Table

Summary of Experiments

Shear rate

Experiments

Coagulant

Measurements

Experiment Set 1 (lake water and montmorillonite)

Alum

Initial conditions, charge neutralization, sweep floc 10, 20, 30 min n/a-tested supernatant after mixing (during settling) 20, 80

Experiment Set 2 (latex particles) (Expts. 1-8)

Alum

Experiment Set 3 (latex particles, Buffalo River) (Expts. 9-11)

Polymer

2-150 min

experiments (Experiment Set 2, Section 5.3.2.2), images were taken from the sample while slow mixing was still in progress, that is, particles were photographed in situ.

A schematic of the general experimental setup is shown in Figure 5.2. Images of the suspended particles were illuminated by a strobe light, which provided a coherent backlighting source. Depending on conditions for a particular experiment, the strobe pulse rate and intensity were adjusted to produce one pulse during the time the camera shutter was open. The projected images were captured by a computer-controlled CCD camera (Kodak MegaPlus digital camera, model 1.4) placed on the opposite side of the mixing jar from the strobe. Generally the shutter exposure time was between about 80 and 147 ms. The camera captured digital images on a sensor matrix consisting of 1320 (horizontal) x 1035 (vertical) pixels. Each pixel was recorded using 8 bit resolution, that is, with 256 gray levels. For the present tests, a resolution of 540 pixels per mm was achieved. This was determined by imaging a known length on a stage micrometer and counting the number of pixels corresponding to that length. The camera was mounted on a traversing device so that it could be moved in each of the three coordinate directions, and images were stored on the hard drive of a PC. Camera settings were varied to obtain the best quality (greatest contrast between aggregates and background) for each set of experimental conditions (see Chakraborti25 for further details), but pixel resolution was held constant throughout the tests. Pixel resolution was always sufficient to adequately describe the smallest particles in these experiments.26 Experiments were conducted in a darkened room to eliminate light contamination.

Once saved, images were processed using a public domain image analysis software program (NIH Image). Processing steps included contrast enhancement and thresholding, resulting in a binary image consisting of solids (black) and background (white). Image was then applied to calculate basic geometric properties for each aggregate in the image, which included perimeter and area. In addition, an ellipse was fitted to each aggregate, by matching moment of inertia and area of the original aggregate. This step resulted in the definition of major and minor ellipse axes, and the major axis was taken as the characteristic (longest) length, l, of the aggregate. In order to estimate volumes to calculate D3 (Equation (5.1)), the two-dimensional fitted ellipse was rotated about the long axis. As shown by Chakraborti et al.,27

FIGURE 5.2 Experimental setup consisting of strobe light, CCD camera attached to a computer, and the suspended sample in a mixing jar.

this procedure produces estimates of volume that are preferable to using a spherical encased volume assumption, although there is still obviously greater uncertainly in the volume estimates than in the calculations based on area. Since direct measurements of volume using an image-based method are not available, the ellipse approximation provides a reasonable approach. After application of Image, all data were transferred to a spreadsheet for further processing, including calculations of size distributions, fractal dimensions, and other parameters as described in the following paragraphs.

Before conducting the aggregation experiments, preliminary tests were conducted to ensure that the imaging procedures were providing accurate data. Monodisperse suspensions of several different latex particle sizes with known concentration were photographed to determine the accuracy of size analysis and also to evaluate the degree to which concentration could be reproduced. Initial experiments were conducted by Cheng et al.,28 who showed that both 10 ^.m and 6 ^.m monodisperse latex solutions were correctly analyzed. Chakraborti25 conducted additional tests and, in addition, used the known concentration to evaluate the sampling volume, defined by the field of focus of the camera. The sampling volume was found to be approximately 20 mm square and 3 mm deep. In addition, he conducted a number of sensitivity analyses to further refine the imaging procedures and evaluate the accuracy and reproducibility of the imaging results.

5.3.2 Materials and General Procedures

Three sets of experiments were designed to provide data for analysis of particles resulting from coagulation and flocculation under different process conditions (Table 5.2). In these experiments, alum and polymer were used as coagulants. In Experiment Set 1, particle size distributions and morphology of aggregates obtained from lake water samples and laboratory suspensions of montmorillonite clay were measured. Results of these experiments were reported previously,27 and they demonstrated that fractal dimension could be used to characterize different stages of aggregation, ranging from initial untreated suspensions, to conditions of sweep floc with relatively large alum dose. Experiments were conducted to test the hypothesis that charge neutralization and sweep floc mechanisms produce fundamentally different particle characteristics, including differences in fractal dimension. In Experiment Set 2, images of aggregates were obtained while the suspension was still being stirred during flocculation. Results demonstrated that images could be obtained in situ and also provided direct observation of temporal changes in floc characteristics during mixing. Again, alum was used as a coagulant. The goal of these experiments was to test the hypothesis that changes in fractal dimension are correlated with the physicochemical conditions of a particular experiment. These experiments were reported by Chakraborti et al.26 The third set of experiments was designed to provide a description of the aggregation process over longer periods of time for both inorganic and natural suspensions. Polymer was used as coagulant, to avoid additional mass that may be introduced by alum particles and to focus on fundamental aggregate growth due to primary particles only. The previous flocculation experiments were restricted to durations of only 30 min. For treatment plant operation this length of time may be relevant, but the aggregates were probably still undergoing changes in their shape and size. In particular, disaggregation and restructuring were relatively unimportant during this earlier period, and become more important only for longer times. Results from the second and third sets of experiments provided the basis for model conceptualization and model development described above. Because Experiment Sets 1 and 2 have been previously reported, they are only briefly summarized here.

5.3.2.1 Experiment Set 1

Water samples were collected from a shallow lake located on the campus of the University at Buffalo, Buffalo, New York. The lake has an average depth of 3 m, with maximum depth of 8 m and a total area of 243,000 m2. Experiments also were conducted for clay suspensions prepared by adding montmorillonite clay powder (K-10) to deionized water to produce a sample with a solids concentration of 100 mg/l. Montmorillonite is an aluminum hydrosilicate where the ratio between SiO2 and Al2O3 is approximately 4 to 1. It has a bulk density of 370 g/l, surface area of 240 m2/g and pH 3.2 observed at 10% suspension (Fluka Chemicals, Buchs, Switzerland). For coagulant, a stock solution of alum was prepared by dissolving Al2(SO4)3-18 H2O (Fisher Scientific, Pittsburgh, PA) in deionized water to a concentration of 0.1 M (0.2 M as aluminum). Standard jar tests were conducted with these samples to determine an appropriate dose of alum to generate a "sweep floc" condition. Changes of both surface charge (measured as zeta potential) and residual turbidity with alum dose also were measured. After addition of alum, the suspension was mixed rapidly (@~100 rpm) for 1 min and then slow-mixed for 20 min with a mean velocity gradient, G = 20 sec-1. The mixing was then stopped and images of the resulting aggregated particles in suspension were taken. All experiments were conducted at room temperature (~20°C to 23°C), and the analyzed images were obtained with an alum dose of 20 mg/l, and with pH maintained at 6.5 by manual addition of acid or base as required.

5.3.2.2 Experiment Set 2

Monosized polystyrene latex microspheres with a density of 1.05 g/ml (Duke Scientific Corporation, Palo Alto, CA, United States) were used as the primary particles for these experiments. The nominal particle diameter was 9.975 ^m, with a standard deviation of ±0.061 ^m. Particles were taken from a 15 ml sample of aqueous suspension with 0.2% solids content (manufacturer's specification). The number concentration of particles in the concentrated suspension was 3.66 x 106 particles/ml (±10%). Aliquots of 0.06 ml or 0.1 ml of the suspension were added to the mixing jar along with 1 l of deionized water, resulting in initial number concentrations of 220 and 366 particles/ml, respectively. The higher and lower concentrations yielded total suspended solids of 0.12 and 0.20 mg/l. The images were analyzed to track changes in aggregate morphology for a given test, as well as differences between tests resulting from varying coagulant (alum) dose, particle concentration and mixing speed, or shear rate.

A freshly prepared stock solution of alum was prepared for each test as in Experiment Set 1. For each test, after addition of coagulant and an initial rapid mixing period (with G = 100 sec-1) for 1 min, the mixing speed was reduced to either G = 20 sec-1 or G = 80 sec-1 and continued until the end of the experiment. All tests were conducted at room temperature (20°C to ~23°C) and a constant pH of 6.5 was maintained by adding acid or base as required. Measurements were taken at 10,20, and 30 min after the initial rapid mixing period. Experiments were performed to evaluate temporal changes in the fractal dimensions of aggregates formed during flocculation of the microspheres. Particle size distributions, collision frequency, and aggregate geometrical information at different mixing times were obtained under variable conditions.

5.3.2.3 Experiment Set 3

Typically, two types of suspension were used in the third set of experiments: abiotic (latex) and natural (collected from a local river), containing both inorganic and organic constituents. These experiments were conducted using the same equipment and general procedures as in Experiment Sets 1 and 2. Polystyrene latex particles (Duke Scientific Corporation, Palo Alto, CA) of 6 ¡m diameter (6.038 ¡m ± 0.045 ¡m; density 1.05 g/ml) with a particle concentration of 4000 /ml were used as the primary particles for the abiotic suspensions. Suspensions were prepared by adding a predetermined quantity of particles in deionized water and stirring vigorously to insure homogeneity. These solutions contained 4.52 x 10-5% solids by volume, or 0.5 mg/l. A constant pH = 6.5 was maintained by adding acid or base as required.

The natural suspension was obtained from the Buffalo River (Buffalo, New York). This sample was collected at about 0.5 m below the water surface at a point where the channel is about 50 m wide and total water depth is about 7 m (in the mid-section). The wind velocity recorded on the sampling day was 27 kmph (17 mph), and water temperature was 10.55°C (51°F). The organic content was measured by oven drying a filtered 500 ml sample for 24 h at 105°C, followed by 24 h of oven drying at 550°C. The measured total solids content (TSS) was 14.6 mg/l (measured using a 0.22 ¡m filter), which is fairly typical for rivers, and the volatile organic solids (VSS) was 0.4 mg/l, resulting in a 2.74% organic content.

Since the surface charge of suspended particles is negative, cationic polymer was used for coagulant. Polymer was chosen since it does not form gel or add particles in suspension like alum floc, and it allows quick aggregation. Alken solutions (Alken-Murray Corporation, New York) supplied the polymer Ethanediamine (C193K) for these experiments. This polymer has a molar mass less than one million (~700,000) and contains the quaternary amine (ammonium) group that produces the positive charge. According to the manufacturer, it is completely soluble and has an effective pH range of 0 to 13, with good floc formation at solution pH between 4 and 6. Fresh polymer (0.5% stock solution, per manufacturer's specifications) was prepared for each experiment, and the solution was shaken vigorously before each use.

The selected mixing speeds for the experiments were 15 rpm, 46 rpm, and 100 rpm, resulting in average velocity gradients, G = 10 sec-1, 40 sec-1, and 100 sec-1, respectively. These values were chosen to span the range of mixing environments found in engineered and natural aquatic systems, although some natural environments may have even lower mixing intensities. For each experiment, samples were rapid mixed for 1 min to provide thorough mixing of the particles with the coagulant. The stirring rate was then adjusted to the desired level and images of the flocs were taken at different times throughout the experiment.

Preliminary tests were performed to determine the quantity of coagulant needed to produce particle aggregation at the desired rate. Measurements of zeta potential and resulting residual turbidity (measured at 30 min after stopping the mixing) are shown in Figure 5.3 for different polymer doses added to Buffalo River suspensions. Basic stages of aggregation may be seen with increasing coagulant dose. Initially (Stage I), particle surface charge is reduced as the particles become destabilized. The principle aggregation mechanism in this stage is the reduction of electrostatic repulsion between particles as a result of surface charge reduction. Charge neutralization (Stage II) is reached when sufficient coagulant is added so that the originally negatively charged particles are just neutralized, and aggregates are formed when contacts occur. Further addition of coagulant leads to charge reversal and restabilization (Stage III) where aggregation is not chemically favored. When alum is used as coagulant, Al(OH)3(s) is produced, which coats particles with a gelatinous and "sticky" sheath. At higher doses and for appropriate pH range, "sweep floc" may occur, which causes aggregates to settle more quickly and reduce turbidity. This stage is not present for the polymer, which does not produce "sweep floc."

As the surface charge of the suspended particles changes with increasing coagulant dose, the resulting residual turbidity decreases, reaching a minimum for a dose near the dose corresponding to neutral surface charge (Figure 5.3). The initial turbidity for the Buffalo River suspension was 28 NTU, and the coagulant dose causing charge neutralization was 520 i^g/l. For doses higher than the neutral surface charge dose,

FIGURE 5.3 Zeta potential and residual turbidity measurements as a function of coagulant dose for Buffalo River suspension; different stages of aggregation are indicated with increasing dose.

turbidity increases slightly. Once the dose of coagulants was selected for a particular experimental condition (the charge neutralization dose), the next step was to photograph and analyze images of particles from the mixing jar, as described above.

5.4 RESULTS AND DISCUSSION 5.4.1 Observations and Analysis of Data 5.4.1.1 Coagulation-Flocculation

A time series of floc images formed by Buffalo River particles at different mixing times is shown in Figure 5.4 (these images were obtained as a prelude to the tests in Experiment Set 3). After only 2 min, the initially monodispersed particles are seen to form flocs of various shapes and sizes, and larger flocs formed with additional time. The larger aggregates at later times appear to be more porous and spread out than the smaller aggregates observed at earlier times, and are associated with lower fractal dimensions. The images show that the aggregate structure is an agglomerate of particles/clusters, with a highly irregular surface. Qualitatively at least, this observation supports the characterization of natural aggregates in terms of fractal geometry. The series of images in Figure 5.4 also is consistent with the aggregation

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