Environmental Engineering Reference
In-Depth Information
organics in surface waters cause problems mainly associated with the effects
that result from the action of microorganisms. As the microbes metabolize
organic material, they consume oxygen.
When this process occurs in water, the oxygen consumed is dissolved oxy-
gen (DO). If the oxygen is not continually replaced in the water by artificial
means, the DO level will decrease as the organics are decomposed by the
microbes. This need for oxygen is known as the biochemical oxygen demand
(BOD), which is the amount of dissolved oxygen demanded by bacteria to
break down the organic materials during the stabilization action of the
decomposable organic matter under aerobic conditions over a 5-day incuba-
tion period at 20°C (68°F). This bioassay test measures the oxygen consumed
by living organisms using the organic matter contained in the sample and
dissolved oxygen in the liquid. The organics are broken down into simpler
compounds, and the microbes use the energy released for growth and repro-
duction. A BOD test is not required for monitoring drinking water.
Note:
The more organic material in the water, the higher the BOD exerted
by the microbes will be. Note also that some biodegradable organics can
cause color, taste, and odor problems.
Nonbiodegradable organics are resistant to biological degradation. Good
examples are the constituents of woody plants, including tannic and lig-
nic acids, phenols, and cellulose, which are found in natural water systems
and are considered refractory (resistant to biodegradation). Some polysac-
charides with exceptionally strong bonds and benzene (for example, associ-
ated with the refining of petroleum) with its ringed structure are essentially
nonbiodegradable. Certain nonbiodegradable chemicals can react with oxy-
gen dissolved in water. The chemical oxygen demand (COD) is a more com-
plete and accurate measurement of the total depletion of dissolved oxygen
in water. The
Standard Methods for the Examination of Water and Wastewater
(APHA-AWWA-WEF, 2005) defines COD as a test that provides a measure of
the oxygen equivalent of that portion of the organic matter in a sample that
is susceptible to oxidation by a strong chemical oxidant.
Note:
Chemical oxygen demand (COD) is not normally used to monitor
water supplies but is often used to evaluate contaminated raw water.
Synthetic Organic Chemicals
Synthetic organic chemicals (SOCs) are manmade and are often toxic to
humans. More than 50,000 SOCs are in commercial production today,
including common pesticides, carbon tetrachloride, chloride, dioxin, xylene,
phenols, and aldicarb. Unfortunately, even though they are so prevalent,
little information has been collected on these toxic substances. Determining
definitively just how dangerous many of these SOCs are is rather difficult.
