(459b) DEVELOPING Structure-Tailored Forward Osmosis Membranes for Wastewater Reclamation and Water Production

The three interrelated issues of water, energy and environment, have been affecting the future of mankind due to the growth of overpopulation and the scarcity of exhaustible resources since the last few decades. The pervasive water scarcity is likely to be the most serious constraint on the sustainable development, particularly in drought-prone and environmentally polluted areas. To solve or alleviate the global water scarcity problem, tremendous efforts have been put to identify novel methods of wastewater reclamation and seawater desalination at less energy consumption. The development and application of membrane technologies is one of the most significant recent advances in water production. Recently, the emerging forward (direct) osmosis (FO) process, based on the osmotic pressure as the driving force to produce a net transport of water, has drawn much attention from researchers and its applications have been developed in various fields, such as wastewater treatment, seawater desalination, pharmaceutical and juice concentration, even in the power generation and the potable water reuse in space. The FO process, which is a naturally happened process, utilizes the osmotic pressure gradient generated by a highly concentrated solution (draw solution) to push water to automatically diffuse through a semi-permeable membrane from a saline feed water, which has a relatively low osmotic pressure. FO can operate without high hydraulic pressures which are necessary in the RO process and high temperatures which are necessary in the multistage flash and multi-effect distillation (MSF and MED) for water production. Therefore, less energy is required for the FO process compared to the RO and other thermal processes. The driving force of osmotic pressures in FO can be significantly higher than the hydraulic pressures in RO, subsequently resulting in higher theoretically water flux. In addition, the higher water recovery up to 85% is another advantage for seawater desalination by FO.

Similar to RO, FO utilizes a selective semi-permeable membrane to separate water from dissolved solute molecules or ions. However, one of fundamental hurdles to make this evolutionary process realized is the availability of desirable membranes with the superior separation performance. Presently, almost all membranes used in the FO process are available composite RO membranes except one FO membrane developed by HTI (Hydration Technologies Inc., OR, USA) through coating cellulose triacetate on the woven or non-woven mesh. It is necessary to develop special FO membranes that can adapt for the forward osmosis application. In this study, hydrophilic polybenzimidazole single-layer and dual-layer nanofiltration hollow fiber membranes through the dry-jet wet phase inversion and surface modified patterns were fabricated with different structures, for instances, wall thickness, microstructure and pore size in order to investigate the effects of membrane morphology on the membrane performance during the FO process. It has been acknowledged that the support layer structure have the important effect on the water transport due to the serious internal concentration polarization in the porous support layer. The state-of-the-art for dual-layer membrane fabrication via co-extrusion technology can provide the membrane with an ultra-thin selective dense skin, water channels underneath and microporous sponge-like support structure. Together with its sharp pore size distribution, the dual-layer hollow fiber membrane can achieve a water flux as high as 33.8 liter m-2 hr-1 without elevated operation temperatures at 23ºC and a salt flux less than 1.0 g m-2 hr-1 when using 5M MgCl2 as the draw solution in FO. In addition, we have proposed for the first time that the FO process could also be applied to create a new paradigm for pharmaceutical products enrichment and concentration from the dilute media.

All the asymmetric/composite membranes used in the FO process suffer from the adverse influence of internal concentration polarization (ICP), which seriously counteracts the driving force (forming a sharp concentration gradient within the porous support layer) and subsequently decreases the water permeation flux. The ICP within the membrane porous support cannot be eliminated by enhancing the crossflow velocity and turbulence along the membrane surface. In this work, a proprietary membrane structure design has been proposed and realized to eliminate the adverse influence of ICP within the porous support layer through developing an FO membrane with double selective layers and one middle porous support layer located in-between. The effect of membrane structure on the water and salt transports is investigated together with its potential application in desalination.