A Feasibility Study - Aaltodoc - Aalto-yliopisto

8.3.3 Post-treatment
The permeate from the RO membranes, while with a low salt concentration, also has a
low concentration of substances that make drinking water appealing and acceptable.
Therefore, treating the permeate to meet various drinking water standards is a necessity
(Wetterau 2011). In theory, all the treatment systems downstream from the RO
membranes are considered as post-treatment facilities, whether they treat the permeate
or brine, but in practice post-treatment usually refers to the treatment of permeate
(Bergman 2007).
Post-treatment has not received as much scientific attention as the overall process and
the energy recovery. This is mainly due to the fact that treating drinking water has been
done for a considerable period of time, and that treating desalinated water has not been
considered to be very different from standard knowledge. However, this has sometimes
resulted in poor results with the product water and additional problems e.g. with
corrosion. Therefore, understanding the feed water and permeate quality and how to
affect it, is important in order to develop the most cost-effective and efficient posttreatment
system (Birnhack, Voutchkov & Lahav 2011).
In general, the issues to address in post-treatment include: corrosion control,
decarbonation, alkalinity, hardness and disinfection (Bergman 2007). The permeate is
fairly aggressive due to its low alkalinity and low mineral content and therefore easily
causes corrosion. Decarbonation is necessary to remove excessive amounts of carbonic
acid and thus increase the pH. The alkalinity of the permeate is too low and must be
increased through chemical treatment, usually by adding carbon dioxide. The levels of
calcium and magnesium must be increased to reach acceptable levels of hardness.
Disinfection is also necessary to prevent the growth of harmful micro-organisms. The
interaction of various chemicals is difficult to predict and therefore great care must be
taken in the design phase of the post-treatment system (Birnhack, Voutchkov & Lahav
2011, Wetterau 2011, Bergman 2007).
The standard way to treat permeate is to directly introduce chemicals as slurry,
dissolved in a solution or as a condensed liquid. The benefits of directly dosing the
permeate include: low capital costs, small space requirement, flexibility in which
chemicals to choose and most of all simplicity of operation. Drawbacks are the high
operational costs and the unwanted reactions of added chemicals. Another way to meet
drinking water standards is to blend it with sufficient amounts of feedwater, but it
causes additional problems by adding also unwanted substances and also controlling the
quality of the product water is challenging (Birnhack, Voutchkov & Lahav 2011,
Wetterau 2011, Bergman 2007).
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