A Feasibility Study - Aaltodoc - Aalto-yliopisto

additional sheet of plastic which also creates turbulence and preserves the dimensions of
the feed channel (Kucera 2010).
Figure 31 demonstrates the function of a spiral wound element. The feed water enters
the element flow channels under a high pressure. On one side of the flow channel is the
semipermeable membrane and due to the pressure the water molecules pass through it
while salts are retained in the feed. The separated water spirals to the central tube and is
collected as the product while the feed water retains even more salt ions and becomes
the concentrate (Kucera 2010).
Figure 31. The configuration of a spiral wound element (Kucera 2010).
The standard size of a spiral wound element is 8 inches (203,2 mm) in diameter and 40
inches (1016 mm) in length (Kucera 2010). The present trend is towards larger diameter
elements as they are more cost effective, but also smaller elements are made for
specialized purposes (Bergman 2007). Individual RO membrane elements are packed to
pressure vessels consisting of 6 – 8 elements in series, where the concentrate from the
previous element is the feed for the next.
To maximize the system performance and to minimize the costs, pressure vessels are
typically arranged to 1 – 4 passes or stages, depending on the situation. In a pass the
permeate is the feed for the next, and in a stage the concentrate is the feed for the next
(Greenlee et al. 2009). The number of pressure vessels in following stages or passes is
roughly half of the original number because the preceding stage or pass has removed
roughly 50 % of the feed water (Bergman 2007). The entire RO system of pressure
vessels is usually denoted as the RO train (Greenlee et al. 2009). Design principles have
been developed to help to choose the correct membrane assembly and can be found
from the literature e.g. from Bergman (2007).
43