CONTAMINANTS & WATER TESTING
Particulates
PARTICULATES include silt, plumbing pipe debris, and colloids. These suspended
particles can plug filters, valves, lab tubing, reverse osmosis membranes and conductivity
meters. Particulates are visible as cloudiness or turbidity, and are detected using
filtration and gravimetric means, or microscopic methods. A 10 to 20 micron prefilter
is often placed as the first component in a water purification system to filter
out the larger particles. Smaller particles are removed subsequently by reverse
osmosis, submicron filters and ultrafiltration membranes.
Dissolved Inorganics
DISSOLVED INORGANICS include calcium and magnesium ions dissolved from rock formations
(these two ions make water hard), gases such as carbon dioxide that ionize in water
(carbon dioxide dissolves readily in water to make mildly acidic carbonic acid),
silicates leached from sandy river beds or glass containers, ferric and ferrous
ions from rusty iron pipes, chloride and fluoride ions from water treatment plants,
phosphates from detergents, nitrates from fertilizers, and many others.
There are several tests for identifying specific dissolved inorganics. The simplest
test is a direct measurement of electrical conductivity or resistivity. Most dissolved
inorganics are either negatively charged (anions) or positively charged (cations),
and will transmit a current when a voltage is applied to electrodes inserted in
the water. The more ions present, the greater the conductivity, or the lower the
resistivity of the sample water.
Dissolved Organics
DISSOLVED ORGANICS may include pesticides, herbicides, gasoline, and decayed plant
and animal tissues. Dissolved organics may also include the plasticizers leached
out of plumbing lines, fittings and storage tanks.
Note the sources of the last organic contaminant — all are from improperly designed
water purification systems. Thus, a water purification system must both remove the
contaminants present in the feedwater, and be designed to minimize the addition
of contaminants to the water.
The absence of dissolved organics is very important when performing analyses of
organic substances in HPLC, gas chromatography,electrophoresis, and fluoroscopy,
or in research involving tissue cultures.
There are several ways of measuring dissolved organic levels. The potassium permanganate
(KMnO4) color retention time test is a qualitative organic test that may
be used. The premise of this test is that the bright purple-colored potassium permanganate,
a powerful oxidizing agent, will change color to clear if there are sufficient organics
present in the water to be oxidized. The drawbacks of this test are that it is slow,
it is not sensitive to very low levels of organics (that might still be too high
for HPLC purposes), and it is not quantitative — it doesn’t tell how many parts
per billion of organics are present.
Total Organic Carbon (TOC) analyzers, which oxidize the organics and measure
the CO2 liberated, are being used more and more to determine organic levels in Type
I water due to their low-level detection sensitivities. A low TOC level is very
important for HPLC users.
Microorganisms
MICROORGANISMS constitute another group of contaminants found in water. Surface
water may contain a wide variety of microorganisms, including bacteria, protozoa,
algae, amoebae, rotifers, diatoms and others. Since most laboratory water comes
from municipal water treatment plants, which is extensively treated to remove microorganisms,
the chief microbes of concern for water purification systems are bacteria. A typical
bacterial level for a potable laboratory water supply is one colony forming unit
per milliliter (cfu/ml).
First of all, water is an excellent solvent and the medium of most life processes
on this planet. That is why water gets contaminated with just about everything it
encounters and why microorganisms grow in it so well. The aforementioned is why
purifying water is a much more difficult procedure than it initially might seem
to be.
Microorganisms
- Remove the bacteria present in the feedwater
- Prevent bacteria from entering the system and contaminating it
- Ensure that no bacteria are in the product water
- Inhibit bacterial growth through proper design and operation.
Bacteria are one celled organisms that multiply at exponential rates, thrive in
standing water, and may be present on many surfaces and in the air. Bacteria subsist
on a variety of substrates in purified water including dissolved organics such as
plasticizers and dissolved inorganics such as iron and sulfur.
Pyrogens
Pyrogens, are typically gram-negative bacterial cell wall fragments or lipopolysaccharides.
When injected into a mammal, pyrogens cause a rise in body temperature. Thus pharmaceutical
grade water must be pyrogen-free. Pyrogens also have a detrimental or lethal effect
on tissue cultures.
Pyrogens are detected either by injecting the sample water into
specially bred rabbits and monitoring them for a body temperature rise, or with
the LAL (Limulus Amoebocyte Lysate) test, a sensitive test for very low concentrations
of lipopolysaccharides.