Visualization of hydraulic conditions inside the feed channel of RO using PIV
Comparison of the hydraulic conditions inside the empty and spacer-filled channel is the main objective of this study.
Desalination by Reverse Osmosis (RO) and Nanofiltration (NF) increases is the dominate desalination technology at the moment. DesalData reported that about 78.4% of contracted desalination capacity in 2015 was provided by the RO and NF. The takeover of desalination by RO is mainly due to lower cost of RO compare to other desalination techniques. That is while that the spiral wound modules of RO (SWMRO), by far, is the most applied configuration of RO. The wider application of RO influenced the choice of the feed water as well. Historically, the first generation of RO used for purification of seawater. With passing of time, brackish water, wastewater and freshwater are also indirectly exposed to RO for purification. Application of RO on freshwater is mainly increased due to increase of xenobiotics substances in surface freshwater as the consequence of presence of residual wastewater from industrial, agricultural and medical sections. However, the only important change SWMRO have undergone was increasing of the thickness of their feed spacer, which results in ultralow pressure ROs (second generation). That is partly due to the fact that our understanding of membrane process is limited [1, 2]. For instance, our undressing of the temporal and spatial hydraulic conditions inside the feed channel of SWMRO is incomplete. Our limited understanding of hydraulic conditions of SWM is thanks to visualization by computational models. Visualization by experimental models was limited due to low resolution of available methods in the past and difficulty of application of proper methods at the moment.
Particle image velocimetry is an experimental visualization method, which is used in this study to obtain information about the temporal and spatial conditions inside the empty and spacer-filled channel in the laboratory scale. Some of these results are mentioned below.
Figure 1 illustrates the spatial variations of the velocity averaged over time at the middle of spacer-filled channel (Z2). Figure 8a shows the vector velocity map over the whole field of view, while the Figure 1b and Figure 1c respectively illustrate the cross section X(y= -2.41mm) and Y(x = 4.73mm) of vector velocity values for empty and spacer-filled channel. Figure 1a shows that the greatest diversion of the vector velocity from the flow direction at Z2 can be observed in the direct adjacent of the filaments at the downstream part of each mesh.
Comparing Figure 1a with cross-section in the X-direction (Figure 1b) reveals that the lowest velocity occurs at the nodes, while highest velocity values occur in the distance between the nodes and the middle of mesh. The velocity values in the middle of the meshes, however, are slightly lower than the average velocity. Lau et al.  achieved a comparable velocity map as Figure 1a in the middle of the flow channel they used for a feed spacer of 1.0 mm at flow attract angle of 45o by using a computer model. When comparing Figure 1a with cross-sectional profile parallel to the flow (Figure 1c), the lowest velocity can be observed at the nodes, while the highest velocity can be detected directly at the downstream of the nodes. That is while at the distance of about 25% toward each node from the center of each mesh, the velocity remains more or less constant.
Figure 1: For ease of visualization, the numbers of vectors in Figure 1a are reduced from its original state
Succeeding in better understanding of the hydraulic conditions inside the feed channel of RO leads us to the proper design of membrane modules for each type of the feed water.
At the moment our drinking water companies provide top water quality. This can be remains the same by applying the new trustworthy methods such as RO as a barrier against all kinds of known and unknown pollutants.
 W.G.J.v.d. Meer, Het drinkwaterbedrijf van de toekomst?, in, TU-Delft, Delft University Of Technology, 2013.
 W.G.J. van der Meer, Membraanfiltratie : Presteren onder druk, University of Twente, Enschede, the Netherlands, 2008, pp. 44.