Environmental Engineering Reference
In-Depth Information
natural buffer system which helps to maintain a constant pH. Is also important for biological
primary production, because is necessary in algal photosynthesis to synthetize glucose. Is
important to note that biological activity can also release CO
2
to the media, when fish and
algae breathe. Some bacteria also need CO
2
to maintain its metabolic cycles in constant
function.
Bacteria also release CO
2
, both autotrophs and heterotrophs. For example: The free acid
produced during nitrification reacts with bicarbonate alkalinity in water to release more CO
2
than autotrophs consume. So for every gram of TAN metabolized, 4.6 ppm of oxygen will
be consumed, and 5.9 g of CO
2
will be released. In heterotrophic reactions, for every gram of
O
2
consumed, 1.38 g of CO
2
is released (Summerfelt & Sharrer, 2004).
When CO
2
is dissolved in water, part of it combines to form carbonic acid (H
2
CO
3
). This weak
acid tend to react with calcium carbonate (CaCO
3
) to form calcium bicarbonate [Ca(HCO
3
)],
which is often dissociated on hydrogen ions and carbonate ions (Wheaton, 1982):
+
↔
↔
+
↔
+
These four reactions are mediated by hydrogen concentration. They proceed to the right
when pH rises and they go to the left when pH decreases. Therefore, the carbon dioxide
usually can be found in water under four different forms: As free gas (CO
2
), as carbonic acid
(H
2
CO
3
), as carbonate (CO
3
2-
) and as bicarbonate (HCO
3
-
) (Wheaton, 1982). Thus, the
chemical importance of CO
2
relies in the fact that is a pH buffer, and it has a lot of relation
with other physicochemical parameters, like hardness and alkalinity.
2.2.5 Hardness
The hardness is defined as the total concentration of calcium and magnesium ions,
expressed as calcium carbonated. However, if other metallic ions are presented in the water
(Al, Fe, Mg, Sr, Zn) they can be also considered in the definition (Wheaton, 1982).
The hardness is an important water quality parameter because a direct relation between
water metal content and pH variations exists. When the concentration of Ca and Mg trends
to be higher, the buffering capacity of the water becomes higher, too, and is more capable to
smooth pH variations. In other words, hard water is more stable than soft water.
In the biological perspective, calcium is important in fish metabolism, because is used on the
scale and bond formation, and to keep the adequate balance of Na and K in the blood
(Wurst & Durborow, 1992). Calcium is often required in neural synapses, and in
physiological ion balance. In the case of magnesium, it is used is used by photosynthetic
organism because is embedded in the center of the chlorophyll molecule, and it is also
required as prosthetic group in proteins (Müller-Esterl, 2008)
In the farm, hardness is very important when the organisms are cultivated for reproduction
purposes in hatcheries. If invertebrates are cultivated, hardness becomes an important
variable to consider, because Ca and Mg are very important in the formation of hard parts
(i.e., exoskeleton or shells).
Changes in the water hardness are slowly. They take place typically in weeks or months.
However, is a good management practice to monitoring this variable constantly.
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