Permeate Flux and Rejection Behavior in Submerged Direct Contact Membrane Distillation Process Treating a Low-Strength Synthetic Wastewater

Rejections and membrane fouling of submerged direct contact hollow-fiber membrane distillation as post-treatment for anaerobic fluidized bed bioreactor treating domestic sewage

In this study, submerged direct contact membrane distillation (SDCMD) with a hollow-fiber membrane was applied as a post-treatment for an anaerobic fluidized bed bioreactor (AFBR) treating domestic sewage. The rejection efficiency of organic contaminants and nutrients, such as ammonia nitrogen and phosphate in SDCMD were investigated. As the transmembrane temperature difference increased, the permeate flux of SDCMD increased, while the rejection efficiency of ammonia nitrogen decreased. Regardless of the transmembrane temperature applied in this study, rejection efficiencies greater than 90% were achieved for organics and phosphate by SDCMD treatment of the AFBR effluent. A higher solution pH resulted in a lower ammonia nitrogen rejection efficiency, probably because nitrogen dominantly exists in the gaseous form and can easily pass through the hollow-fiber membrane. Long-term operation with the integrated AFBR-SDCMD process over 50 d at a transmembrane temperature of 30 °C and solution pH of 5.5 showed rejection efficiencies of 98.7%, 98.1%, and 90.5% for ammonia nitrogen, phosphate, and dissolved organic carbon (DOC), respectively. During the entire integrated process for treating domestic sewage, both DOC and nutrients present in the bulk solution of the SDCMD reactor were effectively removed to a concentrate. However, the permeate flux produced by the SDCMD membrane decreased over time, mainly because of the progressive biofouling.

Advances in Membrane Distillation Module Configurations

Membrane Distillation (MD) is a membrane-based, temperature-driven water reclamation process. While research emphasis has been largely on membrane design, upscaling of MD has prompted advancements in energy-efficient module design and configurations. Apart from the four conventional configurations, researchers have come up with novel MD membrane module designs and configurations to improve thermal efficiency. While membrane design has been the focus of many studies, development of appropriate system configurations for optimal energy efficiency for each application has received considerable attention, and is a critical aspect in advancing MD configurations. This review assesses advancements in modified and novel MD configurations design with emphasis on the effects of upscaling and pilot scale studies. Improved MD configurations discussed in this review are the material gap MD, conductive gap MD, permeate gap MD, vacuum-enhanced AGMD/DCMD, submerged MD, flashed-feed MD, dead-end MD, and vacuum-enhanced multi-effect MD. All of these modified MD configurations are designed either to reduce the heat loss by mitigating the temperature polarization or to improve the mass transfer and permeate flux. Vacuum-enhanced MD processes and MD process with non-contact feed solution show promise at the lab-scale and must be further investigated. Hollow fiber membrane-based pilot scale modules have not yet been sufficiently explored. In addition, comparison of various configurations is prevented by a lack of standardized testing conditions. We also reflect on recent pilot scale studies, ongoing hurdles in commercialization, and niche applications of the MD process.

Hybrid forward osmosis/membrane distillation integrated with anaerobic fluidized bed bioreactor for advanced wastewater treatment

Forward osmosis (FO)-membrane distillation (MD) process was integrated with anaerobic fluidized bed bioreactor (AFBR) to advance wastewater treatment. Low removal efficiency of nutrients such as ammonia nitrogen was improved significantly by combining FO-MD process with AFBR. The MD membrane was applied to concentrate the draw solution (DS) which can be diluted by FO filtration. By using 1 M of NaCl as DS, about 80% of ammonia nitrogen was further removed by the FO membrane while the phosphorous was removed almost completely (99%). However, the accumulation of ammonia nitrogen in DS and the reverse salt flux through the FO membrane was unavoidable. Nevertheless, combining MD membrane produced excellent removal efficiency yielding only 4 and 5.6 mg/L of ammonia nitrogen and chemical oxygen demand (COD) in MD permeate, respectively at 15 ℃ of transmembrane temperature. Alternatively, there is the possibility that the FO-MD process can be superior to concentrate resources such as nitrogen and phosphorous present in AFBR. The reverse salt flux from DS into AFBR bulk suspension did not show adverse effects on the performances of bioreactor with respect to COD removal efficiency, conductivity and methane production during operational period. Deposit of the fouling layer on FO membrane was also observed, but the fouling on MD membrane was not severe probably because crystallization rate could be retarded by diluting the DS during FO filtration.