Direct Contact Membrane Distillation for Water Reuse Using Nanostructured Ceramic Membranes
Project Number: 07-05
Year Released: 2013
Funding Partner: Bureau of Reclamation
Total Investment: $115,830.43 (Cash: $100,409, In-Kind: $15,421.43)
Principal Investigator: Mark R. Wiesner, Ph.D., Duke University
In contrast with pressure-driven membrane processes such as reverse osmosis, nanofiltration, ultrafiltration, and microfiltration, membrane distillation (MD) is a desalting process that is driven by a thermal gradient. A particularly exciting application of MD is in the area of water reuse through the advanced treatment of wastewater or desalination of waste brines where diffuse heat can be tapped as the driving force for membrane separation. In addition to the ability to use diffuse heat as a driving force, MD offers several advantages over RO or NF as a desalting process.
Goals and Objectives
The project increases our knowledge base on a technology that holds significant promise for water reuse—membrane distillation (MD). The objectives of this project were to (a) develop and characterize ceramic membranes to have the necessary chemical and physical properties for use in direct contact MD (DCMD), (b) integrate these membranes into a laboratory-scale unit, and © evaluate the performance of these membranes alongside a polytetrafluoroethylene (PTFE) polymeric counterpart during treatment of different synthetic solutions containing organic foulants as well as wastewater from the North Durham Water Reclamation Facility.
Prospective coatings for ceramic membranes were tested for adequate hydrophobicity, liquid entry pressure, and chemical degradation resistance. Successful coatings were then compared to established polymeric PTFE membranes via parallel membrane distillation studies using a lab-scale module. A model was developed to assess their relative performances. In addition the membranes were subject to a variety of feed waters, synthetic wastewaters, and actual municipal 2o clarified effluent. Membrane performance over time was monitored, and fouled membranes were evaluated after tests.
Findings and Conclusions
Ceramic membrane coating: The surface chemistry of ceramic membranes can be successfully modified to yield a hydrophobic surface suitable for use in MD. The PFS coating is resistant to exposure to high concentrations of chlorine, and maintains the requisite hydrophobicity for MD use.
Ceramic/polymeric performance: The modified ceramic membranes provide a water flux approximately 45% lower than that of PTFE membrane during MD operation. This lower flux is attributed to a combination in the differences between the thermal conductivity and porosity of the membranes. The uniform pore size and structure of the ceramic membranes provide a higher LEP, which make it more a more viable alternative than PTFE for MD applications where high-pressures or low surface tension feed waters are present.
Membrane fouling: The foulant layers had no significant effect on the LEP of either the PTFE or PFS anodiscTM membranes. This suggests that the foulants did not enter the pore matrix of the membranes, which means that membrane rejection properties are not likely to degrade over time. Complete water flux recovery of membranes fouled after treating wastewater was achieved using either sodium hypochlorite solutions or DI water.
The project is intended to inform the water reuse community on the benefits and tradeoffs of using ceramic membranes in DCMD as well as to provide performance information on MD as an advanced treatment technology.