Virus removal filtration systems

In terminal areas, the air volumes are much greater, and typical HVAC filtration systems do not remove aerosol particles from the air as efficiently as do aircraft Environmental Control Systems (see Chapter 2). Thus, the costs and benefits of various enhanced filtration and air-cleaning strategies would have to be carefully assessed. An ancillary benefit to be considered would be the reduction of the transmission of common ills such as cold and flu viruses (or more serious viruses, such as the severe acute respiratory syndrome [SARS] virus) among airport patrons. 

Disinfection alone, or a combination of disinfection and filtration, can achieve the minimum mandatory removals and/or inactivation of 99.9% Giardia cysts and 99.99% enteric viruses. Primary disinfection systems that use ozone, chlorine, or chlorine dioxide can achieve greater than the above-stated inactivation of enteric viruses when 99.9% inactivation of Giardia cysts is attained. Therefore, achieving sufficient Giardia cyst inactivation can ensure adequate inactivation of both types of organisms. This is not the case, however, when using chloramination because it is such a poor virucide. 

The DLVO-Lifshitz theory should be regarded as a principal mechanism governing the adsorption of viruses on various inorganic surfaces. This finding has direct application to problems concerning transport of viruses in aquatic systems and soils. It is possible that it could lead to the design and optimization of adsorption-filtration processes for removing viruses and other particulates from contaminated water. 

Bearing in mind the previous discussion, there are basic official or unofficial sterilization procedures, all of which are overkills, designed to kill or get rid of the very last and most resistant organism in the system being treated. Filtration is, of course, designed to physically remove all bacteria present. It does not usually remove viruses or mycoplasms and, as noted above, some of the assumptions made during a filtration process need to be very carefully evaluated by the operator. 

Microfiltration. Various membrane filters have been used to remove viral agents from fluids. In some cases, membranes which have pores larger than the viral particle can be used if the filtration is conducted under conditions which allow for the adsorption of the viral particle to the membrane matrix. These are typically single-pass systems having pore sizes of 0.10—0.22 m. Under situations which allow optimum adsorption, between 10—102 particles of poliovirus (28—30 nm) were removed (34—36). The formation of a cake layer enhanced removal (35). The titer reduction when using 0.10—0.22 Jim membrane filters declined under conditions which minimized adsorption. By removal standards, these filters remove viruses at a rate on the low end of the desired titer reduction and the removal efficiency varies with differences in fluid chemistry and surface chemistry of viral agents (26). 

In the last few years, a third type of microfiltration operating system called semi-dead-end filtration has emerged. In these systems, the membrane unit is operated as a dead-end filter until the pressure required to maintain a useful flow across the filter reaches its maximum level. At this point, the filter is operated in cross-flow mode, while concurrently backflushing with air or permeate solution. After a short period of backflushing in cross-flow mode to remove material deposited on the membrane, the system is switched back to dead-end operation. This procedure is particularly applicable in microfiltration units used as final bacterial and virus filters for municipal water treatment plants. The feed water has a very low loading of material to be removed, so in-line operation can be used for a prolonged time before backflushing and cross-flow to remove the deposited solids is needed. 

The performance of each filter type depends on the quality of the influent and proper design and operation. The range of influent characteristics for which various filters are effective has been provided by the US EPA (3) in Tables 5 and 6. According to the two tables, DE filtration is an established process mostly for small water systems with good influent quality (less than 5 NTU turbidity, less than 5 color units, and less than 50/100-mL conform count) and low influent capacity (below 100 MGD). The removal capacities for Giardia cysts and viruses of the above seven filter systems are presented in Table 7. It is important to note that DE filtration is better than conventional filtration and direct filtration in terms of removal efficiency of Giardia cyst and viruses. 

The physical process of filtration is rather effective in removing pathogens from water. During the 1800s, before chlorine came into widespread use, filtration with simple sand filters employed in just a few cities cut down significantly on the incidence of waterborne cholera in those cities. With modern membrane technology (see Section 5.10), ultrafiltration can remove even viruses from water. Small amounts of chlorine or chloramines (see Section 5.11.3) can be added to maintain sterile water in distribution systems, but much less of these agents are required for membrane-filtered water than are required for total disinfection. 

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