Science of Water : Concepts and Applications

258 The Science of Water: Concepts and Applications
The advantage of a meter/probe is that DO and temperature can be quickly read at any point where
the probe is inserted into the stream. DO levels can be measured at a certain point on a continuous
basis. The results are read directly as milligrams per liter, unlike the titration methods, in which the
fi nal titration result might have to be converted by an equation to milligrams per liter.
DO meters are more fragile than fi eld kits, and repairs to a damaged meter can be costly. The
meter/probe must be carefully maintained, and must be calibrated before each sample run, and if
many tests are done, between sampling. Because of the expense, a small water/wastewater facility
might only have one meter/probe, which means that only one team of samplers can sample
DO, and they must test all the sites. With fi eld kits, on the other hand, several teams can sample
simultaneously.
Biochemical Oxygen Demand Testing
As mentioned, BOD measures the amount of oxygen consumed 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 specifi ed 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 DO in water bodies. The greater the BOD, the more rapidly
oxygen is depleted in the water body, leaving less oxygen available for higher forms of aquatic
life. The consequences of high BOD are the same as those for low DO: aquatic organisms become
stressed, suffocate, and die. Most river waters used as water supplies have a BOD less than 7 mg/L;
therefore, dilution is not necessary.
Sources of BOD include leaves and wood debris; dead plants and animals; animal manure;
effl uents from pulp and paper mills, wastewater treatment plants, feedlots, and food-processing
plants; failing septic systems; and urban stormwater runoff.
√ Note: To evaluate raw water’s potential for use as a drinking water supply, it is usually sampled,
analyzed, and tested for BOD when turbid, polluted water is the only source available.
Sampling Considerations
BOD is affected by the same factors that affect DO. Aeration of stream water—by rapids and waterfalls,
for example—will accelerate the decomposition of organic and inorganic material. Therefore,
BOD levels at a sampling site with slower, deeper waters might be higher for a given column of
organic and inorganic material than the levels for a similar site in high aerated waters.
Chlorine can also affect BOD measurement by inhibiting or killing the microorganisms that
decompose the organic and inorganic matter in a sample. If sampling in chlorinated waters (such
as those below the effl uent from a sewage treatment plant), neutralizing the chlorine with sodium
thiosulfate is necessary (see Standard Methods).
BOD measurement requires taking two samples at each site. One is tested immediately for DO,
and the second is incubated in the dark at 20°C for 5 days, and then tested for the DO remaining.
The difference in oxygen levels between the fi rst test and the second test [in milligrams per liter
(mg/L)] is the amount of BOD. This represents the amount of oxygen consumed by microorganisms
and used to break down the organic matter present in the sample bottle during the incubation
period. Because of the 5-day incubation, the tests are conducted in a laboratory.
Sometimes by the end of the 5-day incubation period, the DO level is zero. This is especially
true for rivers and streams with a lot of organic pollution. Since knowing when the zero point was
reached is not possible, determining the BOD level is also impossible. In this case, diluting the
original sample by a factor that results in a fi nal DO level of at least 2 mg/L is necessary. Special
dilution water should be used for the dilutions (see Standard Methods).