Flocculation is not only an important part of sediment dynamics within coastal marine waters, but is also a factor of sediment transport within small freshwater streams in Blacksburg, Virginia. The goal of this project was to develop a relationship between floc sizes and stream characteristics (temperature, salinity, chlorophyll-a, organic content, TSS, pH) and to compare how that relationship varies seasonally and spatially across two streams in the same watershed with a similar drainage area but different land uses within these areas. Microscopic images of flocs and water samples were taken within two local streams every two to four weeks throughout the span of one year. The images were analyzed to obtain the floc sizes and the water samples were tested in a lab for various stream properties. The compiled data from the entire year were analyzed to determine if there was a seasonal relationship between floc sizes and the various properties of the water. The process was also repeated at multiple locations along the entire length of both of the streams once in the summer and once in the winter to see if there was a spatial relationship within a single stream. Our study found that significant rainfall events tend to have the greatest effect on floc size in the small headwater streams. However, many of the individual variables alone do not correlate strongly with floc size and a combination of variables may be the best way to analyze the floc size. / Master of Science / Flocculation is the process of single particles coming together to form larger aggregated particles called "flocs". This project focuses on flocculation of sediment within local streams and how the sizes of these flocs may vary throughout the year. The rate of flocculation and the size of these flocs can have a large effect on the movement of sediment within freshwater streams. Images of flocs and water properties such as water temperature, salinity, and pH, were analyzed every two to four weeks throughout the span of a year to determine if there was a relationship between floc size and any of the measured water properties. While a relationship between rainfall and floc size was noticed, it is apparent that multiple variables should be factored into the analysis to get the most accurate results.

Stokes’ law that is traditionally used for modeling the sedimentation of flocs, incorrectly assumes that the floc is solid and spherical. Consequently the settling rates of flocs cannot be estimated using the Stokes law. The application of fractal dimensions to study the internal structure and settling of flocs formed in lime softening process was investigated. An optical microscope with motorized stage was used to measure the fractal dimensions of lime softening flocs directly on their images in 2 and 3D space. The fractal dimensions of the lime softening flocs were 1.15-1.27 for floc boundary, 1.49-1.90 for cross-sectional area and 2.55-2.99 for floc volume. Free settling tests were used for indirect determination of 3D fractal dimension. The measured settling velocity of flocs ranged from 0.1 to 7.1 mm/s (average: 2.37 mm/s) for the flocs with equivalent diameters from 10µm to 260µm (average: 124 µm). Floc settling model incorporating variable floc fractal dimensions as well as variable primary particle size was found to describe the settling velocity of large (>60 µm) lime softening flocs better than Stokes’ law. Settling velocities of smaller flocs (<60 µm) could still be quite well predicted by the Stokes’ law. The variation of fractal dimensions with lime floc size in this study indicated that two mechanisms are involved in the formation of these flocs: cluster-cluster aggregation for small flocs (>60 µm) and diffusion-limited aggregation for large flocs (<60 µm). Therefore, the relationship between the floc fractal dimension and floc size appears to be determined by floc aggregation mechanisms. The settling velocity of lime softening flocs was also modeled by a general model that assumes multiple normally distributed fractal dimensions for each floc size. The settling velocities were in the range of 0-10mm/s and in good agreement with measured settling velocities (0.1-7.1mm/s). The Stokes’ law overestimates the settling velocity of lime flocs. It seems that the settling velocity of flocs is mainly controlled by aggregation mechanisms and forming large floc does not guarantee improved sedimentation. The multifractal analysis of lime softening flocs showed that these aggregates are multifractal and a spectrum of fractal dimensions is required to describe the structure of an individual floc.

Stokes’ law that is traditionally used for modeling the sedimentation of flocs, incorrectly assumes that the floc is solid and spherical. Consequently the settling rates of flocs cannot be estimated using the Stokes law. The application of fractal dimensions to study the internal structure and settling of flocs formed in lime softening process was investigated. An optical microscope with motorized stage was used to measure the fractal dimensions of lime softening flocs directly on their images in 2 and 3D space. The fractal dimensions of the lime softening flocs were 1.15-1.27 for floc boundary, 1.49-1.90 for cross-sectional area and 2.55-2.99 for floc volume. Free settling tests were used for indirect determination of 3D fractal dimension. The measured settling velocity of flocs ranged from 0.1 to 7.1 mm/s (average: 2.37 mm/s) for the flocs with equivalent diameters from 10µm to 260µm (average: 124 µm). Floc settling model incorporating variable floc fractal dimensions as well as variable primary particle size was found to describe the settling velocity of large (>60 µm) lime softening flocs better than Stokes’ law. Settling velocities of smaller flocs (<60 µm) could still be quite well predicted by the Stokes’ law. The variation of fractal dimensions with lime floc size in this study indicated that two mechanisms are involved in the formation of these flocs: cluster-cluster aggregation for small flocs (>60 µm) and diffusion-limited aggregation for large flocs (<60 µm). Therefore, the relationship between the floc fractal dimension and floc size appears to be determined by floc aggregation mechanisms. The settling velocity of lime softening flocs was also modeled by a general model that assumes multiple normally distributed fractal dimensions for each floc size. The settling velocities were in the range of 0-10mm/s and in good agreement with measured settling velocities (0.1-7.1mm/s). The Stokes’ law overestimates the settling velocity of lime flocs. It seems that the settling velocity of flocs is mainly controlled by aggregation mechanisms and forming large floc does not guarantee improved sedimentation. The multifractal analysis of lime softening flocs showed that these aggregates are multifractal and a spectrum of fractal dimensions is required to describe the structure of an individual floc.

To reduce energy consumption, and to achieve the desired denitrification, the activated sludge process sometimes needs to operate at low dissolved oxygen concentrations. The ASM2d model describes the activated sludge process, if nitrification and denitrification occur during different phases in a sequencing batch reactor (SBR). Despite the widespread study of enhanced biological phosphorous removal, comprehensive sludge floc distribution model remains lacking that would better describe this process. Consequently, the integrated system model has been developed to understand the impact of floc at low DO concentrations, and during the process of biological nitrogen and phosphorous removal. In a wastewater treatment plant used in this study, the dissolved oxygen was controlled at a low concentration, 1.7〖gO_2 m〗^(-3), and the dispersion coefficient into the floc was found to be 〖D=1.2×10〗^(-4) m^2/day. Introduction of a number of effectiveness factors contributed to the development of the ASM2d model described herein. This developed model could be valuable for predicting process behaviours applicable under various configurations. Moreover, parameters and effectiveness factors for the model could be calibrated using a genetic algorithm approach. Optimisation was then achieved by controlling the solids retention time during the activated sludge process.

By shearing activated sludge using a high shear rotor stator device, bioavailable proteinaceous material can be produced. Operation at elevated temperatures, serves to increase the amount of material that is rendered soluble (<0.45 um) and biodegradable. The storage of sludge under anoxic condition prior to shearing does not appear to enhance solublization of solids, though deflocculation and deterioration of dewaterablility was observed. Anaerobic digestibility appears to be enhanced by the addition of a high shear as shown by increases in gas production and volatile solids destruction. The dewatering properties of activated sludge, measured by capillary suction time, deteriorated with the addition of sheared solids, but reaeration resulted in near complete recovery. The role of iron and iron chemistry plays a critical role in the activated sludge. Iron apparently selectively removes protein, in particular material ranging in the 1.5 um to 30K size range. The addition of ferric iron was found to increase SVI and decrease zone-settling velocity, when added to reactors with mechanically disintegrated sludges. Similar trends were not observed in reactors dosed with ferrous iron. Preliminary results suggest that the ferric/ferrous redox chemistry may serve to enhance floc structure, as observed by increased settling velocity and shear resistance for sludges dosed with ferrous sulfate. / Master of Science

The accuracy of sediment transport models depends on identifying an appropriate sediment settling velocity. Determining this value for mud suspensions can be difficult because cohesive mud particles can aggregate, forming flocs whose sizes are a function of hydrodynamic and physiochemical conditions of the suspension. Here we present a new model refining the predicted floc size based on hydrodynamic conditions and inherited floc sizes, as well as on the salinity of the fluid environment. The improvements come from modifications made to the Winterwerp (1998) (W98) model. These improvements include: limiting floc size to the Kolmogorov microscale and including an initial salinity dependence. Limiting floc size in this way brings the model predictions more in line with flocculation theory and experimental observations. The salinity dependence was introduced based on a preliminary set of experiments that were conducted to examine floc growth rate and equilibrium size under different salinity conditions. In these experiments, increasing salinity from 2.5 to 10 PSU did not affect equilibrium floc size. However, the increases in salinity did result in longer times to equilibrium and an apparent increase in floc density or fractal dimension. The modified W98 model allows calibrated aggregation and breakup coefficients obtained under one set of concentration values (for both sediment and salinity) to be used to predict floc size under other concentration conditions. Comparing the modified W98 model with laboratory data shows more accurate predictive values, indicating that the modified W98 equation is a promising tool for incorporation into larger sediment transport models. / Master of Science / In order to assist in efforts to improve the environments of rivers, oceans, and estuaries, it is essential for scientists to have the ability to model all of the processes involved. One of the more difficult processes to model is sediment transport, specifically, cohesive sediment transport. How quickly a particle settles out of the water, and thus, how far a particle travels, is related to the density, size, and shape of the particle. With non-cohesive particles, this results in a relatively simple model as the particles stay the same size, shape, and density throughout time and space; however, cohesive particles are constantly changing as they can grow and shrink depending on the properties of the water at any given time. This process is called flocculation, and resulting particles are called flocs. This study aims to improve the modeling of cohesive sediment in the water column. Using existing data from [Tran et al., 2018], improvements were made to the existing Winterwerp [1998] model to account for the dependency of the particle size on sediment concentration. Tests were then run to collect data on how salinity impacts flocculation. This data was then used to further modify the Winterwerp [1998] model in order to account for salinity. These modifications resulted in predictions that better matched laboratory data, indicating that the modified Winterwerp [1998] equation is a promising tool for incorporation into larger sediment transport models.

The coagulation/flocculation process is an important part of surface water treatment. It has direct impact on the reliability of plant operations and final water qualities together with cost control. Low water temperature has a significant impact on the operation of drinking water treatment plants, especially on coagulation/flocculation processes.<p> A microscopic image technique has been used to study the coagulation and flocculation process in recent years, but it requires sample handling that disturbs the floc characteristics during measurement. A high resolution photographic technique was applied to evaluate flocculation processes in the present work. With this technique, the images of the flocs were obtained directly while the flocculation process was taking place. In combination with camera control software and particle size analysis software, this procedure provided a convenient means of gathering data to calculate size distribution. Once the size distribution was calculated, the floc growth and floc size change in the aggregation process could be analyzed. Results show that low water temperature had a detrimental impact on aggregation processes. A water temperature of 0 °C resulted in a slow floc growth and small floc size. Although the floc growth rates at 4 °C and 1 °C were less than those at 22 °C, they were higher than at 0 °C. To improve aggregation processes at low water temperature, adding the coagulant aid of anionic copolymer of acrylamide into the water was found to be effective when the temperature was not less than 1 °C. However, it made only a slight impact on aggregation when the temperature approached 0 °C. At water temperatures of 22 °C, 4 °C and 1 °C, the polymer caused the formation of large floc (larger than 0.5 mm2 in projected area). The polymer significantly shortened the required time of flocculation and sedimentation. Three minutes of flocculation and 20 minutes of sedimentation were sufficient for the polymer to achieve good treatment performance, while the flocculation time and sedimentation time had to be 20 and 60 minutes, respectively, without using the polymer. On the other hand, when the temperature was close to 0 °C, the polymer did not cause the formation of the large floc, nor did it shorten the time of flocculation and sedimentation.<p> The experimental results in this research agree with the model for flocculation kinetics given by Argaman and Kaufman (1970). With decreasing water temperature, the aggregation constant (KA) decreased and breakup constant (KB) increased. KA and KB with aluminum sulfate was close to those with ferric sulfate, respectively. <p> In treating the South Saskatchewan River water, an aluminum sulfate or ferric sulfate dosage greater than 50 mg/L resulted in marginal gains in treatment efficiency. Decreasing dosages of aluminum sulfate or ferric sulfate caused lower floc growth rates and smaller floc sizes. Extremely low dosages (5 mg/L or less) resulted in poor floc formation and extremely small sizes.

The coagulation/flocculation process is an important part of surface water treatment. It has direct impact on the reliability of plant operations and final water qualities together with cost control. Low water temperature has a significant impact on the operation of drinking water treatment plants, especially on coagulation/flocculation processes.<p> A microscopic image technique has been used to study the coagulation and flocculation process in recent years, but it requires sample handling that disturbs the floc characteristics during measurement. A high resolution photographic technique was applied to evaluate flocculation processes in the present work. With this technique, the images of the flocs were obtained directly while the flocculation process was taking place. In combination with camera control software and particle size analysis software, this procedure provided a convenient means of gathering data to calculate size distribution. Once the size distribution was calculated, the floc growth and floc size change in the aggregation process could be analyzed. Results show that low water temperature had a detrimental impact on aggregation processes. A water temperature of 0 °C resulted in a slow floc growth and small floc size. Although the floc growth rates at 4 °C and 1 °C were less than those at 22 °C, they were higher than at 0 °C. To improve aggregation processes at low water temperature, adding the coagulant aid of anionic copolymer of acrylamide into the water was found to be effective when the temperature was not less than 1 °C. However, it made only a slight impact on aggregation when the temperature approached 0 °C. At water temperatures of 22 °C, 4 °C and 1 °C, the polymer caused the formation of large floc (larger than 0.5 mm2 in projected area). The polymer significantly shortened the required time of flocculation and sedimentation. Three minutes of flocculation and 20 minutes of sedimentation were sufficient for the polymer to achieve good treatment performance, while the flocculation time and sedimentation time had to be 20 and 60 minutes, respectively, without using the polymer. On the other hand, when the temperature was close to 0 °C, the polymer did not cause the formation of the large floc, nor did it shorten the time of flocculation and sedimentation.<p> The experimental results in this research agree with the model for flocculation kinetics given by Argaman and Kaufman (1970). With decreasing water temperature, the aggregation constant (KA) decreased and breakup constant (KB) increased. KA and KB with aluminum sulfate was close to those with ferric sulfate, respectively. <p> In treating the South Saskatchewan River water, an aluminum sulfate or ferric sulfate dosage greater than 50 mg/L resulted in marginal gains in treatment efficiency. Decreasing dosages of aluminum sulfate or ferric sulfate caused lower floc growth rates and smaller floc sizes. Extremely low dosages (5 mg/L or less) resulted in poor floc formation and extremely small sizes.

Mud makes up a large fraction of sediment transported within rivers to the coasts. Predicting where mud will settle is complicated by the cohesive nature of silts and clays, which can combine to form larger aggregates known as flocs. The size and density, and consequently, the settling velocity, of flocs is highly dynamic and depends on factors such as turbulence levels within the flow and biogeochemical components of the water and sediment. To better predict where mud will deposit, more observations of flocs while in their natural environment is required to better understand the controls on when, where, and to what degree mud is flocculated. However, the need for more field observations is complicated by the dynamic and fragile nature of flocs. This necessitates the need for developing in-situ observation methods to ensure that measured floc sizes are representative of their in-situ size, and not a result of sampling methods. In this thesis, a new instrument for in-situ observation of flocs is presented. In addition, two methods using the data collected from the instrument allow the user to: (1) identify sand within the particle data using a machine learning algorithm, and (2) estimate the mass suspended sediment concentration of the mud and sand fractions of suspended sediment independently. Results from using the instrument in the lowermost Mississippi River reveal differences in floc sizes over the water column, and by season. In addition, a unique observation of flocs in the presence of a salt wedge is presented. Overall, the instrument provided the first known observations of flocs within the Mississippi River, and provides a start to better understanding controls on floc sizes within the fluvial environment. / Master of Science / Flowing water within large rivers carries sediments such as sand and mud to the coasts. Some of the larger sediment carried by rivers can fall to the riverbed if the river does not have enough energy to carry it in the flow. The remaining sediment can be carried to the coasts where it will fall to the bed, providing the material necessary for estuaries or deltas to form and grow. Understanding when and where sediment falls to the bed within rivers, estuaries, and deltas, allows scientists and engineers to predict how these landforms will change over time to better manage them under future climate conditions. Predicting where mud will fall to the bed is particularly difficult because mud has the ability to stick together to form larger aggregates. These aggregates, known as mud flocs, are constantly changing in size depending on the energy in the river and water conditions. As the mud flocs change in size, the speed at which they fall to the bed changes. As such, observing mud flocs while they are in their natural environment is required to understand the conditions under which they form and change in size. This thesis presents a new instrument that can be used to collect images of mud flocs while they are in their natural environment. Results from using the instrument to observe mud flocs in the lowermost Mississippi River are then presented. This new instrument, and observations of mud flocs made with it, provides new insight into mud floc size within the lowermost Mississippi River.

The upper water column (<1 m depth) of freshwater lakes, which includes the surface microlayer (SML; <1 mm depth), is an important microbial habitat as well as an accumulation and dissemination site for chemical and microbial contaminants. This doctoral thesis reports novel insights into how the physical structure and functional capabilities of microbial communities can influence the presence of trace metals and health-relevant bacteria in the upper waters (SML and 0.5 m depth) of freshwater lakes. Two physically and geochemically contrasting lake environments, a remote sheltered boreal lake and a higher energy urban beach on Lake Ontario, were investigated to identify system-dependent physical and biogeochemical factors controlling contaminant-relevant microbial characteristics. The SML was identified as a major site for generation of contaminant-sequestering suspended flocs from a distinct biofilm-forming microbial community over diurnal timeframes via wind and sunlight exposure, with this process being enhanced at the higher energy beach site. More generally, upper waters including the SML were demonstrated to be inhabited by a diverse group of atypical facultative Fe(III)-reducing bacteria (IRB) that exhibited a SML- and lake-specific capacity for solid Fe(III) reduction directly related to floc and Fe(III) availability. Although IRB were hypothesized to be highly resistant to metals and antibiotics relative to other bacteria due to their ability to dissolve metal-rich Fe(III) minerals, this was not found to be the case. Nevertheless, IRB enriched from the SML demonstrated higher antibiotic resistance compared to those from 0.5 m depth and enriched Fe(III)-reducing communities from both depths harboured resistance-mobilizing genetic elements and included potentially pathogenic bacteria. Results of this thesis represent new knowledge concerning how microbial communities regulate the presence of contaminants in the upper waters of lakes. This has important implications for assessing the ecological and human health impacts of contaminants in freshwater systems. / Thesis / Doctor of Philosophy (PhD)