Science of Water : Concepts and Applications

284 The Science of Water: Concepts and Applications
Step 2: Effl uent sample
Because none of the fi lters for the effl uent sample meets the minimum test requirement, the colonies/100
mL must be determined by totaling the colonies on each fi lter and the sample volumes used
for each fi lter.
Totalcolonies105318 colonies
Totalsample 10.0mL1.0mL0. 1mL 11.1mL
18 colonies
colonies/100 mL 100
11.1 mL
162
colonies/100 mL
√ Note: The USEPA criterion for fecal coliform bacteria in bathing waters is a logarithmic mean
of 200/100 mL, based on the minimum of fi ve samples taken over a 30-day period, with not
more than 10% of the total samples exceeding 400/100 mL. Because shellfi sh may be eaten
without being cooked, the strictest coliform criterion applies to shellfi sh cultivation and harvesting.
The USEPA criterion states that the mean fecal coliform concentration should not exceed
14/100 mL, with not more than 10% of the samples exceeding 43/100 mL.
Interferences
Large amounts of turbidity, algae, or suspended solids may interfere with this technique, blocking
the fi ltration of the sample through the membrane fi lter. Dilution of these samples to prevent this
problem may make the test inappropriate for samples with low fecal coliform densities because
the sample volumes after dilution may be too small to give representative results. The presence
of large amounts of noncoliform group bacteria in the samples may also prohibit the use of this
method.
√ Note: Many NPDES discharge permits require fecal coliform testing. Results for fecal coliform
testing must be reported as a geometric mean (average) of all the test results obtained during a
reporting period. A geometric mean, unlike an arithmetic mean or average, dampens the effect
of very high or low values that otherwise might cause a nonrepresentative result.
Apparent Color Testing/Analysis
As mentioned, color in water often originates from organic sources: decomposition of leaves, and
other forest debris such as bark and pine needles. Tannins and lignins, organic compounds, dissolve
in water. Some organics bond to iron to produce soluble color compounds. Biodegrading algae from
recent bloom may cause signifi cant color. Though less likely a source of color in water, possible
inorganic sources of color are salts of iron, copper, and potassium permanganate added in excess
at the treatment plant.
√ Note: Noticeable color is an objectionable characteristic that makes the water psychologically
unacceptable to the consumer.
Recall that true color is dissolved. It is measured colorimetrically and compared against an EPA
color standard.
Apparent color may be caused by suspended material (turbidity) in the water. It is important
to point out that even though it may also be objectionable in the water supply, it is not meant to be
measured in the color analysis or test. Probably the most common cause of apparent color is particulate
oxidized iron.