Page 223 - ro membanes

206
9. MEMBRANE FILTRATION
TABLE 9.3
Temperature
5 10 15 20 25
Temperature Correction Factor
( C)
Flux Correction
(% Increase/Decrease)
55 30 15
0 10
The most prudent approach to determine the effect of algal bloom related biofouling on the design of the membrane pretreatment system is to complete pilot testing during the time of the year, when algal blooms are most likely to occur and are at their highest intensity.
9.6.3 Temperature
Saline water viscosity increases with the decrease of temperature. Viscosity affects the membrane ability to produce filtered water as more pressure (or vacuum for submerged sys- tems) is required to overcome resistance associated with flow across the membrane surface area and through the membrane pores when operating at a constant flux (i.e., producing the same filtered flow). Typically, average design membrane flux is established for average annual temperature, flow, and turbidity and is then adjusted for the minimum monthly average temperature using the correction factor shown in Table 9.3.
For example, if the flux determined for an average annual conditions is 80 lmh, and the average annual temperature is 20C, but the minimum monthly average temperature is 15C, then the design flux should be reduced by approximately 15% (i.e., down to 68 lmh) for the membrane pretreatment system to be able to produce the same filtrate flow during all months of the year at approximately the same recovery and power demand.
The correction factor presented in Table 9.3 is a “rule of thumb” based on practical expe- rience and it may vary from one membrane product to another. Most membrane manufac- turers have recommended capacity compensation factors for their system and should be consulted when such a factor is selected for the site-specific conditions of a given project. The most prudent approach to determine the effect of temperature/viscosity increase on the design of the membrane pretreatment system is to complete pilot testing during the cold- est month of the year.
9.6.4 Experience With Existing Installations
At present, less than 10% of the desalination plants worldwide have membrane pretreat- ment and this type of pretreatment had modest success in terms of performance benefits and cost efficiency. Although the use of UF and MF membranes for saline water pretreatment to date is fairly limited, over 20 years of such full-scale experience exists for fresh water filtration applications. An operations survey completed at 10 fresh drinking water treatment plants in