With water scarcity looming all over the horizon, the generation of pure water from sea/brackish water as well as the saline waste management are becoming important desalination technologies. Desalting is also important for zero to minimal liquid discharge (ZLD) systems that eliminate liquid waste from production facilities by recovering all salts and reusing the purified water. Typical ZLD units use thermal distillation techniques because Reverse osmosis (RO) has limited applicability at high salinity encountered in ZLD.
Compared to conventional thermal distillation, the relatively low temperature operation (50–90°C) and the lower CAPEX (capital expenditure) make membrane distillation (MD) an attractive alternative.
In MD, the driving force is a temperature induced vapor pressure gradient generated by having a hot feed and a cold permeate. The low operating temperature in MD makes desalination possible using low grade heat sources such as low pressure steam and solar energy. Compared to reverse osmosis (RO) a major advantage of MD is that while the former uses dense hydrophilic membranes, MD uses microporous hydrophobic membranes that are less prone to fouling, and MD can be used to treat water with higher salinity.
To make MD commercially viable, it is important to address some of its limitations such as low water vapor flux, fouling at high salt concentrations and high energy consumption. Since MD can be used for treating high salt concentration waters, some major commercial opportunities for MD are the treatment of RO reject, power plant blow downs and water from [16, 27-30]. One of the anticipated problems at high salt concentrations is fouling where flux decreases due to the deposition of suspended or dissolved substances on the membrane.
Membrane processes are susceptible to scaling at high salt concentrations when the ionic product of sparingly soluble salts in the concentrated feed exceeds its equilibrium solubility product. Some common scalants are calcium salts such as calcium carbonate (CaCO3), Calcium sulfate (CaSO4) and barium sulfate (BaSO4). Several approaches such as the use of ultrasound, pH control of feed side, and incorporated gas bubbling into direct contact membrane distillation (DCMD) has been used to reduce fouling in MD.
Recently, we have reported microwave induced membrane distillation (MIMD) where the feed water is heated by microwave instead of conventional thermal heaters. The mechanism of microwave heating is quite different from conventional heating. In general, saline water generates dipoles when placed in a microwave field which then develops orientation polarization, and the lag between the dipole orientation and the electric field leads to heating of the water . In addition, non-thermal effects such as local super heating and generation of nanobubbles are associated with microwave heating. Together these lead to lower temperature polarization, higher vapor pressure gradient and flux. The microwave process is also known to reduce the activation energy of physical and chemical processes, break down hydrogen bonded structures and reduce the average particle size salts in an aqueous environments. The energy consumption in MIMD has also been reported to be significantly lower than that by regular heating. Based on the results so far, MIMD appears to be a promising technique.
Particle size of the dissolved solute is one of the key factors that influences membrane fouling. In general, it has been observed that the particles with smaller size tend to lower the fouling tendency. Since important parameters such as hydrogen bonding and surface tension are affected by microwave radiations, the latter also affects the colloidal behavior of salts. Both crystal growth and decomposition which refers to the breakdown of salt crystals are effected by microwave radiations. Since the mechanism of salt crystals formation in microwave is known to be quite different, therefore, it is expected that the fouling behavior in MIMD will vary from conventional heating. The objective of this research is to explore the effect of microwave heating on fouling in a MD process, especially in the presence of common foulants such as calcium and barium salts.