Development and Characterization of Low-Cost Flat Sheet Ceramic Membranes for Water Desalination by Membrane Distillation

This thesis is concerned with the development and characterization of low-cost flat sheet ceramic membranes for water desalination by membrane distillation. The research covers the beneficiation and characterization of kaolin clay and metakaolin grafted membranes with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFAS). In addition, state-of-the-art research findings on solar-powered membrane technologies for water purification are presented and discussed. The outcomes are presented in three parts.

In the first part, a detailed literature review on solar powered membrane technology is presented in a publication entitled:

‘Solar assisted membrane technology for water purification: A review’

Shortage of safe drinking water is one of the main leading problems in the world. Even in high-income countries where household water treatment systems are present, safe drinking water may not be always available due to the limitations of advanced separation technologies and high energy costs. On the other hand, many urban and rural communities in sub-Saharan Africa and Central Asia situated in semi-arid to arid regions are without reliable access to safe drinking water. It is, therefore, important to explore how solar energy can be coupled/integrated with water treatment systems for clean water production. Membrane-based advanced separation technologies play a major role in water purification by utilization of low-cost heat sources such as low-grade waste heat, geothermal energy, and solar energy to make the process technically viable and economically attractive for small/medium/large-scale water treatment applications. Solar energy can be a viable source of power for water purification facilities in the coming years. Photovoltaic panels and solar thermal collectors are appropriate solar energy collectors for making a solar-powered water treatment system. Solar-assisted membrane-based water purification techniques could hold a viable solution to the existing problems in semi-arid and arid regions. Due to the high quality necessary for potable water, studies have been carried out on solar-assisted membrane-based technologies in water purification. This review considers basic concepts, specific energy consumption, water production cost, and applications of solar-driven membrane-based water purification technologies such as reverse osmosis (RO), forward osmosis (FO), electrodialysis (ED), membrane distillation (MD), and hybrid membrane systems (HMS). This review will allow researchers to have a wider overview of the effort made by several investigators in the area of solar-assisted membrane-based water purification technology.

The second part deals with the characterization and beneficiation of Ethiopian kaolin, and is presented in a publication entitled:

‘Characterization and beneficiation of Ethiopian kaolin for use in fabrication of ceramic membrane’

The use of low-cost ceramic membranes and the ease of construction of a small-scale, membrane-based separation process are attractive for grid-independent desalination where low-cost heat sources such as low-grade waste heat, geothermal energy, and solar energy are readily available could be used. Recently, research in membrane technology is developing high-performance ceramic membranes from local inexpensive raw materials such as kaolin, ball clay, bentonite clay, quartz, bauxite, and others. Ceramic materials are the most commonly used materials for the synthesis of low-cost ceramic membranes/inorganic membranes for membrane-based water purification technology. Over 20 million tons of kaolin deposits are spread over many states in Ethiopia. The kaolin deposits used in the current study were collected from the local area, Debre Tabor Town, located in the state of the Amhara Region, but mining at depth, continuous and progressive research on kaolin deposits and their occurrences, an increased level of mineral impurities makes commercial applications not viable. To improve the quality of kaolin deposits to possibly meet the requirement of ceramic materials, the coloring impurities must be removed through efficient, economical, and environmentally friendly beneficiation methods. A kaolin ore from Debre Tabor, Ethiopia was physically, chemically, and thermally treated for possible industrial use, especially for ceramic membrane fabrication. The leaching experiments were carried out using oxalic acid solutions as leaching reagents for the iron extraction process. The effect of acid concentration, reaction temperature, and contact time on iron leaching was investigated. It was determined that the rate of iron extraction increased with the oxalic acid concentration, leaching temperature, and contact time. The treated kaolin raw material was used to fabricate a low-cost ceramic membrane. Finally, it was demonstrated that the characteristics of the sintered ceramic membranes such as mass loss, water absorption, shrinkage, and chemical stability have thus properties comparable to commercial ceramic membranes.

Therefore, this study will serve as a stepping stone for other researchers who will be interested in ceramic membrane fabrication and characterization techniques and advanced separation technology. Moreover, this study can generate baseline data that may give an insight for further studies in other areas.

The third part deals with the fabrication of flat sheet metakaolin based ceramic membrane and is presented in a publication entitled:

‘Flat sheet metakaolin ceramic membrane for water desalination via direct contact membrane distillation’

In this study, locally available and inexpensive Ethiopian kaolin was used as the primary raw material for the preparation of metakaolin-based flat sheet membranes. Hydrophobic metakaolin-based flat sheet membranes were developed via phase inversion and sintering and modified through 1H,1H,2H,2H-perfluorooctyltriethoxysilane grafting agents. This study also explores the use of flat sheet ceramic membrane for membrane distillation application and the means to optimize the membrane preparation parameters such as metakaolin loading, sintering temperature, the concentration of the additives, and mixing practice for enhancing mechanical strength to make it competitive with existing ceramic membrane modules such as hollow fiber and tubular. The prepared membranes were characterized using a Scanning Electron Microscope (SEM), Dynamic Mechanical Analysis (DMA), Fourier Transform Infrared (FTIR) spectroscopy, and contact angle and open porosity measurement. Their results indicated that the wettability, structural and mechanical properties of the prepared membrane confirm the suitability of the material for membrane distillation (MD) application. The prepared flat sheet membrane acquired hydrophobic properties after surface modification. Afterwards, the membrane performance was tested for different sodium chloride aqueous solutions (synthetic seawater) and various operating parameters such as feed temperature and feed flow rate using direct contact membrane distillation (DCMD). Based on the findings, the newly developed membranes achieved a good performance in terms of salt rejection and permeate flux. It can be concluded that further optimization of the membrane fabrication process is required to maximize the permeate flux and salt rejection.