CFD Modeling And Optimization Of Primary Sedimentation Tank - DiVA Portal

understandable since it ignores the hydrodynamic and turbulent of the flow. The separation process taking place is strongly driven by the settling capacity of particles that depends on flow conditions, retention times, particle characteristics, temperature and more. The tank performance can be affected by the eddy currents generated by the inertia of the influent, the circulation zone at the water-air interface (only for the uncovered tank) and the density currents caused by temperature difference between the incoming water and water inside the tank. Due to these factors, phenomenon of the short circuiting in different extent will be observed. (Tchobanoglous, Burton, Stensel, & Eddy, 2003) When designing a primary clarifier, decisions to the configuration, depth, inlet and outlet layout, collection and withdrawal mechanisms of sludge and scum will be made with the aim of maximizing flocculation, controlling biological activities and minimizing the possibilities of undesired hydraulic phenomena such as, eddy and density currents. (Water Environment Federation, 2005) Related concepts of the design considerations for rectangular primary sedimentation tank consist of inlet configuration, sludge hopper size and arrangement, effluent launder requirements, scum withdrawal and collector drive arrangement, covered or uncovered, collector type, flight depth and spacing, flight speed and constant-speed or variable- speed sludge collectors. A primary process functions irrespective of the shape of the tank (rectangular or circular), consists of inlet zone, settling zone and outlet zone. Good sediment performance is primarily determined by an optimal design of the inlet zone and the inlet flow. At the inlet, the flow velocity and turbulence intensity should be reduced to achieve proper flow conditions, so that the sediment diverges from the flow and settles to the bottom. To explore and optimize the flow pattern and mixing regime at the inlet zone of primary tank, CFD (Computational Fluid Dynamic) is an option to achieve that. There are some examples of the simulations that other people have done for primary sedimentation tank which are (Emad Imam, 1983), (Liu B.-C. , Ma, Huang, Chen, & Chen, 2008), (A. Razmi, 2008), (A. Tamayol, 2008), (Fatemeh Rostami, 2010), (Xiaofeng Liu and Marcelo H. García, 2011), (Mahdi Shahrokhi, Fatemeh Rostami, & Said, 2013). However, all the previous works done for rectangular primary tank are for the cases where the inlet aperture either is located the bottom of the tank or just below the water surface and the sludge hopper is ignored. While the inlet of the original tank at Syvab is an overflow orifice by which the water flows into the primary clarifier from the distribution channel and the sludge hopper will be involved in the model. That is different with other cases involved in the literatures. 1.2 Aim and Objectives In this project, with the help of CFD modeling simulations, this thesis aims to investigate and improve the design of the primary sedimentation tank at Syvab and especially the inlet zone under the operational load conditions (Christian Baresel and 2

Anders Björk, 2016). There are three main objectives in the project, as listed below. to identify the flow pattern and contour of turbulence kinetic at the inlet zone including the sludge hopper area of the primary sedimentation tank at Syvab. to find the effective ways to reduce the velocity head and turbulent intensity of the influent. to compare the velocity and turbulence kinetic energy profiles in 4 cases. The performance of settling tank is primarily determined by the flow pattern and mixing regime in the tank. In other words, accurately simulating of the flow pattern and mixing regime is crucial to predict the tank efficiency under the different operational conditions. (Fatemeh Rostami, 2010) So, the final intention of the project is to achieve proper sedimentation performance. 3

2. Literature review The methods applied to this project mainly include literature review, consultation with experts at IVL and CFD implementation. The literature review mainly focuses on selecting a completed and verified model and method and the proposed optimization approaches for inlet zone and inlet flow. Because of the differences on the characteristics of the receiving water of the primary sedimentation tank and secondary sedimentation tank and the different roles they played in sewage treatment plant, the predominantly hydraulic phenomena in each tank are different and that results in the corresponding modeling components and methods applied to build the CFD model are different. (Peter Krebs, 1995) pointed the design of inlet zone for primary and secondary sedimentation tank cannot be optimized in the same way since primary clarifier inlet design mainly focuses on the kinetic energy dissipation, while the secondary clarifier need to consider the effects of density currents because of the differences in temperature or mean suspended solids concentration between the influent and the original wastewater in the tank (D. A. Lyn, 1992) on the flow field and the particle flocculation. From the description above, a distinction between primary clarifier and secondary clarifier has to be made when doing a literature review. It naturally led to that the published articles about CFD of primary tank will be read in a more details. 2.1 Selection of the model and method to be employed First, the decision on the model and method that will be adopted in this project has to be made. All the numerical modeling of primary sedimentation tank already been published were checked. The following is a summary regarding that. Modeling of the velocity field and solid concentration distribution in the tank are the main task of the numerical simulation. It implies there are two sub-models to describe the hydrodynamic and solid transport phenomena, respectively. In the work of (Emad Imam, 1983), the vorticity transport and stream-function equations of the governing unsteady Eulerian equations with a constant turbulent eddy viscosity applying a partial slip condition on the wall constitutes the hydrodynamic sub-model. Solids concentration distribution in the tank is obtained by solving convective-diffusion equation. Similar work was done by (Liu B.-C. , Ma, Huang, Chen, & Chen, 2008), where the turbulent flow and mass transfer were taken into the consideration. The suspended solids concentration is an important characteristic of the sewage. Good knowledge of the characteristics of the wastewater is the initial step to achieve the proper design of primary clarifier. (Water Environment Federation, 2005) For the reason of low suspended solid concentration in primary clarifiers, the concentration field only has a relatively little effect on flow filed and buoyancy effects can be left out. 4

(A. Tamayol, 2008) The same judgement can be found in the article of (A. Razmi, 2008) which is that the secondary sedimentation tank is placed after aeration tank so activated sludge will be included in the influent and that results in the growing of particle size and the simulation of flow field in secondary tank has to take the solid concentration distribution into account while not in primary tank. Even some experiments already been done to investigate the flow pattern in primary clarifier and been applied to calibrate and validate the numerical modeling, are pure-water tests, for instance the experiments of (Weidner 1967), (Larsen 1977), (Lyn and Rodi 1990), (A. Razmi, 2008), (Liu B., et al., 2010). (Peter Krebs, 1995) Then, (Fatemeh Rostami, 2010) built a 2D one phase (pure water) CFD to simulate the flow pattern of the settling tank which contains the streamlines, velocity fields, and turbulence kinetic energy. VOF (Volume Of Fluid) was selected to model the air-water multi-phase. RNG (Re-Normalization Group) k-ε model was applied to calculate the turbulent quantities, i.e. the kinetic energy(k) and its dissipation rate (ε). It was applied to study the effects of different positions and numbers of inlet apertures on the flow pattern in primary clarifier. One year later, (Mahdi Shahrokhi F. R., 2011) used the same model but with a different turbulent model, Standard k-ε model instead of RNG k-ε model to explore the effects of different configuration of baffles on flow pattern in primary tank. The difference between the two turbulent models and the reason of our choice will be described in numerical modeling section. Compared to VOF model, two-fluid model is the most complicated multiphase model involved in ANSYS Fluent since each phase in two-fluid model has a set of continuity and momentum equations. In the research of (Xiaofeng Liu and Marcelo H. García, 2011), the behavior of suspended solids-water mixture in the primary circular sedimentation tanks in Chicago were simulated by two-fluid model. Turbulence kinetic energy and the dissipation rate are calculated by the k-ε turbulence model. In the same time, the sludge flow and hindered particle settling process were studied by the relevant expressions. Concerning the aim and objectives of this project which only focuses on the velocity head and turbulent intensity of the influent and its variability in inlet zone and the license available to student, the CFD code ANSYS Fluent is selected and the multiphase flow modeling and turbulence modeling will be achieved by VOF method and RNG k-ε turbulence model, respectively. 2.2 Proposed optimization methods in literatures Inlet design of the primary sedimentation tank should focus on dissipating the kinetic energy or velocity head of the sewage, avoiding short circuiting, alleviating the density currents effects and lowering the blanket disturbances. (Fatemeh Rostami, 2010) Baffles have been extensively applied to dissipate the kinetic energy of the influent and distribute the incoming flow over the whole cross section to stabilize the flow and avoid 5

short circuiting, especially when a buoyant density current exists. (Emad Imam, 1983) (A. Tamayol, 2008) pointed that the baffle should be installed in a proper location with an appropriate submergence depth, otherwise it will even worsen the tank performance instead of improving and concluded the baffle should be put where the circulation zone exists to destroy it. The numerical model developed by (Emad Imam, 1983) was taken to study the effects of relative baffle submergence on a real rectangular tank performance. The relative baffle submergence was defined as the ratio of the baffle length and the tank depth. When the ratio decreased from 0.67 to 0.27, the removal rate increased from 0.65 to 0.75, instead. However, if continuing decreasing the relative baffle submergence, the removal rate does not increase any more. In addition, it should be noticed that an even high baffle submergence may disadvantageously the tank performance since a high submergence results in a ‘shallow tank’ where a bulky dead zone over the live stream exists. One can conclude that probably a ratio around 0.3 should be selected to minimize the incoming flow jet. 5 cases (as shown below, a-e) with the different positions and numbers of the inlet apertures of the settling tank were simulated with the aim of reducing high-velocity currents and averting the flow jets. Based on the simulation of flow pattern, velocity profiles, turbulence kinetic energy, flow through curves and hydraulic efficiency, (Fatemeh Rostami, 2010) concluded that the inlet aperture in the bottom is better than in the surface and a middle aperture (case b) is preferred if only one is available; otherwise, the configuration of two apertures (case d) is better. Figure 1. 5 cases of velocity inlet. (Fatemeh Rostami, 2010) Closely following the research of (Fatemeh Rostami, 2010), (Mahdi Shahrokhi F. R., 2011) investigated the various numbers of baffles with the same height on the tank performance by analyzing the simulated flow patterns and the Flow Through Curves. The results show that the proper placement of a suitable number of baffles results in a minimum volume of recirculation region, kinetic energy dissipation and then produce a uniform flow field in the tank so the sedimentation capacity of the tank is increased, finally. (A. Razmi, 2008) found that the optimal distance (d) between the inlet slot and a single baffle is 12.5% of the tank length (L). With this finding, (Mahdi Shahrokhi F. R., 2011) concluded that the volume of dead zone would decrease as the number of baffles increases however, the positions of these baffles are more important but it’s unpractical with too much baffles so finally suggestions of two baffles located at d/L 0.125 and 0.388 and three baffles located at d/L 0.125, 0.3 and 0.388 are given 6

because of the lowest volume of circulation zone. By (Liu B. , et al., 2010), the two-dimensional laser Doppler velocimetry (2D LDV) was applied to measure the flow velocity of the primary rectangular settling tanks in the horizontal and vertical directions under 5 cases with various flow rates and reaction baffle heights. Suggested values for the relative baffle submergence height and the ratio of tank length to height were given in a low suspended solid (LSS) concentration conditions (LSS 150-200mg/L). A large circulation zone behind the baffle was observed both in their experiment and numerical simulation. The recirculation length increases with an increased flow rate or an increased submerged depth of the reaction baffle. However, if we compare the influence of the variation of flow rate and submerged depth of reaction baffle on flow field, the latter one is stronger. So, providing an optimal relative submergence depth under the various flow rate, which is around 0.2 – 0.5, is more worthy and practical. In addition, to improve the removal rate and make full use of the tank dimension, the length-to-height ratio for a rectangular sedimentation tank should be within the range of 8-12. 7

3. Case study of Himmerfjärd Himmerfjärd is a wastewater treatment plant owned by SYVAB and located north of Hörningsnäs in Botkyrka. Over here the wastewater from Botkyrka, Huddinge, Nykvarn, Salem, Southwestern Stockholm and Södertälje will be treated and released into Himmerfjärden, a bay situated between Mörkö and Södertörn in the south archipelagos of Stockholm. (WIKIPEDIA, 2011) There are 14 “Squircle” primary sedimentation tanks at the plant, each of which is a rectangular tank equipped with a circular sludge hopper in the influent end. According to the latest board documents from Syvab, the plant receives and processes 130000m3 wastewater every day. (Syvab, 2017) Primary sedimentaion tanks Figure 2. Processes map of Syvab. (SYVAB, 2017) As shown above, this is the whole facilities installed in Syvab wastewater treatment plant. Color blue represents all the transport and treatment installations for wastewater and the corresponding flow direction. The wastewater treatment mainly includes socalled pre-treatment, primary treatment, secondary treatment and tertiary. The pretreatment mainly refers to Number 4 to separate the solids larger than 20mm by coarse bar screens, Number 7 to settle large particles, Number 9 to remove the materials larger than 2mm by 2 fine bar screens. What is called primary treatment step in the whole processes refers to Number 11, as circled in Figure 2, 14 parallel 50-meters primary sedimentation tanks to remove the suspended solids and COD or BOD. Aeration tanks, number 13 and secondary sedimentation tanks, number 15 constitute the secondary treatment system. In aeration tanks, the organic materials and phosphorous as the ‘food’ will be eaten by microorganisms. In addition, nitrogen removal processes, nitrification where the ammonium will be oxidized to nitrate and denitrification where the nitrates will be converted to nitrogen gas will take place, too. In secondary sedimentation tanks, the biological sludge is settled and part of them will be sent back to aeration tank and almost all the rest of them will be treated as excess sludge. 8

The drawings of the tank were provided by supervisors at IVL and staff at Syvab to define the computational domain. However, to fully understand how does the water flow into and drain out from the tank and collect the relevant data, an on-site visit was executed on 27th of March 2017. 3.1 Construction of 4 cases Firstly, the flow characteristics predominantly existed in the origin

processes involved in the plant, primary sedimentation tank has a greater potential to remove more TSS (Total Suspended Solids), COD or BOD with a less operational cost. (Water Environment Federation, 2005) This project will only focus on primary sedimentation tank. The earliest theory applied to design the sedimentation unit was the