CHAPTER 4: RESULTS AND DISCUSSION
4.1. Material Composition Tests
4.1.1. Particle Size Analysis
Sediment samples were extracted from the bulk water samples collected from the Mfolozi River and Lake St Lucia for particle size analysis. Particle size analysis tests were performed to classify the sediment. The Malvern Mastersizer 2000 was used to size the primary particles present. Tests were performed on virgin water samples and on dried and combusted sediments. The results were found to differ.
Table 4-1: Malvern particle size analysis results
Sample d5 d10 d50 d90
Virgin Mfolozi water sample 0.836 1.145 4.018 16.558 Dried Mfolozi sediment 1.240 1.781 10.523 44.129 Dried and Combusted
Mfolozi Sediment 2.934 5.795 31.170 69.357
Virgin Charters Creek water
sample 1.680 2.576 8.220 28.682
Dried Charters Creek
sediment 1.865 2.791 14.416 40.937
Dried and Combusted
Charters Creek sediment 3.001 6.229 42.453 111.908
*All figures in μm
Table 4-2: Sediment Classification
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Sample Clay %
(<2μm) Silt %
(2-63μm) Sand % (63-2000μm) Virgin Mfolozi water sample 24.8 72.6 2.6
Dried Mfolozi sediment 12.0 84.9 3.1
Dried and Combusted Mfolozi
Sediment 3.3 83.2 13.5
Virgin Charters Creek water
sample 6.7 90.4 2.9
Dried Charters Creek
sediment 5.7 92.1 2.2
Dried and Combusted
Charters Creek sediment 3.3 65.0 31.7
The virgin water sample results of Mfolozi sediment shows a large clay fraction of 24.8%. This is not reflected in the results of the dried and combusted samples. The reduction in clay sized materials may be attributed to the high organic content of Mfolozi sediment. Both the Mfolozi and Charters Creek virgin samples in table 4-2 show a reduction in the clay fraction when dried, and a further reduction when combusted. There are two likely explanations for this behaviour:
(1) The loss of organic matter during combustion. A significant reduction in particles less 2μm with ignition suggests high organic matter content.
(2) Water samples were dried in order to remove the sediment from suspension. When sediment is dried, finer particles and organic matter adhered to each other, forming aggregates. These aggregates required separation prior to testing. This was done using a pessel and mortar. It was difficult to break down all of these small aggregates without excessive grinding which causes the particles to shear. The dried and combusted sediments were mixed into a solution of de-ionized water for the particle size analysis. It is uncertain whether all aggregates have been separated in the solution. Therefore the clay- sized fraction and the lower end of the silt fraction may be under-represented after drying and combustion. Note that these sediments remained in solution for a few days prior to testing.
The sand fraction for both sediments was observed to increase after drying and combustion.
This seems reasonable in light of the decrease in organic fines and possible aggregate formation. It is likely that the sand fraction is overestimated, particularly for the Charters Creek sediment. The estimate of 31.7% seems high in comparison to images of the sediment taken during flocculation experiments, where few sand-sized particles are visible. The median sizes of Mfolozi and Charters Creek sediments were 31.17μm and 42.453μm respectively after combustion. These estimates were also higher than expected. This is perhaps an indication that aggregates that formed when the sediment was dried were still present during the test. The
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particle size distributions of the virgin and combusted samples presented in figure 4-1 (A-D) show a disproportionately high increase in the sand fraction after combustion. Images of unflocculated suspended sediment show very few sand-sized particles but rather an abundance of undetectable fine particles. This supports the explanation of aggregate formation during drying.
Plate 4-1: Microscope image of fine unflocculated sediment. The solid bar represents500μm.
This result was not unexpected. In a previous study similar behaviour was observed (Maine, 2010). This behaviour of dried cohesive sediment is accepted and general conclusions may still be drawn from the particle size analysis results.
General conclusions:
Mfolozi and Charters Creek sediments may be classified as silt-dominant. The virgin samples are dominated by silt-sized particles. Combusted sediment samples were silt-dominant. The clay fractions of both sediments were less than 5% after combustion. The clay fractions of the two different sediments were similar. The low clay fractions will likely limit the flocculation potential of the two sediments. The particle size analysis results also indicate that the organic matter content of Mfolozi sediments is high. This may influence its flocculation potential in comparison to Charters Creek sediments.
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Figure 4-1: Particle size distributions for: A: virgin Mfolozi water sample. B: combusted Mfolozi sediment. C: virgin Charters Creek water sample. D:
combusted Charters Creek sediment
77 4.1.2. Organic Matter Content
The organic matter content of the bulk water samples was measured over the period of the flocculation experiments. The OMC was also measured for the dry sediment extracted from the bulk samples. The results in table 4-3 show the average OMC for both sediments to range between 12.8-18.6%. The organic matter content was higher than expected. This may influence the flocculation potential of the sediments. The average OMC varied for the Mfolozi bulk water samples. The high OMC of the Mfolozi water and dried sediment corresponds with observations in the particle size analysis above. It is evident that the OMC of Mfolozi sediment was higher than that of Charters Creek sediment at the time of the particle size analysis tests.
Table 4-3: Organic Matter Content of sediments
Sample Date SSC
(mg/L) OMC
Mfolozi bulk water sample
(%)(mean) 06-Oct 211 13.3
Mfolozi bulk water sample
(mean) 07-Nov 174 16.1
Charters bulk water
sample (mean) 31-Oct 615 14.4
Charters bulk water
sample (mean) 07-Nov 638 13.3
Mfolozi dried sediment
(mean) 08-Nov 18.6
Charters dried sediment 08-Nov 12.8
Reasonable steps were taken to avoid bacterial growth within the water samples. The water samples were stored in a dark laboratory and not exposed to significant temperature changes.
Most organisms such as phytoplankton and zooplankton present during sampling were expected to die. It is reasonable to expect small variations of organic matter content between filtration tests considering the small sample sizes. The filtration tests were performed on 100ml samples.
For the purpose of the study it may be concluded that the organic matter content both sediment types was high, varying between 12 and 19% during the time of the flocculation tests.
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