NAMS 2002 Workshop - ICOM 2008

Osmotically Driven Membrane Processes – 3
Tuesday July 15, 10:00 AM-10:30 AM, O’ahu/Waialua
A Novel Hybrid Forward Osmosis Process for Drinking Water
Augmentation using Impaired Water and Saline Water Sources
C. Lundin (Speaker), Colorado School of Mines, Golden, CO, USA
T. Cath, Colorado School of Mines, Golden, CO, USA, tcath@mines.edu
J. Drewes, Colorado School of Mines, Golden, CO, USA
As water resources become more contaminated and over allocated, new sources
of water must be developed. While many coastal areas are turning to reverse
osmosis (RO) desalination, the energy requirements can be a large drawback.
The large amounts of energy required for RO desalination is mainly due to the
need to overcome the osmotic pressure of seawater. The high osmotic pressure
of seawater limits the maximum recovery possible by RO systems. There are
only a few ways of reducing the energy required, one of which is by reducing the
osmotic pressure of the feed water, for example through dilution, thereby
reducing the needed applied high pressure.
Concurrently in many coastal areas, treated wastewater effluent is being
discharged to the ocean without providing any beneficial use; wasting a valuable
resource. In some areas the effluent is put into non-potable reuse systems, and
recently, some very progressive utilities have started using reclaimed water for
indirect potable reuse. Indirect potable reuse can work well in some areas, but it
requires a large natural water storage area (lakes or aquifer) and further
treatment after extraction from the aquifer and before ultimate potable use. Thus,
it might be more efficient to pursue direct potable reuse in certain circumstances.
The two problems of energy demand and wasted reclaimed water could be
synergistically solved if the impaired water stream could be safely used to dilute
seawater before RO desalination.
In a newly patented approach, forward osmosis (FO) uses a saline stream
(seawater or brackish water concentrate) to extract purified water from a source
of impaired water. FO uses an osmotic pressure differential as the driving force;
drawing water through a semi-permeable membrane and rejecting almost all
dissolved contaminants in the process. Because FO uses only osmotic pressure
as a driving force, its energy demand is very low. The diluted seawater is then
processed through an RO desalination system which provides rejection of salts,
as well as further rejection of dissolved contaminants that may have crossed the
FO membrane from the impaired water source. Most importantly, because the
saline water is diluted during FO, the energy required for subsequent RO
desalination of the diluted saline water is reduced. Thus, the energy demand of