout......................70 
                  3.6.1. Design Guidelines for the University, Ota.................73 
                         3.6.1.1 Temperature (T)......................73 
                         3.6.1.2 Population (P).........................73 
                         3.6.1.3 Wastewater generation (Q) and Design for 20 years period........73 
                         3.6.1.4 BOD Contribution per capita per day (BOD).........73 
                         3.6.1.5 Total Organic Load (B)...................74 
                         3.6.1.6 Total Influent BOD Concentration (Li).................74 
                         3.6.1.7 Volumetric organic loading (λv) ...................74 
                         3.6.1.8 Influent Bacteria Concentration (Bi).............74 
                         3.6.1.9 Required effluent standards....................74 
3.7    Waste stabilization pond design.....75 
             3.7.1 Design of Anaerobic Pond. .....................75 
             3.7.2 Design of Facultative pond..76 
             3.7.3 Design of Maturation Pond..77 
3.8  Design of Laboratory scale model..........................79 
             3.8.1 Modeling of the Anaerobic Laboratory-scale pond.. ..............79 
                3.8.2 Modeling of the Facultative Laboratory-scale pond...................81 
             3.8.3 Modeling of the Maturation Laboratory-scale pond................82 
 3.9 Laboratory Studies..........85 
            3.9.1 Construction of the laboratory-scale waste stabilization ponds........85 
            3.9.2 Materials used for the construction of the inlet and outlet structures.....86 
            3.9.3 Design of inlet and outlet structures of the WSP...............91 
            3.9.4 Operation of the Laboratory-Scale waste stabilization pond...........94 
            3.9.5 Sampling and data collection.................95 
                 3.9.5.1 Water temperature....................95 
                 3.9.5.2 Influent and effluent samples...............95 
3.10    Laboratory methods ....95 
           3.10.1     Feacal coliform.....96 
           3.10.2     Chloride....96 
           3.10.3     Sulphate......96 
           3.10.4     Nitrate.........96 
           3.10.5     Phosphate.......96 
           3.10.6     Total Dissolved Solids.........................96 
           3.10.7     Conductivity......97 
           3.10.8     pH....... ..97 
3.11    Tracer Experiment.... ....97 
              3.11.1   Determination of First Order Kinetics (K value) for Residence time  
                        distribution (RTD) characterization................. ..99 
              3.11.2    The gamma extension to the N-tanks in series model approach...... 101 
3.12   Methodology and application of Computational Fluid Dynamics model...... 103 
           3.12.1 Introduction. ..103 
           3.12.2 CFD Model Application ...................... .106 
                      3.12.2.1 Simulation of fluid mechanics fecal coliform inactivation.. 106 
                      3.12.2.2 Constants used in the application modes.........109 
                      3.12.2.3 Mesh generation for the computational fluid dynamics model..110 
                          3.12.2.4 Model test for the simulation of residence time distribution 
                                     curve in the CFD....................113 
                     3.12.2.5 Model test for the simulation of faecal coliform inactivation in 
                                     the unbaffled reactor.......................114 
                     3.12.2.6  Model test for the simulation of faecal coliform inactivation in 
                                    the baffled reactors..................116 
             3.12.3 Application of segregated flow model to compare RTD prediction  
                      and the CFD predictions for feacal coliform reduction........122  
           3.12.4 Summary of the CFD model methodology.....................124 
  3.13.1 Optimization methodology and application.................125    
             3.13.1.1 Integration of COMSOL Multiphysics (CFD) with  
                           ModeFRONTIER optimization tool....................125 
3.13.1.2 The workflow pattern..........................126 
             3.13.1.3 Building the process flow....................127 
             3.13.1.4 Creating the application script.............128 
             3.13.1.5 Creating the data flow.........................129 
             3.13.1.6 Creating the template input.................130 
             3.13.1.7 Mining the output variables from the output files........131 
3.13.2 Defining the goals.......................132 
             3.13.2.1 The Objective functions for the optimization loop..............132 
             3.13.2.2 The constraints for the optimization loop................133 
             3.13.2.3 Cost objective Optimization .......................133 
             3.13.2.4 The DOE and scheduler nodes set up..136 
             3.13.2.5 Model parameterization of input variables ................137  
             3.13.2.6 DOE Algorithm........140 
             3.13.2.7 Simplex algorithm......140 
              3.13.2.8 Multi-Objective Genetic Algorithm II (MOGA-II) .........................141 
             3.13.2.9 Faecal coliform log-removal for transverse and longitudinal  
                           baffle arrangements.143 
     3.13.3 Sensitivity Analysis on the model parameters..................145      
     3.13.4 Running of output results from modeFRONTIER with the CFD tool............146 
     3.13.5 Summary of the optimization methodology.........................146 
 
Chapter 4: Modeling results and Analysis  

    4.1 Model results for the RTD curve and FC inactivation for unbaffled reactors...147  
    4.2 Initial Evaluation of baffled WSP designs in the absence of Cost using CFD..151 
        4.2.1 Application of segregated flow model to compare the result of RTD  
                 prediction and the CFD predictions for feacal coliform reduction......163  
   4.3 Results of the N-Tanks in series and CFD models ....................166 
       4.3.1 General discussion on the results of the N-Tanks in series and CFD 
                Models.....173 
   4.4 Results of some selected simulation of faecal coliform inactivation for 80% 
            Pond-width baffle Laboratory- scale reactors.................176 
4.5 Optimization model results.....181 
       4.5.1 The single objective SIMPLEX optimization configuration results......181 
       4.5.2 The Multi-objective MOGA II optimization configuration results.......195  
       4.5.3 Scaling up of Optimized design configuration..............216 
                 4.5.3.1 Scaling up of Anaerobic Longitudinal baffle arrangement.........216 
                 4.5.3.2 Scaling up of Facultative Transverse baffle arrangement........218 
                 4.5.3.3 Scaling up of Maturation Longitudinal baffle arrangement.....219 
                 4.5.3.4 Summary of results of scaling up of design configuration........220 
      4.5.4 Results of sensitivity analysis for Simplex design at upper and lower  
                boundary....220 
     4.5.5 Results of sensitivity analysis for MOGA II design at upper and lower  
                     boundary........235 
     4.5.6 Summary of the optimization model result.......................249 
 
Chapter 5: Laboratory-Scale WSP post-modeling results and verification of the  
                   Optimized models........................250 
5.1 Introduction....250 
5.2 Microbial and physico-chemical parameters................251  
     5.2.1 Feacal coliform inactivation in the reactors..................251 
     5.2.2 Phosphate removal.256 
     5.2.3 Chloride removal.........................258 
     5.2.4 Nitrate removal.........................259 
     5.2.5 Sulphate removal.........................260 
     5.2.6 pH variation..265 
     5.2.7 Total dissolved solids removal...................266 
     5.2.8 Conductivity variation.......................266 
     5.2.9 Summary of laboratory experimentation................267 
 
Chapter 6: Discussion of results.....................269
 
         6.1 Experimental results of Laboratory-scale waste stabilization ponds  
               in series.......269 
6.2 Hydraulic efficiency of CFD model laboratory-scale waste stabilization 
               ponds in series...270 
         6.3 Optimization of laboratory-scale ponds by Simplex and MOGA II  
              Algorithms......274 
         6.4 Summary of discussion.......................275 
 
Chapter 7: Conclusions and recommendations for further work........277 
        7.1 Conclusions.277 
        7.2 Contributions to knowledge..................278 
        7.3 Recommendation for further work....................279 
 
References......280 
   
Appendix A.......298 

  A1     COMSOL Multiphysics Model M-file for Transverse baffle  
          anaerobic reactor......................,.......298 
  A2     COMSOL Multiphysics Model M-file for longitudinal baffle  
          anaerobic reactor.....302 
  A3     COMSOL Multiphysics Model M-file for Transverse baffle  
           facultative reactor..306 
  A4     COMSOL Multiphysics Model M-file for longitudinal baffle  
             facultative reactor.........................310 
  A5     COMSOL Multiphysics Model M-file for Transverse  
          Maturation reactor........................314 
  A6     COMSOL Multiphysics Model M-file for longitudinal 
          Maturation reactor.......................318 
 
Appendix B..322 
B1     Transverse baffle arrangement scripting................322 
B2     Longitudinal baffle arrangement scripting.............324 


List of Plates 
Plate 3.1       Tanker dislodging wastewater into the treatment chamber.........67 
Plate 3.2       The water hyacinth reed beds showing baffle arrangement  
                      at opposing edges.68 
Plate 3.3       The inlet compartment showing gate valve..............68 
Plate 3.4       The Outfall waterway leading into the valley below the cliff.......69 
Plate 3.5       Effluent discharging through the outfall into the thick  
                     vegetation valley.........................69 
Plate 3.6       Front view of the laboratory-scale pond.....88 
Plate 3.7        Areal view of the laboratory-scale pond close to source of sunlight...........88 
Plate 3.8        An elevated tank serving as reservoir........89 
Plate 3.9        Inlet-outlet alternation of laboratory-scale WSP...........89 
Plate 3.10      Laboratory-scaled anaerobic ponds...............90 
Plate 3.11      Laboratory-scaled facultative ponds.................90 
Plate 3.12      Laboratory-scaled maturation ponds...................91 
Plate 3.13       Inlet and outlet structure of the laboratory-scale  
                       waste stabilization pond...................92 
Plate 3.14     Two 25-mm PVC hoses linked with the T-connector............92 
Plate 3.15     Control valves screwed to position for wastewater flow.......93 
Plate 3.16     Outlet structures connected to two pieces of ½ inch hoses  
                     for effluent Discharge......................93 
Plate 3.17      Tracer experiment with Sodium Aluminum Sulphosilicate...........97 
Plate 3.18     Tracer chemical diluting with the wastewater before  
                     getting to the outlet.........................98 
Plate 3.19      Improvement in wastewater quality along the units.............98

List of Figures 
Figure 2.1      Waste stabilization pond configurations                                                    12 
Figure 2.2      Operation of the Anaerobic Pond                                                               14 
Figure 2.3      Operation of the facultative pond                                                               23 
Figure 3.1      Bar chart of staff and student population trend                                          63 
Figure 3.2      Template for calculating the per-capita water use                                     65 
Figure 3.3      A sketch of the laboratory-scale WSP and operating conditions               72 
Figure 3.4      Configuration of the designed WSP for Covenant University                  79 
Figure 3.5      Different baffle arrangements with 70% pond width  
                        anaerobic pond                                                                                         99 
Figure 3.6       Different baffle arrangements with 70% pond width  
                       facultative pond                                                                                       100 
Figure 3.7       Different baffle arrangements with 70% pond width 
                        maturation pond                                                                                     100 
Figure 3.8      Data conversion for reactor length to width ratio to N for 
                       N-tanks in series model                                                                          102 
Figure 3.9      Description of length to width ratio for the laboratory-scale  
                       model                                                                                                      102 
Figure 3.10     Triangular meshes for the model anaerobic reactor                               111 
Figure 3.11     Triangular meshes for the model facultative reactor                             111 
Figure 3.12     Triangular meshes for the model maturation reactor                            112 
Figure 3.13    Model Navigator showing the application modes                                  113 
Figure 3.14    Correlation data of the predicted-CFD and observed effluent Faecal  
                       coliform counts in baffled pilot-scale ponds                                         115 
Figure 3.15   General arrangements of conventional longitudinal baffles of  
                     different  lengths in the anaerobic pond                                                  117 
Figure 3.16   General arrangements of conventional longitudinal baffles of  
                     different lengths in the facultative pond                                                  117 
Figure 3.17   General arrangements of conventional longitudinal baffles of  
                     different lengths in the maturation pond                                                 118 
Figure 3.18      Mesh structure in a 4 baffled 70% Transverse Anaerobic reactor            118     
Figure 3.19      Mesh structure in a 4 baffled 70% Longitudinal Anaerobic reactor         119 
Figure 3.20      Mesh structure in a 4 baffled 70% Transverse Facultative                       119 
Figure 3.21      Mesh structure in a 4 baffled 70% Longitudinal Facultative  
                         reactor                                                                                                       120 
Figure 3.22      Mesh structure in a 4 baffled 70% Transverse Maturation  
                         reactor                                                                                                       120 
Figure 3.23      Mesh structure in a 4 baffled 70% Longitudinal Maturation   
                         reactor                                                                                                       121 
Figure 3.24      Workflow showing all links and nodes in the user application  
                         interface                                                                                                    127 
Figure 3.25      Logic End properties dialogue interface                                                   128 
Figure 3.26      Data variable carrying nodes and the input variable properties  
                         Dialogue interface                                                                                    129 
Figure 3.27      Template for the calculator properties and JavaScript  
                         expression editor                                                                                      130 
Figure 3.28      Output variable mining interface and input template editor                    131 
Figure 3.29      DOS Batch properties and batch test editor for mined data                    132 
Figure 3.30      Constraint properties dialogue in the workflow canvas                          135 
Figure 3.31      Objective properties dialogue in the workflow canvas                           135 
Figure 3.32     DOE properties dialog showing the initial population of designs           136 
Figure 3.33     Scheduler properties dialog showing optimization wizards                    137 
Figure 3.34     Designs table showing the outcomes of different reactor  
                       configurations                                                                                          144 
Figure 3.35     History cost on designs table showing the optimized cost     &a

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