Drainage Services Department Research & Development Forum 2017 – Smart Primary Sedimentation Tanks Design using Three-Dimensional Computational Fluid Dynamic Modeling 14 th November 2017 Date: By: Professor Kin-Man Ho Kingsford Environmental
CONTENTS 1. Uses of 2/3D CFD Modelling for Smart STW Flow & Process Hydraulic Design in HK 2. Smart PST Design Using 3D CFD Modelling 3. Process Hydraulic Enhancement Features by 3D CFD PST Modelling 4. Technical Background of the 3D CFD PST Model 5. Application of the 3D CFD PST Model 6. Full-Scale R&D Trial – Shatin STW 7. Recommendations & Further R&D Studies 8. Acknowledgement 9. Q&A
Uses of 2/3D CFD Modelling for Smart STW Flow & Process Hydraulic Design in HK CFD Modelling has been extensively used in STW Flow & Process Design in Hong Kong since 1996: - ➢ The Shek Wu Hui STW – Upgrading Existing and New FSTs with 2D CFD only in 1996 and 2009; ➢ The Tai Po STW – Upgrading Existing and New FSTs with 2D CFD only in 2007 and 2013; ➢ The Sai Kung STW – Existing FSTs with 2D CFD only for Operation and Performance Evaluation only in 2005; ➢ The Shatin STW – Upgrading Existing and New FSTs with 2D CFD only in 1998 and 2006; ➢ The Stonecutters Island STW – Upgrading Existing and New CEPT Tanks with Integrated 2D PST and 3D Flow Distribution CFD Modelling in 2008 ➢ The Shatin & Tai Po STWs – Enhance the hydraulic performance of the new effluent UV Channels with 3D CFD Model in 2009 ➢ The Shatin STW – Existing PSTs 3D CFD only in 2016 - 17
Uses of 3D CFD Modelling for Smart STW Flow & Process Hydraulic Design (cont’) Role of 3D CFD Models ➢ Very powerful commercial packages, FLOW3D, FLUENT etc., are now available for flow distribution modelling ➢ For Solids separation, integrated with sub-routine(s) of proprietary Liquid-Solids Separation Equations/Program ➢ To design and or evaluate Grit Channel, PST/FST, CEPT, Mixing and Anaerobic Digestors ➢ To simulate the flow hydraulic and process hydraulic (with solids) performance ➢ To facilitate the design and upgrading & allow evaluation of options to optimize the system/process hydraulics or to relieve any capacity constrains identified ➢ To investigate likely overflow and maintenance events under different development scenarios during dry & wet seasons ➢ To serve as a planning tool for investigation options to modify, improve & operate the new/existing systems for better performance & power consumption
Smart PST Design Using 3D CFD Modelling Why these works are important in Smart PST Design and upgrading? ➢ Higher PST TCOD, TSS & TKN removal to cope with variations of influent sewage characteristics lead to ➢ lower aeration power consumption ➢ more biogas production ➢ larger process capacity ➢ Less MLSS concentration increases the capacity of the FSTs which is the bottle-neck of the biological process ➢ More compact PST design/upgrade with simple in-tank hydraulic enhancement features simulated and designed by the 3D CFD PST Model ➢ Target to increase TCOD removal of ~20 - 25%.
Process Hydraulic Enhancement Features by 3D CFD PST Modelling Inlet Energy In-Board Dissipating & Mid-Tank Launders Flocculation Baffles Baffles Transitional Uniform Velocity Effluent Zone Inlet Zone Zone Zone Lamella Plates/Tubes Settled Primary Sludge Protection Baffles
Technical Background of the 3D CFD PST Model ➢ Powerful 3D PST Model ➢ To give complete & precise representation of water & solids movements in the PST in terms of flow, energy/velocity & solids concentration contours,; ➢ The flocculation & settling patterns from the inlets to the outlets of both settled sewage & sludge lines. ➢ Incorporates a special technique, known as the FAVOR TM (Fractional Area Volume Obstacle Representation) method, which is used to define general geometric regions within the rectangular grid. ➢ Uses special numerical method to track the location of surfaces & to apply the proper dynamic boundary conditions at those surfaces. ➢ Offers multi-block meshing, which add more flexibility & efficiency to the finite difference meshing technique.
Application of the 3D CFD PST Model ➢ 1 st Step -Development and Calibration ➢ Review the design and operation details of the new or existing PSTs to develop & calibrate the 3D CFD PST model. ➢ Prepare 3D CAD drawings of the PSTs & convert to Standard Triangle Language (STL) format as input to the commercial 3D CFD package. ➢ Check the numerical stability & convergence of the built model. ➢ Use water surface elevation measurements to calibrate the model.
Application of the 3D CFD PST Model (cont’) ➢ 2 nd Stage Design and Capacity Evaluation and Optimization ➢ Further stimulations to evaluate the effectiveness of hydraulic enhancement feature(s) for optimizing the process & hydraulic performance could then be commenced. ➢ Use the modified PST model to test different “what if” (extreme) operational scenarios. ➢ 3 rd Stage Full-Scale Validation ➢ Conduct full-scale tests with dye distribution method combined with water surface elevations measurements to validate & fine-tune the model.
Full-Scale R&D Trial – Shatin STW The existing Shatin STW has been selected for the trial. Flow-3D integrated with HACM was selected for this project. This Study aims to confirm the applicability of the 3D CFD Model for Smart PST Design in Two (2) Phases: - 1 st Phase (This Presentation): - To develop the 3D CFD Model; To quantify the performance improvement of the existing PSTs with and without SAS co-settling; To identify most optimal upgrading option with different combinations of the hydraulic improvement features; To evaluate the economic viability of these features in OPEX savings. 2 nd Phase (If Proceeded): - Full-scale upgrading of a selected PST; Performance comparison of the PSTs with and without upgrading; Validation of the 3D CFD PST Model Applications for new PST (and FST, others) design for future projects
1 st Stage of the 3D CFD PST Model To develop and calibrate the 3D CFD PST Model ➢ As-built 2D drawings of Unit 3 PSTS 3D drawings & STL CFD Model input files; ➢ 12 months historical operation data of the crude sewage, & settled sewage (Jan to Dec 2015); ➢ On-Site Data Collection of the primary sludge characteristics 3-D AutoCAD Drawing of Existing PST No. 9
1 st Stage of the 3D CFD PST Model (cont’) On-site Data Collection (with & without co-settling) ➢ 5-day raw & settled sewage characterization study ➢ Flocculation tests with determination of the non-settleable solids concentration ➢ Solids profile tests ➢ Sludge settling velocity distribution tests
2 nd Stage of the 3D CFD PST Model To determine the optimal achievable performance improvement in terms of COD, TSS & TKN removal using the 3D CFD PST Model, with the process In-Board Mid-Tank hydraulic enhancement features & combination(s) Launders Baffles below: - Inlet Baffles ➢ Inlet Energy Dissipating & Flocculation Baffles ➢ Sludge Hopper Protection Baffles ➢ Mid-Tank Re-flocculation Baffles ➢ In-Board Launders Transitional Uniform Velocity Effluent Zone Inlet Zone ➢ Lamella Plates/Tubes (Not considered due to limitation of Zone Zone Lamella their applications in the existing PST geometry). Settled Primary Plates/Tubes Protection Braffles
Application of the 3D CFD Model in Configuring the Process Hydraulic Enhancement Modification Inlet Energy Dissipating & Flocculation Baffles ➢ Improve the energy dissipation at the inlet & solids flocculation & hence the solids removal performance. ➢ Solids settled gradually along the full length of the PST ➢ Settled solids re-suspended close to the effluent launder Without Inlet Baffle
Application of the 3D CFD Model in Configuring the Process Hydraulic Enhancement Modification (cont’) Inlet Energy Dissipating & Flocculation Baffles + Mid-Tank Re-Flocculation Baffles ➢ A set of Mid-Tank Re-Flocculation Baffles introduced & simulated. ➢ Significant removal of solids was evident. ➢ Mid-Tank Baffles provided sufficient quiescent velocity distribution on its downstream side to promote rapid settling of the remaining solids. ➢ 2 nd set of Mid-Tank Baffles was not considered. ➢ Existing effluent launders do not need to be modified.
Application of the 3D CFD Model in Configuring the Process Hydraulic Enhancement Modification (cont’) Inlet Energy Dissipating & Flocculation Baffles + Mid-Tank Re-Flocculation Baffles + Sludge Hopper Protection Baffles ➢ High velocity near the sludge hopper at the PST front end caused the sludge blanket to re-suspend. ➢ Sludge Hopper Canopy introduced & simulated. ➢ Highly beneficial for separating the bottom velocity near the settled primary sludge layer.
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