Membrane and Desalination Technologies - TCE Moodle Website

36 A.G. (Tony) Fane et al.
4.4. Promising Membrane Systems
With the development of new generations of membrane materials and the improvement in
membrane processes, some membrane applications may be highly attractive in the future for
energy-saving consideration. These promising membrane systems include forward osmosis
(FO), biomimetic membranes, anaerobic MBRs (AMBRs), microbial fuel cell (MFC), decentralized
sanitation systems, etc.
Forward osmosis (FO), also referred to as “osmosis”, is actually a traditional process
which was discovered as the background phenomena of RO (see also Fig. 1.5). In the FO
process, a “draw” solution of high osmostic pressure (relative to that of the feed solution) is
used to induce a net flow of water through a semi-permeable membrane into the draw solution
under an osmotic pressure gradient, thus, effectively separating the feed water from its
solutes. If fresh water is the desired product, a second separation step is required to extract
the fresh water from the less concentrated draw solution. The first separation step of FO,
driven by the osmotic pressure gradient, does not require a significant energy input. The main
energy consumption is involved in the production of fresh water and reconcentration or
regeneration of the draw solution. Forward osmosis has been studied for a range of applications
including water and wastewater treatments, seawater/brackish water desalination, food
processing, drug delivery, and electric power production, but commercial applications are
still limited. It is worth a further exploitation as a promising energy-effective process. The
breakthroughs for FO process require the development of robust and efficient membranes and
modules, and the development of draw solutions that can induce high osmotic pressure with
low energy requirements for regeneration or reconcentration (143).
Nature has developed a most efficient way for water transport across the cell membranes of
microorganisms. Aquaporins or water channel proteins, which were discovered in 1990s by
Peter Agre (2003’s Nobel Laureate for the discovery), are highly permeable to water but
highly retentive to dissolved solutes. They are typically bound in phospholipid cellular
membranes and responsible for water transport across the membrane. The ideal property of
the water channel proteins makes water delivery highly efficient. An artificial membrane may
be developed to mimic the natural cellular membranes by incorporating and immobilizing
aquaporins into an ultrathin amphiphilic polymeric layer to form a similar rejection film
(144). With a robust and carefully engineered microporous support structure, the biomimetic
membrane could be applied to ultrapure water production, water reuse and seawater desalination.
Compared to the conventional RO membranes, the biomimetic membranes offer
potential advantages including better selectivity, improved water permeability, reduced
energy consumption, and improved product water quality because of its remarkable selectivity
and water permeability. However, the biomimetic membranes are only at the stage of
conceptual demonstration for water reuse and desalination. The technical challenges include
the preparation of a suitable membrane support, the incorporation of aquaporin into a copolymer
film and its interaction with the support.
The AMBR uses anaerobic bacteria to degrade organic substrate in the absence of oxygen.
AMBR systems have similar configurations to aerobic MBR systems except for the application
of biogas instead of air in the submerged reactor (Fig. 1.16) (145). Compared to