Hydrodynamic modelling of traffic-related microplastics discharged with stormwater into the G\uf6ta River in Sweden

Microplastics (MP) are transported from land-based sources from rivers to marine waters. However, there is currently little knowledge about MP fate from land sources to marine waters. Traffic is estimated to be one of the largest sources of MP; hence, stormwater is expected to be an important transportation route of MP to marine waters. The aim of this study was to investigate the effect of the size and density of tyre wear particles in road run-off on their fate in the Gota River in Sweden using hydrodynamic modelling. The model of the stretch of Gota River, Sweden\u27s largest river, passing through Gothenburg (Sweden\u27s second largest city) and out to the sea, was set up using MIKE 3 FM software. Literature data were used to define the MP characteristics: concentrations in stormwater, prevalent particle sizes, density of MP commonly occurring in road run-off and settling velocities. Results show that higher concentrations of MP are found on the south side of the river, compared with the north side, due to higher annual average daily traffic loads along the south side of the river. The mixing processes in the river and the MP concentrations were generally influenced by the vertical water density gradient caused by saline water from the Kattegat strait. While most MP with higher density and larger size settle in the river, smaller MP with density close to 1.0 g/cm(3) do not settle in the river and therefore reach the Kattegat strait and the marine environments. Further research is needed to describe the fate and transport of microplastics in the stormwater system, including treatment facilities, i.e. biofouling, aggregation, degradation and/or further fragmentation and settling

Temperature-dependent mechanisms of DOM removal by biological activated carbon filters

Seasonal variability in the removal of dissolved organic matter (DOM) by drinking water biological activated carbon (BAC) filters is often attributed to temperature changes. However, it can be rather difficult to directly relate temperature to treatment efficiency at full scale due to seasonal variations in other influential parameters like DOM concentration and character, and microbial activity. Furthermore, processes in BAC filters include adsorption, desorption and biodegradation within biofilms while each respond differently to temperature. This study aimed to decouple these processes by studying the removal of various DOM fractions from coagulated and settled drinking water when in contact with aged (>3 years) BAC filter material at different water temperatures. DOM removal was measured as changes in dissolved organic carbon (DOC), ultraviolet absorbance at 254 nm (UV254) and fluorescence. Under the particular experimental conditions there was little evidence of biological removal; instead, removal of DOM fractions emitting at longer wavelengths ("humic-like", >430 nm) was consistent with chemisorption, removal of DOM emitting at intermediate wavelengths ("humic-like", 390-420 nm) was consistent with physisorption, and multiple mechanisms were indicated for "protein-like" (<380 nm) DOM. Non-biological mechanisms of DOM removal by aged BAC filters are often assumed to be unimportant; however, these results suggest they are important for some DOM fractions, especially during periods of reduced microbial activity

Data-driven models for predicting microbial water quality in the drinking water source using E. coli monitoring and hydrometeorological data

Rapid changes in microbial water quality in surface waters pose challenges for production of safe drinking water. If not treated to an acceptable level, microbial pathogens present in the drinking water can result in severe consequences for public health. The aim of this paper was to evaluate the suitability of data-driven models of different complexity for predicting the concentrations of E. coli in the river G\uf6ta \ue4lv at the water intake of the drinking water treatment plant in Gothenburg, Sweden. The objectives were to (i) assess how the complexity of the model affects the model performance; and (ii) identify relevant factors and assess their effect as predictors of E. coli levels. To forecast E. coli levels one day ahead, the data on laboratory measurements of E. coli and total coliforms, Colifast measurements of E. coli, water temperature, turbidity, precipitation, and water flow were used. The baseline approaches included Exponential Smoothing and ARIMA (Autoregressive Integrated Moving Average), which are commonly used univariate methods, and a naive baseline that used the previous observed value as its next prediction. Also, models common in the machine learning domain were included: LASSO (Least Absolute Shrinkage and Selection Operator) Regression and Random Forest, and a tool for optimising machine learning pipelines – TPOT (Tree-based Pipeline Optimization Tool). Also, a multivariate autoregressive model VAR (Vector Autoregression) was included. The models that included multiple predictors performed better than univariate models. Random Forest and TPOT resulted in higher performance but showed a tendency of overfitting. Water temperature, microbial concentrations upstream and at the water intake, and precipitation upstream were shown to be important predictors. Data-driven modelling enables water producers to interpret the measurements in the context of what concentrations can be expected based on the recent historic data, and thus identify unexplained deviations warranting further investigation of their origin

Numerical Modelling of Dissolved Air Flotation

Dissolved Air Flotation (DAF), a well-established treatment method for water containing e.g. dissolved organic matter and Cryptosporidium, has previously been examined experimentally. However, most measuring techniques still suffer from disturbances from air bubbles and intrusion of the measuring equipment into the flow. Consequently, researchers have turned to Computational Fluid Dynamics (CFD) and numerical models of flotation tanks have been developed with the aim of increasing knowledge and efficiency of the process.In this licentiate thesis, one- and two-phase, steady-state numerical models of a down scaled DAF tank are evaluated. All modelling choices are made with a compromise struck between computational cost and the required accuracy of the flow. The geometry of the tank is modelled in GAMBIT and the flow simulated in the CFD software FLUENT 6.3. The turbulence is modelled using the standard k-epsilon model with standard wall functions in the one-phase model and using the realizable k-epsilon model with non-equilibrium wall functions for the two-phase model. The multiphase flow of air and water is solved in the Eulerian-Lagrangian frame of reference. The simulated flow is compared to experimental measurements for validation. The option of making a two- or a three-dimensional model is discussed further in the study and parameters influencing the modelling, such as the outlet geometry, the inlet flow and the air bubble size, are also examined.A two-dimensional model requires adjustments in the geometry and in the parameters governing the flow since the models do not take into account the three-dimensional effects. Simulations show that a two-dimensional, one-phase model can capture the flow in the separation zone reasonably well, but that a three-dimensional model is required if the flow in the contact zone is to be studied. Although a stratified flow shows both steadiness in time and two-dimensional nature of the flow, a two-dimensional, two-phase model should be used with caution. A three-dimensional, two-phase model would reflect the flow in a DAF tank more truthfully, but requires more computer capacity and results indicate that a transient solver is required

Numerical Modelling of Dissolved Air Flotation

Dissolved Air Flotation (DAF), a well-established treatment method for water containing e.g. dissolved organic matter and Cryptosporidium, has previously been examined experimentally. However, most measuring techniques still suffer from disturbances from air bubbles and intrusion of the measuring equipment into the flow. Consequently, researchers have turned to Computational Fluid Dynamics (CFD) and numerical models of flotation tanks have been developed with the aim of increasing knowledge and efficiency of the process. In this licentiate thesis, one- and two-phase, steady-state numerical models of a down scaled DAF tank are evaluated. All modelling choices are made with a compromise struck between computational cost and the required accuracy of the flow. The geometry of the tank is modelled in GAMBIT and the flow simulated in the CFD software FLUENT 6.3. The turbulence is modelled using the standard k-epsilon model with standard wall functions in the one-phase model and using the realizable k-epsilon model with non-equilibrium wall functions for the two-phase model. The multiphase flow of air and water is solved in the Eulerian-Lagrangian frame of reference. The simulated flow is compared to experimental measurements for validation. The option of making a two- or a three-dimensional model is discussed further in the study and parameters influencing the modelling, such as the outlet geometry, the inlet flow and the air bubble size, are also examined. A two-dimensional model requires adjustments in the geometry and in the parameters governing the flow since the models do not take into account the three-dimensional effects. Simulations show that a two-dimensional, one-phase model can capture the flow in the separation zone reasonably well, but that a three-dimensional model is required if the flow in the contact zone is to be studied. Although a stratified flow shows both steadiness in time and two-dimensional nature of the flow, a two-dimensional, two-phase model should be used with caution. A three-dimensional, two-phase model would reflect the flow in a DAF tank more truthfully, but requires more computer capacity and results indicate that a transient solver is required

Dissolved Air Flotation

Dissolved Air Flotation (DAF) is a well-established treatment method for water containing low density particles. The density-driven process is operated by the injection of water saturated with air under high pressure. At the pressure reduction microscopic air bubbles are formed. The bubbles attach to the particles and the aggregates created rise to the surface of the unit where they are removed. Researchers working with DAF units struggle with disturbances in their measurements caused by air bubbles and the intrusion of the measuring equipment into the flow. In order to increase the knowledge and efficiency of the process researchers are now turning to Computational Fluid Dynamics (CFD). The aim of this thesis is to investigate the applicability to use CFD for modelling DAF. The work is carried out by examining single and multiphase, steady-state numerical models of a pilot tank where the flow is simulated with ANSYS Fluent. The options of making a two- or a three-dimensional model and the choice of turbulence and multiphase models are examined. The simulations are compared to experimental measurements for validation. A model describing the formation of the aggregates in the flotation unit is derived and a field study on flotation units in operation in Finland and Sweden is carried out. The work demonstrates that a single-phase, two-dimensional model can capture the flow in the separation zone reasonably well, but that a three-dimensional model is required if the flow in the contact zone is to be studied. A two-phase flow can be captured in a two-dimensional geometry if it carries the characteristics of a stratified flow, suggesting that a two-dimensional model should be used with caution. A three-dimensional, two-phase model will reflect the flow in a DAF tank more truthfully, but requires more computer capacity and results indicate that a transient solver is required. The implementation of the aggregate model demonstrates the creation and motion of the aggregates within the contact zone of the DAF unit. The field study proves the acceptance of the DAF process and indicates that a numerical model would be valuable for investigating the performance of a flotation unit in operation