Vast quantities of coal combustion residues (ash) and effluents are produced
simultaneously in the coal processing facilities of South African parastatals, Eskom
and Sasol. The handling and disposal of saline effluents is a difficult and complex
problem. The current practice used by these parastatals is to co-dispose the
effluents with the ash in landfills; so-called ash dams. Although this practice provides
a potentially elegant approach, at least from the viewpoint of the generator of both
the ash and effluent, the co-disposal of ash and brine in a landfill could have dire
consequences on the environment of the landfill site. This applies in particular to the
release of environmentally deleterious and toxic constituents of the ash into the air,
soil, surface and groundwater which can lead not only to environmental and land-use
problems, but also jeopardize the health of organisms living in the surrounding
ecosystem. The question therefore arises as to how Eskom and Sasol better
manage their ash dams, to not only satisfy all legal requirements and possible
pressure from social awareness groups, but also more importantly, prevent, or at
least limit, pollution of the natural environment.
The present investigation arose from a request by Eskom and Sasol to provide them
with a detailed proposal for a framework with a view to increasing the competency of
both organizations in the management of the co-disposal of ash and brine and the
dissemination of knowledge with regard to the impacts of the sites. Two sites were
selected by the organizations for this purpose: the Tutuka Power Station and the
Secunda Synthetic Fuel Plant.
The application of geohydrological models to assess the behaviour of a waste
disposal site has historically often been viewed as an attempt to predict the future
behaviour of the site. However, this would require information on relational
parameters and known interactions whose behaviour far into the future cannot be
determined with certainty. A geohydrological model should therefore never be
viewed as an attempt to predict the future of a given waste site, but rather as an aid
to assess how effectively the site is managed and controlled. The best way to
achieve this is to investigate the waste site systematically, preferably utilizing wellestablished
and accepted international methodology. Unfortunately, at this time, no documents exist that describe such a methodology, its implications and the steps
necessary to implement it in practice in a way that can also be understood by
interested members of the public.
A two-dimensional transient model for flow through saturated/unsaturated porous
ash dump media has been developed. This model numerically solves the governing
partial differential equations, which are highly non-linear. The model code uses
quadrilateral finite elements for the geometrical assembly: the bilinear Galerkin
interpolation for the spatial integration, and the Gaussian elimination for the solution
of the resulting matrix equations. In addition to the usual constant-flux and constanthead
boundary conditions, the code is capable of applying pressure-dependent
boundary conditions at the ground surface. Thus, infiltration into, or seepage from,
this surface may be simulated. Each element may be assigned different material
properties that allow the investigation of layered geologic formations.
The results discussed in this work are all based on conclusions derived from a
generic model for the Tutuka ash dumpsites. While such a model can provide
valuable insight into the physical behaviour of such a study area, it can never replace
field observations. Field observations and models must complement each other.
An attempt was also made to provide guidelines for the development of site-specific
data investigation methodology, based on the ISAM Safety Assessment
Methodologies for Near Surface Disposal Facilities.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-12152010-121738 |
| Date | 15 December 2010 |
| Creators | Menghistu, Mehari Tewolde |
| Contributors | Prof JF Botha |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-12152010-121738/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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