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222 10. COMPARISON OF GRANULAR MEDIA AND MEMBRANE PRETREATMENT
technology evaluation and selection. The following issues have to be taken into consider- ation when selecting between granular media and membrane pretreatment filtration for a specific application.
10.2 EFFECT OF SOURCE WATER QUALITY ON PERFORMANCE
Membrane filtration has a wider spectrum of particle removal capabilities than conven- tional media filtration. Because particulate separation process is based on filtration through a membrane with a fairly uniform pore size, particulate removal efficiency is higher and more consistent than the more randomly porous granular media filtration bed. Single or dual-media filters usually have a lower removal efficiency in terms of raw source water organics in suspended form, disinfection byproduct precursors, fine particles, silt, and pathogens.
Membrane filtration technologies are less prone to upsets caused by seasonal changes in source water turbidity, color, pathogen contamination, and size and type of water particles because their primary treatment mechanism is a mechanical particle removal through fine- pore membranes (Pearce, 2011). Therefore, the upstream chemical coagulation and floccula- tion of the source water particles is of a lesser importance for their consistent and efficient performance.
In contrast, pretreatment performance of granular media filtration systems is very depen- dent on how efficient chemical coagulation and flocculation of the source water are ahead of the filtration process. Therefore, for applications where intake source water quality experi- ences significant seasonal variations and presents a challenge in terms of high level of path- ogens and elevated concentration of fine particles and particulate organics, membrane filtration technologies are likely to offer performance benefits. However, if the source water for the desalination plant is collected from an open intake located far from the tidally influ- enced zone and at adequate depth to be exposed to only limited seasonal variations (typically 10 m/33 ft or deeper), granular media filtration may offer a very cost-effective pretreatment alternative to membrane filtration.
Source water temperature is a very important factor when selecting pretreatment system. Application of vacuum-driven membrane pretreatment systems is usually less cost effective than conventional granular media filtration for source water of temperature lower than 15C (59C) because the productivity (flux) of vacuum-driven membrane filtration is dramatically reduced by the significant increase in viscosity of source water at low temperature (AWWA, 2007).
Another condition under which the use of sand media filtration may have certain addi- tional benefits is when a source water is exposed to sudden and unpredictable changes of spe- cific contaminants such as very high or low pH, chemical spills, oil and grease spills, frequent exposures to very high source water temperature, or contaminants that may damage the membrane filtration (MF) or ultrafiltration (UF) pretreatment membranes irreversibly, if they are used for this application. If the membrane elements are permanently damaged, the cost of their replacement could be significant, especially for large reverse osmosis (RO) desalination plants.