Water Sample Filtration Unit Marvin W. Skougstad and George F. Scarbro, Jr. U. S. Geological Survey, Denver, Colo. 80225
B A readily portable, all plastic, pressure filtration unit is
described which greatly facilitates rapid micropore membrane field filtration of up to several liters of water with a minimum risk of inorganic chemical alteration or contamination of the sample. The unit accommodates standard 10.2-cm. (4-inch) diameter filters. The storage and carrying case serves as a convenient filter stand for both field and laboratory use.
R
eported chemical-quality characteristics of water samples normally include only those substances which are in solution at the time of analysis. This implies that all suspended particulate matter has been removed from that portion of the sample from which aliquots for analysis are taken. Turbid samples obviously require a preliminary separation of the suspended material. Many apparently clear samples, however, may contain a surprising amount of filterable material. Questions may arise as to the certainty of removal of particulate matter prior to analysis and, equally important, as t o the criteria used t o distinguish particulate material from substances in true solution. While these questions may be of little significance when only the major constituents are determined, they are of real concern when the sample is analyzed for minor or trace constituents. A small amount of finely dispersed particulate material may contribute the greater part of the total determined amount of a given trace constituent. The removal of suspended solids may be accomplished by any one of a number of techniques, and there is presently little uniformity among water analysis laboratories as t o the method used. Slow settling, centrifuging, and filtration are all commonly used. Within recent years, however, micropore membrane filters of controlled, uniform pore size and consistent high quality have become commonplace in most analytical laboratories. As a consequence, many analysts now remove suspended particulate matter from water samples by filtration through micropore membrane filters having an average pore diameter of 0.45 micron. Such practice, indeed, has much t o recommend it. The accurate control of pore sizes in the membrane material assures consistently reproducible removal of particulate matter from the sample. Filtration 298 Environmental Science and Technology
rates are reasonably good because of the high ratio of pore area to total filter area. Although the membrane filters are somewhat fragile, they are sufficiently durable so that, with reasonable care, they can be easily handled. The choice of a membrane filter of an average pore diameter of 0.45 micron permits a reasonable and practical distinction between true solute material and material that may be considered particulate or not in true solution. From the foregoing, one can infer the desirability of establishing a standard water sample filtration procedure using a micropore membrane filter of 0.45-micron average pore size. If a standard procedure were followed, the users of waterquality data could be assured that exactly comparable samples had been analyzed by all laboratories. One additional important factor must be considered. In the process of collecting and analyzing a water sample, significant chemical changes in the sample may occur during the time interval between collection of the sample and its later analysis in the laboratory. Such changes may involve both an increase in the amount of dissolved material, as occurs when finely divided solids are slowly dissolved by the solvent or solution, and a decrease in dissolved material, as when precipitation occurs. For this reason, it is desirable t o filter each sample at the time it is collected. If this is done, any solids which have appeared in the sample by the time the analysis is begun can only be concluded t o have not been present as such in the original sample. Such material must have been in solution at the time of collection and, therefore, should be included in the analysis and be reported as a part of the total solute concentration of the original sample. Depending upon the determinations t o be made, any such solids may be either redissolved or uniformly dispersed throughout the sample before sample aliquots are withdrawn for analysis. Immediate filtration of the sample at the time of collection also eliminates, or at least greatly minimizes, chemical and solvent action between solution and solids. Hence, it would seem desirable t o standardize not only the filtration technique for removal of particulate matter from water samples prior t o analysis, but also the entire sample collection operation, even t o the extent that the filtration be performed immediately at the time the sample is collected. Admittedly, even such a procedure has limitations. It does, however, eliminate many of the uncertainties encountered in
any attempt t o provide a water sample which reliably represents conditions at the time of collection.
Field Filtrcition Heretofore, micropore membrane field filtration of the several liters of water frequently needed for a complete chemical analysis has been neither a convenient nor a simple operation. The usual glass micropore-filter funnel and suction flask arrangement accommodates only a comparatively smalldiameter filter disk, making the filtration of large volumes, particularly of turbid samples, a tedious and slow process. Glass apparatus items are easily broken when used in the field, and metal funnels and holders cannot be used when minor constituents are determined because of the possibility of contamination of the sample. Even stainless steel and monel tnay contribute detectable amounts of metallic contaminants. Finally, vacuum filtration is neither efficient nor practical when the operation must be accomplished in the field away from a good vacuum supply. A pressure filter can be more easily operated in the field and is much more effective. This report describes the design and use of a portable allplastic pressure filter which is uniquely suitable for the field filtration of up to several liters of water. Since it is constructed entirely of inert materials, there is a minimum risk ofinorganic contamination or alteration of the sample. It is lightweight and therefore readily portable. Moreover, it is strong and durable and is not easily damaged or broken, even under hard field use. Filtration pressures up to 65 pounds (p.s.i.g.) can be applied with adequate margin of safety. The unit accepts a standard 10.2-cm. (4-inch) diameter filter disk, and although designed especially for micropore membrane filters, any similar material can be used. A filter of this diameter permits reasonably rapid filtration of all but the most turbid samples and is a compromise between a larger diameter filter, which would speed the filtration of turbid samples but would also be more costly and would necessitate a heavier, more bulky apparatus, and a smaller diameter filter, which would lack the necessary capacity t o handle large volumes.
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Figure 1. Pressure filter-parts
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and construction
Description The filter unit (Figure 1) consists of three separable sections: a bottom flange and filter support, A , a sample reservoir, B , and a cover, C . When assembled and fastened with nylon Volume 2, Number 4, April 1968 299
thumb screws, I , the two silicone O-rings, 2, 2', provide a pressure-tight seal. The micropore membrane filter, 3, is supported on a porous polyethylene disk, 4 , set in a recess, 6, cut in the bottom flange section, IO. This disk must fit with perfect smoothness into its recess in the flange t o provide a n even, uniform surface for the support of the membrane filter. Any irregularity will cause rupture of a wet membrane filter when pressure is later applied. The drain stem, 5, is a short length of polyvinyl chloride pipe, cut and threaded into the bottom flange section, IO. The sample reservoir, B , consists of a section of a lucite cylinder of 6.4-mm. (l/,-inch) wall thickness cemented into shallow recesses cut into the upper, 8, and lower, 9, lucite flanges. The reservoir (12.5-cm. I.D. X 20.5 cm. long) has a capacity of about 2.5 liters-ample for the collection of a filtered 2-liter sample with a liberal allowance for prerinsing of both the apparatus and the collection bottle. The dimensions of the reservoir may be adjusted according to individual needs or preference, although probably very little would be gained, for most applications, by any considerable deviation from the dimensions given here. Note that the inner surface of the lower flange, 9, of the reservoir is tapered t o a 1.9-cm. ("*-inch) hole in its center. This permits complete drainage of the sample from the reservoir and, more important, provides a protective baffle which prevents damage to the fragile filter disk when sample is poured into the reservoir. The underside of this flange is also tapered toward the center hole t o facilitate escape of air bubbles trapped above the filter disk. A removable lucite flange, 7, is provided to simplify the addition of sample and for ready access to the reservoir for cleaning. A suitable length of rubber pressure tubing is attached at the polyvinyl chloride hose-connector fitting, I I , and an ordinary automobile tire valve is sealed into the opposite end of the pressure tubing to permit easy attachment of a pressure pump. The tire valve also serves as a necessary pressure release valve for relieving the interior pressure before disassembling the unit or removing its cover. Only thoroughly cleaned pressure tubing should be used t o avoid contamination of the sample. A small air pump, such as is used to inflate volleyballs, will provide adequate pressure for rapid filtration of most samples. Moreover, its small, handy size makes it particularly suited for field use since it can be fitted into the filter carrying case along with the filter unit. When an athletic air pump is used, a short length of adapter hose containing a a screw fitting for attachment to the tire valve must be provided. Alternatively a regular or heavy-duty automobile tire pump may be used t o obtain the necessary filtration pressure. Whereas the athletic air pump easily produces a working pressure of 40 t o 50 pounds (p.s.i.g.), a tire pump will provide up t o about 65 pounds (p.s.i.g.). The filtration unit may be operated at pressures up to 85 pounds (p.s.i.g.) with complete
300 Environmental Science and Technology
safety. Because the maximum pressure that can be obtained even with a heavy duty tire pump is less than 85 pounds, the operator is always assured of safe operation. Hence, when the filter unit is used normally in this manner, a safety valve or pressure gage is unnecessary. Incorporating these items would only make the simple unit unduly complex and would significantly increase its construction cost. Indeed, a pressure gage would be intolerable if attached to the unit itself, because of the sample contamination that would certainly result. A word of caution is necessary, however: Whenever the filter is operated from any pressure source that may exceed 85 pounds (p.s.i,g.), such as from a cylinder of compressed air or nitrogen, or from a high pressure air line, it is absolutely essential that adequate precautions are taken to make certain that safe operating pressures are never exceeded. Normally, this requires that a pressure regulator and a pressure indicating gage be installed ahead of the filter unit. In actual practice, working pressures in excess of 65 pounds (p.s.i.g.) are of no advantage when filtering turbid samples. Once the filter membrane becomes caked with accumulated residue and the filtration rate becomes excessively slow, a further increase in pressure only compresses the filter cake and further restricts the flow of filtrate. At this point it is best to interrupt the filtration and replace the filter disk with a new one. Operution The carrying case (Figures 2 and 3) protects the filter unit when not in use and serves also as a convenient filter stand for field filtrations. When the case is placed on end and the plastic plug, I 2 , is removed from the top opening, 13, the filter unit may be placed firmly on the case with its drain stem, 5, extending through the opening in the case and into the neck of a sample-collection bottle. A sliding shelf, I5, may be inserted into any one of three positioning slots, 14, to accommodate 1-, 2-, or 4-liter, narrow-mouthed polyethylene bottles. Use of the pressure filter is straightforward. Careful attention to a few details, however, will ensure satisfactory operation and filtration. The low wet strength of membrane filters makes them susceptible t o tearing during assembly of the unit. For this reason, it is essential that the polyethylene filter disk support, 4 , the entire bottom flange section, I O , and the O-ring, 2, be thoroughly dry when a membrane filter is placed in position. Drying of these parts may best be accomplished by wiping and blotting with a clean towel or absorbent tissue. The unit can be assembled and a pressure-tight seal achieved by drawing the six nylon thumb screws to finger tightness. The three screws on each of the upper, 7 , and lower, 9, flanges should be tightened evenly and firmly to avoid undue strain and distortion and to ensure uniform pressure on the O-ring seals. Forceful tightening of the thumb screws is not
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Figure 2. Storage and carrying case
necessary and should be avoided. Since the seals are readily visible through the clear plastic flanges, a quick inspection indicates whether or not the O-rings are uniformly sealed. Disassembly of the unit or removal of the cover is best accomplished after the air pump has been disconnected and the pressure inside the unit has been released. The authors recommend that the first three 50- to 75-ml. portions of filtrate he discarded, using them only t o rinse a newly installed filter, the filter funnel itself, and the receiving bottle. It is desirable, whenever possible, to rinse the reservoir with one or two portions of the sample before filling it with the solution to be filtered. With these precautions and by thoroughly rinsing the apparatus before beginning the collection of a filtered portion of a sample, the possibilities of sample contamination are greatly minimized. Lucite is a moderately soft plastic material and is easily scratched. Therefore, reasonable care should he used in handling and cleaning the filter unit. Abrasive cleaners should not be used, and all sediment residues should be removed immediately by flushing and thorough rinsing. Organic solvents which attack the plastic should, of course, be avoided. Mild detergent solutions or dilute mineral acids may be used without damage t o the filter unit. With proper treatment, the unit will give long and satisfactory service. Perfurmnnce
A 0.45-micron average pore diameter membrane filter effectively removes particulate matter of dimensions exceeding 0.45 micron. Indeed, many smaller particles are either trapped within the interstices of the membrane, held to the membrane by electrostatic forces, or retained on the surface as the residue accumulates. The over-all range of sizes of particulate matter removed depends on the amount and the nature of the suspended material present, and there can he no sharp distinction between the size of particles retained and passed. Although no significant amount of material greater than 0.45 micron appears in the filtrate, at least some material of smaller size is also removed. Field filtration rates using the pressure filter are at least 10 times faster than those obtained with a suction filter operated from either a hand pump or an automobile exhaust manifold. I n many cases, use of the pressure filter makes practical the field filtration of samples which otherwise would require a n unreasonably long filtration time. As an example, one 200..ml. sample which required more than 2 hours to filter by suction filtration was filtered in less than 10 minutes using the pressure filter and a hand pump. When field filtration is necessary, it is estimated that a savings of at least 50% in sample collection costs are realized. Figure 3. Field filtration
Received fur review December 13, 1967. Accepted Februury 23,1968. Voliime 2, Number 4, April 1968 301
