Environmental Engineering Reference
In-Depth Information
than field kits, however, and repairs to a damaged meter can be costly. The
meter and probe must be carefully maintained and must be calibrated before
each sample run or, if many tests are done, between sampling. Because of the
expense, a small waterworks might only have one meter and probe, which
means that only one team of samplers can sample DO and they must test
all the sites. With field kits, on the other hand, several teams can sample
simultaneously.
Laboratory Testing of Dissolved Oxygen
If a meter and probe are used, the testing must be done in the field because
dissolved oxygen levels in a sample bottle change quickly due to the decom-
position of organic material by microorganisms or the production of oxygen
by algae and other plants in the sample, which can lower the DO reading. If
a variation of the Winkler method is used, it is possible to fix the sample in
the field and then deliver it to a lab for titration. This might be preferable if
sampling is conducted under adverse conditions or if time spent collecting
samples is an issue. Titrating samples in the lab is a little easier, and greater
quality control is possible because the same person can do all the titrations.
What is biochemical Oxygen Demand and Why is it important?
Biochemical oxygen demand (BOD) measures the amount of oxygen con-
sumed by microorganisms in decomposing organic matter in stream water.
BOD also measures the chemical oxidation of inorganic matter (the extraction
of oxygen from water via chemical reaction). A test is used to measure the
amount of oxygen consumed by these organisms during a specified period
of time (usually 5 days at 20°C). The rate of oxygen consumption in a stream
is affected by a number of variables: temperature, pH, the presence of certain
kinds of microorganisms, and the type of organic and inorganic material
in the water. BOD directly affects the amount of dissolved oxygen in rivers
and streams. The greater the BOD, the more rapidly oxygen is depleted in
the river or stream, leaving less oxygen available to higher forms of aquatic
life. The consequences of high BOD are the same as those for low dissolved
oxygen: Aquatic organisms become stressed, suffocate, and die. Most river
waters used as water supplies have a BOD of less than 7 mg/L; therefore,
dilution is not necessary. Sources of BOD include leaves and woody debris;
dead plants and animals; animal manure; effluents from pulp and paper
mills, wastewater treatment plants, feedlots, and food-processing plants;
failing septic systems; and urban stormwater runoff.
Note:
To evaluate the potential of raw water for use as a drinking water
supply, the water is usually sampled, analyzed, and tested for biochemical
oxygen demand when turbid, polluted water is the only source available.
