Water treatment residue (WTR), a by-product of the water treatment process,
consists primarily of precipitated hydroxides of the coagulants used in the water
treatment process, along with sand, silt, clay, humic compounds, and dissolved
organic matter. It is usually disposed of by landfill, a technology with numerous
problems that include dwindling landfill capacity, extensive dewatering
requirements for the WTRs, high costs of transportation, and potential liability for
landfill clean-up. Therefore, land disposal (or land treatment) presents a popular
alternative disposal method based on the principle that the physical, chemical,
and microbial properties of the soil can be used to assimilate applied waste
without inducing any negative effects on soil quality.
The objective of this study was to investigate the effects of land disposal of the
WTR generated by Umgeni Water, a local water treatment authority, on soil
quality. These effects were investigated using depth samples from soil profiles of
Westleigh and Hutton soil forms at field trials located at Ukulinga Research Farm,
near Pietermartizburg and Brookdale Farm, Howick, KwaZulu-Natal, South
Africa, respectively. Four rates of WTR (0, 80, 320, and 1280Mg ha-1
incorporated into the soil) were investigated at both trials, in addition to mulched
treatments at rates of 320 and 1280Mg ha-1 at Brookdale only. Sampling of plots
was carried out in September 2001 and May 2002, and all treatments were
investigated under fallow and grassed cultivation. Laboratory measurements
used to assess soil quality included pH, electrical conductivity (EC), organic
carbon (QC), and microbial activity using f1uorescein diacetate (FDA) hydrolysis.
At both trials in September 2001 WTR-amended plots displayed higher pH in the
0-200mm soil in comparison to the controls, whereas by May 2002 pH had
returned to the condition of the controls. Addition of WTR at Ukulinga resulted in
higher QC in September 2001, but in May 2002 this was similar to the controls.
However, at Brookdale QC was unaffected by WTR. At Ukulinga and Brookdale the effect of WTR on EC was variable, and microbial activity in the soil profile
was unaffected by WTR addition.
Observations at Ukulinga and Brookdale reflected long term changes (3 and 5
years, respectively) to soil quality following WTR addition. To examine the initial
changes in soil quality a laboratory experiment was set up using the field trial
soils. Research objectives were also extended to include WTRs from Rand
Water (Johannesburg), Midvaal Water Company (Stilfontein), Amatola Water
(East London), and two samples from the Faure Water Treatment Plant (near
Cape Town). The second Faure sample (Faure2
) was collected when blue green
algal problems were experienced at the plant. The measurements used to
investigate these short term effects on soil quality were soil pH, EC, and
microbial activity as indicated by respiration rate.
Each of the WTRs added to the Hutton and Westleigh soils increased soil pH by
varying increments, and the higher the WTR application rate, the higher was the
pH recorded. With the exception of the Rand and Umgeni WTRs that clearly
increased soil EC, the effect of the otherWTRs on EC was variable. The Faure1
and Amatola WTRs appeared to have no effect on microbial activity, whereas the
Umgeni, Rand, Midvaal, and Faure2 WTRs stimulated microbial activity by Day 2
following the addition of WTR, but this had declined by Day 14. As for pH, higher
microbial activity was recorded at higher WTR application rates.
Changes in microbial community structure of the Hutton soil only, following the
addition of WTR were examined using denaturing gradient gel electrophoresis
(DGGE) analysis. Community profiles of the different WTRs proved to be
markedly different. However, WTR-amended soil retained banding patterns
consistent with the control soil indicating that dominant populations in the Hutton
soil had been retained. The field trials indicated that long term effects of land disposal of WTR were not
detrimental to the measured indicators of soil quality namely, pH, EC, QC, and
microbial activity. The laboratory assessments of the short term response of the
Hutton and Westleigh soil forms to WTR addition suggested that the tested
variables were altered by WTR, but not significantly changed to the detriment of
soil quality. Microbial community analysis indicated that the community structure
of the Hutton soil was not significantly altered by WTR amendments. Present
findings provide no evidence to suggest that land disposal of WTR is detrimental
to soil quality. It is therefore regarded as a feasible disposal option although
there are some aspects that should be investigated further. These include
investigations into rhizosphere/microbial interactions and the feasibility of
growing cash crops. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2003.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/3596 |
| Date | January 2003 |
| Creators | Pecku, Shantel. |
| Contributors | Hunter, Charles H., Hughes, Jeffrey C. |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
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