Environmental Engineering Reference
In-Depth Information
pollution by organic matter, the destruction of organic substances and level of self-
purification of the water. It is usually determined based on electro-chemical
principles by oxygen probes.
2.2.2
Inorganic constituents - nutrients
Nutrient enrichment (enhanced eutrophication) is a major theme in freshwater ecology.
Some themes come and go, but the inevitable release of phosphorus and nitrogen that
accompanies human presence seems to ensure that eutrophication will not soon become
an outmoded subject of study (Lewis & William 1984). Eutrophication raises issues that
range from the pressingly practical problems of phosphorus removal to the very
fundamental ecological questions surrounding biological community regulation by
resource supply and the conservation of the ecological habitat. Excessive levels of
nitrogen and phosphorus in storm water could promote the growth of undesirable aquatic
plants, including algae and floating macrophytes, which form a mat on the water surface.
Eutrophied water reduces the possible recreational use of lakes and reservoirs, and
increases the water treatment cost, when eutrophied water is used for potable purposes.
2.2.2.1 Nitrogen compounds
Nitrogen is essential for living organisms as an important constituent of proteins,
including genetic material. Plant and microorganisms convert inorganic nitrogen to
organic forms. In the environment, inorganic nitrogen occurs in a range of oxidation
states as nitrate (NO
3
) and nitrite (NO
2
), ammonia in gaseous or ionic form, and
molecular nitrogen (N
2
). It undergoes biological and non-biological transformations in
the environment as part of the nitrogen cycle. The major non-biological processes involve
phase transformations such as volatilization, sorption and sedimentation. Biological
transformations consist of:
i. Assimilation of inorganic forms (ammonia and nitrate) by plants and microorganisms
to form organic nitrogen e.g., amino - acids;
ii. Reduction of nitrogen gas to ammonia and organic nitrogen by microorganisms;
iii. Complex heterotrophic conversions from one organism to another;
iv. Oxidation of ammonia to nitrate and nitrite (nitrification);
v. Ammonification of organic nitrogen to produce ammonia during the decomposition of
organic matter;
vi. Bacteria reduction of nitrate to nitrous oxide (N
2
O) and molecular nitrogen (N
2
) under
anoxic conditions (denitrification)
• Ammonia - it is formed by bacterial enzymes that hydrolyze urea as well as by
oxidation of proteins, excretion by biota and from gas exchange with the atmosphere.
It is also discharged into water bodies by some industrial processes (e.g. ammonia
based pulp and paper production) and also as a component of municipal or community
waste. At certain pH levels, high concentrations of ammonia are toxic to aquatic life
and, therefore, detrimental to the ecological balance of water bodies. Moyo (1997)
explains the massive fish death in Lake Chivero in 1996 as due to anoxia, presence of
ammonia and other causes. In aqueous solution, unionized ammonia exists in
