Nanofibre Membrane Distillation for Brackish Water Treatment in Offshores and Small Islands

Introduction

Many areas situated in the vicinity of the sea or ocean face limited access to potable water. For instance, the offshore platforms, small islands, and isolated areas along the coastlines. The water resources in these areas have limited freshwater; thus the non-conventional water sources mainly come from saline water or brackish groundwater resources, both of which may likely contain salts and deemed unsuitable for drinking. Typically, non-conventional water sources require state of the art technologies for treatment which are often complex to operate and consume high energy.

Offshore platforms are ever more compelled to work in increasingly challenging and remote environments as the oil explorations progress. As a result, consistent potable water supply from the bunkering tanks carried from the mainland may be insufficient due to limited onboard normal storage capacities to support around one week of operation. Scarce water supply for drinking and operational uses may render the offshore operation to cease and faces the consequence of loss of production. Thus, water treatment technology on the offshore platform emerges as one of the most critical challenges faced by the offshore oil and gas industry. Besides, groundwater supplies which are largely relied upon at small islands and remote coastal areas exhibit brackish or saline properties and may be unsuitable for drinking without further treatment. The brackish properties are usually caused by the contamination of saltwater into the underground water reserves. Because of that, suitable treatments to produce potable water in these areas are of the interests for many researchers to come up with practical solutions. Many types of research are focusing on alternative technology approach as opposed to conventional techniques such as reverse osmosis, but are found to be less robust and less consistent.

Reverse osmosis is a membrane filtration process to separate the solvent from a solution, leaving behind a more concentrated solution. In an osmosis process with a semipermeable membrane separating a dilute solution from concentrated solution, the solvent moves through the membrane from the dilute to concentrated side in a bid to balance the concentrations. In this case, the concentration gradient drives the movement phenomenon of liquid. To prevent this flow of solvent, an opposing hydrostatic pressure is applied to the concentrated solution. The extent of pressure needed to thoroughly inhibit the solvent flow is called osmotic pressure. The reverse osmosis phenomenon is achieved when the applied hydrostatic pressure exceeds the osmotic pressure, whereby solvent flows from the concentrated solution to the dilute solution. Although reverse osmosis is capable of producing purified water, the downside is that it consumes a very large amount of energy to operate. More importantly, the high-pressure operation leads to the build-up of a concentrated layer of rejected solutes on the surface of the membrane. This phenomenon is called concentration polarization or cake formation, which is the main cause of fouling, bacteria generation, and flux decline. Moreover, high energy utilization and brine disposal issues are encountered because of limited water recovery. Also, reverse osmosis application would one day become complicated as energy demand and brine desalination will both become unsustainable in the future. Table 1 shows a general comparison between membrane distillation and reverse osmosis in terms of its principle, driving force, and energy requirement.

Promising technology with low energy consumption, high efficiency, and the prospect of utilizing alternative energy sources such as solar energy, geothermal energy, and low-grade waste heat is the main focus to address this issue. One of the interesting alternative technologies that tick the checklist is membrane distillation, which is an emerging technology for separation application that is traditionally achieved via conventional distillation or reverse osmosis. Membrane distillation and reverse osmosis are both closely associated with the desalination process with membrane application. It is attractive because it allows the retention of liquid water via surface tension while permitting the passage of water vapours across the membrane pores. Ideally, only a more volatile component passes through the membrane as permeating fluid, which allows high purity of recovered component. Currently, membrane distillation is still in the research and developmental stage. In desalination application, membrane distillation involves water vapour transport from a brackish solution across a porous hydrophobic membrane.