Pressure driven technology, such as reverse osmosis (RO), is still the most common desalination technology. However, the water recovery rate and salinity limit are not as high as with membrane distillation. Thus, MD can be coupled with RO in order to increase the overall water recovery of the desalination system. This can be done by using residual electrical energy to circulate the water at low pressures and by using thermal energy for solar thermal collectors.
Since MD can operate at much higher salinity then RO, the intended setup is to use the concentrated brine from RO to feed the MD system. This can reach a TDS of 150,000 mg/L. This means that the overall potable water recovery of this system could be as high as 75% compared to the usual 50% of the RO alone. That results in a brine volume reduction by a factor 4 (instead of 2). Additionally, since for the RO system some pre-treatment is already in place to avoid scaling in the membranes, no additional pre-treatment is necessary for the MD system.
Moreover, a zero liquid discharge (ZLD) approach can be more easily applied in order to further increase the overall water recovery. With already 2 streams of potable water exiting the RO and the MD systems, the concentrated brine from MD can for example be further sent to a crystallizer and a fluid bed dryer to extract the salts from this very saline stream. This results in more then 99% of brine volume reduction, which reduces considerably the impact on soil and sea.
With this application of membrane distillation, incorporated with other desalination technologies and solar energy in remote sunny locations, it becomes possible to provide such an essential need to these communities in a higher amount and with very little reject, as well as in an economical and sustainable manner.