WATER QUALITY: Field filtering of water samples for chemical analysis

                                                  April 5, 1978


Subject: WATER QUALITY: Field filtering of water samples for 
         chemical analysis

Technical reviews of District water-quality activities have 
revealed that a number of types of filtering devices and pressure 
sources are being used in the WRD to filter water samples. In most 
instances, correct procedures and equipment are being used; 
however, upon occasion incorrect procedures and equipment likely 
to result in sample contamination or possible injury to personnel 
have been encountered (see WRD Memo's Nos. 77.29 and 77.51).

The purpose of this memorandum is to establish guidelines for 
filtering water samples for chemical analysis. Biological sample 
filtering guidelines already have been established and are 
presented in "Methods for Collection and Analysis of Aquatic 
Biological and Microbiological Samples" (Greeson, P. E., and 
others, 1977, TWRI, Book 5, Chapter A4).


By Federal interagency convention, the standard mean pore diameter 
size of filters used for separating water and sediment for 
chemical analysis is 0.45 micrometer (um). A prefilter with a 
larger pore diameter size may be used for samples containing large 
amounts of suspended materials; however, the final filter pore 
diameter size must be 0.45 um when the results of the chemical 
analysis are to be stated in terms of "dissolved" or "suspended" 
chemical constituents in reports or in the WATSTORE or STORET data 
systems. Thus, it is imperative that data resulting from analyses 
of samples passed through filters with pore diameter sizes other 
than 0.45 um be qualified separately in WRD publications to 
distinguish them from standard data. Such data should not be 
published in annual basic data reports unless they are clearly and 
adequately qualified, and the data cannot be stored in WATSTORE 
under the parameter codes now in use for the standardized data. 
These instructions should not be construed to discourage the use 
of finer filters where required for geochemical solubility 
studies. The Quality of Water Branch is currently investigating 
ways of storing data resulting from use of filters with pore sizes 
other than 0.45 um.

Membrane filters made of mixed cellulose acetate and nitrate 
(Millipore HAWP, or equivalent) and polycarbonate film (Nuclepore, 
or equivalent) are recommended for processing water samples for 
inorganic analysis. The most common prefilters, when used, are the 
glass-fiber type (Gelman type E, or equivalent), having a mean 
pore size range from 1 to 5 um. It is recommended that the use of 
prefilters be kept to a minimum to reduce the possibility of 
contamination and sample alteration resulting from the possible 
presence of wetting agents and the ion exchange capacity of the 

Most filters are known to leach contaminants in small quantities 
and some exhibit ion exchange properties for some of the 
substances in water; therefore, care must be taken to thoroughly 
equilibrate the filter with sample water when the filtrate is to 
be analyzed. Generally, 100-250 milliliters of sample passed 
through the filter is sufficient for equilibration. This rinse 
water should then be discarded and an additional quantity of 
sample filtered to provide the filtrate for chemical analysis.


Basically there are three types of filtering assemblies available 
for general purpose filtration work; plate-, funnel-, and 
pressure-type systems. Whichever type is used it should be 
constructed of nonmetallic materials such as plexiglass (lucite), 
polyvinylchloride, teflon, or polycarbonate. For some filtrating 
applications, for example when trace metal analyses are not 
required, the filter assemblies can be made of high grade 
stainless steel. Internal components of filtering assemblies, like 
the support screens, should be made of noncontaminating materials 
such as nylon, polyester, teflon, or high grade stainless steel. 
However, stainless steel should not be used where it will come in 
contact with water that is to be analyzed for trace metals. 
Gaskets and o-rings should be made of silicone, teflon, or Buna-N 
materials. Silicone and teflon are the preferred materials and are 
especially recommended for high precision/low concentration 
research-type work. Nylon or high grade stainless steel are 
recommended for filter clamps and bolts.

The plate filter is the recommended filter assembly. It consists 
of two retainer plates with the filter, filter screen, and 
prefilter sandwiched between. When used the complete system is 
filled with liquid and no trapped compressed air is present. Plate 
filters are available in a variety of sizes and configurations. 
Normally a size from 102mm to 293mm diameter is used. Some smaller 
sizes such as the 47mm type can be purchased as complete 
disposable assemblies ready for immediate use. The larger types 
are designed for quick filter change capability without undue loss 
of sample. Some types feature a backflushing operation which 
enables repeated use of the same filter membrane in heavy sediment 
laden waters. For further information on backflushing filters, see 
open-file report 76-126 "Backflushing filters for field processing 
of water samples prior to trace-element analyses" by V. C. Kennedy 
and others. (See QW Branch Memo No. 77.04).

Funnel filter systems are of the types used for microbiological 
filtration work. For samples from which trace metals are to be 
analyzed, the funnel assembly and filter screen should be plastic 
such as polycarbonate, PVC, etc. The stainless steel 
microbiological assembly can be used to filter samples for 
inorganic constituents other than trace metals. It should be 
thoroughly cleansed of all sterilization liquids and gases before 
use. Funnel filter systems use vacuum as the power source. This is 
discussed in more detail in the next section. This method is best 
used for filtering small volumes of water that do not have high 
concentrations of filter clogging material.

Pressure filtering systems use compressed nitrogen or air to force 
the water out of a reservoir through the filter. Most are similar 
in design to the recalled barrel filter unit which is no longer in 
use (see WRD Memo No. 77.147). Pressure filter units are usable 
for filtering moderate volumes of water without too much filter 
clogging material present. For a safety standpoint the pressure on 
these systems should be kept as low as possible and should not 
exceed 30 psi. Pressure filtration is useful for filtering samples 
for dissolved organic substances (see next section) and for 
certain biological applications such as ATP determinations where 
it is critical that a controlled low pressure be used to prevent 
rupture of algal cells.

Pressure filter units are not recommended for general use unless 
the type and design is proven to be safe and reliable. Units that 
are used must have approved safety devices to prevent over 
pressurization and reliable regulation of the amount of pressure 
applied to the unit. Several types are available from the major 
filter manufactures. You should check with your Regional Safety 
Officer before purchasing any pressure units.

The Ohio District has designed a PVC low pressure unit using a 
membrane safety valve and hand squeeze bulb as a pressure source. 
This unit has been approved for general field use. At present, 
though there is no commercial source from which the unit may be 
purchased. A description and plans of the unit can be obtained 
from the Ohio District.


Newly purchased filter assemblies should be thoroughly cleaned 
with a nonphosphate laboratory detergent to remove residual 
fabrication products such as oils, polishes, glues, and cleaners. 
Follow the cleaning with a tap-water rinse and an overnight 
soaking in 5 percent hydrochloric acid. Then rinse the filter 
assembly with tap water followed by distilled or deionized water. 
The filter assembly must receive the same thorough cleaning after 
each use in heavily polluted waters. Cleaning of the assembly 
after other types of usage consists of at least a thorough rinse 
in distilled/ deionized water or a detergent (nonphosphate) wash
followed by a distilled/ deionized water rinse. wash porous 
plastic filter retainer pads and screens with distilled or 
deionized water after each use. Then, with the filter in place, 
the initial rinse water passing through the filter will serve to 
rinse and equilibrate both the filter itself and the filter 
support. Periodically clean all external components of the filter 
in order to avoid contamination of the filter assembly and the 
filter supports.

Filter tubing should be of surgical silicone, Tygon, or other 
equivalent inert material. Discard thick-walled rubber tubing that 
often accompanies commercial filters and replace it with one of 
the above mentioned types. Inspect all tubing before each use for 
signs of possible contamination. Replace tubing that shows signs 
of residue accumulation, discoloration, weakness, or other 

New tubing, regardless of quality, price, and manufacturing 
controls should be assumed to contain contaminants. Therefore, 
thoroughly clean before use all tubing used to transfer gas from a 
pressure source to the filter inlet, and all tubing used to 
transfer water for filtering from a sample container, stream, or 
well. Ideally, this consists of rinsing with the sample water to 
be filtered or, as an alternative, by soaking the tubing 
thoroughly in distilled or deionized water overnight before use. 
Do not use new uncleaned tubing for transferring water for 


Filtering of samples for the determination of organic constituents 
requires the use of equipment constructed from lnorganic 
materials. Acceptable filtration equipment consists of a stainless 
steel assembly (Gelman No. 4280 filter assembly or equivalent) 
with a 0.45 um silver filter (Selas F.I47 or equivalent). Bear in 
mind that most stainless steel filter assemblies received new from 
the company are contaminated with grease. All parts must be 
cleaned sequentially with acetone, methylene chloride, and ethyl 
alcohol. After cleaning with these three organic solvents, wash 
the filter assembly carefully using a good quality laboratory 
detergent followed by a thorough rinsing with distilled water. 
with infrequent use, the stainless steel filter assembly may 
develop a few rust spots. These spots can be removed by rinsing 
the filter assembly in a dilute solution of sodium dithionite, 
Na2S204, (also called sodium hydrosulfite). After cleaning the 
filter assembly with the chemical, rinse it thoroughly with 
distilled water before using. Use extreme care when handling the 
stainless steel filter support screen in the bottom of the 
assembly. Burrs and projections on this screen may puncture the 
silver filter. Rinse the inside of the filter assembly thoroughly 
with the sample water before filtering. After use, rinse the 
assembly with distilled water and store in a plastic bag to avoid 
contamination. Handle the silver filters only with forceps.


Field filtration can be conducted using pump, compressed gas, or 
vacuum transfer methods. The preferred procedure is the 
peristaltic pump (pump transfer method) in conjunction with the 
plate filter assembly. The peristaltic pump operates on a squeeze 
and release principle using rollers and flexible tubing.

In recent years many different designs and sizes of peristaltic 
pumps have become available on the market. They can be purchased 
as laboratory units running off AC power or as portable field 
pumps that are battery power. Many have reversible direction and 
variable speed motors which are desirable and necessary features 
for certain filtration methods such as backflushing filtration 
(see QW Branch Memo No. 77.04).

Filtering with peristaltic pumps us done by placing the intake end 
of the tube directly into the sample-compositing container, such 
as the churn splitter, or directly into the stream, well, or lake. 
The discharge end is attached to the filtration unit and then the 
sample is pumped through the system using the peristaltic pump. 
The operating speed of the pump should be kept slow speed. As the 
filter becomes clogged, pressure will slowly build up to about 35 
psi (depending on the type and size of tubing used) when the 
filter is completely clogged. Flow through a clogged filter can be 
reestablished by backflushing, if a backflushing plate filter unit 
is used, or by changing the filter membrane.

Use of a compressed inert gas to force the liquid through the 
filter membrane is the primary method recommended for filtering 
organic carbon samples. It can also be used to filter other water 
samples when the proper approved equipment is used. The preferred 
gas for filtering is ultra-high purity, low moisture content 
nitrogen gas. This grade of nitrogen gas contains less than 0.5 
ppm hydrocarbon, is water scrubbed, and is satisfactory for 
filtering all types of water samples. It is the same type of gas 
that is currently approved for use in manometer stage recording 
systems (bubble gages).

Clean compressed air can be used if it is known that the presence 
of oxygen will not adversely effect the sample. The clean air can 
be pumped by either a peristaltic pump equipped with dry, clean 
tubing or a hand squeeze bulb such as used on blood-pressure 
measuring units. Bicycle pumps or air compressor sources should 
not be used because they can easily contaminate the water sample 
with their lubrication fluids. Air should not be used for 
filtering samples of water with low dissolved oxygen content, for 
example, waters highly polluted with oxygen-demanding substances, 
water at deeper depths in lakes, or ground waters.

Cylinders of compressed gas must be equipped with an adjustable 
pressure regulator and the output line must have a safety pop-off 
valve set at 30 PSIG (see WRD Memo No. 77.113). Use the minimum 
pressure that will force water through the filter at an acceptable 
rate. Pressures above 15-20 PSIG have little effect on filtration 
rate when filters have become clogged.

It should be remembered that some types of algal cells can rupture 
at pressures above 5 PSIG, releasing their cellular contents into 
the filtrate. However, many types of algal cells can withstand 
pressures greater than 5 PSIG, and in many waters the 
concentrations of algae ill not be great enough to influence the 
chemical quality of the water even if cells do rupture, except in 
cases where ATP is to be determined . Nevertheless, for samples 
from lakes, ponds, and estuaries, the presence of ruptured algal 
cells should be assumed and their influence on the chemical 
quality of the water considered.

Vacuum or negative pressure also may be used to draw water through 
a filter. For very small volumes, the Millipore funnel and hand 
vacuum system may be used. For larger volumes, a vacuum pump or 
the engine manifold vacuum of an automobile may be needed; when 
using such equipment, a check valve should be installed in the 
vacuum line to prevent contamination by engine fumes.

Vacuum filtering is most useful for the collecting of filterable 
("suspended") material on a filter membrane for subsequent 
processing. If it is used to collect the filtrate, special care 
must be taken to avoid contamination. The filtrate vacuum flask 
must be thoroughly cleaned before use and rinsed with clean 
filtrate. The initial filtrate through a new filter membrane 
should be discarded, and all traces removed from the collection 
flask before the required filtrate can he collected.

Remember that the purpose of filtering water is to remove 
suspended material larger than a known size without contaminating 
the sample. Care in both sample handling and filtering is required 
to insure against contamination. Common-sense practices and 
procedures will help insure that water samples collected and 
filtered by WRD personnel remain representative of the body of 
water that was sampled.

                             R. J. Pickering 
                             Chief, Quality of Water Branch

WRD Distribution: A, B, S, FO, PO