A Feasibility Study - Aaltodoc - Aalto-yliopisto

Figure 29. UF pre-treatment. Adapted from (Wolf, Siverns & Monti 2005).
The choice of a pre-treatment system for AaltoRO is not an easy one. This is mainly due
to the fact that in regular systems the fundamental operating principle is different than in
AaltoRO. In standard RO systems, the pre-treatment can be done at the point most
convenient for it, typically onshore, and before the pressurization to a high pressure. In
AaltoRO the pressurization dictates all other activities as it occurs at the bottom of the
sea in the WEC, typically at 15 meters of depth and at 500 meters distance from the
shore.
Pre-treating water at a high pressure is practically impossible and therefore it has to be
done before the WEC. But pre-treating water at the bottom of the sea is also extremely
difficult. The pre-treatment for AaltoRO cannot be done either by conventional or by
membrane pre-treatment. Yet, there are two mindsets for determining the suitable pretreatment
system. Either to produce pre-treated water with excellent quality to minimize
the maintenance of the RO membranes, or to have allowing pre-treatment and to wash
RO membranes frequently to maintain the desired performance (Taniguchi 1997). Due
to the challenges posed by AaltoRO, the second mindset is adopted in this thesis.
Two main challenges for the AaltoRO pre-treatment system are small particles (e.g.
sand particles) and biological fouling. With a carefully designed intake structure it is
possible to minimize the incoming foulants, and with coarse and fine screens the
filtration can be taken up to a level of 100 microns. Filtration to a better degree has been
deemed too difficult, and therefore the system will have to be able to handle small
particles of under that size.
Biological fouling and scaling of soluble components can be taken care of with various
chemicals. Antiscalants have been a standard option in RO systems for a long time, but
recently there have been encouraging reports of antibiofouling chemicals as well
(Matsumoto 2012). With the use of these chemicals, it has been estimated that scaling
and biofouling will not pose a major threat to AaltoRO.
The proposed pre-treatment system for AaltoRO is very light compared with the typical
RO pre-treatment systems. The filtration level is not high, and depending only on
chemicals with biofouling is also somewhat speculative. It also remains to be seen how
well the mechanical parts of the system (the WEC and the APG) will perform with the
chosen pre-treatment. At least, the RO membranes will require more maintenance and
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