This special issue of Desalination – under Guest Editor Professor Enrico Drioli of the Institute on Membrane Technology at the University of Calabria in Italy – collects a series of papers that explore the potential of membrane distillation (MD) and related membrane operations.
Membrane distillation is an innovative membrane-based process with promising applications such as water desalination, treatment of industrial effluents, the pharmaceutical and food industries, and the removal of organic compounds from aqueous streams.
The process is based on the temperature gradient created across a microporous membrane that separates vapour–liquid/liquid–liquid phases in equilibrium. The process has many advantages over conventional distillation or pressure-driven membrane based processes: theoretically complete rejection of non-volatile components, lower operating temperatures and pressures, the acceptance of membranes with relatively poor mechanical properties etc.
MD has the capability to replace reverse osmosis if cheap or low-grade ‘waste’ energy is available. There is tremendous potential for the use of MD in processes such as water desalination in areas with a hot climate. For example, in the Middle East, half of the domestic oil consumption is associated with water purification. MD can play a crucial role in significantly reducing this, utilising the region’s abundant solar energy.
MD can be easily adapted to other systems. It can be used separately or integrated with other processes like membrane bioreactors (MBRs), forward osmosis (FO), microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) to improve process efficiency in terms of purity and recovery factor.
In the case of desalination plants using NF and/or RO as the separation technology, MD and its extension membrane crystallisation (MCr) can be used to recover water from residual brine, obtain crystals of different salts, and separate metals present in various effluents and sea water. MD-based processes, such as membrane crystallisation and osmotic membrane distillation, have opened an innovative gateway to obtain crystals from salt solutions and concentrate various solutions.
Another interesting application of MD is its potential to recover toxic or/and useful compounds and heavy metals from industrial effluents, which can be reused in the process, or separated from the effluent to avoid their injurious environmental impact. Hence, MD can provide an innovative, cost-effective and easy-to-adopt pathway with a significant impact in terms of ‘green’ chemistry.
These innovative thermal membrane operations overcome most drawbacks and limitations of traditional thermal desalination systems, such as multi-stage flash (MSF), multi-effect distillation (MED), and vapour compression (VC).
But the potential of MD is much greater than the areas now being explored. Membrane crystallisation (MC) and membrane condensing (MC) are two important examples of process engineering operations, which might successfully redesign conventional crystallisers and condensers following the principles of Process Intensification, and are well suited to MD. The hardware is basically the same; different software will allow them to realise their optimum performance.
