Long-term Membrane Distillation Research Articles

Mechanism of biofilm formation on a hydrophobic polytetrafluoroethylene membrane during the purification of surface water using direct contact membrane distillation (DCMD), with especial interest in the feed properties

The impact of feed water quality on biofilm formation during membrane distillation (MD) was investigated in this study, particularly emphasizing the interrelationship between organics, salts, and microbes. Two types of typical natural surface waters in Nanjing, China, were chosen as feed solutions for long-term MD operation, including the Qinhuai River and Xuanwu Lake. The biofilms that developed under different feed water qualities exhibited distinct Foulant compositions and structures, causing different flux decline trends for the MD system. Accordingly, two typical patterns of biofilm formation were suggested for the MD operation of the two different kinds of surface waters in this study. Organics from a primal feed solution and dead bacteria were the key to the establishment of a biofilm on the membrane, and this needs to be effectively removed from the MD system through pre-treatment and process control strategies. Finally, a feasible strategy for MD biofouling control was suggested.

Morphological changes and creep recovery behavior of expanded polytetrafluoroethylene (ePTFE) membranes used for membrane distillation

Existing studies of membrane distillation (MD) have shown changes in membrane surface microstructure along with decreased system performance during long-term MD system operation; however, changes in feed-side morphology during long-term operation have not been demonstrated in the absence of foulants. Also, the net effect of morphological changes alone on performance during long-term operation has not previously been quantified. Three 30-day direct contact MD experiments were operated at different feed temperatures and salinities with a flat-sheet expanded polytetrafluoroethylene (ePTFE) membrane. Results showed that changes in membrane morphology (surface morphology, thickness, and bulk porosity) in the absence of foulants can result in decreasing water flux over time. Higher temperature (65 °C) experiments showed greater membrane morphology changes compared to lower temperature (45 °C) experiments. Creep recovery tests performed at different temperatures demonstrated membrane deformation over time under low and constant stress with the potential for permanent deformation after stress removal; these tests also demonstrated that higher temperatures result in greater deformation. Correlation between short-term creep recovery test results and long-term MD experimental results indicates that creep recovery tests may provide a useful shorter-term method for assessing and comparing propensity of different membranes to deform over time and at different temperatures.

Intrinsically superhydrophobic PVDF membrane by phase inversion for membrane distillation

Superhydrophobic surfaces have been considered as preferable property for desalination by membrane distillation (MD). Highly porous and intrinsically superhydrophobic PVDF membrane was prepared by phase inversion method using alcohols, namely methanol and ethanol, as nonsolvent on a nonwoven support. The water contact angle for the top and bottom surface of membrane and subsequent mechanical scratching demonstrated that not only surface of the membrane whole membrane film intrinsically possess the superhydrophobic behavior. The prepared membrane was characterized by using AFM, SEM, contact angle measurement, capillary flow porometer and water entry pressure test. The methanol phase induced PVDF membrane showed much superior results compared to conventional water phase induced PVDF membrane for the desalination of salt water by direct contact membrane distillation (DCMD) with water vapor flux up to 90LMH at ΔT 60°C and salt rejection of >99.99%. The superhydrophobic membrane showed excellent salt rejection up to 200,000ppm albeit with reduction in vapor flux. The long-term MD showed that the superhydrophobic membrane exhibited reasonably stable flux for a week time without compromising flux or NaCl salt rejection for 35,000–80,000ppm range.

Distillation membrane constructed by TiO2 nanofiber followed by fluorination for excellent water desalination performance

The hydrophobicity and module characteristic of membrane are key factors affecting the performance of the direct contact membrane distillation. In this paper, the superhydrophobic membrane, constrained by high porosity and large pore size, was prepared. Titania is regarded as a promising candidate material for environmental application, due to its high photocatalytic ability, relatively low cost, remarkable photostability, and toxicity. The membrane, consisted of titania nanofibers, was designed and fabricated by vacuum filtration and fluorination modification. Compared with ceramic particle aggregated membrane distillation (MD) membrane, an interconnected pore structure was constructed by entangled nanofibers to endow the prepared membrane with porosity higher than 80%. During direct contact membrane distillation process, the prepared membrane displayed an excellent desalination performance with flux of 12 LMH and salt rejection of 99.92%. Importantly, the flux was much higher than those of ceramic membranes with particle aggregation structure. Moreover, the prepared membrane possesses a good stability for long-term MD operation in pure water and even desalinating high saline water. The superhydrophobic titania nanofibrous ceramic membrane modified by fluorinated holds promise for practical applications due to its excellent performance for water desalination.