Environmental Engineering Reference
In-Depth Information
the fraction of toxic ammonia increases at elevated temperature and high values of pH
(Durborow et al., 1997a), as it can be seen in the following equation:
=
10
+
Where
k
eq
is the negative logarithm of
pK
(
=−log(
)
), and
=0.09018+2727.9−
(+273.1)+(0.1552+0.0003142)
,
T
is temperature (°C) and
I
is Ionic strength (
M
)
(Eshchar et al., 2006). For more convenient purposes, computations of ammonia toxic
fractions can be made with the tables provided by Durborow et al. (1997a). Measures of
TAN, pH and temperature are required.
2.2 Important water quality parameters
The dynamic of the important parameters occurs slower than the reactions implicit on
critical ones, so they can be monitored with less frequency. Solids, nitrite (NO
2
), nitrate
(NO
3
), carbon dioxide (CO
2
), hardness and alkalinity can be measure once a week. Lethal
effects on the fish at high concentrations of this parameters are not common, but their
accumulation can affects directly and indirectly the fish growth. Also, if unfavourable
conditions are presented, an enhanced risk to infections or diseases can be presented.
2.2.1 Solids
In aquaculture, the solids are placed into a special category because in most of the cases they
can be controlled by good management practices. Solids in pond are presented as uneaten
food, faeces, fish scales, dead bacteria and algae, dust, and dead fish (Cripps and Bergheim,
2000). The adverse consequences driven by the presence of organic solids in the system are
caused mainly by bacterial processes. It implies additional oxygen consumption and carbon
dioxide release, among other effects. In some anoxic environments, hydrogen sulfide (H
2
S)
and ammonia can be release to water by bacterial bacterial anaerobic processes.
In general, the solids can be classified in three major divisions. 1) Settleable solids, wich can
be easy separated by sedimentation or decantation (for example, uneaten feed, fish scales,
faeces); 2) Suspended solids, which are very fine solids without the capacity of rapid
sedimentation (about 1-10
μ
m on diameter); and 3) dissolved solids, which are nano-scale
elementary forms as molecules or atoms. (Losordo et al., 1999; Malone, 1991).
In general, the impacts of the solids in aquaculture systems are negative. For example, in the
surface when the water turbidity rises, the photosynthetic activity in algae decreases. In
bottom, accumulation of solid wastes causes anaerobic zones, in which undesirable bacteria
can proliferate. In ponds, high clay turbidity usually causes acidity, low nutrient levels, and
limited light penetration for photosynthesis (Yi et al., 2003). Dead fish are solids too, and its
presence on culture water can be a factor for pathogenic propagation inside the farms
(Cripps & Bergheim, 2000).
Is useful to think that solids on aquacultural environments are mainly composed by organic
material. If they remain in the water, they became basics nutrients, like nitrogen and
phosphorus that could be easy assimilated by microscopic organism. So the total oxygen
budget will rise (Timmons et al., 2002).
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