Environmental Engineering Reference
In-Depth Information
most common. However, paddle wheels aerators and propeller-aspirator-pumps are the
most efficient devices (Boyd, 1998).
There are some useful indications about the use of aerators. In semi-intensive culture, is
common to turn on mechanical aeration at night, when is a lack of photosynthesis and the
respiration rate is in a maximum value. However, is important to note that if the aerators are
operated on sunny afternoons when water is supersaturated, oxygen will be lost (degassed)
from the water (Tucker, 2005)
When commercial aerators are not available, a degassing column can be constructed with
local available materials. Wheaton (1982) mentioned how to build a simple aeration tower.
Tucker (2005) gives photos and a short description about it uses on farm systems.
Theoretical background can be found in Boyd (1998), Summerfelt et al. (2000) and Vinatea &
Carvalho (2007)
3.4 Nitrogen compounds
Ebeling et al. (2006) pointed out the following: for every gram of ammonia-nitrogen
converted to nitrate-nitrogen, 4.18 of dissolved oxygen and 7.05 g of alkalinity (1.69 g of
inorganic carbon) are consumed and 0.20 g of microbial biomass (0.105 g organic carbon)
and 5.85 g of CO 2 (1.59 g inorganic carbon) are produced. So the nitrogen cycle in pond
water affects the physical, chemical and biological components present in the system. To
control the accumulation of every kind of components in the pond the most usual method is
the addition of make-up water. However, there are alternatives to manage the levels of
nitrogen compounds. Some of them are described below.
3.4.1 Ammonia
The most extended method to control ammonia inside aquacultural ponds is to keep a
good feeding schedule based on nutrimental tables. If the feeding regimen in the fish farm
is intensive, then pH control is also recomended. If pH can be maintained lower than 8,
the toxic fraction of TAN will remain in minimal percentage (Hargreaves and Tucker,
2004).
A common method to control ammonia in recirculating aquaculture systems is a biofilter
addition in the system. Biofilters design and theoretical foundations are already in literature
(Bazil (2006), Drennan II et al. (2006), Eding et al. (2006), Gutierrez-Wing and Malone (2006),
Kuo-Feng & Kuo-Ling (2004)). Examples of applications and affordable designs can be
found in Soto-ZarazĂșa et al. (2010), Timmons et al. (2006), Al-Hafedh et al. (2003) and Ridha
& Cruz (2001)
Another method to control ammonia is the increment of microbiological activity inside the
pond (Hargreaves and Tucker, 2004). The addition of organic carbon in relation with nitrogen
concentration is very useful (Crab et al., 2009). Theoretical background of this technique (also
named bio-floc technology) is already available in Avnimelech (2006, 2003 and 1999).
3.4.2 Nitrite
A common practice to reduce nitrite toxicity is the elevation of chloride concentration in the
culture water (Losordo et al., 1998). For this purpose, common salt (sodium chloride) is
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