Membrane distillation improvements

Leader

Challenge

With increasing demand for water, desalination has become an increasingly attractive process, particularly in locations where traditional sources of water are insufficient. For landlocked areas, inland desalination of brackish water has become a potential solution. However, disposal of concentrated brine produced from the desalination process poses a major challenge to inland desalination plants as the option of discharging the brine into the ocean is not available. Conventional brine disposal methods, such as deep well injection and evaporation ponds can be costly and are often ineffective for handling a higher flow of concentrated brine. Consequently, a solution which achieves close to zero liquid discharge is most ideal.

Membrane distillation (MD) has been increasingly investigated for processing high salinity solutions. A typical MD system involves salt solution at a higher temperature on one side of the membrane and cooling water or gas on the other side. The vapour pressure difference caused by the temperature and concentration differences results in water vapour transport from the hot to the cold side, where it condenses. Since MD is driven by vapour pressures, it is not affected by the osmotic pressure gradient across the membrane; therefore highly concentrated salt solutions close to the saturation point can be obtained.

Investigation

Compare the desalination performance of different MD configurations and investigate the feasibility of using a submerged vacuum MD system for the treatment of brine produced by inland desalination processes. Examine the effect of transverse vibration on MD performance and the anti-scaling performance of transverse vibration compared to air backwash and aeration.

Outcomes

In comparison to conventional cross-flow configurations, submerged vacuum MD distillation offered lower energy consumption while achieving relatively similar permeate flux. However, scaling remained an issue. The application of transverse vibration in submerged vacuum MD did not significantly limit salt crystal deposition, but did increase permeate flux by up to 21%. Long-term operation indicated that while submerged vacuum MD coupled with transverse vibration consistently produced higher flux for both single and mixed salt feed solutions, wetting occurred at a faster rate compared to when agitation was not present. This was attributed to changes in the structure of the scaling layer on the membrane.

The implementation of periodical air backwash and aeration in conjunction with transverse vibration showed that bubble generation in mixed feed solutions containing sparingly soluble salts may encourage scaling in submerged vacuum MD.

Future Direction

Further studies are needed for looking at strategies for managing (prevention, reduction, cyclic operation) crystallisation, membrane fouling and wetting over long term operation, such as periodic operation, including crystal stabilisation period, crystal filtration, membrane distillation coupled with crystallisation operation with agitation, membrane cleaning.

The long term effect of different operation modes on the integrity of the membrane such as pore size, surface property and performance recovery by cleaning is another aspect of research that is also needed.

In the area of waste mitigation, application of dense film for protection of membrane surface in sever crystallising solution for prevention of pore penetration and wetting in the submerged vacuum membrane distillation system is also worth pursuing.

Partners


 

Total Value: $731,714 (cash and in-kind contributions)

Principal Investigator: Professor Vicki Chen

Title: Transverse vibrational motion enhanced submerged hollow fibre membrane crystalliser

Length: 43 months

Personnel: 11 collaborators contributing 4.16 FTE

Further Information

FR3 UNSW Chen (Crystallisers) Summary Poster

Project Summary Poster – MD improvements

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