are very poorly degradable substances, such as polychlorinated biphenyls (PCBs), that
tend to accumulate in sediments and in the lipid (fat) tissues of fish and birds that eat fish.
Organic water pollutants are addressed briefly here.
Oxygen-Demanding Substances
One of the most common water pollutants consists of substances that are not
toxic, but serve as excellent food sources for bacteria in water. When such substances,
represented here as {CH 2 O}, undergo biodegradation,
{CH 2 O} + O 2
Microorganisms
CO 2 + H 2 O (7.13.1)oxygen is consumed. The resulting low oxygen levels are detrimental to fish and some
other forms of aquatic life and also include the likelihood that odorous reduced species,
particularly H 2 S, will be evolved. High levels of oxygen-demanding substances are
associated with the water pollution phenomenon of eutrophication discussed above.
The oxygen-consuming potential of biodegradable materials in water is called
biochemical oxygen demand, BOD. BOD is commonly expressed as the amount of
oxygen consumed in biodegrading the organics in a liter of water.
Sewage
One of the most common sources of BOD is sewage from domestic, commercial,
and industrial sources. In addition to BOD from fecal matter and food wastes, sewage
contains oil, grease, grit, sand, salt, soap, detergents, degradation-resistant organic
compounds, and an incredible variety of objects that get flushed down the drain. Sewage
used to be simply discarded to the nearest handy stream or body of water, an unfortunate
practice that still continues in many parts of the world. There are several reasons that this
should not be done, the most obvious of which is that sewage stinks. But it also carries
disease-causing (pathogenic) microorganisms (bacteria, virus, protozoa) and it exerts
a high BOD in the water to which it is discarded. Sewage treatment to reduce BOD is
addressed in Section 7.17.
Detergents in Sewage
A major problem with sewage that was solved by the application of green chemistry
before it was known as such was the presence of detergents in the sewage. After detergents
came into widespread use, in the 1940s, massive layers of foam that sometimes covered
most of the treatment plant developed at sewage treatment plants in which air is blown
through sewage to promote biodegradation and at the outfalls where treated sewage
was discharged into streams. There were even cases of treatment plant workers who
walked into the foam, fell, and were asphyxiated by the gases entrained by the foam.
The phenomenon even extended to surface water and groundwater contaminated with
treated sewage, which developed troublesome heads of foam upon being drained from a
178 Green Chemistry, 2nd ed