WATER QUALITY: Field filtering of water samples for chemical analysis
April 5, 1978
QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 78.06
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).
FILTER PORE SIZE
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 Such data should not be
published in annual basic data reports unless they are clearly and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
filter.
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.
FILTERS AND FILTERING EQUIPMENT FOR INORGANIC CHEMICAL
CONSTITUENTS
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.
CLEANING FILTER EQUIPMENT
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
deterioration.
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
filtration.
FILTERS AND FILTERING EQUIPMENT FOR ORGANIC SOLUTES CHEMICAL
CONSTITUENTS
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.
METHODS OF FILTERING
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