658 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 80øC) are used to reduce the population of gram-negative organisms. Theo- retically, a temperature of 80øC held for 30-45 minutes will kill all vegetative organisms, but not spores. In practice, not even all the vegetative organisms are killed, for there are always pockets of cooler water where microorganisms survive. Condensation droplets falling back into the tank are another source of recontamination. Filtration is often used to achieve sterile or low-count make-up water. Sev- eral filter materials are available that have been termed "bacterial retentive." However, only a filter that removes 100% of all bacteria can be considered a true sterilizing filter, for even if a few viable organisms penetrate, they can soon reinfect an entire filtered batch. Thus, success in this critical procedure hinges on selection of the ideal filter properties. Filter media may be grouped into two broad categories, depth or screen. Screen filters have uniform pore openings and retain particles on their sur- faces, primarily by a sieving mechanism. Examples of screen filters are woven wire mesh or photoetched screens. The most useful characteristic of a screen filter is that pore openings can be controlled during production with such pre- cision that it is possible to specify precisely the maximum size of particles which will pass through the filter. A screen filter is an absolute filter it will retain 100% of all particles larger than its maximum pore size. Depth filters consist usually of fibers, particles, or other fragmented materi- als that have been pressed or bonded together to form a tortuous maze of flow passages. Examples are asbestos pads, ceramic candles, porous sintered glass, and complex laminates of asbestos impregnated with a resinous bonding agent to increase porous density. Although the flow channels in a depth filter can be made coarse or fine, de- pending on the degree of compaction or the size of the fibers, pore size dis- tribution is inherently random and therefore unpredictable. The principal mechanisms of retention are random adsorption and entrapment. Therefore, a filter that is 99% efficient at retaining 0.5-/xm organisms may also retain many that are much smaller, while at the same time passing a substantial number of particles and microorganisms larger than 1 it/All. For very small particles and most microorganisms, adsorption is the most important retention mechanism, since some flow channels invariably are large enough to let them pass. But, because the molecular forces of adhesion are usually strong enough to offset the small viscous drag at the laminar layer of the fluid boundary, a microorganism will adhere if it comes in contact with the filter medium. Unfortunately, some filter manufacturers attempt to assign "absolute" rather than "nominal" ratings to depth filters. This is contradictory because these fil- ters cannot, by their very nature, be absolute even though they may appear so under certain carefully limited conditions (8). But if flow conditions are al-

MICROBIAL CONTROL 659 tered by filter clogging or "channeling" (fibers parting under pressure), or if the filter is shocked by the opening or closing of a valve, particles and micro- organisms may be dislodged and dragged into the effluent. Grow-through of organisms trapped in a depth filter matrix is also a prob- lem. If conditions are favorable, microorganisms will reproduce within the filter and successive generations will penetrate the filter. This is most likely to occur during long filtration runs. Because contaminants become trapped throughout the matrix of a depth filter, its dirt-holding capacity is relatively high. This is an advantage that manifests itself either in long life or in the ability to handle highly contaminated fluids. In this respect, depth filters complement the screen filters, which have a more limited dirt-holding capacity owing to surface collection only. Depth and screen filters are therefore usually used together in high-volume filtration applications-the depth filter as prefilter, and the screen filter as an absolute filter. Functioning as a prefilter, the depth filter removes most of the burden of contaminants and the screen filter then removes the comparatively small number which inevitably traverse the depth filter. A properly selected depth filter will greatly extend the life of the screen filter, thereby cutting the costs of absolute microfiltration. The only true screen filter of practical interest to the fluid processor con- cerned with the removal of bacteria is the membrane filter (9). Membrane filters are thin, porous sheets of pure, polymeric substances. The surface of a membrane filter contains millions of precisely uniform pores per square centi- meter. The high porosity and ability of membrane filters to withstand high pressures enable them to handle flow rates an order of magnitude greater than those possible through a sterilizing grade depth filter. Grow-through is not a problem since the pores are chosen to be smaller than the collected microorganisms and their succeeding generations. Another unique advantage of the membrane filter is its ability to be "integ- rity" or bubble tested (10). Unlike conventional filters, a membrane has a capillary pore structure. If the pores are filled with liquid, the amount of air pressure required to overcome the surface tension is related directly to ' pore size. Therefore, the inlet pressure at which air bubbles will appear in an outlet tube submerged in water (bubble point) is a reliable, nondestructive means of detecting imperfections in the assembled filter or faulty assembly of the holder. A "bubble test" may be run immediately before, during, or after filtration as an in-process control for checking the efficiency of filtration. It is somewhat analogous to the measurement of temperature to ensure the effectiveness of heat sterilization. Other methods, such as depth filtration or uv light, require 24 hours for verification of effective processing. This is done by microbiologi- cal examination, during which time the water is held in storage.