Search

The floc size distribution (FSD) is crucial to predict cohesive sediment dynamics in aquatic environments. Recently, increasing attention has been given to biofilm effects on the FSDs of suspended particles since the presence of biofilms on particle surfaces may lead to larger flocs and thus higher settling velocities. In this study, results from a settling column experiment conducted by Tang and Maggi (2018; https://doi.org/10.1002/2017JG004165) under nutrient‐free and biomass‐free, nutrient‐affected and biomass‐free, and nutrient‐affected and biomass‐affected conditions, with different suspended sediment concentrations, shear rates, and nutrient concentrations, have been used to validate modeled FSDs that is based on the population balance equation solved by the quadrature method of moments. In addition to the processes of aggregation and breakage, the effects of biofilm are expressed in the growth term of the population balance equation. The logistic growth pattern is used to account for an increase in biomass, which is primarily controlled by the specific growth rate and the carrying capacity. In this study, the biofilm growth rate is assumed nutrient dependent, and the carrying capacity of floc size is hypothesized to be proportional to the Kolmogorov microscale. With eight size classes to interpret a simulated FSD, the predicted and observed FSDs exhibit a reasonable match for all nutrient‐free and biomass‐free, nutrient‐affected and biomass‐free, and nutrient‐affected and biomass‐affected conditions. This simplified bioflocculation model fills the gap between the simulations of the FSDs of cohesive sediments without and with biofilms and has the potential to be included in large‐scale models in the future. Plain Language Summary: In estuaries or adjacent coastal regions, the transport of suspended sediment is responsible for many environmental and engineering issues, for example, siltation and dredging in navigation channels and harbors, water quality, water clarity, pollutants transport, and ecosystem responses. Suspended sediment particles can flocculate and thus can form aggregates with size, shape, density, and settling velocity largely different from the building particles. A challenge to predict the particle behaviors originates from a lack of flocculation models that are able to address the variations in floc size distributions. The aim of this study is to develop a flocculation model that includes besides the "classical" aggregation and breakage driven by turbulence also a biological process, which is biofilm growth. The biofilm growth and its impact on flocculation and thus floc size are simulated in a similar way as the growth of microbes but with different growth rates. The model is validated with laboratory experiments that have shown that the sizes of flocs made solely with sediment particles largely increase when incubated microbes are present. This model provides a sound basis to simulate the behavior of natural particles (minerals, organic, and biological particles) and particles from human origin (plastics) in future environmental risk assessment studies. Key Points: The quadrature‐based multiclass population balance model was used to model the floc size distributions of cohesive sedimentsThe effects of aggregation, breakage, and biofilm growth were included in the modelThe net increase in floc size due to biofilm effects is assumed to follow the logistic growth pattern [ABSTRACT FROM AUTHOR]

Highlights • The biomineral flocs in the Belgian coast were investigated during two contrasting periods. • The observed floc size distributions were decomposed to represent microflocs, macroflocs and megaflocs. • A simple flocculation model that can describe the biofilm growth of flocs was developed. • This model was successfully implemented in the open TELEMAC with five passive tracers. Abstract The Floc Size Distributions (FSDs) of biomineral suspended particles are of great importance to understand the dynamics of bio-mediated Suspended Particulate Matters (SPMs). Field observations were investigated at Station MOW1 in Belgian coastal waters (southern North Sea) during two typical periods with abundant and reduced biomass. In addition, the Shen et al. (2018) [Water Res. Vol 145, pp 473–486] multi-class population balance flocculation model was extended to address the occurrence of suspended microflocs, macroflocs and megaflocs during these contrasting periods. The microflocs are treated as elementary particles that constitute macroflocs or megaflocs. The FSD is represented by the size and mass fraction of each particle group, which corresponds to a temporal and spatial varying mass weighted settling velocity. The representative sizes of macroflocs and megaflocs are unfixed and migrated between classes mainly due to the effects of turbulent shear, differential settling and biofilm growth. The growth of an aggregate because of bio-activities is allotted to each elementary particle. It is further hypothesized that the growth kinetics of biomineral particles due to biofilm coating follows the logistic equation. This simple bio-flocculation model has been successfully coupled in the open source TELEMAC modeling system with five passive tracers in a quasi-1D vertical case. Within an intra-tide scale, the settling velocity (w s) is large during slack tides while it is small during maximum current velocities because of variations in turbulence intensities. Nonetheless, the w s may be largely underestimated when the biological effect is neglected. For a seasonal pattern, the w s is higher in biomass-rich periods in May than in biomass-poor periods in October. While the mean sizes of megaflocs are close during the two periods, the macroflocs during algae bloom periods are more abundant with a larger mean size. This study enhances our knowledge on the dynamics of SPMs, especially the biophysical influences on the fate and transport of estuarine aggregates. [ABSTRACT FROM AUTHOR]

Organic matter in the water environment can enhance either flocculation or stabilization and, thus, controls the fate and transportation of cohesive sediments and causes seasonal variation in the turbidity of river water, determining floc morphology and settling velocity. The aim of this study was to elucidate the way that biological factors change the organic matter composition and enhances either flocculation or stabilization in different seasons. Jar test experiments were performed using a mixture of standard kaolinite and the filtered river water samples collected (bi-)weekly or monthly from April to December 2015 upstream a constructed weir in Nakdong River, to estimate the flocculation potential of the seasonal river water samples. Chlorophyll-a concentration, algae number concentration, and the fluorescence characteristics of organic matter were used to represent the biological factors. Our results revealed that flocculation potential depended not only on the algal population dynamics, but also the origins (or chemical composition) of organic matter in the river water. Extracellular polymeric substances (EPS), as algal organic matter, enhanced flocculation, while humic substances (HS), as terrestrial organic matter, enhanced stabilization, rather than flocculation. Since flocculation potential reached its maximum around the peaks of algal population, algae-produced EPS likely enhanced flocculation by binding sediment particles in the flocs. This observation supports previous findings of seasonal variation in EPS production and EPS-mediated flocculation. However, when HS was transported from the surrounding basin by a heavy rainfall event, cohesive sediments tended to be rather stabilized. Supplementary flocculation potential tests, which were performed with artificial water containing refined EPS and HS, also showed the opposing effects of EPS and HS. [ABSTRACT FROM AUTHOR]

Abstract: Bimodal flocculation of marine and estuarine sediments describes the aggregation and breakage process in which dense microflocs and floppy macroflocs change their relative mass fraction and develop a bimodal floc size distribution. To simulate bimodal flocculation of such sediments, a Two-Class Population Balance Equation (TCPBE), which includes both size-fixed microflocs and size-varying macroflocs, was developed. The new TCPBE was tested by a model-data fitting analysis with experimental data from 1-D column tests, in comparison with the simple Single-Class PBE (SCPBE) and the elaborate Multi-Class PBE (MCPBE). Results showed that the TCPBE was the simplest model that is capable of simulating the major aspects of the bimodal flocculation of marine and estuarine sediments. Therefore, the TCPBE can be implemented in a large-scale multi-dimensional flocculation model with least computational cost and used as a prototypic model for researchers to investigate complicated cohesive sediment transport in marine and estuarine environments. Incorporating additional biological and physicochemical aspects into the TCPBE flocculation process is straight-forward also. [Copyright &y& Elsevier]

To manage coastal and estuarine waters, it is critical to accurately predict the movements of cohesive and non-cohesive sediments. There are well-established methods to estimate the behavior of non-cohesive sediments; however, without extensive knowledge on flocculation processes it remains difficult to predict the behavior of cohesive sediments. Flocculation is one of the main processes (e.g., erosion, deposition, settling, consolidation and flocculation) in cohesive sediment dynamics. The study of flocculation is an interdisciplinary work since it relates to various physical (e.g., transport, settling and deposition), chemical (e.g., contaminant uptake and transformation) and biological (e.g., community structure activities and metabolism) activities. Nevertheless, a widely-accepted flocculation model that can quantitative simulating the Floc Size Distributions (FSDs) for a relatively large study domain has not yet been fully developed. In this study, a multi-class population balance flocculation model was developed to address the occurrence of suspended microflocs, macroflocs and megaflocs in Belgian coastal waters (southern North Sea). The floc size distributions were represented by the size and mass fraction of each particle group. The representative sizes of macroflocs and megaflocs are unfixed and migrated between classes mainly due to the effects of turbulent shear, differential settling and biofilm growth. Specifically, the growth of an aggregate because of biofilm attachment and extracellular polymeric substance glue is averaged to each elementary particle, with its growth rate response to various bio-activities. This simple bio-flocculation model has been successfully coupled in the open source TELEMAC modeling system with five passive tracers in a quasi-1D vertical case. It was validated by observation data at the station MOW1 close to Zeebrugge harbor during both peak algae bloom and low biomass periods. It shows that when the biomass is abundant the predictions of the mean settling velocities are largely underestimated when the biological effect is neglected. This model will enhance our knowledge of the dynamics of suspended particulate matters, especially the biophysical influences on the fate and transport of estuarine aggregates. [ABSTRACT FROM AUTHOR]

Abstract: Divalent cations have been reported to develop bridges between anionic polyelectrolytes and negatively-charged colloidal particles, thereby enhancing particle flocculation. However, results from this study of kaolinite suspensions dosed with various anionic polyacrylamides (PAMs) reveal that Ca2+ and Mg2+ can lead to colloid stabilization under some conditions. To explain the opposite but coexisting processes of flocculation and stabilization with divalent cations, a conceptual flocculation model with (1) particle-binding divalent cationic bridges between PAM molecules and kaolinite particles and (2) polymer-binding divalent cationic bridges between PAM molecules is proposed. The particle-binding bridges enhanced flocculation and aggregated kaolinite particles in large, easily-settleable flocs whereas the polymer-binding bridges increased steric stabilization by developing polymer layers covering the kaolinite surface. Both the particle-binding and polymer-binding divalent cationic bridges coexist in anionic PAM- and kaolinite-containing suspensions and thus induce the counteracting processes of particle flocculation and stabilization. Therefore, anionic polyelectrolytes in divalent cation-enriched aqueous solutions can sometimes lead to the stabilization of colloidal particles due to the polymer-binding divalent cationic bridges. [Copyright &y& Elsevier]

Abstract Estuarine and coastal regions are often characterized by a high variability of suspended sediment concentrations in their waters, which influences dredging projects, contaminant transport, aquaculture and fisheries. Although various three-dimensional open source software are available to model the hydrodynamics of coastal water with a sediment module, the prediction of the fate and transport of cohesive sediments is still far from satisfied due to the lack of an efficient and robust flocculation model to estimate the floc settling velocity and the deposition rate. Single-class and sometimes two-class flocculation models are oversimplified and fail to examine complicated floc size distributions, while quadrature-based or multi-class based flocculation models may be too complicated to be coupled with large scale estuarine or ocean models. Therefore, a three-class population balance model was developed to track the sizes and number concentrations of microflocs, macroflocs and megaflocs, respectively. With the assumption of a fixed size of microflocs and megaflocs, only four tracers are needed when coupled with the open-source TELEMAC system. It enables better settling flux estimates and better addresses the occurrence and concentration of larger megaflocs. This tri-modal flocculation model was validated with two experimental data sets: (1) 1-D settling column tests with the Ems mud and (2) in-situ measurements at the WZ Buoy station on the Belgian coast. Results show that the flocculation properties of cohesive sediments can be reasonably simulated in both environments. It is also found that the number of macroflocs created, when a larger macrofloc breaks up, is a statistical mean value and may not be an integer when applying the model in the field. Graphical abstract Image Highlights • The sizes of flocs are dynamically altered because of particle aggregation and breakage. • A three-class population balance flocculation model was developed. • This flocculation model has been successfully implemented in the open source TELEMAC. • Four tracer variables, representing the floc size distributions, were included in TELEMAC. • The model was validated with settling column tests and field measurements. [ABSTRACT FROM AUTHOR]

[Display omitted] The quasi-Monte Carlo (QMC) method was enhanced to solve the population balance model (PBM) including aggregation and fragmentation processes for simulating the temporal evolutions of characteristic sizes and floc size distributions (FSDs) of cohesive sediments. Ideal cases with analytical solutions were firstly adopted to validate this QMC model to illustrate selected pure aggregation, pure fragmentation, and combined aggregation and fragmentation systems. Two available laboratory data sets, one with suspended kaolinite and the other with a mixture of kaolinite and montmorillonite, were further used to monitor the FSDs of cohesive sediments in controlled shear conditions. The model results show reasonable agreements with both analytical solutions and laboratory experiments. Moreover, different QMC schemes were tested and compared with the standard Monte Carlo scheme and a Latin Hypercube Sampling scheme to optimize the model performance. It shows that all QMC schemes perform better in both accuracy and time consumption than standard Monte Carlo scheme. In particular, compared with other schemes, the QMC scheme using Halton sequence requires the least particle numbers in the simulated system to reach reasonable accuracy. In the sensitivity tests, we also show that the fractal dimension and the fragmentation distribution function have large impacts on the predicted FSDs. This study indicates a great advance in employing QMC schemes to solve PBM for simulating the flocculation of cohesive sediments. [ABSTRACT FROM AUTHOR]