[0001] The present invention generally relates to filtertechnology and, more specifically, to filtering units hav-ing flat filter members, such as membrane filters, depthfilters or prefilters used in laboratory or research facili-ties.[0002] A wide variety of filtering apparatus exists forseparating particles from liquid. The choice of whichtype of filtering apparatus touse ina given applicationdepends mainlyon particle size and throughput param-eters. Generally, particles ofa size greater than 10gmmay be separated from liquids usinga macrofiltrationprocess, typically involving the use of depth filters. Thedepth filters used in macrofiltration may be madeofpa-per, asbestos, plastic or glass fiber. Macrofiltration is fre-quently used in series with microfiltrationto remove larg-er particles priorto the microfiltration process. This typeof multi-stage filtering process can increase the rate atwhich liquid can be passed througha membrane filteror, in other words, increase the throughput rate. In mi-crofiltration, particles greater than about200 Angstromsbut less than about 100gm are separated froma liquidtypically usinga membrane filter as a final filter.[0003] Conventional membrane filters are much thin-ner and more fragile than depth filters and are typicallyavailable ina wide variety of particle retention capabili-ties and ina variety of materials, shapes and sizes.Membrane filter material may be, for example, nylon,polyethersulfone, PTFEor celluloseacetate. Membranefilters are available generally as either thin discs or inafolded cartridge configuration. Cartridge filters are usu-ally recommended for more quickly filtering large vol-umesof liquid, such as volumes greater than about400liters. These operations may be carried out withthe car-tridge filter attached in-line and with the liquid flowingfrom one end ofthe cartridge to the other.[0004] Often, laboratory or research facility needsmay be met using disc-shaped membrane filters havinga diameter of, for example, 50 mm to 75mm. These fa-cilities may not require the throughput rate provided bya cartridge filter, but high throughput is still desirable inorder to carry out the filtering procedure as quickly aspossible. Larger diameter membrane filters providealarger throughput rate due to their increased surface ar-ea, however, the increase in throughput rate comes ata much greater incremental cost. Therefore, the largerdiameter membrane filter units can be undesirable fromthis standpoint. Also, these types of filtering units typi-cally come preassembled with the disc-shaped mem-brane filter permanently affixed beMeen upperand low-er containers or, alternatively, in an unassembled con-dition. In this latter case, the user must handlethe frag-ile, disc-shaped membrane filter during assembly. Eachof these options has disadvantages in that the pre-as-sembled filtering unit does not allow versatility in the se-lection of filter size or pore size following purchase bythe user. On the other hand, requiring the userto handle10203040502in damage or contamination. Finally, increasing thethroughput rate by prefiltering the liquid is not easilyac-complished as it generally necessitates the use of mul-tiple filtering units.[0005] To address various problems in this area, suchasthe problems mentioned above, it wouldbe desirableto providea filtering unit having an easily assembledandreplaceable filter, such asa membrane filter, and a moreefficient and less costly manner of providinga greaterthroughput rate while retaining the ability to use smallerdiameter filters.A filtering unit witha flat filter memberheld in an upper container for filtering liquid to be col-lected ina lower container is known from DE 3711735.f5[0006] The present invention therefore providesa fil-tering unit includinga first fluid receiving member, suchas an upper container, for receiving fluid to be filteredand a second fluid receiving member, such asa lowercontainer, for receivingthe fluid fromthe upper containerafter the filtration process has taken place. In accord-ance withthe invention,a filter cassette includinga sup-port member connected witha flat filter member is con-figured for disposition between the fluid to be filtered inthe upper container and the lower container. The filter25cassette is separateand may be removable with respectto each ofthe upperand lower containers. The cassetteis capable of forminga selectively engageable seal be-tweenthe fluid in the upper containerand the lower con-tainerto ensure that fluid in the upper container passesthrough the filter member before passing into the towercontainer. Preferably,a threaded connection is formedbeMeenthe upperand lower containers. This allowsthelower containertobe separated fromthe upper contain-er aftera filtering operation and thereby allows the con-35tentstobe stored or used in further processing. Ina pre-ferred embodiment, the flat filter membermay includeamembrane filter and/ora prefilter or depth filter. Thesefilter members may be made from conventional materi-als as discussed above. The use of both of these typesof filter members can allowa multi-stage filtering oper-ation to be performed witha single filtering unit.[0007] Specifically, for multi-stage filtering processes,a plurality of the separately attachable filter cassettesmay be connected ina serial fashion beMeenthe upper45and lower containers. As an option,a single filter cas-sette could be constructed withMo different filter mem-bers, such asa prefilter or depth filter and a membranefilter connected in series. As still another option, one fil-ter membermay be permanently affixed to the filteringunit while anothermay be partofthe inventive filter cas-sette. These types of multi-stage filtering unit may beusedto increase throughput volume in an easy, efficientand relatively low cost manner.[0008] The lower end ofthe upper container prefera-55bly includesa stepped-in portion for centering and en-gaging the peripheral sealing member ina liquid-tightmanner duringa filtering operation. This unique config-uration, coupled with the use ofa peripheral, selectively
