Cross-Flow Filtration 329 7.4 Production of Bacteria-free Water Bacteria are living organisms composed of a single cell in the form of straight or curved rods(bacilli), spheres(cocci) or spiral structures. Their chemical composition is primarily protein and nucleic acid. Bacteria can be classified by particle sizes in the range of about 0. 2 to 2 um. Some forms of bacteria can be somewhat smaller (-0. 1 um)or somewhat larger, up to 5 um Microfiltration can be an effective means of bacteria removal since the pores of a microfilter are small enough to retain most forms of bacteria while maintaining relatively large flow rates for the transport of aqueous solution across the membrane barrier. [59 the level of bacterial contamination and downstream processing require- ments. The filter ability to retain bacteria is commonly expressed in terms of the Log Reduction Value(LRv. The LRv is defined as the logarithm of the ratio of total microorganisms in the challenge to the microorganisms in the filtered fluid when a filter is subjected to a specific challenge. a0. 2 um filter is challenged with Pseudonomas diminuta microorganisms and a 0.45 um filter is challenged with Serratia marcescens using guidelines recommended by the Health Industry Manufacturers Association(HIMA) Although cross-flow filtration can be effectively used for sterile filtration, dead end filtration can adequately serve these applications when the amount of contaminant is generally small (less than 1000 bacteria/mL) Cross-flow filtration may be more useful when high loads(107microorgar isms/mL)of bacteria are involved requiring removal efficiency with a lrv value greater than 7, 160 At high bacterial loadings, there may be significant membrane fouling and/or concentration polarization which could reduce flux and cause irreversible fouling. At high bacterial loadings, microporou membrane filters operating in the dead end configuration may be limited by low flux and require frequent cartridge replacement due to rapid pore plugging Table 12 shows the typical LRV values obtained using a polymeric and ceramic microfilter, Sterile filtration requires 100% bacteria retention by the membrane, whereas in many industrial bacteria removal applications the presence of a small quantity of bacteria in the filtrate may be acceptable. For example, drinking water obtained by microfiltration may contain nominal counts ofbacteria in the filtrate which is then treated with a disinfectant such as chlorine or ozone. The use of ceramic filters may allow the user to combine thesterile filtration with steam sterilization in a single operation. This process can be repeated many times without changing filters due to their long service life(5 years or longer)Cross-Flow Filtration 329 7.4 Production of Bacteria-free Water Bacteria are living organisms composed of a single cell in the form of straight or curved rods (bacilli), spheres (cocci) or spiral structures. Their chemical composition is primarily protein and nucleic acid. Bacteria can be classified by particle sizes in the range of about 0.2 to 2 pm. Some forms of bacteria can be somewhat smaller (-0.1 pm) or somewhat larger, up to 5 pm. Microfiltration can be an effective means of bacteria removal since the pores of a microfilter are small enough to retain most forms of bacteria while maintaining relatively large flow rates for the transport of aqueous solution across the membrane barrier.[59] The relative efficiency of bacteria removal will, however, depend on the level of bacterial contamination and downstream processing require￾ments. The filter ability to retain bacteria is commonly expressed in terms of the Log Reduction Value (LRV). The LRV is defined as the logarithm ofthe ratio of total microorganisms in the challenge to the microorganisms in the filtered fluid when a filter is subjected to a specific challenge. A 0.2 pm filter is challenged with Pseudonomas diminuta microorganisms and a 0.45 pm filter is challenged with Serratia marcescens using guidelines recommended by the Health Industry Manufacturers Association (HIMA). Although cross-flow filtration can be effectively used for sterile filtration, dead end filtration can adequately serve these applications when the amount of contaminant is generally small (less than 1000 bacteridml). Cross-flow filtration may be more usefd when high loads (>1 O7 microorgan￾isms/mL) of bacteria are involved requiring removal efficiency with a LRV value greater than 7.L6Ol At high bacterial loadings, there may be significant membrane fouling andor concentration polarization which could reduce flux and cause irreversible fouling. At high bacterial loadings, microporous membrane filters operating in the dead end configuration may be limited by low flux and require frequent cartridge replacement due to rapid pore Table 12 shows the typical LRVvalues obtained using a polymeric and ceramic microfilter, Sterile filtration requires 100% bacteria retention by the membrane, whereas in many industrial bacteria removal applications the presence of a small quantity of bacteria in the filtrate may be acceptable. For example, drinking water obtained by microfiltration may contain nominal counts of bacteria in the filtrate which is then treated with a disinfectant such as chlorine or ozone. The use ofceramic filters may allow the user to combine the sterile filtration with steam sterilization in a single operation. This process can be repeated many times without changing filters due to their long service life (5 years or longer). plugging