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    Water quality indices for water distribution network optimisation
    (University of the Witwatersrand, Johannesburg, 2025-07) Nyirenda, Mercy Sibongile; Ndiritu, John
    Safe drinking water is the basis for human health, survival, growth and development. The provision of safe drinking water is additionally recognised as a basic human right and even though numerous global initiatives exist to this end; it is reported that at least 1.9 billion people rely on an unimproved source or an improved source that is faecal contaminated for drinking water. The provision of safe drinking water is protected by standards for drinking water quality where water supply agencies must ensure when planning, designing and running water treatment and supply infrastructure. These standards are not always upheld as reports of higher levels of salinity, microbial contamination, eutrophication and endocrine disrupting compounds are being reported at treatment plants in South Africa and worldwide. The design and optimisation of water distribution/reticulation networks using computational methods is a widely researched area in water design and management and is gaining more popularity as technological advances increase. Water distribution network design and optimisation typically includes selecting pipe elements, determining nodes, selecting tank locations and sizes, selecting pumps and pumping schedules while considering cost efficiency; however, inclusion of water quality has generally been excluded due to the increase in complexity resulting in increased computational time and space. However, to ensure efficient and safe provision of clean water; water quality has to be included in the design of water distribution networks to minimise potentially harmful effects. This research dissertation developed and tested the use of water quality indices that were historically used for studies on surface water bodies. The water quality indices are capable of transforming and combining multiple water quality constituents into a single aggregated value that can be incorporated into optimisation algorithms. Five water quality indices were developed and used here to assess the incorporation of water quality into the optimization of service reservoir designs. A set of sub-indices were developed for the four chosen water quality constituents: water age, chlorine, tri-halomethanes and halo-acetic acids and were then aggregated into indices that quantified the water quality for each reservoir. This formulation provided an estimate of the overall water quality in a reservoir where multiple water quality constituents with various units of measurement are involved. The formulation was intended to reduce computational effort for the optimisation of water distribution systems which include water quality as an objective.
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    Dissolution Of Copper from Waste Printed Circuit Boards (PCBs) in Acidic Cupric Chloride Media (H2SO4-CuSO4-NaCl)
    (University of the Witwatersrand, Johannesburg, 2025) Nxumalo, Duduzile Nontobeko; Sibanda, Vusumuzi; Basson, Petrus
    This study investigated the dissolution of copper from a high-grade waste printed circuit board (PCB) sample in acidic, cupric chloride media (H2SO4-CuSO4-NaCl + Air). The effects of particle size, solids concentration, acidity, temperature, concentrations of cupric ion, chloride and ferrous ion as well as pre-leach calcination were evaluated in agitated reactors. Copper dissolutions of up to 98% were obtained under optimised conditions. The findings revealed that higher solution potential and dissolved oxygen concentrations significantly enhance the dissolution rate, while reducing particle size and solids concentration improved mass transfer efficiency. Calcination as a pretreatment significantly improved the leaching kinetics, with 93% copper dissolutions achieved after 2 hours. Under the best conditions, comparable results were achieved at both bench and pilot scale, demonstrating potential for large-scale application. Overall, copper dissolution is therefore primarily governed by solution potential, dissolved oxygen concentration, particle size, solids concentration, and acidity. PCB heterogeneity affects mass balance accuracy, highlighting the need for homogenisation procedures to ensure consistent and representative results. A key recommendation is the optimisation of solution potential through enhanced oxygen availability, and higher initial cupric ion concentrations which can substantially improve the rate of copper dissolution.
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    Sizing Leachate Dams and Treatment Infrastructure in South African Landfills
    (University of the Witwatersrand, Johannesburg, 2025-08) Nkwane, Khutso; Biyela, Precious
    Adequate leachate management through leachate collection, storage, and treatment is vital for preventing groundwater and surface water pollution during landfilling. The storage of leachate using lined pollution control dams, referred to as leachate dams, before treatment is common in landfills. However, leachate dams are at a high risk of leakage and spillage if they are not designed and sized appropriately. Furthermore, the size of a leachate dam directly informs the sizing of the leachate treatment processes that follow it by establishing a required average treatment rate. The purpose of this research is to provide a framework for the adequate design and sizing of leachate dams and leachate treatment infrastructure in South Africa, by investigating the regulations for the design of leachate dams and leachate treatment; investigating applicable processes for treating leachate; and analysing the suitability of hydrological models and methods to estimate leachate discharge and leachate evaporation for leachate dam size water balance modelling. A thorough literature review was conducted to study: (1) the regulatory requirements for leachate dam design and leachate treatment; and (2) the different methods and processes used for leachate treatment. Analyses were conducted to verify whether the Hydrologic Evaluation of Landfill Performance model (HELP model) is suitable for estimating leachate generation and discharge; predicted leachate discharge values computed by the HELP model using climatic records of an existing landfill were compared to leachate discharge values observed and recorded at the landfill. Furthermore, a method for estimating leachate evaporation was derived in this research; the method’s suitability was analysed by comparing its predicted leachate evaporation values, computed using climatic and leachate composition records at a landfill, to the actual leachate evaporation values observed and recorded at the landfill. To prevent excessive leachate seepage into groundwater, South African regulations mandate that leachate dams be designed with a liner system that is adequate for the waste disposed in the landfill. Moreover, the regulations mandate that leachate dams must be designed with a storage capacity large enough that they are not likely to spill more than once every 50 years. Finally, there is a requirement for leachate to be adequately treated for permanent containment or discharge into the environment; the regulations provide minimum requirements for treated effluent’s quality, as well as its allowable temperature and discharge rate if it is to be discharged into the environment. Studies on leachate treatment systems discuss the use of physical and chemical processes, such as flotation and precipitation; biological processes, such as activated sludge processes; and advanced treatment processes, such as reverse osmosis. The studies recommend the use of reverse osmosis as a final leachate treatment step, with a biological treatment and/or chemical treatment step preceding it. The percent error between the observed average daily leachate discharge rate and the average daily leachate discharge rate predicted by the HELP model was found to be 9.780 %. This proves this research’s hypothesis of 10 % was correct. However, this was achieved through the use of an apparent saturated hydraulic conductivity that was significantly higher than what is reported in existing literature for landfilled waste. This variance is considered to be attributable to the methods used in the existing literature for determining saturated hydraulic conductivity; the methods do not take into account large impenetrable pieces of waste, such as drums and containers, creating large flow paths. The percent error between the observed monthly leachate evaporation and the monthly evaporation predicted by the derived leachate evaporation method was found to be 7.91 %. This slightly surpassed the research’s hypothesis of 10 %. The derived method also predicted monthly leachate evaporation more accurately in the wet season than in the dry season. Both the HELP model and derived leachate evaporation method were found to be suitable for use in leachate dam sizing to ensure that leachate dams fulfil South Africa’s regulatory requirements on leakage and spillage. Further research is required using data recorded over longer periods in order to further verify the HELP model’s suitability. Given the significant variability in the chemical and biological composition of leachate, further research is also required using leachates with different compositions in order to further verify the suitability of the derived leachate evaporation method.
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    The Occurrence, Distribution, and Concentration levels of Critical Raw Materials from the Coal Discard of the Witbank Coalfield
    (University of the Witwatersrand, Johannesburg, 2025-06) Nkwamza, Xolisa; Malumbazo, Nandi
    This study investigates the modes of occurrence, distribution, and concentration levels of critical raw materials (CRMs) within coal discard dumps from the Witbank Coalfield, South Africa. Traditionally regarded as waste, these coal discards have been found to contain economically significant concentrations of CRMs, including Ba, Ga, Sr, Zr, and other valuable elements. Two contrasting coal discard dumps an older, decommissioned dump (CD1) and a younger, active dump (CD2) were selected for sampling and analysed using a suite of analytical techniques such as thermogravimetric analysis (TGA), bomb calorimetry, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), automated mineralogy (TIMA), X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP-MS). Sampling at CD1 was conducted systematically by depth profiling, while CD2 samples were collected unsystematically at the surface. Samples were categorized into four lithological groups: bright coal discard, moist coal discard, clay-rich coal discard, and reddish oxidized coal discard. Results indicate that both CD1 and CD2 contain high-volatile bituminous coal discards, with CD1 exhibiting average values of 4.49 wt% moisture, 21.91 wt% volatile matter, 38.53 wt% fixed carbon, 34.78 wt% ash, and a calorific value of 18.11 MJ/kg. CD2 showed slightly different values: 6.46 wt% moisture, 20.29 wt% volatile matter, 38.14 wt% fixed carbon, 35.11 wt% ash, and 17.55 MJ/kg calorific value. FTIR analysis revealed characteristic absorption bands including O-H, C H, C=C, C-O, C-C, M-O, Si-O, and Si-O-Si in samples from both dumps. Mineralogically, quartz and kaolinite dominate, accompanied by minor amounts of barite, zircon, apatite, illite, muscovite, and ilmenite. Major oxides in both dumps are dominated by SiO2 and Al2O3, however, Al2O3 content remains below 40%, classifying these as low-alumina coal discards. CD1 is comparatively enriched in Fe2O3, CaO, MnO, MgO, Ba, Ga, Sr, Cu, Ni, and rare earth elements plus yttrium (REY), whereas CD2 displays higher concentrations of SiO2, Fe2O3, P2O5, SiO2, Na2O, Be, Co, Li, Nb, Mo, Pb, Rb, Zr, V, and Zn. On average, CRM concentrations are higher in CD1 compared to CD2. Vertical variability of CRMs in CD1 was clearly evident, with depth-dependent trends noted for Ba and Sr, especially within 0–1 m and 3–4 m depth intervals. Sr is also widely distributed throughout CD2. Ba predominantly occurs in barite, while Sr exists mostly in barite in CD1 and is primarily associated with phosphate and carbonate minerals in CD2. Zr associates strongly with zircon, Ga occurs within kaolinite and organic matter, and Li shows intimate organic associations. REYs are hosted by Fe-bearing and phosphate minerals in CD1 but by Ca-rich aluminosilicates in CD2. Nb associates with both aluminosilicate and Ti-bearing minerals in CD1, but shows organic associations in CD2. Mo is mainly found in Ti-bearing minerals in CD1 and organic matter in CD2. Co occurs in Mn-, Ca-, Na- and Ti-bearing minerals in CD1 but is limited to Ca, Fe, Mn carbonate in CD2. Cu exhibits strong organic affinity in CD1 and occurs in phosphate and chromite minerals in CD2. Pb associates with organic matter in CD1 and is found in apatite, chromite, Ti-, and Mg-bearing minerals in CD2. Ni, Be, and Zn show organic affinity in both dumps, while Rb is mainly hosted by illite and muscovite. The study highlights the influence of hydrogeochemical processes and igneous intrusions on CRM enrichment and mineral associations within coal discards. The findings underscore the potential for coal discard dumps to serve as secondary sources of CRMs, offering promising opportunities for sustainable resource recovery and economic value addition. Furthermore, CRM extraction from coal discards could help mitigate the environmental risks posed by stockpiled waste materials. Based on results, it is recommended that further research focus on optimizing flotation and selective leaching techniques to enhance CRM recovery potentials.
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    The Influence of Footwall Rock Materials on the Rock Mass Response to Seismicity at Mponeng Gold Mine
    (University of the Witwatersrand, Johannesburg, 2025) Nkomo, Darlington Brilliant; Watson, Bryan P.
    Mponeng Mine is currently the deepest mine in the world, mining at a depth of 3500 m below the surface. The active mining stopes are in a very high-stress environment, and the mine is highly seismically active. Seismicity has been the main driver of the fall of ground incidents recorded at the Mponeng Mine over the years. The seismically induced falls of ground have contributed to multiple fatal accidents and loss of production. Between 2013 and 2023, there were 315 seismic related accidents, of which 13 resulted in fatalities. Of the 13 fatalities, 12 were in the stopes and one was in an off-reef development excavation. There are two footwall zones at the mine: quartzite on the west and shale to the east. Of the 12 fatalities in the stopes, 8 were in the quartzite footwall zone and 4 in the shale footwall zone. Some conclusions were drawn following laboratory strength analyses of the two different footwall rock types at Mponeng Mine. The mechanical properties of the lava hanging wall are almost constant throughout the mine. However, the footwall rock types are different. The mechanical properties vary significantly between the quartzite and shale footwalls. In particular, the quartzite footwall is significantly more brittle than the shale. The different footwall properties play a role in the stress distribution around the stopes, which influences the ground conditions of the hangingwall and footwall. The hangingwall above in the quartzite footwall was significantly more fractured than in the areas with a shale footwall. The quartzite footwall was also highly fractured and undulating. At the current depth of mining, the hangingwall was expected to be highly fractured, except where the face was de-stressed by pre-conditioning or over-stoping. Preconditioning in both footwall zones was done properly, but the deeper fracture distance in the shale footwall contributed to a better ground condition. The quartzite footwall zone generated a high number of small magnitude events ahead of the face with higher damaged volume, while the shale footwall zone generated a high number of larger magnitude events with far less damage. However, Map3D elastic numerical modelling software showed a higher energy release rate in the shale area. The softer Booysen’s shale appears to absorb more strain energy during static and dynamic loading than the quartzite zones. The shale is also more prone to creep, allowing closure and reducing fracturing in the hangingwall. The quartzite material fractures more easily, generating smaller magnitude events. The highly fractured rock in the quartzite zone suffers greater shakedown damage during the vibrations caused by a seismic event. Based on the findings of this research, the support resistance in Booysen’s shale footwall stopes may be lower and the extraction ratio higher than in areas with a quartzite footwall.