Environmental Engineering Reference
In-Depth Information
2.2.6 Alkalinity
The alkalinity is the amount of acids (H
+
) that water can neutralize before to reach a given
pH. It is defined as the stoichiometric sum of the bases in a solution. Common bases found
in fish ponds include carbonates, bicarbonates, hydroxides, phosphates and berates.
Carbonates and bicarbonates are the most common and the most important components of
alkalinity. Because total alkalinity can be expressed as ppm of CaCO
3
, is common to confuse
it with hardness (Wurts & Durborow, 1992).
Like another water quality parameters, alkalinity can be affected by the biological activity of
systems. Because photosynthesis of phytoplankton requires CO
2
to synthesize glucose, pH
in the water increases due to inorganic carbon adsorption in water (mainly H
2
CO
3
and CO
3
2-
). During long periods of intensive photosynthesis, the release of carbonate can elevate the
pH levels over 9. These effects can be observed if water has low alkalinity (20 to 50 ppm) or
low bicarbonate (75 to 200 ppm). High photosynthetic activity can be presented when the
sodium and potassium carbonates are dissolved in water, because they are more soluble
than calcium and magnesium bicarbonates (Wurts & Durborow, 1992).
3. Low-tech water quality management
Currently, the aquacultural engineering gives a considerable number of solutions to control
the variables involved in aquaculture water quality. In general, they are available
commercial devices and chemical products to control water quality. However, there are
alternative techniques used by scientist that considered the pool as a bioreactor. New
aquaculture techniques are bio-flocs (Avnimelech, 2006), Integrated Multi-trophic
Aquaculture (IMTA) (Chopin, 2003), greenwater systems (Hargreaves, 2006), Zero-Exchange
water systems (Panjaitan, 2010; Olvera-Olvera et al., 2009) and aquaponics (Rakocy &
Hargreaves, 1993), among others.
3.1 Temperature
The temperature control on aquaculture could be difficult, because the high specific heat
capacity of water (4.18 kJ kg
-1
°K
-1
) implies a high amount of energy when water is heated or
cooled. In addition, the water volume used on aquacultural facilities is frequently high, so
the monetary and environmental cost to raise water temperature could be unaffordable
(Seginer & Mozes, 2008).
There are several alternatives to increase thermal stability on aquaculture. The most
effective system is the use of greenhouse to cover aquaculture ponds or tanks (Soto-Zarazúa
et al, 2011; Fuller, 2007). Some farms use heating pumps, thermosolar systems, fossil fuel
heaters or electric resistances, but all of them are expensive ways to heat water, both by
initial and operational cost. Besides, some of them imply negative environmental impacts
caused by greenhouse gas emissions (Mohanty et al., 2010).
Nevertheless, there are alternatives for the small-scale producers. The most effective
strategy is a good planning on farm building. Then, the most important thing is to pick a
good geographical location. The climate and the temperature of the make-up water must be
adequate for the cultivated fish. An analysis of the local climatologically data, if available,
can give us an idea if the selected location is proper to our intentions.
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