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.

660 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Experience accumulated during the past 16 years in literally millions of sterile filtration runs has indicated that a filter of 0.22-/xm pore size will, un- der all conditions, remove any viable cellular organisms that could pose a spoilage or health hazard (11). Of the smaller strains of problem organisms, Pseudomonas aeruginosa (ATCC 19146) has become the indicator of filter sterilization efficiency. It has a smallest dimension in the order of 0.3 and is completely removed from water using a 0.22-/xm filter. An effective filtration system, currently in use at a large cosmetics manufac- turing plant consists of 12 prefilter cartridges each containing 11 ft 2 of 0.5-/xm (nominal retention) depth filter media and twenty 0.22-/zm (absolute) pore size membrane filter cartridges, each containing 1.3 ft 2 of membrane filter. The inlet and outlet connections are 1% inch Tri-Clamp©.* The maximum flow rate for this system is 115 gal/min. The unit is sanitized nightly using 60øC water. The cost of make-up water filtration at this plant is $2.70 per 1,000 gallons. Raw Materials' Raw materials such as dyes, oils, and petrolatum often contain large num- bers of microorganisms and are difficult to sterilize. Any liquid or soluble ma- terial may be filtered to remove pathogenic or spoilage bacteria. Again, a 0.22-/tin pore size membrane filter is recommended. For example, 40 gallons of soluble dye may be filtered through a single 293- mm diameter 0.22-/zm filter at a cost of approximately seven cents per gallon. Data indicate that filtered dyes do not deteriorate during a three-month stor- age period, whereas unfiltered dyes rapidly deteriorated. Petrolatum and oils are often sterilized by holding at temperatures of 150øC for one hour or longer. In the absence of moisture, the extended time and high temperature are necessary to ensure inactivation of all microorgan- isms. Unfortunately, this sort of treatment causes some degradation and dis- coloration of compounds such as petrolatum. It also necessitates cooling, with the subsequent risk of recontamination, prior to the addition of other com- pounds. A filtration system consisting of a single cartridge housing containing a 0.22-/xm pore size membrane filter cartridge is shown in Fig. 1. This cartridge contains 1.3 ft 2 of membrane and prefilter (depth-type) media. The heating belt provides a temperature of 65øC during the filtration run. Petrolatum is melted in the tank and pumped through the filter, at 35 psig, at a flow rate of 8 kg/min. Lanolin oil at 70øC is filtered at the rate of 7 1./min. at 20 psig. The cost of filtering these compounds is nine cents/kg and nine cents/1. re- spectively. *Ladish Co., Kenosha, Wis.