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Systematic comparison of the effectiveness of water treatment processes

This project is based on the hypothesis that existing processes for treatment of wastewater streams are not necessarily the most effective, partly because both the range of contaminants and treatment methods is very broad. This project is focussed on testing this hypothesis by modelling (on a single simulation platform) a wide range of proven water treatment technologies, combining these into both existing and new processes, and performing multi-criteria evaluations and comparisons of these processes (also using the available tools on the simulation platform). From the findings of this analysis, it is expected that new heuristic principles for the design of water treatment processes will be derived. In addition, a framework will have been developed in which models of emerging technologies can be incorporated and evaluated.

An Investigation into the Kinetics and Mechanisms of Fe (III) Oxyhydroxides Precipitation in Lime Neutralisation Processes

Acid Mine Drainage (AMD) and hydrometallurgical wastewater are products of metallurgical activities that pose environmental risks as they contain heavy metals. The abundance of Fe in these waste streams and its notorious tendency to precipitate as a sludge has drawn attention to the mechanisms and kinetics of Fe (III) oxyhydroxide precipitation. The lack of a process which meets the effluent discharge limits whilst also producing a sludge-free precipitate that is amenable to metal value recovery has resulted in downstream challenges in terms of sludge disposal in landfills and the significant loss of water associated with this practice. In this study, the mechanisms and kinetics of Fe (III) oxyhydroxide precipitation will be investigated using lime.

The kinetics of lime dissolution in acid mine drainage neutralization

Acid mine drainage (AMD) contains high concentrations of dissolved heavy metals and sulphates, with pH values as low as 2.5. Such acidic wastewater streams pose a threat to plant and aquatic life. It is therefore important to treat acidic effluents before their disposal to the environment. The treatment of AMD involves neutralization using a caustic material or basic solution to adjust pH to acceptable environmental standards. Some of the commonly used bases or alkaline solutions for neutralization are sodium hydroxide (NaOH), calcium oxide (lime, quicklime or CaO), slaked lime (Ca(OH)2), limestone (CaCO3), and soda ash (Na2CO3), with lime being the most commonly used. Lime has been adopted because it is cheap, safe to handle and reacts readily with all types of acids from the strongest to weakest, whether organic or inorganic.

The lime neutralisation process involves two steps: slaking or reaction of quicklime (calcium oxide) with water to form hydrated lime, which is a solid-liquid reaction, and reaction of hydrated lime with an acidic solution to be treated. It has been established from literature that the reaction of quicklime with water is an exothermic reaction; therefore, there are many factors that are anticipated to affect this process. This includes thermodynamic factors, mass transport and phase transition. It is therefore important to establish a strong base of understanding for lime dissolution kinetics.