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Three-Dimensional Hydrodynamic Behavior of an Operational Waste-Stabilization Pond
Abstract
Waste stabilization ponds (WSPs) are a common wastewater treatment approach throughout the world. Typically, WSPs are designed with the aid of empirical equations that may not incorporate complex hydrodynamics. Whereas numerical models have been presented as an alternative, they have been limited to two dimensions or lacked validation. In the present study, field monitoring and a high-resolution three-dimensional Delft3D model, incorporating inflows, changing water levels, and wind, are combined to deepen the understanding of WSP hydrodynamics. Observed water levels varied considerably and rapidly, changing by up to 0.5 m within 36 h, and velocities were typically very low (<0.01 m/s), reaching peak speeds of 0.03 m/s during strong winds. Modeled velocities were in good agreement with observations in open water areas, with a root mean square error (RMSE) of 0.003 m/s and an R2 of 0.58. The results indicate that circulation was primarily driven by wind, with a smaller contribution from varying water levels, and circulation patterns were classified into four hydraulic regimes. A dimensionless empirical equation was developed relating the longitudinal current to wind speed, direction, and WSP outflows, providing a tool to predict hydraulics in shallow and enclosed water bodies and representing an important step toward incorporating hydraulic complexity into WSP design.
... The governing equations were integrated using an implicit finite difference scheme (ADI) on the staggered grid. Recent applications of DELFT3D to model lake hydrodynamics include Baracchini et al. (2020), Amadori et al. (2020), and Rey et al. (2021). ...
Patagonian lakes are one of the most unexplored aquatic environments on Earth, and little is known about their thermo-hydrodynamics and current trophic state. Meanwhile, increasing urbanization and industrialization in their catchments compromise their health. Here, we investigate Lake Llanquihue, one of the Earth's great freshwater bodies in Northern Patagonia, Chile. Still considered a pristine environment, Llanquihue has experienced recent contamination events along its littoral, whose impacts remain unknown. In response, public and private agencies have started to develop tailor-made monitoring plans to survey water quality. However, without comprehensive knowledge of the lake's functioning, it is impossible to determine the fate and effects of contaminants in the water body. Here, we characterize via numerical simulations the basin-scale hydrodynamics of Lake Llanquihue, crucial information for diagnosing the transport of dissolved and suspended matter within its basin. Aiming to delimit the regions impacted by contaminated discharges, we pose a fundamental question that applies to any lake: What is the physical connectivity between two zones of interest within a lake? To address this question, we introduce a framework that characterizes the preferential pathways and quantifies the timescale tracers take to stream from one zone to another within an aquatic system. This framework is applied to investigate the physical connectivity among the main urban and rural settlements connected to the littoral zone of Lake Llanquihue. Our results show that the physical connectivity between long-distance littoral regions is primarily controlled by a time-persistent, large-scale gyre and seasonal mesoscale gyres. The above information will enable lake managers to monitor the health of the water basin, identify and notify potential risk zones impacted by polluted effluents, and advance toward a more sustainable relationship with Lake Llanquihue.
... Wind is the main stress for driving currents in large water bodies (Hutter et al., 2011;MacIntyre et al., 2020;Rey et al., 2021;Schoen et al., 2014). Wind stress on the water surface has received considerable research attention in the field of hydrodynamics (Jeffreys, 1925;Munk, 1955;Wu, 1980;Shchepetkin and McWilliams, 2005;Chen et al., 2020). ...
Wind stress, wind waves, and turbulence are essential
variables and play a critical role in regulating a series of physical and
biogeochemical processes in large shallow lakes. However, the
parameterization of these variables and simulation of their interactions in
large shallow lakes have not been strictly evaluated owing to a lack of
field observations of lake hydrodynamic processes. To address this problem,
two process-based field observations were conducted to record the
development of summer and winter wind-driven currents in Lake Taihu, a large
shallow lake in China. Using these observations and numerical experiments, a Wave and Current Coupled Model (WCCM) is
developed by
rebuilding the wind drag coefficient expression, introducing wave-induced
radiation stress, and adopting a simple turbulence scheme to simulate
wind-driven currents in Lake Taihu. The results show that the WCCM can
accurately simulate the upwelling process driven by wind-driven currents
during the field observations. A comparison with a reference model indicates
a 42.9 % increase of the WCCM-simulated current speed, which is mainly
attributed to the new wind drag coefficient expression. The WCCM-simulated
current direction and field are also improved owing to the introduction of
wave-induced radiation stress. The use of the simple turbulent scheme in the
WCCM improves the efficiency of the upwelling process simulation. The WCCM
thus provides a sound basis for simulating shallow lake ecosystems.
... Computational fluid dynamics (CFD) overcomes a number of inherent traditional design method limitations and provides tools to predict hydrodynamics, turbulence, treatment, and visualization. Especially with significant advances in computational hardware, processing, and numerical algorithms, CFD is now widely used for design analysis and optimization in mechanical, biomedical, nuclear, wind, and chemical application and is increasingly documented for municipal applications of urban water treatment engineering (Stovin and Saul 1996;Persson et al. 1999;Adamsson 1999;Li et al. 2004;Liu and García 2011;Zhang et al. 2014b;Yan et al. 2014;Garofalo and Sansalone 2018;Spelman and Sansalone 2018a;Sonnenwald et al. 2018Sonnenwald et al. , 2019bLiu and Zhang 2019;Yan et al. 2020;Li et al. 2021c;Nakhaei et al. 2019;Rey et al. 2021). For example, Liu and García (2011) applied CFD to improve the design of primary clarifiers in the Metropolitan Water Reclamation District of Greater Chicago's Calumet Water Reclamation Plant. ...
Stormwater basins, which are subject to highly unsteady flows and loads, function as clarifiers for particulate matter (PM) and PM-bound constituents. Quasi-steady flow hydrodynamics in water and wastewater treatment plant units, from grit to contact chambers, are known to impact treatment. In contrast, there is sparse basin design guidance for unsteady hydrodynamics and clarification interactions. Basins can become impaired with needed retrofits. Lumped models, such as surface overflow rate (SOR), are less suitable for complex geometries, hydrodynamics, and loadings. A computational fluid dynamics (CFD) and evolutionary optimization framework as higher-fidelity tools is proposed for relatively shallow basins with permeable baffle retrofits. This study extends previous research and optimizes baffle configuration by further considering routing effects and unsteady loading on clarification. For an influent heterodisperse particle size distribution (PSD), baffles improve PM clarification by 58.8% based on baffle number. Strategically altering the baffle position, length, and angle yields an optimal configuration with improved clarification of 32% in comparison to conventional baffle configurations. Baffle optimization leads to construction costs that are one-third to one-fifth those of conventional baffle configurations.
Stratification in constructed urban stormwater wetlands is one of the fundamental physical processes that affect hydrodynamics, transport and fate of stormwater pollutants. Adverse effects of stratification include decreasing pollutant retention capacity, causing the water at lower depths to become anoxic, degrading water quality and increasing stress on the downstream aquatic communities. The current study reports on a comprehensive field monitoring program of stratification and hydrodynamics in two ice-free seasons (May - October) in two constructed urban stormwater wetlands in Calgary, Canada, with different inlet, outlet, morphometric and vegetation designs. Despite their small sizes of 0.5 and 1.2 ha and shallow water depths of 0.8 m, stratification was strong and persistent in the wetlands. The response of stratification and mixing to atmospheric forcings (e.g., air temperature, atmospheric instability, rainfall depth, wind speed) and the impact of design characteristics (inlet/outlet design, water depth, surface area and aquatic vegetation) were examined and discussed. Thermal stratification, defined as a vertical temperature gradient >1 °C/m, was found to be significantly higher (up to ten times) near the inlets and last longer (up to twice) than in the main cells and the outlet basins due to the relatively cold summer inflows. The wetland with twice the permanent water volume and surface area and half the length-to-width ratio had denser submerged aquatic vegetation, higher (by up to 2 °C) water temperature and more severe (up to eight times) thermal stratification. Strong densimetric stratification and low wind stress on the water surface caused hypoxic conditions near the bed, potentially adversely affecting water quality and downstream aquatic communities.
Stratification is one of the fundamental physical processes that may have a significant impact on water quality in stormwater wet ponds. However, the role of thermal and chemical stratifications in governing water quality processes is not fully understood. This is in part due to the lack of detailed field measurements of sufficient governing parameters over time periods that span a wide range of environmental conditions. To fill this gap, a comprehensive 2-year field program was undertaken in two stormwater wet ponds in Calgary, Alberta, Canada, during the ice-free season from May to November in 2018 and 2019. At different locations in each pond, thermal and chemical stratifications were observed, thermocline depth and strength were determined, and continuous water velocity profiles were measured. In addition, the effect of local weather conditions on stratification, thermocline, and hydrodynamics was investigated. The results showed that the ponds had vertical water temperature differences >1 °C for 99% of the time, May to August. In addition, salt-laden inflows from road deicing salts led to strong chemical stratification up to five times stronger in the sediment forebays than in the main cells in spring. Wind-induced surface currents were insignificant, scaling at 0.3% of the wind speed with negligible impact on vertical mixing in the ponds. Our results demonstrate that the ponds' strong and prolonged stratification decreased pollutant retention capacity and caused the water at depth to become anoxic, degrading the quality of the water discharged downstream. Hence, additional consideration of stratification is required when designing new stormwater ponds.
A three-dimensional coupled hydrodynamic-biogeochemical model was applied to simulate and investigate causes of eutrophication in two stormwater ponds in the City of Edmonton (Alberta, Canada). The ponds differ in shape, surface area, depth and trophic state. Strong spatial gradients, in water column thermal structure and water quality state variables were observed (e.g., ∼10 °C; 0.1 mg L⁻¹ PO4, 0.5 mg L⁻¹ total P and 50 μg L⁻¹ chlorophyll-a over ∼3m), showing the ponds were incompletely mixed systems. Using a single set of calibration parameters, which differed from calibrations to larger systems, the model accurately simulated these gradients, with errors being highest for NO3 and total chlorophyll-a (RMSE <64.3 μg L⁻¹ and NRMSE<0.94. Simulation of total chlorophyll-a was at times visually inaccurate, particularly in the more eutrophic pond, but the normalized metrics (e.g., NRMSE) were consistent between ponds and with literature values. The model accurately predicted the trophic state (in more than 60% of the observations) and the distribution of phytoplankton community structure was simulated with marginal error (average ∼7%). Chlorophyll-a alone was not a suitable index to define trophic state in the ponds; suspended algae and macro-algae reduced pond aesthetics, while competing with phytoplankton for nutrients and was coincident with lower phytoplankton biomass and increased water clarity. Simulated remediation strategies, involving reduction of influent phosphorus and nitrogen fractions, showed that the trophic state of each pond could be improved from mesotrophic/eutrophic to oligotrophic/mesotrophic states by reducing nutrient loads at least 50%.
Waste stabilization ponds (WSPs) rely on natural processes to improve water quality. WSPs require less capital or operational resources than traditional treatment and are commonly used in rural communities or for secondary disinfection. Typically, WSPs designs use empirical equations without three-dimensional (3D) circulation, thermal stratification, or varying water levels. In the present study, a 3D numerical model (Delft3D version 4.04) was applied combined with observations of an operational WSP for nine months (May–November 2017). After calibrating light attenuation and eddy viscosity, model results were in agreement with temperature profiles at five locations,
with an average root mean square errors of 1.3°C. The results indicate minimal effects from inflows temperatures and vertical temperature differences of up to 8.0°C between the surface and bed, which inhibited vertical mixing. The buoyancy frequency, a measure of density stratification, was inversely proportional to wind speed and was used to determine that the threshold wind speed required for mixing varied seasonally between 1 and 3 m=s. The results from a simulated tracer indicate that empirical methods for estimating hydraulic retention times—the average length of time water remains in a WSP—are inaccurate and varied by an average of 22% from the simulated tracer.
Facing an unprecedented population growth, it is difficult to overstress the assets for wastewater treatment of waste stabilization ponds (WSPs), i.e. high removal efficiency, simplicity, and low cost, which have been recognized by numerous scientists and operators. However, stricter discharge standards, changes in wastewater compounds, high emissions of greenhouse gases, and elevated land prices have led to their replacements in many places. This review aims at delivering a comprehensive overview of the historical development and current state of WSPs, and providing further insights to deal with their limitations in the future. The 21st century is witnessing changes in the way of approaching conventional problems in pond technology, in which WSPs should no longer be considered as a low treatment technology. Advanced models and technologies have been integrated for better design, control, and management. The roles of algae, which have been crucial as solar-powered aeration, will continue being a key solution. Yet, the separation of suspended algae to avoid deterioration of the effluent remains a major challenge in WSPs while in the case of high algal rate pond, further research is needed to maximize algal growth yield, select proper strains, and optimize harvesting methods to put algal biomass production in practice. Significant gaps need to be filled in understanding mechanisms of greenhouse gas emission, climate change mitigation, pond ecosystem services, and the fate and toxicity of emerging contaminants. From these insights, adaptation strategies are developed to deal with new opportunities and future challenges.
In this paper, the energy transfer phenomena of a secondary facultative lagoon and its relationship with environmental conditions such as wind, and solar radiation were studied. To this end, a three dimensional Computational Fluid Dynamics model was constructed within a conventional facultative lagoon located in the municipality of Ginebra-Valle del Cauca, Colombia. The model included the transport of suspended solids from inlet to outlet, the penetration of incident solar radiation into the body of water, surface wind, the transfer of heat through lateral walls and the temperature profiles of the wastewater. The model was validated using tracer studies and experimental temperature data. This demonstrated that simulated data of temperature and solar radiation penetration have an acceptable degree of consistency compared with the experimental data. The model demonstrated that temperature profiles were significantly affected by the wind at 0.45 m and 1.40 m in depth, while no effect on the surface.
In 2012, Environment Canada updated the Wastewater System Effluent Regulations in an effort to reduce the 150 billion litres of untreated wastewater being discharged into Canadian waters annually. The revised regulations will result in the commissioning of over $20 billion in wastewater infrastructure upgrades for municipalities. Many small, rural and remote municipalities use passive wastewater treatment systems, such as wastewater stabilization ponds (WSPs), as sustainable alternatives to conventional wastewater treatment due to their ease of operation, minimal energy input and low costs. WSPs can effectively attenuate nutrient loads while also providing environmental conditions suitable for the removal of pathogens through naturally occurring biological, chemical and physical treatment mechanisms. However, since WSPs are open systems, they are susceptible to variations in external conditions. In particular, they are conducive to algal blooms and high pH events during the summer seasons, with warmer temperatures and higher hydraulic retention times. Water chemistry parameters, temperature, pH, dissolved oxygen, Escherichia coli, nitrate and total phosphorus, were collected from a WSP system in eastern Ontario, with excessive algal growth, over a five-year period. The removal efficiencies of various water quality parameters and indicator organisms for each season were used to determine seasonal treatment and disinfection performance of the system. Multivariate statistical tests and time series analyses were used to determine the strength and type of relationships influencing the WSP treatment for different seasons. Nitrate and E. coli removal were shown to be lowest during the winter periods at 95.6% and 27.9%, while total phosphorus remained consistent throughout the monitoring period. E. coli removal was shown to be significantly negatively correlated with pH (ρ =-0.268, p=0.05) and DO (ρ=-0.390, p=0.01), using Spearman's correlation coefficient. Seasonal Kendall tests revealed dissolved oxygen levels and nitrate concentrations both significantly decreased during the fall period. This research will be used directly to inform the monitoring program for the WSP system at the site and contributes to the continued improvement of WSP design and performance. The multivariate statistical methodologies presented in this research offer an insightful approach to the monitoring of water treatment systems where large datasets are generated and the extraction of key relationships is critical in informing system design and operation.
Waste stabilization ponds (WSPs) are used extensively for the treatment of wastewater in Australia, mostly in regional and remote areas. Wastewater treatment plants (WWTPs) using pond technologies are also distributed over the full geographical extent of Australia, encompassing many climatic zones. Predominantly used to service small to medium‐sized communities, WSPs are also used to service large metropolitan Australian populations, up to 2.5 million people. When well‐maintained, WSPs are a sustainable and resilient treatment option, and treatment is achieved at significantly lower cost when compared with conventional WWTPs. Increasing population, changing regulations, and climate variability are placing increasing pressure on Australian WSP systems. Sludge accumulation over time presents a significant challenge to pond maintenance, along with increasing occurrence of toxic cyanobacterial bloom events. These challenges are only enhanced by the wide geographical distribution and by increasing operational and maintenance costs. Increased demand for recycled water is placing further pressure on Australian WSP systems, as higher value treated water is expected from WSP infrastructure that is often overloaded or under‐designed. This increased demand for high‐quality treatment presents an opportunity for operators and researchers to develop a better understanding of the coupling between hydraulics and microbial ecology of these systems. With more stringent guidelines for greenhouse gas emissions (GHGs), a better understanding of biophysicochemical processes in WSPs will lead to better estimates of GHG fluxes and variability. This information will become critical for the future planning, maintenance and operation of WSPs, and will result in a better understanding of WSP systems overall.
This article is categorized under:
Engineering Water > Water, Health, and Sanitation
Engineering Water > Sustainable Engineering of Water
Waste stabilisation ponds (WSP) are designed to treat wastewater through natural microbiological, photosynthetic, biochemical, physico-chemical and hydrodynamic processes. They are used throughout the world for wastewater treatment due to their minimal technical requirements, low cost and minimal energy consumption. It is well recognised that WSP hydrodynamics plays a crucial role influencing WSP treatment efficiency. In this study, published WSP literature is reviewed with a focus on the importance of environmental forces, pond configurations and pond loadings on WSP hydrodynamics and pond treatment efficiency. The findings are used to propose future WSP design and modelling requirements. It has been found that the interrelated effects of multiple factors such as pond length-to-width ratio, inlet/outlet configuration, temperature, solar radiation and wind on WSP treatment performance are not investigated sufficiently at present. It is suggested that a system based on numerical pond modelling and field measuring analyses be formulated to address the interacting influences of these factors on WSPs. Consequently, field studies on full-scale WSPs are required to obtain a complete dataset for validation purposes. In this regard, a combination of three-dimensional pond water temperature survey and tracer concentration mapping is suggested. Details of specific modelling components, such as sludge accumulation, turbulence mechanisms and the assumption of constant influent conditions, require further attention. As a result of this review process, a strategic evaluation framework together with the required dataset for the modelling and field work activities are proposed. This paves the way for subsequent studies on WSP hydrodynamics and treatment efficiency, thus benefitting pond design and operation processes.
A better design instruction for waste stabilization ponds is needed due to their growing application for wastewater purification, increasingly strict environmental regulations, and the fact that most of previous design manuals are outdated. To critically review model-based designs of typical pond treatment systems, this paper analyzed more than 150 articles, books, and reports from 1956 to 2016. The models developed in these publications ranged from simple rules and equations to more complex first-order and mechanistic models. From a case study on all four approaches, it appeared that rules of thumb is no longer a proper tool for pond designs due to its low design specification and very high output variability and uncertainty. On the other hand, at the beginning phase of design process or in case of low pressure over land and moderate water quality required, regression equations can be useful to form an idea for pond dimensions. More importantly, mechanistic models proved their capacity of generating more precise and comprehensive designs but still need to overcome their lack of calibration and validation, and overparameterization. In another case study, an essential but often overlooked role of uncertainty analysis in pond designs was investigated via a comparison between deterministic and uncertainty-based approaches. Unlike applying a safety factor representing all uncertainty sources, probabilistic designs quantify the uncertainty of model outputs by including prior uncertainty of inputs and parameters, which generates more scientifically reliable outcomes for decision makers. Based on these findings, we advise engineers and designers to shift from the conventional approaches to more innovative and economic tools which are suitable for dealing with large variations of natural biological systems.
Urban lakes provide many ecosystem services, e.g., flood control, nature protection, coolness island, recreation. Hydrodynamic models will increasingly be used to enhance these benefits. We present the first validation of a three-dimensional (3D) hydrodynamic model on a small shallow lake with high resolution and high frequency measurements. Lake Créteil, France (area 0.4 km2, mean depth 4.5 m, and catchment area 1 km2) is a former gravel pit and now part of a regional park. The model Delft3D-FLOW was calibrated on a one-month period, with continuous measurements of temperature at five depths at the center of the lake and at three depths at two other stations, and with current speed profiles at the centre of the lake. The model was then verified on 18 1-month periods with similar temperature measurements. The model reproduced very well the temperature dynamics, including the alternation between mixing and stratification periods and internal wave patterns. The mean absolute errors over the five depths at the central point remained below 0.55∘C in spring and summer, the most favorable seasons for phytoplankton growth. Horizontal temperature differences, which rose up to 3∘C at the beginning of stratification periods, were also well reproduced, as well as current speeds. These results are very promising for assessing nutrient and pollutant diffusion, settling and resuspension, as well as for understanding how phytoplankton blooms start in small shallow lakes.
Treatment in a wastewater stabilization pond (WSP) relies on natural purification processes, which can be sensitive to both location and climate. This study investigated the effects of three environmental factors, pH, dissolved oxygen (DO) and temperature, on disinfection efficiency in a WSP system consisting of three facultative cells, and operated in a temperate climate region, in Eastern Ontario, Canada. Indicator organism (Escherichia coli (E. coli)) removal in WSP systems is driven by a combination of different factors. Elevated pH and DO concentrations, which are attributed to the presence of algae, are important factors for effective disinfection. Therefore, the presence of algae in natural wastewater treatment systems can contribute appreciably to disinfection. Consequently, based on algal concentrations, removal efficiencies of pathogenic microorganisms during wastewater treatment over the course of a year can be highly variable, where higher removal efficiencies would be expected in summer and fall seasons.
Knowledge of the hydraulic behaviour is very important in the characterization of a stabilization pond, since pond hydrodynamics plays a fundamental role in treatment efficiency. An advanced hydrodynamics characterization may be achieved by carrying out measurements with tracers, dyes and drogues or using mathematical simulation employing computational fluid dynamics (CFD). The current study involved experimental determinations and mathematical simulations of a full-scale facultative pond in Brazil. A 3D CFD model showed major flow lines, degree of dispersion, dead zones and short circuit regions in the pond. Drogue tracking, wind measurements and dye dispersion were also used in order to obtain information about the actual flow in the pond and as a means of assessing the performance of the CFD model. The drogue, designed and built as part of this research, and which included a geographical positioning system (GPS), presented very satisfactory results. The CFD modelling has proven to be very useful in the evaluation of the hydrodynamic conditions of the facultative pond. A virtual tracer test allowed an estimation of the real mean hydraulic retention time and mixing conditions in the pond. The computational model in CFD corresponded well to what was verified in the field.
Current velocity profiles in combined wave-current flows are investigated on the basis of available laboratory and field data. From the laboratory measurements of Van Doorn's (1982) S10RAL, S20RAL, M10RAL and M20RAL, it is found that there exists a boundary interaction layer (which includes the wave boundary layer) where the current eddy viscosity ϵc is dominated by the wave motion inside the boundary interaction layer and then by the current upwards. Based on this finding, a new and simple model is developed to predict current velocity profiles in the presence of waves. The input parameters used in the model are wave height H, wave period T, water depth h, bed roughness Ks, and a reference current velocity . In comparison with the former models of Christoffersen and Jonsson (1985), Coffey and Nielsen (1986) and Sleath (1991), the present model provides a simple method to explicitly calculate the wave friction factor, current friction velocity and current velocities without laborious iterations involved. Another new aspect of the model is that calculation of current friction velocities and current velocity profiles is quite simple and can be carried out with a simple scientific calculator. Results from the model are sufficiently accurate for engineering purposes.
The spatial and temporal variation of physical, chemical, and biological parameters was determined, in summer and winter, at nine sites in a large (112 ha) waste stabilisation pond (WSP) at the Bolivar Wastewater Treatment Plant. Each site was extensively sampled over the course of one day, with the nine sites sampled over successive days at exactly the same times of day, progressing in the direction of bulk flow through the pond. Analyses of covariance were used to test the independent impact of site and climate on the way in which the mean values and stratification gradient of the physical, chemical, and biological parameters varied diurnally at each site. In both winter and summer studies there was a very strong correlation at all sites between changes in temperature, pH and dissolved oxygen (DO). Mean pond temperatures were higher in summer than winter, and thermal stratification was more common in summer. In summer, during the day at each site, concentrations of chlorophyll-a, DO, suspended solids and pH increased with higher solar radiation levels. This relationship was less evident in winter. There was no systematic depth or temporal variation identified in either the summer or winter study for the broad range of chemical parameters measured. Mean values for these parameters, and to a lesser extent their stratification gradients, increased by varying extents throughout the day at the different sites in both summer and winter, irrespective of changes in climate when the different sites were sampled. Sites nearer the inlet to the WSP recorded lower NH4N and higher NO2N and NO3N concentrations than the rest of the WSP. This was indicative of nitrification. Somewhat surprisingly, high DO concentrations were also recorded at these sites near the inlets. Computational fluid dynamics (CFD) modelling, incorporating the predominant wind conditions, offers a rationale for these observations. Recirculation was evident, which may increase the residence time for the slow growing autotrophic nitrifying bacteria and recirculate oxygen rich water around these sites - conditions which would enhance nitrification. Understanding the effect of these variations, overlaid by the influence of hydraulic and temporal scenarios, assists in developing a mechanistic understanding of pond operation.
Over the past fifty years, considerable research in waste stabilization pond operation has led to the development of a number of models used to describe the hydraulic regime and predict treatment efficiency. Models range in complexity from plug or completely mixed simplifications to computational fluid dynamics (CFD) models which are able to predict flow hydraulics at a local level. Information about the exit age of pond effluent can be used to estimate pollutant decay. However, a mechanistic approach to understanding pond operation highlights the importance of knowing both the time and spatial history of pond effluent. A CFD model of a large pond system was constructed to demonstrate various hydraulic scenarios under different boundary conditions. Two scenarios were compared to visually demonstrate the effects of differing hydraulic conditions. Typical mechanistic models were applied to each condition to quantify biological differences. This simple example indicates that integrating biological and localised flow models will lead to a more holistic understanding of pond operation and treatment efficiency.
Stormwater ponds (SWPs) are widely utilized for flood and water quality control. Low-flow rates are common in SWPs, sometimes causing wind-driven currents to become the dominant hydrodynamic force during ice-free periods. Hence, it is essential to understand the influence of the wind-induced flow on stratification and dissolved oxygen (DO) concentrations in shallow SWPs to predict the performance and water quality of these systems. The objective of this study is to evaluate the influence of wind-driven circulation on the spatial distribution of DO in an SWP using a numerical model. A bottom-mounted acoustic Doppler current profiler (ADCP) was utilized to measure small wind-induced currents and to validate a hydrodynamic model, which suggested that a wind-dominated circulation was generated even with the moderate wind speed. Countercurrents opposite in the direction to surface wind-generated flow were also present. The DO model demonstrated that complete mixing can be produced by higher wind speed, leading to fully oxic conditions throughout the water column (7.00 mg/L DO or higher), wherein low DO water at depth was carried to the surface by upwelling circulation and was possibly replenished during the surface transportation. This sheds some light on the impact of wind-induced mixing on the water quality in shallow SWPs.
A novel dynamic and unified 3D Computational Fluid Dynamics (CFD) model to hydrodynamics studies of waste stabilization ponds is proposed. Two full-scale maturation ponds, at different operational settings, were modelled. This work attempted to consider gaps existing in literature regarding CFD modelling in ponds, such as the real geometric conformation of the accumulated sludge on the bottom; dynamic models representing environmental conditions (wind, solar radiation, air temperature), approximation of the physical properties of the fluid to real conditions, buoyancy etc., all together in a unified model. For validation/calibration purposes, monitoring data were used and experimental tests were performed in the field, using tracers, dyes, temperature and water quality measurements, hydraulic devices and local weather data. Results of field tests agreed well with each other and with the results of the CFD models. From the model proposed, more detailed studies and important analyses regarding the hydrodynamic behavior of ponds could be accomplished, so that interventions to improve treatment efficiency can be better studied and oriented, with low cost and without the need for field or laboratory experiments. The model developed in this study certainly has potential for application in the modelling of ponds in the future, under more realistic environmental conditions.
Per- and polyfluoroalkyl substances (PFAS) are presently essential ingredients in aqueous film forming foam (AFFF) used for fire-fighting, but are also pervasive environmental contaminants. The use and subsequent release and transport of AFFF in the ocean environment from marine vessels has not been studied to date. A numerical model (Delft3D) was rigorously calibrated and validated for the hydrodynamics, and used to predict the transport of PFAS released instantaneously into a large harbour (Halifax Harbour, Nova Scotia) that is representative of coastal environments in eastern Canada and other parts of the world. The numerical model results indicate that PFAS released in the presence of strong winds and waves during a storm will travel up to 31 km in 2 days, approximately 40% farther than PFAS release during a time period dominated by tidal currents with light winds and small waves (<1 m). After a 10 day simulation, PFAS levels from release sites in the Inner Harbour were higher (40–60 μg/L) compared to PFAS levels from the Outer Harbour release site which had decreased to low levels (<1 μg/L) during a non-storm period. Along shorelines within the Harbour, PFAS concentrations remained elevated after 12 h (40–500 μg/L) and 48 h (2–300 μg/L). These concentrations are within the range of PFAS guidance values for recreational water use. The methods described here are relevant to studies of PFAS dispersion and transport in other coastal areas, and could be used to determine best practices for applications of AFFF in the coastal environment.
Wastewater stabilization ponds (WSPs) have been proven to be economical alternatives to conventional wastewater treatment technologies due to their unique advantages including ease of operation, minimal energy input, and minimal maintenance requirements. Their reported high pathogen removal efficiencies have made WSPs a popular choice for wastewater treatment, especially as tertiary lagoons. This paper provides a critical overview of the various disinfection processes and mechanisms that occur in WSPs. A thorough review of the removal or attenuation mechanisms for bacterial, viral, protozoan, and helminthic pathogens is presented. Factors that impact the removal efficiency of pathogenic organisms may include sunlight, pH, dissolved oxygen, temperature, sedimentation, attachment, hydraulic retention time, pond depth, predation and nutrient availability; the relationship between these factors is also discussed. The purpose of this review paper is to utilize the current understanding of pathogen removal mechanisms in pond systems to improve the operation and design of WSPs, and more importantly, to provide guidance for the definition of regulations with respect to pathogen removal in eco-engineered wastewater treatment systems such as WSPs.
CFD (Computational Fluid Dynamics) is a numerical modelling tool for flow analysis, whose use in research and applications in stabilization ponds has been consolidated over the past few years. However, current literature presents a lack of review articles focusing on this particular application. Based on previous works in the literature, after adaptations, additions and updates, this review presents a new compilation of 28 publications focusing on the use of CFD in non-mechanized stabilization ponds (anaerobic, facultative, maturation and polishing). Emphasis is given to the main features of the models and methods used, as well as the main results and conclusions of these studies. After an introduction to the tool and a summary of the publications, a brief discussion is held on the key points of the modelling practices adopted, with the intention of identifying the main methods used, gaps, bottlenecks and trends, as well as to suggest good modelling practices when possible. It is therefore expected to contribute to the development and consolidation of the CFD technique in modelling of stabilization ponds and assist new researchers to identify possible starting points for future works.
Waste stabilisation ponds (WSP) are used worldwide for wastewater treatment, and throughout their operation require periodic sludge surveys. Sludge accumulation in WSP can impact performance by reducing the effective volume of the pond, and altering the pond hydraulics and wastewater treatment efficiency. Traditionally, sludge heights, and thus sludge volume, have been measured using low resolution and labour intensive methods such as “sludge judge” and the “white towel” test. A sonar device, a readily available technology, fitted to a remotely operated vehicle (ROV) was shown to improve the spatial resolution and accuracy of sludge height measurements, as well as reduce labour and safety requirements. Coupled with a dedicated software package, the profiling of several WSP has shown that the ROV with autonomous sonar device is capable of providing sludge bathymetry with greatly increased spatial resolution in a greatly reduced profiling time, leading to a better understanding of the role played by sludge accumulation in hydraulic performance of WSP. The high resolution bathymetry collected was used to support a much more detailed hydrodynamic assessment of systems with low, medium and high accumulations of sludge. The results of the modelling show that hydraulic performance is not only influenced by the sludge accumulation, but also that the spatial distribution of sludge plays a critical role in reducing the in treatment capacity of these systems. In a range of ponds modelled, the reduction in residence time ranged from 33% in pond with a uniform sludge distribution to a reduction of up to 60% in a pond with highly channelized flow. The combination of high-resolution measurement of sludge accumulation and hydrodynamic modelling will help in the development of frameworks for wastewater sludge management, including the development of more reliable computer models, and could potentially have wider application in the monitoring of other small to medium water bodies, such as channels and recreational and commercial ports.
The wastewater stabilization ponds (WSPs) at a wastewater treatment facility in eastern Ontario, Canada, have experienced excessive algae growth and high pH levels in the summer months. A full range of parameters were sampled from the system and the chemical dynamics in the three WSPs were assessed through multivariate statistical analysis. The study presents a novel approach for exploratory analysis of a comprehensive water chemistry dataset, incorporating principal components analysis (PCA) and principal components (PC) and partial least squares (PLS) regressions. The analyses showed strong correlations between chl-a and sunlight, temperature, organic matter, and nutrients, and weak and negative correlations between chl-a and pH and chl-a and DO. PCA reduced the data from 19 to 8 variables, with a good fit to the original data matrix (similarity measure of 0.73). Multivariate regressions to model system pH in terms of these key parameters were performed on the reduced variable set and the PCs generated, for which strong fits (R(2) > 0.79 with all data) were observed. The methodologies presented in this study are applicable to a wide range of natural and engineered systems where a large number of water chemistry parameters are monitored resulting in the generation of large data sets.
The implications of climatic change on Lake Constance are studied by long-term hydrodynamic model simulations. In contrast to earlier studies that mostly have applied one-dimensional models, this investigation utilizes a three-dimensional hydrodynamic model and thus profits from the advantages of a spatial representation of lake bathymetry and hydrophysical processes. Model adaptation and validation are based on half a century of vertically resolved temperature recordings (1961 to 2011). Three different horizontal grid layouts are used to test the sensitivity of the thermal stratification and effective vertical turbulent diffusivities (DVeff) determined with the heat-budget method to grid resolution. DVeff calculated from observations and from simulations with different grid resolutions agree rather well. However, in the deep water simulated DVeff are overestimated if the basin is resolved only by few grid cells. The investigation of the effect of climatic changes on Lake Constance is focused on the effects of altered air temperatures and wind velocities on deep-water renewal. Numerical tracers are used as indicators of the winter vertical transport and mixing. Effects on stratification and mixing strongly depend on the seasonal course of the climatic changes. Warmer winter temperatures result in reduced deep-water exchange and more frequent years with incomplete mixing. Increased air temperatures in summer have almost no effect on deep-water renewal in winter but increase water-column stability during summer and autumn. Increased wind speeds influence vertical mixing and surface heat fluxes and cause higher deep-water temperatures if wind speeds are higher during the summer.
Waste stabilization pond (WSP) technology has been an active area of research for the last three decades. In spite of its relative simplicity of design, operation and maintenance, the various processes taking place in WSP have not been entirely quantified. Lately, modelling has served as an important, low-cost tool for a better description and an improved understanding of the system. Although several papers on individual pond models have been published, there is no specific review on different models developed so far. This paper aims at filling this gap. Models are compared by focussing on their key features like the presence and comprehensiveness of a water quality sub-model in terms of aerobic/anoxic and anaerobic carbon removal and nutrient removal; the type of hydraulic sub-model used (0D, 1D, 2D or 3D); the software used for implementation and simulation; and whether or not sensitivity analysis, calibration and validation were done. This paper also recommends future directions of research in this area. In-depth study of the published models reveals a clear evolution over time in the concept of modelling, from just hydraulic empirical models to 3D ones and from simple first-order water quality models to complex ones which describe key biochemical processes as a set of mathematical equations. Due to the inherent complexity, models tend to focus only on specific aspects whilst ignoring or simplifying others. For instance, many models have been developed that either focus solely on hydrodynamics or solely on biochemical processes. Models which integrate both aspects in detail are still rare. Furthermore, it is evident from the review of the different models that calibration and validation with full-scale WSP data is also scarce. Hence, we believe that there is a need for the development of a comprehensive, calibrated model for waste stabilization ponds that can reliably serve as a support tool for the improvement and optimization of pond design and performance.
In analysis of the hydrodynamics of large and small water bodies, a relation is frequently required between surface velocity, Us, and wind speed, U. Here a theory is developed to explain the observed relationship between surface velocity in a water body and both wind speed and latitude.
This paper describes a comprehensive model of wastewater treatment in secondary facultative ponds, which combines 3D hydrodynamics with a mechanistic water quality model. The hydrodynamics are based on the Navier–Stokes equation for incompressible fluids under shallow water and Boussinesq assumptions capturing the flow dynamics along length, breadth and depth of the pond. The water quality sub model is based on the Activated Sludge Model (ASM) concept, describing COD and nutrient removal as function of bacterial growth following Monod kinetics, except for Escherichia coli removal, which was modelled as first order decay. The model was implemented in the Delft3D software and was used to evaluate the effect of wind and the addition of baffles on the water flow pattern, temperature profiles in the pond and treatment efficiency. In contrast to earlier models reported in the literature, our simulation results did not show any significant improvement in COD removal (based on the ASM concept) with addition of baffles or under intermittent wind-induced mixing. However, E. coli removal efficiency, based on a first order decay approach, showed a fair improvement in the presence of baffles or intermittent wind-induced mixing. Furthermore, simulations with continuous wind effect showed a decrease in removal efficiency for COD but a further increase in E. coli removal efficiency. Such contrasting results for two different approaches in modelling could indicate that the first order decay concept might not be appropriate to describe all the interactions between biochemical processes in a pond. However, these interpretations remain theoretical, as the model needs validation with field data.
Surface layer coefficients for wind profiles, wind stress, and heat flux in typical open sea conditions are briefly reviewed. Businger-Dyer flux-gradient relationships and a Charnock wind stress formula fit the empirical data and are dimensionally consistent. These have been solved by an iterative method, and the results are presented in a tabular form suitable for climatological calculations from marine wind and temperature data.
Measurements show that wastewater stabilization ponds although often only 1–2 m deep stratify and destratify intermittently depending primarily on weather. Stratification can be observed in vertical profiles of water temperature, dissolved oxygen, pH and other water quality parameters. In three stabilization ponds of a small Minnesota town, stratification develops primarily by differential heating of the pondwater through its surface and, in the absence of artificial aeration or mixing devices, by insufficient wind mixing. The resulting water temperature stratification affects other parameters in a variety of ways through chemical, microbial and planktonic kinetics and reduced vertical mixing. To gain a better understanding of stabilization pond water quality dynamics, temperature profiles were monitored at 20-min intervals, and a dynamic lake water quality model was modified and applied to simulate the temperature stratification. A 12-h timestep option was incorporated into the program in order to capture the diurnal variation in stratification. Measurements and simulations were made for three wastewater stabilization ponds at Harris, Minnesota. The level of agreement between field measurements and numerical simulations demonstrated that water temperatures and stratification dynamics in a shallow and small pond can be simulated on a diurnal timescale with a standard error from 1.0 to 1.5°C between simulation and measurements. The model includes wastewater inflow in the form of a vertical jet and water transfer between ponds in the form of non-surface inflow and outflow. Three types of stratification were identified and their respective durations were determined. Stratification occurred on about 55% of all days from 1 April to 30 November. Information on pond stratification presented can be used to guide field studies and reactor modeling of ponds which can lead to further improvements in design and operation.
Computer modeling of sediment transport patterns is generally recognized as a valuable tool for understanding and predicting morphological developments. In practice, state-of-the-art computer models are one- or two-dimensional (depth-averaged) and have a limited ability to model many of the important three-dimensional flow phenomena found in nature. This paper presents the implementation and validation of sediment transport formulations within the proven DELFT3D three-dimensional (hydrostatic, free surface) flow solver. The paper briefly discusses the operation of the DELFT3D-FLOW module, presents the key features of the formulations used to model both suspended and bedload transport of noncohesive sediment, and describes the implemented morphological updating scheme. The modeling of the three-dimensional effects of waves is also discussed. Following the details of the implementation, the results of a number of validation studies are presented. The model is shown to perform well in several theoretical, laboratory, and real-life situations.
This study analyses how the hydraulic performance differs between 13 ponds with hypothetically different layouts. The paper also includes a discussion of short-circuiting, hydraulic efficiency and suitable parameters for measuring hydraulic performance. The ponds were studied by using a 2-D vertically integrated numerical model. Tracer studies were simulated and then evaluated and compared. The results confirm that length-to-width ratio, location of in- and outlets, and subsurface berm have a large impact on pond hydraulic performance. There is also an indication that an island placed in front of the inlet improves the hydraulic performance, and that a curved pond or an island placed near the side does not decrease the performance.
As treatment processes are kinetic-dependent, a consistent description of water residence times is essential to the prediction of waste stabilization ponds performance. A physically-based 3D transient CFD model simulating the water velocity, temperature and concentration fields as a function of all influent meteorological factors--wind speed and direction, solar radiation, air temperature and relative humidity--was used to identify the relationships between the meteorological conditions and the hydrodynamic patterns and water residence times distributions in a polishing pond. The required meteorological data were recorded on site and water temperatures recorded at 10 sampling sites for 141 days. Stratification events appear on very calm days for wind speeds lower than 3 m s(-1) and on sunny days for wind speeds lower than 5 m s(-1). De-stratification is related to two mixing processes: nightly convection cells and global mixing patterns. Numerical tracer experiments show that the results of the flow patterns can be evaluated using the dispersed flow regime approximation and, for wind speeds exceeding 6 m s(-1), the completely stirred tank reactor assumption.
Pond hydraulic behaviour is influenced by the inlet/outlet configuration, baffles and wind, but design information relating to these factors is still very limited. This paper reviews the development of "Guidelines for the Improved Hydraulic Design of Waste Stabilisation Ponds" and summarises some of the key findings and recommendations. This work was based on review of previous research, laboratory experimentation, field studies and mathematical modelling using computational fluid dynamics. The inlet design can have a significant influence on the flow regime in a pond. Poorly considered positioning of the inlet and the outlet can create hydraulic short-circuiting problems. As an example of the nature of the work undertaken in this project, the use of a small horizontal inlet pipe was compared against a vertical inlet design. A practical method of assessing the relative significance of wind versus inlet power input was presented. The application of this analysis may allow engineers to size inlet pipes to help control the flow patterns in ponds for efficient performance. Extensive testing has been undertaken on a wide range of baffle configurations. An example of this research showed how short stub baffles could provide similar improvements to longer "traditional" baffle designs, potentially offering significant savings in construction costs. For traditional baffle designs a minimum of two baffles is recommended. For the pond modelled in this work, it was found that any more than four baffles gave only marginal improvements.
The city of Nelson, New Zealand, has a 27 hectare oxidation pond as its primary wastewater treatment facility. Recent changes in the configuration of the pond and installation of a mixer/aerator raised concerns that pond treatment rates and effluent quality may be affected by high internal pond velocities and short retention times.
This paper presents the findings of an investigation into wastewater velocity and movement within the pond using qualitative dye dispersion and tracking of small-scale “holey-sock” drogues. Simultaneous deployment of drogues and dye allowed methods to be compared, since small-scale drogues have not commonly been used in wastewater ponds. Dye dispersion was assessed using low-level aerial photography from a tethered helium blimp to track short term movement and mixing, while a datalogger and fluorometer were used to measure pond retention time. Drogue movement was tracked in conjunction with the dye study from a small boat using hand-held GPS.
The dye study found that: (i) the first portion of pond influent discharged from the pond after 37.5 hours, substantially quicker than the theoretical pond retention time of 27 days. However, the measured retention time was with a mixer in place and the theoretical time was without a mixer; (ii) the position of the paddle wheel mixer/aerator was not optimally placed for mixing the influent and a quiescent region existed adjacent to the influent point; and (iii) the low-level aerial photography was an effective method of evaluating larger pond systems.
The “holey-sock” drogue studies showed that: (i) the drogues accurately followed the movement and velocity of dyed influent within the pond; (ii) wastewater velocity and movement was dominated by the paddle wheel mixer/aerator; and (iii) wind direction had a minor influence on wastewater velocity and movement in areas not directly affected by the paddle wheel mixer/aerator.
The study demonstrated that the combined use of dye and drogues was a relatively low-cost and effective means of determining internal pond velocities and movement. Future studies using similar methods will be useful in helping validate computer-modelled movement and velocity.
Wind effects on residence time in waste stabilization lagoons
- Y Fares
- B J Lloyd
Three-dimensional investigation of retention time distribution of waste stabilisation ponds
- M H Li
- C Zhang
- Z Lemckert
- L Lu
- H Lei
- Stratton
Three-dimensional simulation of hydrodynamics and water quality in a wastewater stabilization pond
- F A Mahyari
- L Rey
- P Boegman
- R P Champagne
- G Mulligan
- A Hall
- Y Silva
- Filion
The role of algae in disinfection processes in wastewater stabilization ponds
- L Liu
Water quality spatial heterogeneity in wastewater stabilization ponds at elevated pH.” Master’s thesis Dept
- M R Schueder
Principles of design and operations of wastewater treatment pond systems for plant operators engineers and managers
- Usepa