Page 240 - ro membanes

10.2 EFFECT OF SOURCE WATER QUALITY ON PERFORMANCE 223
As indicated previously, source water naturally contains sharp particles, which could damage MF and UF membranes upon contact (Ransome et al., 2015). To remove these sharp source water particles, the RO plant intake system should incorporate a microscreening sys- tem of mesh size of 120 mm or less ahead of the membrane pretreatment system (see Chapter 5 for more details). Performance and reliability of conventional granular media pretreatment systems are not sensitive to the content of sharp objects in the source water and do not require elaborate and costly microscreening ahead of the filters. Typically, mechanical traveling screens of 3e10 mm openings provide an adequate protection of conventional granular me- dia pretreatment systems.
A particular challenge for seawater pretreatment systems with open intakes could be the content of barnacle plankton in the source water of size small enough to pass through the microscreening system and to grow to adult barnacles on the walls of the wet well of the pumps feeding the pretreatment system. When some of the barnacles enter the membrane pretreatment feed pumps, the shells are broken by the pumps into small sharp particles, which are then pumped (or drawn by vacuum) against the membrane fibers causing occa- sional punctures and over time resulting in the loss of membrane integrity and performance. Typically, such challenges can be addressed by matching the size of the microscreens up- stream of the membrane pretreatment system with the size of the smallest barnacle (or other shellfish) plankton species that may occur in the saline source water. This issue warrants a detailed year around investigation as the size and type of plankton species contained in the source water typically change seasonally. If such a challenge is likely to occur for a partic- ular project, it may be prudent to consider the installation of cartridge filters downstream of the membrane pretreatment system, which would be able to capture particles and other im- purities that may pass through the punctured pretreatment fibers.
Because installation and operation of a microscreening system downstream of source wa- ter intake screens is only needed if membrane filtration is used for pretreatment, the cost for microscreening of the source water should be taken under consideration when comparing conventional and membrane filtration pretreatment.
On the other hand, the use of membrane pretreatment would eliminate the need and costs for installation and operation of cartridge filter system ahead of the RO feed pumps. Car- tridge filters are needed when granular filtration system is used for pretreatment to protect the downstream RO membranes from damage caused by fine sand particles, which may be conveyed occasionally with the pretreated water. The size of MF and UF membranes is usually several orders of magnitude smaller than that of cartridge filters, and therefore, if the pretreatment membrane integrity is maintained intact, cartridge filtration would not be necessary. However, if the membrane pretreatment system employs low-quality MF or UF membranes, which have limited durability and could easily break during hydraulic pressure surges or get punctured by shells due to inadequate prescreening of the source water, the use of cartridge filters is desirable and recommended.
Occurrence of frequent and prolonged algal blooms in the area of the source water intake is another important factor to consider when selecting the type and configuration of source water pretreatment system. As indicated previously (see Chapter 7), many of the marine microalgae, which grow excessively during algal blooms, cannot withstand external pressure of more than 0.3e0.6 bars (4e8 psi) and their cells could break when exposed to pressure or vacuum-driven MF or UF filtration.