February 7, 1977 QUALITY 0F WATER BRANCH TECHNICAL MEMORANDUM NO. 77.04 Subject: WATER QUALITY: Filtering of Water Samples Current standard practice for collection of samples for water quality analysis requires field filtering of samples through a 0.45 micrometer membrane filter for the determination of most dissolved inorganic constituents. Geochemical equilibrium studies commonly require field filtering through filters with smaller openings. Several methods have been employed for field filtering in the past. The current and most popular method utilizes the barrel-type filter that was designed by M. W. Skougstad, and which is currently available from the Leonard Mold & Die Works (LM&DW) in Denver, Colorado. A new method, which has been developed by WRD researchers at Menlo Park, California, utilizes a backflushing plate filter. The filter is similar to the LM&DW in-line filter (the barrel filter without the barrel), except that it allows backflushing to remove excessive sediment from the filter. The backflushing filter uses a 142 millimeter membrane filter, which provides a 94% larger filtering area than the 102 millimeter filter used in most barrel filters. The unit has been in limited use in several districts as well as in the research program during the past few years and has been found to be a reliable and efficient filtering system. The backflushing capability and larger filter size enable more water to be filtered before clogging necessitates the replacement of the membrane filter. The backflushing filter commonly is used in conjunction with a reversible peristaltic pump. Peristaltic pump heads can be obtained from many laboratory supply houses. Some sources offer them with drive units capable of operating from various power sources. Twelve volt DC, reversible, variable-speed pumps are under development by some companies and should be available in the near future. The backflushing filters have been used with compressed nitrogen gas as the driving force for the water sample, but generally those methods do not provide for easy backflushing capabilities. Proper operation of the unit is described in USGS Open-File Report 76-126, "Backflushing filter for field processing of water samples prior to trace-element analyses", by V. C. Kennedy, E. A. Jenne, and J. B. Burchard (copy attached for district offices). The Quality of Water Branch approves the use of this type of system for routine field filtration of water-quality samples to be submitted for inorganic chemical analysis. Some modifications to the backflushing filter have been suggested and approved for routine field work. They are: (1) replacing the nylon bolts and nuts with stainless steel swing-out bolts and wing nuts, (2) use of Vitron o-rings instead of silicone rubber o-rings and, (3) installation of shorter, stockier legs. Researchers should consider whether these modifications will have an adverse affect on their experimental results before adopting them. It is not the intent of the Branch of require the use of this filtering method for all field work, but rather to make field offices aware of the availability of this system and the Branch's evaluation of it. You may continue to use other approved filtering equipment. You are reminded that the barrel filter or other gas pressure filtration devices, when used, should be equipped with a pressure-relief safety valve to prevent over- pressurization. The need for this safety precaution was emphasized recently by a tragic accident in the field. The filters should be operated only with a clean, inert gas like nitrogen in order to avoid inducing changes in the sample before analysis. The non-backflushing plate filters that are available from several manufacturers are also very useful when working with waters having low concentrations of suspended sediment. A Quality of Water Branch Technical Memorandum describing acceptable field filtering techniques and equipment is in preparation and will be issued within the next few months. Presently the Branch knows of only one company, Leonard Mold & Die Works, Denver, Colorado, that is manufacturing the backflushing plate filter, as designed by Mr. Kennedy, and offering them for sale. As other sources become available we will pass the information on to you. R. J. Pickering Attachment WRD Distribution: A, B, S, PO (w/o attachment) FO-L (w/attachment)