A Population Balance Model for Shear-Induced Polymer-Bridging Flocculation of Total Tailings

Shear-induced polymer-bridging flocculation is widely used in the solid–liquid separation process in cemented paste backfill, beneficial to water recycling and tailings management in metal mines. A flocculation kinetics model based on Population Balance Model (PBM) is proposed to model the polymer-bridging flocculation process of total tailings. The PBM leads to a system of ordinary differential equations describing the evolution of the size distribution, and incorporates an aggregation kernel and a breakage kernel. In the aggregation kernel, a collision frequency model describes the particle collision under the combined effects of Brownian motions, shear flow, and differential sedimentation. A semi-empirical collision efficiency model with three fitting parameters is applied. In the breakage kernel, a new breakage rate coefficient model with another three fitting parameters is introduced. Values of the six fitting parameters are determined by minimizing the difference between experimental data obtained from FBRM and modeling result through particle swarm global optimization. All of the six fitting parameters vary with flocculation conditions. The six fitting parameters are regressed with the flocculation factors with six regression models obtained. The validation modeling demonstrates that the proposed PBM quantifies well the dynamic evolution of the floc size during flocculation under the given experimental setup. The investigation will provide significant new insights into the flocculation kinetics of total tailings and lay a foundation for studying the performance of the feedwell of a gravity thickener.

Modelling the Effects of Multiple Fractal Dimensions on the Flocculation and Resuspension Processes of Cohesive Sediment

The flocculation of cohesive sediments represents a critical process in coastal sediment transport, with its appropriate representation in numerical models crucial for the prediction of contaminant transport, coastal morphodynamics and engineering problems. In this study, a flocculation model considering the effects of multiple fractal dimensions is incorporated into a two-phase numerical modelling framework and used to investigate the effects of spatio-temporal variations in sediment concentrations on the temporal evolution of local floc sizes. Initially, the model is applied to simulate the aggregation of clay suspensions in a vertical grid-stirred settling column, with results confirming the importance of multiple fractal dimensions when predicting the time evolution of floc sizes. The adoption of multiple fractal dimensions, in particular, allows the two-phase numerical model to better match the measured settling column data with improved overall correlation. This is especially the case when predicting initial floc size growth during the early period of settling when the flocs tend to adjust more rapidly to their equilibrium sizes. The two-phase model is then applied to simulate field measurements of mud resuspension process in a tidally driven channel. Again, by considering multiple fractal dimensions within the flocculation model, better agreement is obtained between observed and modelled suspended sediment concentrations, while predicted floc sizes are also in general accord with previous field measurements made within the same estuary.

Super-bridging Fibrous Materials for Water Treatment: Impacts on Removal of Plastic Particles, Phosphorus and Natural Organic Matter

To deal with issues of process sustainability, cost, and efficiency, we developed materials reengineered from fibers to serve as super-bridging agents, adsorbents, and ballast media. These sustainable fiber-based materials considerably increased the floc size (~6630 µm) compared to conventional physicochemical treatment using a coagulant and a flocculant (~520 µm). The materials also reduced coagulant usage (up to 40%) and flocculant usage (up to 60%). These materials could be used in synergy with coagulants and flocculants to improve settling in existing water treatment processes and allow facilities to reduce their capital and operating costs as well as their environmental footprint. Moreover, the super-sized flocs produced using fiber-based materials (up to ~13 times larger compared to conventional treatment) enabled easy floc removal by screening, eliminating the need for a settling tank, a large and costly process unit. The materials can be effective solutions at removing classical (e.g., natural organic matter (NOM) and phosphorus) and emerging contaminants (e.g., microplastics and nanoplastics). Due to their large size, Si- and Fe-grafted fiber-based materials can be easily recovered from sludge and reused multiple times.

Super-bridging Fibrous Materials for Water Treatment: Impacts on Removal of Plastic Particles, Phosphorus and Natural Organic Matter

Aggregation combined with gravitational separation is the most commonly used method to treat water globally, but it carries a significant economic and environmental burden as the chemicals used in the process (e.g., coagulants) generate ~8 million tons of metal-based sludge waste annually. To simultaneously deal with the issues of process sustainability, cost, and efficiency, we developed materials reengineered from pristine or waste fibers to serve as super-bridging agents, adsorbents, and ballast media. This study shows that these sustainable fiber-based materials considerably increased the floc size (~6630 µm) compared to conventional physicochemical treatment using a coagulant and a flocculant (~520 µm). The fiber-based materials also reduced coagulant (up to 40%) and flocculant usage (up to 60%). Moreover, the unprecedented size of flocs produced using fiber-based materials (up to ~13 times larger compared to conventional treatment) enabled easy floc removal by screening, thereby eliminating the need for a settling tank, a large and costly process unit. Our results show that fiber-based materials can be effective solutions at removing classical (e.g., natural organic matter (NOM) and phosphorus) and emerging contaminants (e.g., microplastics and nanoplastics). Due to their large size (> 3000 µm), some Si-grafted and Fe-grafted fiber-based materials can be easily recovered from settled/screened sludge and reused multiple times for coagulation/flocculation. Our results also show that these materials could be used in synergy with coagulants and flocculants to improve settling in existing water treatment processes. Furthermore, these reusable materials combined with separation via screening could allow global water treatment facilities to reduce their capital and operating costs as well as their environmental footprint.

Effect of hydrodynamic breakage on floc evolution and turbidity reduction in flocculation and sedimentation processes

Coagulation and sedimentation process is one of the most popular processes in drinking water treatment. Hydrodynamic breakage has a significant impact on the evolution of floc characteristic and the efficiency of turbidity removal. In this work, the effects of hydrodynamic breakage on floc size, fractal dimension, and floc morphology were investigated with an in-situ recognition system. The experiments were conducted in a continuous flocculation and sedimentation reactor equipped with perforated plates to provide different hydrodynamic breakage conditions. The experimental results indicated that the hydrodynamic conditions significantly influenced the floc destabilization and restructuring processes. A low hydrodynamic shear forces provided by P1 led to the increase of both bigger sized flocs but accompanied with small particles (0–10 μm). Excessive velocity gradient provided by P3 produced smaller and looser flocs. An appropriate velocity gradient (i.e., the flow velocity through the perforated plate P2 at 18.9 × 10−3m s−1) was conducive for the formation of larger and more compact structures, with higher average floc size and fractal dimension. This flocculation condition in turn resulted in effective improvements in the turbidity removal efficiency. Floc evolution models were described based on the mechanism of the breakage and restructuring process.

Suspended sediment floc size in the Cam - Nam Trieu estuary (Hai Phong, Vietnam), in wet season

Median diameters (D50) of suspended particles were inferred in the wet season in the Cam - Nam Trieu estuary (Hai Phong, Vietnam) based on the particle size distribution measured by LISST-100X on five transects along the river from 23 to 26 September 2015. The results showed that floc diameters varied between 3.6 μm to 146.5 μm and averaged 49.14 μm. At high tide, the average floc size D50 was lowest (42.66 ± 11.55 µm). It reached the highest value in the ebb tide (62.87 ± 23.34 µm) and then decreased to intermediate values in the flood tide (48.75 ± 15.72 µm). The coefficient of variation of the mean floc size D50 was lowest in the high tide (27.05%), highest in ebb tide (37.13%), then intermediate in the flood tide (32.12%).

An Extended Entropic Model for Cohesive Sediment Flocculation in a Piecewise Varied Shear Environment

In this study, an extended model for describing the temporal evolution of a characteristic floc size of cohesive sediment particles when the flocculation system is subject to a piecewise varied turbulent shear rate was derived by the probability methods based on the Shannon entropy theory following Zhu (2018). This model only contained three important parameters: initial and steady-state values of floc size, and a parameter characterizing the maximum capacity for floc size increase (or decay), and it can be adopted to capture well a monotonic pattern in which floc size increases (or decays) with flocculation time. Comparison with 13 literature experimental data sets regarding floc size variation to a varied shear rate showed the validity of the entropic model with a high correlation coefficient and few errors. Furthermore, for the case of tapered shear flocculation, it was found that there was a power decay of the capacity parameter with the shear rate, which is similar to the dependence of the steady-state floc size on the shear rate. The entropic model was further parameterized by introducing these two empirical relations into it, and the finally obtained model was found to be more sensitive to two empirical coefficients that have been incorporated into the capacity parameter than those in the steady-state floc size. The proposed entropic model could have the potential, as an addition to existing flocculation models, to be coupled into present mature hydrodynamic models to model the cohesive sediment transport in estuarine and coastal regions.

Silylated Thiol-Containing Cellulose Nanofibers as a Bio-Based Flocculation Agent for Ultrafine Mineral Particles of Chalcopyrite and Pyrite

A considerable amount of very fine particles can be found, e.g., stored in tailing ponds, and they can include valuable or hazardous minerals that have the potential to be recovered. Selective flocculation, i.e., the formation of larger aggregates from specific minerals, offers a promising approach to improve the recovery of ultrafine particles. This study focuses on the use of a new bio-based flocculation agent made of silylated cellulose nanofibers containing a thiol-functional moiety (SiCNF). Flocculation was performed in separated systems of ultrafine mineral dispersions of pyrite, chalcopyrite, and quartz in aqueous alkaline medium. The flocculation performance of SiCNF was addressed in terms of the turbidity reduction of mineral dispersions and the floc size, and the results were compared with the performance of a commercial anionic polyacrylamide. SiCNF exhibited a turbidity removal efficiency of approximately 90%–99% at a concentration of 4000–8000 ppm with chalcopyrite and pyrite, whereas the turbidity removal of quartz suspension was significantly lower (a maximum of approximately 30%). The sulfide particles formed flocs with a size of several hundreds of micrometers. The quartz in turn did not form any visible flocs, and the dispersion still had a milky appearance after dosing 12,000 ppm of the flocculant. These results open a promising path for the investigation of SiCNF as a selective flocculation agent for sulfide minerals. Graphical Abstract

Sustainable Fiber-Based Materials as Super-bridging Agents, Adsorbents, and Ballast Media

Aggregation combined with gravitational separation is the most commonly used method to treat water globally, but it carries a significant economic and environmental burden as the chemicals used in the process (e.g., coagulants) generate ~8 million tons of metal-based sludge waste annually. To simultaneously deal with the issues of process sustainability, cost, and efficiency, we developed materials reengineered from pristine or waste fibers (e.g., cellulose, polyester, cotton, and keratin) to serve as super-bridging agents, adsorbents and ballast media. This study shows that these sustainable materials (fibers, microspheres, and flakes functionalized with Si, Al and/or Fe) considerably increased the floc size (~6630 µm) compared to conventional physicochemical treatment (~520 µm; using alum and polyacrylamide). The fiber-based materials also reduced chemical usage (20–60 %) and improved contaminant removal during settling by increasing floc size and density. Moreover, the unprecedented size of flocs produced using fiber-based materials (13 times larger compared to conventional treatment) enabled easy floc removal by screening, thereby eliminating the need for a settling tank, a large and costly process unit used to treat more than 70% of water globally. Our results show that fiber-based materials can be effective solutions at removing classical (e.g., natural organic matter (NOM) and phosphorus, via electrostatic affinities) and emerging contaminants (e.g., microplastics and nanoplastics). Due to their large size (> 3000 µm), some Si-grafted and Fe-grafted fiber-based materials were easily recovered from settled/screened sludge and reused multiple times for coagulation/flocculation. These reusable materials combined with separation via screening could allow global water treatment facilities to reduce their capital and operating costs as well as their environmental footprint. Finally, our results also show that these materials could be used in synergy with coagulants and flocculants to improve existing water treatment plants for the removal of NOM, phosphorus, turbidity, total suspended solids and microplastics.