A Feasibility Study - Aaltodoc - Aalto-yliopisto

The performance of the RO membranes depends upon numerous factors, including the
total dissolved solids (TDS) concentration, membrane material and thickness as well as
the feed pressure, feed flow rate and feed temperature. In a standard RO system the
operating pressure is maintained at the desired level and only start-ups and shut-downs
cause significant changes in the pressure (Bergman 2007) .
It has been estimated that pressure variations will cause losses in the life-time and the
performance of the membranes, although the matter is still under discussion (Folley,
Peñate Suarez & Whittaker 2008). With wave power being fluctuating by nature, it
could cause a problem for the AaltoRO system, but the Adaptive Pressure Generator
(APG) will be able to stabilize the pressure by using pressure accumulators. Thus,
pressure fluctuations are not considered to be problematic for AaltoRO.
The choice of various RO system components depends mainly on the desired output –
its quantity and quality. For example, drinking water has strict standards, although
varying between countries, which have to be met. World Health Organization does not
give a strict standard for the TDS level of drinking water, instead it states that water
with TDS concentrations below 1000 mg/l is acceptable (WHO 2003). The United
States Environmental Protection Agency has set a secondary maximum contaminant
level of 500 mg/l for TDS, which usually is the target for water treatment (Bergman
2007, EPA 2012).
Measuring the efficiency of the RO membranes and the overall membrane configuration
is usually done by giving out certain key figures. The most important key figure is the
recovery rate, usually denoted with R. It explains how much of the incoming feed water
is transformed into product water. Typically the recovery rate will be kept at the highest
possible value taking into account the desired water quality, fouling of the membranes
and the capital and maintenance costs (Bergman 2007). The membrane system
performance is measured by the membrane salt rejection capability, which can be as
high as 99,7 % (Greenlee et al. 2009).
Depending on the desired output, the input flow rate velocity, required maintenance
schedule and various factors depending on the project in question, the decision of the
suitable design configuration is made. It can be a single pass, dual-pass or any other
configuration. Still, some key design principles should be thought of. First, the RO
system has a minimum energy requirement when the recovery is between 50 – 55 %.
Second, maximum recovery is achieved when only one pass system is used (Greenlee et
al. 2009). Third, novel practices and membranes can improve the efficiency and lifetime
of the membranes thus resulting in significant savings (Peñate, García-Rodríguez 2012).
Therefore, the choice of the design configuration should be made only after a careful
consideration (Kucera 2010). AaltoRO will utilize a single pass system, as it is typical
in small seawater reverse osmosis systems (DOW 2010).
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