So given the above challenges why then bother to have solved them? The answers again are not so
straight forward and like all membrane processes can be site specific. For brevity the two main
advantages over conventional reverse osmosis are lower fouling propensity and lower energy
consumption, which are discussed in more detail later
The significantly lower fouling potential of the forward osmosis process compared to reverse osmosis
membranes, operating under the same feed conditions, has been shown by a number of academic
researchers. However more importantly under real world conditions on challenging feedwaters, Modern
Water has demonstrated that on one particular site in Oman no chemical cleaning was required over
several years operation, yet the conventional process required cleaning every few weeks with several
membrane changes [ 10]. This of course also means a reduction in the use of membrane cleaning
chemicals and improved availability.
The potential for a lower energy consumption, which is sometimes not so readily understood and
requires careful explanation, comes about in a number of ways:
If we consider that a ‘state of the art’ reverse osmosis system has a similar energy consumption
to a ‘state of the art’ FO system in the clean state. Then given that FO fouls at a lower rate than
RO then it is clear that to maintain the same output the RO based process will require more
energy. The degree of energy saving depends on the degree of fouling.
Now consider the regeneration step, which is where the bulk of the energy is consumed for this
forward osmosis desalination process. The osmotic agent is free of all particulates and large
molecular weight organics from the feedwater, as the solution is made from permeate and as
such a parallel should be drawn with the normal design criteria for second pass RO systems. The
result is that the degree of irreversible flux decline is reduced. Additionally the recovery rate
(50% for minimum energy), the membrane selection and configuration can be fully optimised
and with conventional energy recovery systems applied to the high pressure concentrated
osmotic agent stream.
The regeneration step, again because it is fed with a ‘perfect’ solution has improved salt rejection
compared to conventional RO. This may eliminate the use of a second pass and as such, energy
is saved.
Boron in desalinated water produced by reverse osmosis has long presented a challenge to the
membrane industry with very poor rejection and the necessity for special high pH second passes
or the use of ion exchange columns, to meet the relevant in country standards. These of course
add not just complexity, capital cost and operational cost but increased energy consumption. The
advantage of having a re-circulating osmotic agent is that its properties can be controlled and as
such, in combination with the forward osmosis membranes, has improved boron rejection
characteristics.
The regeneration system operates at an elevated temperature compared to the feedwater as a
result of the recirculating osmotic agent and therefore again energy is saved.