Page 24 - ro membanes

1.2 MEMBRANE-FOULING MECHANISMS 7
pumps (which typically is within 10%e20% from their average capacity) and the maximum physical pressure the RO membrane elements are designed to handle (typically for most commercially available SWRO membrane elements, such pressure is 83e85 bars).
Because of these practical limitations, the typical impact of increase in concentration polar- ization on desalination plant operations is the reduction of plant’s freshwater production (Herzberg and Elimelech, 2007).
The salt transport rate through the RO membranes could be presented as follows:
Qs 1⁄4 BSðbCb CpÞ (1.3)
As it can be deducted from Eq. (1.3), the increase in concentration polarization will result in an increase in salt passage because the salt transport through the membranes is propor- tional to the difference between salinity on both sides on the membranes. Since the salinity at the feed side of the membranes is higher than the salinity in the feed solution, the salt trans- port is proportionally higher as well. This means that as the Beta value increases, the mem- brane salt rejection is reduced and the salinity of the produced freshwater is increased.
Due to the salinity gradient and particulate solids accumulation within the boundary layer the hydraulic resistance of this layer is higher than that of the feed water. As a result the head losses associated with the movement of freshwater through the membranes are increased, and assuming the same feed pressure is applied, the membrane flux is reduced. This flux reduction is the consequence of elevated hydraulic resistance of the membranes caused by concentrate polarization compounds with the flux reduction resulting from the elevated os- motic pressure that has to be overcome by the RO feed pumps.
If salt concentration in the boundary layer exceeds the solubility of sparingly soluble salts (such as calcium carbonate and sulfate) contained in the source water, these salts would begin precipitating on the membrane surface and would form mineral scale. Membrane scaling in turn will result in reduced membrane permeability and flux.
The magnitude of the concentration polarization factor, Beta, is driven by three key factors:
1. permeate flux;
2. feed flow; and
3. configuration and dimensions of feed channels and of feed spacer.
Increase in permeate flux (i.e., freshwater production) increases exponentially the quantity
of salt ions and particulate solids conveyed to the boundary layer and, therefore, exacerbates concentrate polarization and particulate fouling of the membranes. Increase in feed flow, however, intensifies turbulence in the boundary layer and, as a result, decreases the thickness and salt and solids concentration of this layer. Depending on its configuration and geometry, RO-membrane feed/concentrate spacer and feed/concentrate channel may cause more or less turbulence in the concentrated boundary layer and, therefore, may reduce or enhance concentration polarization.
Since feed/concentrate spacer configuration and feed/concentrate channel size are con- stant for a given standard RO-membrane element, permeate flux and feed flow are the two key factors that determine the magnitude of concentrate polarization.
As indicated previously, the ratio between the permeate flow and the feed flow of a given RO-membrane element is defined as the permeate recovery rate of this element. Similarly,