A compartmental model to describe hydraulics in a full-scale waste stabilization pond

The advancement of experimental and computational resources has facilitated the use of computational fluid dynamics (CFD) models as a predictive tool for mixing behaviour in full-scale waste stabilization pond systems. However, in view of combining hydraulic behaviour with a biokinetic process model, the computational load is still too high for practical use. This contribution presents a method that uses a validated CFD model with tracer experiments as a platform for the development of a simpler compartmental model (CM) to describe the hydraulics in a full-scale maturation pond (7 ha) of a waste stabilization ponds complex in Cuenca (Ecuador). 3D CFD models were validated with experimental data from pulse tracer experiments, showing a sufficient agreement. Based on the CFD model results, a number of compartments were selected considering the turbulence characteristics of the flow, the residence time distribution (RTD) curves and the dominant velocity component at different pond locations. The arrangement of compartments based on the introduction of recirculation flow rate between adjacent compartments, which in turn is dependent on the turbulence diffusion coefficient, is illustrated. Simulated RTD’s from a systemic tanks-in-series (TIS) model and the developed CM were compared. The TIS was unable to capture the measured RTD, whereas the CM predicted convincingly the peaks and lags of the tracer experiment using only a minimal fraction of the computational demand of the CFD model. Finally, a biokinetic model was coupled to both approaches demonstrating the impact an insufficient hydraulic model can have on the outcome of a modelling exercise. TIS and CM showed drastic differences in the output loads implying that the CM approach is to be used when modelling the biological performance of the full-scale system.     

On CFD simulation of wind-induced airflow in narrow ventilated facade cavities: Coupled and decoupled simulations and modelling limitations

Heat and mass transfer modelling in building facades with ventilated cavities requires information on the cavity air change rates, which can be a complex function of the building and cavity geometry and the meteorological conditions. This paper applies Reynolds-averaged Navier–Stokes (RANS) CFD to study wind-induced airflow in the narrow (23 mm) ventilated facade cavities of an isolated low-rise building. Both coupled and decoupled simulations are performed. In the coupled simulations, the atmospheric boundary layer wind-flow pattern around the building and the resulting airflow in the cavities are calculated simultaneously and within the same computational domain. In the decoupled simulations, two separate CFD simulations are conducted: a simulation of the outdoor wind flow around the building (with closed cavities) to determine the surface pressures at the position of the cavity inlet and outlet openings, and a simulation of the cavity airflow, driven by these surface pressures. CFD validation is performed for the external and internal (cavity) flows. It indicates an important modelling limitation: while both laminar and turbulent cavity airflow can be accurately reproduced with low-Reynolds number modelling, this method fails in the transitional regime. The valid CFD results (outside the transitional regime) are analysed in terms of cavity airflow patterns and cavity air change rates per hour (ACH) for different cavity positions, wind speeds and wind directions. The CFD results of cavity air speed and ACH compare favourably with values from previous experimental studies. The coupled and decoupled simulation results are compared to provide an indication of the local losses. It is concluded that future work should focus on adapting RANS CFD low-Reynolds number models to accurately model cavity flow in the transitional regime.     

Measurement and three-dimensional simulation of flow in a rectangular detention tank

There are two main ways to obtain better knowledge of the hydraulics of ponds, namely measurements and simulations. In this study, the applicability of using three-dimensional simulations as an engineering tool in stormwater pond design was investigated. To do this, three-dimensional simulations were compared with measurements of flow pattern and residence time in a large physical model of a detention tank (13 × 9 × 1 m). The agreement between measurements and simulations concerning both flow pattern and residence time distribution curves was found to be good for high flow rates.     

城市的蓄水囊--滞留池和储水池在美国园林设计中的应用

滞留池和储水池作为雨水控制结构,几乎用于美国的每一个新建项目中.阐述了滞留池、扩展型滞留池和储水池的特点、用途和设计结构.并通过实例说明滞留池尺寸的基本计算方法.在实践中,风景园林师通过湿地植物配置,可以将这种雨水控制结构与园林设计相结合,创造出雨水湿地.介绍了雨水湿地的种类、特点和植物选择.     

Modelling of coliform removal in 186 facultative and maturation ponds around the world

The paper presents a very extensive evaluation of the coliform decay in facultative and maturation ponds, based on data from 186 different ponds in the world. The ponds encompass a very wide diversity in terms of physical and operating conditions, covering most situations encountered in practice. The median values for the coliform removal efficiencies were 1.8 log units (98% removal) for primary facultative ponds, 1.0 log units for secondary facultative ponds (90% removal) and 1.2 log units (94% removal) for each maturation pond in the series. Two equations to be used for design purposes were derived for estimating the die-off coefficient K(b) (dispersed flow, 20 degrees C) in facultative and maturation ponds. The first equation led to a slightly better fitting with the observed logarithm of the effluent coliform concentrations (R2 = 0.874), and related K(b) with the pond detention time t and depth H (K(b) = 0.682 H(-1.286) t(-0.103)). The other equation also led to a satisfactory fitting (R2 = 0.845), but was slightly simpler, depending only on the pond depth (K(b) = 0.549 H(-1.456)).     

Bod removal rates of waste stabilization ponds as a function of loading, retention time, temperature and hydraulic flow pattern

This study evaluates the influence of loading, detention time and temperature on the first order BOD removal coefficient K₁. Numerical values of K₁ are derived from semi-continuous-flow laboratory units of sewage ponds and introduced into the design formula for a multi-stage continuous flow reactor. There was a good correspondence (r = 0.92) between the computed BOD removal rates and the empirical results obtained from three large-scale multi-stage pond systems.     

Mixing characterisation of full-scale membrane bioreactors: CFD modelling with experimental validation

Membrane Bioreactors (MBRs) have been successfully used in aerobic biological wastewater treatment to solve the perennial problem of effective solids-liquid separation. The optimisation of MBRs requires knowledge of the membrane fouling, biokinetics and mixing. However, research has mainly concentrated on the fouling and biokinetics (Ng and Kim, 2007). Current methods of design for a desired flow regime within MBRs are largely based on assumptions (e.g. complete mixing of tanks) and empirical techniques (e.g. specific mixing energy). However, it is difficult to predict how sludge rheology and vessel design in full-scale installations affects hydrodynamics, hence overall performance. Computational Fluid Dynamics (CFD) provides a method for prediction of how vessel features and mixing energy usage affect the hydrodynamics. In this study, a CFD model was developed which accounts for aeration, sludge rheology and geometry (i.e. bioreactor and membrane module). This MBR CFD model was then applied to two full-scale MBRs and was successfully validated against experimental results. The effect of sludge settling and rheology was found to have a minimal impact on the bulk mixing (i.e. the residence time distribution).     

A novel tracer technique for the assessment of fine sediment dynamics in urban water management systems

Urban storm water run off can reduce the quality of receiving waters due to high sediment load and associated sediment-bound contaminants. Consequently, urban water management systems, such as detention ponds, that both modify water quantity through storage and improve water quality through sediment retention are frequently-used best management practices. To manage such systems effectively and to improve their efficiency, there is a need to understand the dynamics (transport and settling) of sediment, and in particular the fine sediment fraction (<63 μm) and its associated contaminants within urban storm water management systems. This can be difficult to achieve, as modelling the transport behaviour of fine-grained and cohesive sediment is problematic and field-based measurements can be costly, time-consuming and unrepresentative.The aim of this study was to test the application of a novel cohesive sediment tracer and to determine fine sediment transport dynamics within a storm water detention pond. The cohesive sediment tracer used was a holmium labelled montmorillonite clay which flocculated and had similar size and settling velocity to the natural pond sediment it was intended to mimic. The tracer demonstrated that fine sediment was deposited across the entire pond, with the presence of reed beds and water depth being important factors for maximising sediment retention. The results of the sediment tracer experiment were in good agreement with those of a mathematical sediment transport model. Here, the deposited sediment tracer was sampled by collecting and analysing surface pond sediments for holmium. However, analysis and sampling of the three dimensional suspended tracer ‘cloud’ may provide more accurate information regarding internal pond sediment dynamics.     

Impact of design and operation variables on the performance of vertical-flow constructed wetlands and intermittent sand filters treating pond effluent

With the aim of improving the quality of the effluent from a waste stabilization pond (WSP) different types of vertical-flow constructed wetlands (VFCWs) and intermittent sand filters (ISFs) were tested at a pilot plant in Aurignac (France). The effectiveness of each design at upgrading the pond effluent was studied over a period of 2 years. Physicochemical parameters were monitored by taking composite samples over 24 h and grab samples every week. The hydraulic behaviour of the filters was studied using (NaCl) tracer tests and monitoring the infiltration rate. This paper describes the influence on the performance of the beds of: (a) the characteristics of the medium (type of sand, depth, and presence of Phragmites); (b) feed modes; and (c) the presence of an algae clogging layer. The study demonstrates the viability of VFCWs and ISFs as means of upgrading effluent from WSPs. For hydraulic loads (HL) of up to 80 cm/day, both technologies effectively retain algae, complete organic matter degradation, and nitrify the pond effluent. The presence of plants did not significantly affect the performance of the filters although it was important in terms of maintenance. The deeper filters presented better removals for all the parameter tested, due to higher hydraulic detention times (HDTs). The dosing regime and resting period duration all affected the hydraulic performance and purification efficiency of the filters.     

Dynamic thermal building analysis with CFD – modelling radiation

In an attempt to reduce the high computational effort required for dynamic thermal simulation of buildings using computational fluid dynamics (CFD) the authors have recently developed an adaptive freeze-flow method (i.e. freezing of flow equations over variable time periods). This article documents the work that has been carried out to predict the surface heat transfer in dynamic thermal building processes using CFD with particular focus on radiation. The Monte Carlo (MC) and discrete transfer (DT) radiation models were investigated and results compared with analytical solutions. The DT model has shown good performance whereas an unrealistic radiation distribution on the surfaces was observed when using the MC model. A further investigation of the DT model for the cooling of a solid wall has shown that the adaptive freeze-flow method is an efficient and accurate means of conducting dynamic thermal CFD simulations which involve radiation. Finally, application of the technique to a more realistic space comprising an uneven distribution of solar gain showed very good results when compared with a zonal dynamic thermal simulation program.     

Comparison of coarse grid lattice Boltzmann and Navier Stokes for real time flow simulations in rooms

Coarse grid lattice Boltzmann models (LBM) and computational fluid dynamics (CFD) are compared in building simulations. Three simulations are used to assess the different numerical models: (i) a 2D isothermal flow case, (ii) a 3D isothermal flow case and (iii) a 2D non-isothermal flow case. Both models predicted the correct flow patterns and temperature field and could be used for real time (RT), near real time (NRT) and faster than real time (FRT) simulations without loss of accuracy on multi-core processors. The results indicate that the coarse grid CFD is both faster and more time accurate than the LBM for unsteady simulations.     

Development of draft tube in hydro-turbine: a review

The efficiency of a hydraulic reaction turbine is significantly affected by the efficiency of its draft tube. The shape (profile) and velocity distribution at the inlet affect the performance of the draft tube. So far, the design of draft tubes has been improved through experimental observations resulting in ‘rules of thumb’ and empirical formulae. In the last two decades the use of computational fluid dynamics (CFD) for research and designing complex profiles has improved significantly due to its flexibility and cost-effectiveness. A CFD-based design can further be aided with robust and user-friendly optimisation. Numerical analysis of fluid through a draft tube is challenging and time consuming due to complex flow features. Hence there is a need for developing accurate and reliable CFD models together with efficient optimisation. Studies of the principles of draft tube, internal flow pattern, various turbulence models and associated divergence along with results have been presented in this paper. The objective of this paper is to present the application of CFD simulation in design and flow analysis of the draft tube and also find out the factors which influence the deviation of CFD results with experimental results. From the literature, it has been observed that there are several factors (accurate inlet conditions, turbulent models selected for simulation, modification in geometric parameters and accuracy in measurement of experimental results) that influence the draft tube design and performance. Thus, there is a scope of research for optimisation of geometrical parameters of the draft tube for its best performance at full load condition using CFD simulation. It is carried out by applying 3D velocity as an inlet boundary condition measured with particle image velocimetry/laser Doppler velocimetry.     

Elimination of Salmonella in stabilization ponds

While the reduction of putrescible organic matter is important in sewage treatment, the microbiological quality of the effluents is of great public health significance, since these are usually discharged into streams or onto land. The performance of two types of stabilization ponds located at Nagpur, India, was studied with respect to the reduction of Salmonella, Coliforms, E. coli and faecal streptococci. The two ponds were operated at different BOD loading and detention time. Furthermore while one had two cells with submerged interconnection the other had three cells interconnected with a surface overflow arrangement. Comparing the results of these two ponds, treating domestic sewage, it is found that Salmonella does not get eliminated in the two celled pond while they are absent in 1 litre aliquots of effluent samples from the three celled pond. The indicator organisms are reduced to a great extent in the latter pond as compared to the former. From this it appears that the number of cells and the interconnecting arrangements are very important besides the loading, detention period and other design features.     

CFD study to determine the optimal configuration of aerators in a full-scale waste stabilization pond

Aerated lagoons (ALs) are important variants of the pond wastewater treatment technology that have not received much attention in the literature. The hydraulic behaviour of ALs and especially the Facultative aerated lagoons (FALs) is very complex since the aeration in these systems is designed for oxygen transfer but not necessarily to create complete mixing. In this work, the energy expenditure of the aerators was studied by means of a scenario analysis. 3D CFD models (one phase and multiphase) of a 3 ha FAL in a waste stabilization pond system in Cuenca (Ecuador) were built for different configurations of aerators. The thrust produced by the aerators was modelled by an external momentum source applied as velocity vectors into the pond fluid. The predictions of a single phase model were in satisfactory agreement with experimental results. Subsequently, a scenario analysis assessing several aeration schemes with different numbers of aerators in operation were tested with respect to velocity profiles and residence time distribution (RTD) curves. This analysis showed that the aeration scheme with all 10 aerators switched on produces a similar hydraulic behaviour compared to using only 6 or 8 aerators. The current operational schemes comprise of switching off some aerators during the peak hours of the day and operating all 10 aerators during night. This current practice could be economically replaced by continuously operating 4 or 6 aerators without significantly affecting the overall mixing. Furthermore, a continuous mixing regime minimises the sediment oxygen demand enhancing the oxygen levels in the pond.     

Optimisation of structured grid spacing parameters for separated flow simulation using mathematical optimisation

This paper describes the use of computational fluid dynamics (CFD) and mathematical optimisation techniques to minimise the error in predicting the recirculation zone for a separated flow topology. Grid spacing parameters are varied in the optimisation process. The accuracy of separated flow solutions is known to be dependent on the grid resolution and clustering. Although general guidelines have been developed for grid generation of separated flow topologies, the flow solutions using the resulting grids often under-predict features like recirculation zones. This study addresses this aspect by providing an automatic tool for optimising the grid for solution accuracy. This approach has until recently been too expensive, but is becoming more viable with ever-increasing computer power. A two-dimensional sinusoidal hill is used as an example of a separated flow topology. The CFD simulation employs the commercial CFD solver STAR-CD to solve the Reynolds-Averaged Navier–Stokes equations with the RNG k– turbulence model. CFD solution time is drastically reduced by making use of initial field restarts. The optimisation is carried out by means of Snyman's DYNAMIC-Q method, which is specifically designed to handle constrained problems where the objective or constraint functions are expensive to evaluate. Six design variables (grid spacing parameters) are considered in this study. The results indicate that the re-attachment point of the recirculation zone is predicted to within 1% of the specified experimental value in four optimisation iterations and therefore represents a cost-effective way to determine grids based on solution accuracy.     

Computational analysis of reduced-mixing personal ventilation jets

In this paper we develop a detailed computational fluid dynamics (CFD) model of a personal ventilation (PV) setup comprising a PV nozzle, seated thermal manikin and floor diffuser, then use experimental velocity and tracer gas concentration data for the same setup to validate the CFD model. Specifically, we compare CFD results with the experimental results obtained with both a conventional round nozzle and a novel low-mixing co-flow nozzle directing a PV fresh air jet toward the breathing zone (BZ) of a seated thermal manikin in a thermally controlled chamber ventilated also by a floor diffuser behind the manikin. The CFD model shows excellent agreement with the experimental data. We then exercise the CFD model to study the effect of nozzle exit boundary conditions such as turbulence intensity and length scale, flow rate and temperature, and manikin temperature on the air quality in the BZ of the heated manikin. It is shown that the air quality of the novel PV system is sensitive to the nozzle exit turbulence intensity and flow rate, and insensitive to jet temperature within the 20–26 °C range, and to body temperature within a clo range of 0–1. A companion paper presents in detail the experimental set up and results used to validate the CFD model discussed in this paper.     

Calculation and design of tunnel ventilation systems using a two-scale modelling approach

This paper develops a novel modelling approach for ventilation flow in tunnels at ambient conditions (i.e. cold flow). The complexity of full CFD models of flow in tunnels or the inaccuracies of simplistic assumptions are avoided by efficiently combining a simple, mono-dimensional approach to model tunnel regions where the flow is fully developed, with detailed CFD solutions where flow conditions require 3D resolution. This multi-scale method has not previously been applied to tunnel flows. The low computational cost of this method is of great value when hundreds of possible ventilation scenarios need to be studied. The multi-scale approach is able to provide detailed local flow conditions, where required, with a significant reduction in the overall computational time. The coupling procedures and the numerical error induced by this new approach are studied and discussed. The paper describes a comparison between numerical results and experimental data recorded within a real tunnel underlining how the developed methodology can be used as a valid design tool for any tunnel ventilation system.This work sets the foundations for the coupling of fire-induced flows and ventilation systems where further complexities are introduced by the hot gas plume and smoke stratification.     

Treatment of stormwater using a detention pond and constructed filters

A stormwater treatment plant, consisting of a detention pond, a constructed filter system and a constructed wetland, has been investigated according to stormwater quality improvement, sediment and heavy metals accumulation and potential toxicity of the stormwater and sediment. The reduction of metal content in the detention pond was on average 26?–?84%. No acute toxicity in the stormwater was detected although heavy metal levels often exceeded guideline values during storm events. Pore water samples of the collected sediments were not toxic but the whole sediment was toxic when assessed with the Microtox^® Solid-phase test. The constructed filter system became clogged due to cementation of the filter substrate.     

Development of a fast fluid dynamics-based adjoint method for the inverse design of indoor environments

The computational fluid dynamics (CFD)-based adjoint method may be appropriate for the inverse design of indoor environments, considering both accuracy and efficiency, but a single design still requires tens of hours with the use of a personal computer. To speed up the inverse design process, this study evaluated four fast fluid dynamics (FFD) models in terms of solving the Navier–Stokes equations, integration with turbulence models, and solving the adjoint equations. This study implemented the FFD solvers in OpenFOAM and validated them for predicting steady-state and transient indoor airflow. This study then validated the FFD solvers for solving the adjoint equations and the FFD-based adjoint method for inverse identification problems and inverse designs in indoor environments. The results showed that FFD was 20 times faster than CFD in predicting transient indoor airflow, and similar computational accuracy could be maintained; the FFD-based adjoint method was 4–16 times faster than the CFD-based adjoint method in the inverse design process.     

Importance of flow stratification and bubble aggregation in the separation zone of a dissolved air flotation tank

The importance of horizontal flow patterns and bubble aggregation on the ability of dissolved air flotation (DAF) systems to improve bubble removal during drinking water treatment were explored using computational fluid dynamics (CFD) modeling. Both analytical and CFD analyses demonstrated benefits to horizontal flow. Two dimensional CFD modeling of a DAF system showed that increasing the amount of air in the system improved the bubble removal and generated a beneficial stratified horizontal flow pattern. Loading rates beyond a critical level disrupted the horizontal flow pattern, leading to significantly lower bubble removal. The results also demonstrated that including the effects of bubble aggregation in CFD modeling of DAF systems is an essential component toward achieving realistic modeling results.