Aquaculture Water Quality for Small-Scale Producers - Aquaculture and the Environment: A Shared Destiny - page 135

used. Calcium chloride can also be used. A high chloride:nitrate ratio of 10:1 is used to
prevent brown blood disease. In catfish, for example, is recommended to maintain at least
100 ppm chloride in pond waters (Durborow et al., 1997).
When brown gill disease is not presented, a good practice is to flush water. In general, the
absorption of NO 2 by bacteria is presented in tanks and ponds. To enhance its activity,
biofiltration is recommended.
3.4.3 Nitrate
In general, nitrate is a non-toxic form of nitrogen in pond. However, nitrate management is
usually carried out by water exchanges. Phytoplankton and bacterial uptake is another
method to assimilate nitrate into cellular tissue (Gross et al., 2000). Aquaponics is another
way to assimilate NO 3 into fresh, marketable plant biomass, as lettuce, tomato, basil and
other crops (Graber & Junge, 2009; Rico-García et al., 2009; Savidov et al., 2005).
Denitrification is also an alternative to eliminate aquacultural nitrate. Theory and practice
examples can be found in van Rijn et al., 2006.
3.5 Solids
The management of solids include feed design and management, flow regulation and
separation treatment technology (Cripps and Bergheim, 2000). In small scale aquaculture, a
good start point is to asses an adequate feed schedule. In our experience, we have noted that
overfeeding is usual in new managers, so the detriment of the water quality is faster.
Fish also waste a lot of feed. In the case of tilapia, even 50% of their feed can be wasted
(Avnimelech, 2003). So the experience of the manager is fundamental in solids control.
A complementary approach to treat solid wastes is a quickest remotion of them (Timmons et
al., 1998). In general, the settleable solids can be easily remove from the culture water. If
circular tanks are used, the centrifugal forces and conical bottoms must help to accumulate
the solids in the center drain. (Timmons et al., 1998; Summerfelt et al., 1998). If rectangular
tanks are used, is recommended to adjust length/width ratios to increased bottom velocities
and reduced biosolid accumulation (Oca and Masaló, 2007). In some cases additional
components can be used, like settling basins or hydroclones (Wheaton, 1982). For
aquaponics, the size reduction of solid wastes from pellets to fine particles or even until
basic organic compounds is desirable (Rakocy & Hargreaves, 1993).
If extensive ponds are used, the use of chemicals to enhance flocculation of organic
particulate material could be useful. The addition of gypsum helps to diminish the negative
electrical charges between particulate material, enhancing the flocculation and
sedimentation of suspended solids. The recommended amounts are between 100 to 300 mg
per liter (Hargreaves, 1999).
3.6 Carbon dioxide (CO 2 )
In general terms, rarely CO 2 concentration cause problems in fish ponds with sufficient
alkalinity. However, if the pond is deeper than 5 feet and poor mixing is presented, there is
a risk of water stratification (Hargreaves and Brunson, 1996). Vigorous aeration can prevent