A Feasibility Study - Aaltodoc - Aalto-yliopisto

9.3 Determining an optimum pressure level
Reverse osmosis occurs when the pressure of the system exceeds the osmotic pressure
of seawater (29 bar). However, in order to maintain continuous and smooth operation,
RO systems are typically operated so that the pressure difference between seawater and
the osmotic pressure is substantial, e.g. 20 – 30 bar. Standard operating pressures are
therefore around 50 – 70 bar.
The choice of the optimum operating pressure for AaltoRO must be done, since
AaltoRO is not a typical commercial system, but has unique properties, which affect the
operation. The most important of these is the chosen minimum pre-treatment. Due to
this, the feed water is more challenging than in typical systems and is expected to
damage the membranes faster than in standard systems.
One way to increase the lifetime of the membranes is to operate the system with a lower
recovery rate, i.e. limiting how much of the feed water is turned into fresh water. In
essence, the membranes are not pushed to their operational limits, which ensures a
longer lifetime for the membranes.
Essentially the choice of the optimum pressure level is a question of optimization,
which issues are the most important ones: a good recovery rate, the lifetime of the
membranes, the cost of the water produced or a combination of these and other
parameters. For AaltoRO, it has been determined that the most important parameter is
the cost of water. The second parameter to optimize is the recovery rate. If the cost of
water is the same, the pressure level with the lowest recovery rate is chosen to maximize
the lifetime of the membranes.
The calculations for determining the optimum operational pressure level have been done
for Site 1 only, but it has been assumed that the same results would be get from other
Sites as well. Therefore the results from Site 1 will be generalized to all sites. To reach a
conclusion, the following values have been calculated: the permeate production
(m³/day), the load factor (suitable wave power / available wave power), the cost of
water (€/m³) and the recovery rate in each pressure level. The results are presented in
the following Figures 38, 39, 40 and 41, and the conclusions drawn from each figure are
presented right afterwards The conclusion of the optimum pressure level is presented
lastly.
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