Microbiological
Disease-causing (pathogenic)
microorganisms, like bacteria, viruses and protozoa can cause swimmers to
get sick. Fish and shellfish can become contaminated and people who eat
them can become ill. Some serious diseases like polio and cholera are
waterborne.
- Chemical
A whole variety of chemicals
from industry, such as metals and solvents, and even chemicals which are
formed from the breakdown of natural wastes (ammonia, for instance) are
poisonous to fish and other aquatic life. Pesticides used in agriculture
and around the home-- insecticides for controlling insects and herbicides
for controlling weeds-- are another type of toxic chemical. Some of these
can accumulate in fish and shellfish and poison people, animals, and birds
that eat them. Materials like detergents and oils float and spoil the
appearance of a water body, as well as being toxic; and many chemical
pollutants have unpleasant odors. The Niagara River, between the US and
Canada, even caught fire at one time because of flammable chemical wastes
discharged into the water.
- Oxygen-depleting
Substances
Many wastes are biodegradable,
that is, they can be broken down and used as food by microorganisms like
bacteria. We tend to think of biodegradable wastes as being preferable to
non-biodegradable ones, because they will be broken down and not remain in
the environment for very long times. Too much biodegradable material,
though, can cause the serious problem of oxygen depletion in
receiving waters.
Like fish, aerobic bacteria that live in water use oxygen gas which
is dissolved in the water when they consume their "food".
(The oxygen in the compound H2O, water, is chemically bound,
and is not available for respiration (breathing)). But, oxygen is
not very soluble in water. Even when the water is saturated with dissolved
oxygen, it contains only about 1/25 the concentration that is present in
air. So if there is too much "food" in the water, the bacteria
that are consuming it can easily use up all of the dissolved oxygen,
leaving none for the fish, which will die of suffocation.
Once the oxygen is gone (depleted), other bacteria that do not need
dissolved oxygen take over. But while aerobic microorganisms--
those which use dissolved oxygen-- convert the nitrogen, sulfur, and
carbon compounds that are present in the wastewater into odorless-- and
relatively harmless-- oxygenated forms like nitrates, sulfates and
carbonates, these anaerobic microorganisms produce toxic and smelly
ammonia, amines, and sulfides, and flammable methane (swamp gas). Add in
the dead fish, and you see why we don't want large amounts of
biodegradable materials entering lakes and streams.
- Nutrients
The elements phosphorus and
nitrogen are necessary for plant growth, and are plentiful in untreated
wastwater. Added to lakes and streams, they cause nuisance growth of
aquatic weeds, as well as "blooms" of algae, which are
microscopic plants. This can cause several problems. Weeds can make a lake
unsuitable for swimming and boating. Algae and weeds die and become
biodegrable material, which can cause the problems mentioned above (and
below). If the water is used as a drinking water source, algae can clog
filters and impart unpleasant tastes and odors to the finished water.
- Suspended matter
Some pollutants are dissolved
in wastewater, meaning that the individual molecules or ions
(electrically charged atoms or molecules) of the substance are mixed
directly in between the molecules of water. Other pollutants, referred to
as particulate matter, consist of much larger-- but still very
small-- particles which are just suspended in the water. Although
they may be kept in suspension by turbulence, once in the receiving water,
they will eventually settle out and form silt or mud at the bottom. These sediments
can decrease the depth of the body of water. If there is a lot of
biodegradable organic material in the sediment, it will become anaerobic
and contribute to problems mentioned above. Toxic materials can also
accumulate in the sediment and affect the organisms which live there and
can build up in fish that feed on them, and so be passed up the food
chain, causing problems all along the way . Also, some of the particulate
matter may be grease-- or be coated with grease, which is lighter than
water, and float to the top, creating an aesthetic nuisance. |