CHAPTER 4: RESULTS AND DISCUSSION
4.2. Flocculation Tests
4.2.5. Volume-based floc size distributions
The image analysis results of the aggregation tests show that the floc population is dominated by finer particles. The mass and volume-base distributions do not follow the same trend as the population distribution. Floc volume distributions were calculated from the areas of flocs measured at 70 minutes during the aggregation tests. The results are shown in figures 4-14 to 4-17 below. The floc size results from the aggregation tests indicated that macrofloc population is significantly less than the microfloc population. The macrofloc population however constitutes a greater proportion of the floc mass and volume. This is evident in the figures below. The figures also show that the floc size at which the mass is concentrated changes with concentration, salinity and shear rate. For simplicity all flocs are assumed to have equal density. Floc volume and mass are therefore assumed to be linearly proportional to the area of the floc. It is acknowledged that these assumptions may be inaccurate. If the three dimensional spherical-type shape of the flocs was considered, larger flocs would constitute a greater proportion of the total mass. Measured flocs are separated into 25μm bins.
Figure 4-14: Floc volume distribution from the 200mg/L Mfolozi aggregation tests. Key: C – suspended sediment concentration (mg/L), S- salinity (ppt), G – shear rate (s-1).
The volume distributions of results in figure 4-14 show the floc mass to be concentrated at different floc sizes under different conditions. At 10ppt and 10s-1 the floc mass was concentrated between 100 and 150μm. At 0ppt, the mass was concentrated between 50 and 100mg/L. The mass was concentrated at large floc sizes when the salinity was increased. This result was expected given that a higher population of large flocs was observed in the 10ppt
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% of total volume
Equivalent diameter (μm)
C=200 S=0 G=10 C=200 S=10 G=10 C=200 S=10 G=50
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solution (figure 4-3 B). The floc mass of the 50s-1 solution was concentrated at lower size range than the 10s-1 solution. The mass was concentrated at 50μm. Finer flocs were formed at the high shear rate. These flocs clearly dominate the mass of the floc population. A similar result was observed for the Charters Creek 200mg/L aggregation tests shown in figure 4-15 below.
The flocs formed at 50s-1 were smaller with mass concentrated at 50μm. The mass 10s-1flocs at both 0 and 10ppt was distributed over a greater range of floc sizes. There were imaging difficulties during the 200mg/L tests as previously discussed. These are likely to have affected the distributions in figure 4-15 below.
Figure 4-15: Floc volume distributions of the 200mg/L Charters Creek aggregation tests. Key: C – suspended sediment concentration (mg/L), S- salinity (ppt), G – shear rate (s-1).
The influence of suspended sediment concentration on the floc mass distribution may be observed in figure 4-16 below. The floc mass of the 200mg/L sediment was concentrated at a larger floc size than the 50mg/L sediment. This is because larger flocs were able to form at 200mg/L due to a higher frequency of effective collisions between particles.
Deflocculation tests:
The volume-based distributions of flocs at each stage of the deflocculation test performed on 200mg/L Mfolozi sediment was calculated and presented in figure 4-17. During the test d90 floc size decreased as the shear rate was incrementally increased. The increasing shear stress caused larger flocs to break apart. Figure 4-17 shows that as the shear rate is increased, the floc mass moves from large floc sizes to smaller floc sizes. This implies that at high shear stresses, most of the sediment mass is constituted by smaller flocs. The figures above and
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C=200 S=0.5 G=10 C=200 S=10 G=10 C=200 S=10 G=50
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below all show that at a shear rate of 50s-1 the floc mass is concentrated in microflocs between 25 and 75μm while at 10s-1 it is concentrated in larger flocs (100-150μm for Mfolozi sediments).
Figure 4-16: Floc volume distributions of 50mg/L and 200mg/L Mfolozi aggregation tests. Key:
C – suspended sediment concentration (mg/L), S- salinity (ppt), G – shear rate (s-1).
Figure 4-17: Floc volume distributions for 200mg/L Mfolozi sediment at incrementally increasing shear rates. Key: C – suspended sediment concentration (mg/L), S- salinity (ppt), G – shear
rate (s-1).
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% of total volume
Equivalent diameter (μm)
C=50 S=10 G=10 C=200 S=10 G=10
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Equivalent diameter (μm)
G=10 G=20 G=30 G=50
101 Relevance of volume-based distribution results
The results presented in figures 4-14 to 4-17 all have implications for sediment transport modelling. It is clear that most of the sediment mass is concentrated in particles of 50μm equivalent diameter or greater. In conditions of low shear and high concentration, the sediment mass is concentrated in macroflocs. The observation that macroflocs and large microflocs form a small percentage of the floc population is now less important. Most of the suspended mass will settle in the form of macroflocs or large microflocs. The suitable approach to estimating the mass settling flux would be to find a characteristic settling velocity for the floc size at which the sediment mass is concentrated. This is discussed in more detail later.
Figure 4-17 validates the prior mentioned conclusion that the optimal shear rate is 10s-1. The volume-based distribution for 10s-1 showed a higher proportion (>50%) of mass concentrated in macroflocs the the distributions for 20s-1, 30s-1 and 50s-1. It furthermore shows volume-based distributions concentrated at progressively low floc sizes as shear rate is increased.
Limitations of results:
The aggregation test results did not include flocs less than 20μm in diameter. The influence of these particles is uncertain. If the population of unflocculated material is significant, the volume- based distributions will be inaccurate. This may already be so for test results at G=50s-1. If the mass of unflocculated material is significant, this needs to be considered in sediment transport studies and a characteristic settling velocity for these particles requires estimation. This issue is elaborated upon in section 4.4.8.
It is acknowledged that the distributions above are crudely based on the areas of the observed flocs. Despite this, the distributions were sufficient to demonstrate expected trends. It is incorrect to assume a uniform floc density. Floc effective density decreases when floc size increases. The effective density of smaller flocs is higher than those of larger flocs. This is shown further on in section 4.4. The difference in densities of flocs larger than 50μm is generally not significant (less than 100kg/m3). Correcting for varying densities is not anticipated to significantly change the observed results. It must also be remembered that the distributions are based on images of flocs. Limitations of the image processing technique will reduce the quality of the distributions. Images captured during conditions where flocs are too fine to detect will also lead to poorer quality results if used. Experimental error is discussed in section 4.3 below.
Note that the floc equivalent diameter is used instead of major axis length. The equivalent diameter provides a more accurate approximation of the mass of a floc when used. This parameter is used in the settling tests.
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