published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Desalination of seawater is a widely used technology throughout the world, but is not commonly used in South Africa for domestic water supply. The reasons for this are varied, but very often are based on the assumption that desalination is extremely costly in relation to more traditional water supplies. An economic analysis is undertaken comparing the cost of supplying water to the coastal town of Zinkwazi from various sources including desalination using reverse osmosis. Zinkwazi has an existing borehole water supply that is insufficient to meet current and future demands. The town is also remote from regional bulk surface water infrastructure, which makes it suitable for the investigation of an alternative stand-alone water supply such as desalination. Solving the water supply problems at Zinkwazi is important to Umgeni Water and would support two broad strategic goals of the organisation. Zinkwazi falls within the Ilembe District Municipality, which is an important stakeholder within Umgeni Water's area of jurisdiction. Improving the water supply situation at Zinkwazi is in line with Umgeni Water's goal of assisting Municipalities to meet their developmental objectives. Using desalination to meet this objective is in line with Umgeni Water's goal of using innovative products to alleviate problems of existing customers. Desalination is a multi-billion dollar industry that is growing as traditional surface and goundwater resources become fully utilized and more polluted. Desalination potentially represents a growth opportunity that Umgeni Water, with its expertise in water treatment and supply, could pursue in Africa and Southern Africa in particular. The investigation found that desalination is the most affordable method of supplying water to the town of Zinkwazi and the construction of a desalination pilot plant is recommended for further investigation. / Thesis (MBA)-University of KwaZulu-Natal, Pietermaritzburg, 2005.

Renewable power generation from the controlled mixing of sea and fresh water is relatively unexplored when compared to the development for solar, wind, and other sustainable power alternatives. When global river discharge was taken into account, an estimated 2.6 TW of obtainable energy exists in untapped salinity gradients. Reverse electrodialysis is one proposed power-generating mechanism for harnessing energy from brackish environments and relies on the transport of aqueous salt ions through an apparatus of ion-exchange membranes. In this thesis, operational parameters, including flow direction, salinity composition, and membrane selectivity, are investigated. For optimal performance, I have employed counter-current flow mode with monovalent ion selective membranes and pure 0.5 M NaCl saline solution. The results show that a maximum open circuit voltage (OCV) level of 2.01 V is obtained with an active membrane area of 0.0756 m². The presence of multivalent ions in the feed solutions hinders OCV levels, but the effects are reduced with monovalent-selective membranes. Preliminary results are insightful; in order to increase the commercially viability of this technology, future work is needed to enhance the performance properties of the ion exchange membranes.

Abstract available in PDF copy. / Thesis (M.Sc.-Chemical Engineering)-University of Natal, 1988.

Most common inorganic foulants in RO processes operating on brackish water are calcium carbonate, calcium sulphate and silica. However, silica fouling is the recovery limiting factor in RO system. Silica chemistry is complex and its degree of fouling strongly depends on the silica solubility and its polymerization under different operating conditions of RO process. In several studies carried out in batch and dynamic tests, the presence of polyvalent cations and supersaturation of silica in solutions were found to be the important factors (apart from pH and temperature) that affected the rate of silica polymerization and its induction period. Agitation did increased silica solubility; however, its effect was negligible in presence of polyvalent cations. Alkalization of water solution by coagulants particularly sodium hydroxide was found suitable for silica removal during pretreatment. The presence of magnesium in solution played a key role in silica removal mostly by the mechanism of adsorption to the metal hydroxide. The options of inline mixing (high agitation) for 5 to 10 minutes and microfiltration before RO were found suitable for silica pretreatment. During dynamic tests, the most dominant mechanism for salt deposition (mostly CaSO4) was particulate type in high concentration water solution; while crystallization fouling was the prevailing mechanism of deposition (mostly CaCO3 and silica) in low concentration solution. Silica showed significant effect on size and shape of inorganic salt crystals during coprecipitation. Moreover, the presence of common antiscalants promoted silica fouling. This important finding recommends an extra caution while using antiscalants in case feed water contains silica to a level that can attain saturation near membrane during RO process. A model was developed to predict the silica fouling index (SFI) based on the experimental data for induction period of silica polymerization. The model takes into account the effect of polyvalent cations and concentration polarization near membrane during RO process. It provides a conservative basis for predicting the maximum silica deposition in RO process at the normal operating conditions. A generalised correlation, which was developed for determination of the mass transfer coefficient in RO process, incorporated the effect of temperature change that is usually not considered in previous correlations. A correlation for reduction of silica content in feed water, down to a safe limit of 15 ppm for RO process, was also formulated and validated by the experimental results.

Thesis (MTech (Chemical Engineering))--Cape Peninsula University of Technology, 2017. / Water scarcity in South Africa (SA) and more specifically Cape Town, Western Cape, has escalated to disaster levels in 2018. Agriculture and irrigation account for 62% of SA’s accessible potable water (Thopil & Pouris, 2016), and although the agriculture sector plays a pivotal role in SA’s socio-economic development, the future of the sector is dependent on critical issues such as climate variability and population growth (Besada & Werner, 2015). Wine production in SA is an important agricultural activity, contributing great economic value to the agri-food sector. However, despite this, the wine industry is responsible for vast water consumption and the unsafe disposal of winery wastewater, which are critical issues from an environmental and economic standpoint. The ever-imminent crisis pertaining to the limited supply of fresh water from conventional water resources has necessitated the need to develop alternative water resources to supplement an increased water supply, which include the reuse of wastewater, ground water, brackish water (BW) and seawater (SW) desalination. When fresh water supplies are limited, agricultural irrigation is penalised. The reuse of agricultural wastewater as a substitution for potable water irrigation may prove beneficial in areas where water shortages are severe. Forward osmosis (FO) is a developing desalination technology that has received increased attention as a promising lower-energy desalination technology. FO technology relies on the natural osmotic process, driven by a concentration gradient as opposed to significant hydraulic pressures like reverse osmosis (RO). Water is extracted from a lower concentrated feed solution (FS) to a highly concentrated draw solution (DS). The term “lower energy” is only applicable for applications where the recovery of the DS is not required. FO technology offers several advantages. However, the lack of suitable membrane modules and DSs hinder its practical application. FO offers novelty applications in which specialised DSs are selected to serve as the final product water, most notably concentrated fertilisers for direct fertigation. The aim of this study was to evaluate the performance and compatibility of commercially available cellulose triacetate (CTA) and aquaporin biomimetic FO membranes with commonly used fertilisers for direct fertigation within the SA wine industry, using a fertiliser drawn forward osmosis (FDFO) system.

Thesis (MTech (Chemical Engineering))--Cape Peninsula University of Technology, 2017. / Similar to the energy crisis, the critical state of the water supply in South Africa (SA) is a combination of (i) resource exhaustion and pollution; (ii) increasing demand; and (iii) poor infrastructure. Despite its importance, water is the most poorly managed resource in the world. The disposal of industrial effluents contributes greatly to the poor quality of water. The textile industry consumes great quantities of water and produces enormous volumes of wastewater which requires appropriate treatment before being released into the environment. In an attempt to address the water issues, research globally has focused on advanced technologies such as desalination to increase limited pure water resources. The need for alternative desalination methods for the production of clean water from alternative water resources, such as seawater and brackish water, has gained worldwide attention. Reverse osmosis (RO) and Nanofiltration (NF) have been used as unswerving approaches to yield freshwater. Forward osmosis (FO) is a developing membrane technology that has increased substantial attention as a possible lower-energy desalination technology. However, challenges such as suitable FO membranes, membrane fouling, concentration polarisation, and the availability of effective draw solutions (DS), limit FO technology. FO is seeking more importance in novel areas where separation and recovery of the DS is not required. The aims of this study was to: i) identify alternative water resources and evaluate their potential as suitable feed solution (FS); ii) Identify dyes and evaluate their potential as suitable draw solutions (DS) at different concentrations; iii) assess the use of aquaporin biomimetic membrane and iv) assess a FO system for the production of dye solutions. Osmotic pressure (OP) is the pressure exerted by the flow of water through semi-permeable membrane, separating two solutions with different concentrations of solute. The DS should always have OP higher than the FS in order to achieve high water flux. Three basic dyes (i.e. Maxilon Turquoise, Red and Blue) and three reactive dyes (i.e. Carmine, Olive Green and Black) were selected, based on their common use in the SA textile industry. The respective dye samples were prepared at different concentrations and dye-to-salt mass ratios ranging from 1:10 to 1:60 and assessed for OP using a freezing point osmometer. A lab-scale FO unit was used for all the studies. Feed and draw channels were circulated in a counter-current flow at a volumetric flow rate of 600 mL/min. Feed solutions(FS) included deionised water (DI) as a control, brackish water (BW), synthetic seawater (SSW) and textile wastewater (TWW) collected from two textile factories. OP of the FS (DI, BW5, SSW and SW, Factory 1 and Factory 2) was 0, 414, 2761, 2579, 1505 and 3308 kPa, respectively. Basic Blue and Reactive Black generated a higher OP compared to other selected dyes in the study and were therefore selected to be used as DS at a 1:10 dye-to-salt ratio and 0.02 M concentration. An aquaporin biomimetic FO membrane (Aquaporin, Denmark) was used for all the experiments conducted in the FO mode.

Water scarcity for agriculture is one of the most important challenges to improve food security worldwide. In this thesis we study the potential to develop a low-cost controller for a small scale brackish desalination plant that consists of proven water treatment technologies, reverse osmosis, cation exchange, and nanofiltration to treat groundwater into two final products: drinking water and irrigation water. The plant is powered by a combination of wind and solar power systems. The low-cost controller uses Arduino Mega, and Arduino DUE, which consist of ATmega2560 and Atmel SAM3X8E ARM Cortex-M3 CPU microcontrollers. These are widely used systems characterized for good performance and low cost. However, Arduino also requires drivers and interfaces to allow the control and monitoring of sensors and actuators. The thesis explains the process, as well as the hardware and software implemented.

Providing sustainable access to water and energy is among the grand engineering challenges in the 21st century. Notably, many pressing issues at the water-energy nexus can be addressed by effective ion separations, such as desalination, nutrient recovery from wastewater, and extraction of energy-related elements from unconventional sources. One tool for such separations is ion-exchange membranes (IEMs), which are charged polymeric thin films. However, conventional IEMs face several performance constraints and fail to achieve ion-specific selectivity. This thesis aims to advance the potential of IEMs for separations at the water-energy nexus. Present-day IEM processes, e.g., electrodialysis (ED) desalination and reverse electrodialysis (RED) power generation, commonly employ the membranes as charge-permselective barriers, transporting oppositely-charged counterions while retaining like-charged co-ions. However, the increase in charge permselectivity is accompanied by a decrease in ionic conductivity, which forms a conductivity-permselectivity tradeoff and crucially limits separation efficiency. This work models IEM conductivity and permselectivity as functions of intrinsic membrane chemical and structural properties, simulating the performance of IEMs in a range of ED and RED operations (Chapter 2). Bulk solution concentration is identified as an external cause for the tradeoff, which confines current IEM applications to sub-seawater salinities. The structure-property-performance analyses reveal membrane water sorption as an intrinsic determinant of the tradeoff, while indicating that increasing ion-exchange capacity and reducing thickness can yield highly selective and conductive IEMs. To depart from this tradeoff, nanocomposite cation-exchange membranes (CEMs) with percolating 1-D sulfonated carbon nanotube (sCNT) network are fabricated (Chapter 3). Membrane conductivity is raised with greater sCNT blending in the polymer matrix (increasing by ≈30% with 20 w/w% incorporation of sCNT), while permselectivity is effectively unchanged (within 2% variation). Further characterization displays sCNT percolation beyond 10 w/w% blending, attributing the conductivity improvement to the interconnected sCNT network. The results imply the potential to advance the conductivity-permselectivity tradeoff with rationally designed nanostructures. Next, this thesis moves from the conventional charge-discriminating selectivity to ion-specific selectivity (Chapter 4), which is urgently needed but underdeveloped due to insufficient understanding of the fundamental transport phenomena. Here, a transport framework is presented to describe counterion migration mobility using an analytical expression based on first principles. The two governing mechanisms are: spatial effect of available fractional volume for ion transport and electrostatic interaction between mobile ions and fixed charges. Mobilities of counterions with different valencies are experimentally characterized, showing high regression R²s with the mobility model. The influence of membrane swelling caused by different counterions is further accounted for, while the frictional effect of electrostatic interaction is quantitatively linked to fixed charged density and dielectric constant of membranes. Additionally, the anion-exchange membrane (AEM) exhibits a weaker electrostatic effect compared to CEMs, which is attributed to the steric hindrance of the quaternary ammonium functional groups. Last, the membrane-level knowledge is extended to two process-level applications, coupling desalination with sustainable energy and desalinating hypersaline brines. This thesis presents a novel low-grade-heat-driven desalination process (Chapter 5), using CEM and AEM Donnan dialysis (DD) stepwise to remove salt ions, e.g. Na+ and Cl-, with a receiver solution of thermally-recoverable solute NH₄HCO₃. NH₄HCO₃ in the streams is later recycled by low-grade heat. The concept is experimentally validated by desalinating brackish water (100mM NaCl) to freshwater salinity (< 17mM). DD desalination of larger ranges of feed and receiver concentrations was then demonstrated, and module-scale analysis quantified the improvements of countercurrent operation to desalination efficiency. Another challenge in water management is the desalination of hypersaline brines. While demand is rapidly increasing, the wider application of hypersaline desalination is held back by considerable technical obstacles. Here, theoretical analyses are carried out to assess the potential of hypersaline ED desalination (Chapter 6). We show that desalination performance is impacted by the interrelated charge-discriminating selectivity and ion-water selectivity of IEMs. The work demonstrates lower energy costs of ED compared to thermal processes, when desalinating 1.0-1.5 M NaCl, and identifies three key performance-determining tradeoffs: conductivity-permselectivity, conductivity-water resistivity, and energy cost-volume reduction factor. To enable highly efficient hypersaline ED, ultra-low swelling IEMs need to be developed. Overall, this work advances mechanistic understanding, membrane development, and process design of IEMs. The thesis contributes insights to breaking conductivity-permselectivity performance constraints and developing highly valued ion-specific selectivity. The informed membrane fabrication and process design provide access to unconventional water sources with less energy input. The findings of the thesis will enable the systematic development of more ion separation processes to address challenges at the water-energy nexus.

High salinity desalination is coming into prominence as pressure on conventional water resources increases. However, high salinity desalination is technologically underserved: the conventional technologies, all of which rely on evaporation of water, are prohibitively energy-intensive and costly. In this work, we examine a promising hypersaline desalination method, temperature swing solvent extraction (TSSE). TSSE uses a switchable solvent to extract water from brine, meaning it can avoid many of the problems faced by conventional evaporation- or membrane-based desalination methods. However, the mechanisms by which water is absorbed and expelled from the switchable solvent, a process that is driven by a change in temperature, are poorly understood. The process by which salts can enter the relatively nonpolar organic phase is likewise mysterious. In this work, we investigate the nanoscale structuring in the switchable solvent-water-salt mixtures used in TSSE. We identify that the aggregation of water inside the solvent phase is key to explaining many of the macroscopic properties of TSSE. For instance, we find evidence of the directional interactions hypothesized as necessary for the unusual temperature-switchable behavior of the amine-water system that is used most often in TSSE. We further find that ions enter the solvent-rich phase inside nanometer-sized aggregates of water, and that the presence of these aggregates accounts for the unusually high viscosity of the amine-water mixture. These results greatly advance our understanding of TSSE, moving us from macroscopic and bulk properties to a fundamental, molecular-level understanding of the process. We also demonstrate several new applications of TSSE. Specifically, we show that TSSE is adept at treating hypersaline brine containing trace amounts of the toxic anions selenate and arsenate. TSSE rejected both trace ions at a far higher rate than the majority ion, chloride. To explain this phenomenon, we conduct the first systematic study of the behavior of different ions in TSSE and develop a straightforward model for predicting salt partitioning in TSSE using fundamental ion-specific parameters. Finally, we demonstrate that TSSE is capable of zero liquid discharge operation, in which only a solid waste is produced.