Addition of organic carbon
Transition of ponds to more heterotrophic dominance could be accomplished
and maintained by providing sufficient amounts of organic carbon or other forms of
carbohydrate. Tacon et al. (2002) reported a gradual transformation of outdoor shrimp
cultivating tanks from phytoplankton based autotrophic food web towards bacterial
based heterotrophic food web after 4 weeks of the daily addition of shrimp diets
containing wheat as a major ingredient. In another report, addition of grain feeds (i.e.,
mixtures of soybean, wheat grain and corn) containing 18 to 22% proteins and
molasses into high-density (120 shrimp/m2
) zero-exchanged shrimp ponds in Belize
could promote the change of pond dynamics from phytoplankton to more heterotrophic
dominance after 8 to 10 weeks of continued supplying organic-mixed shrimp feeds
(Boyd and Clay, 2002). Excessive accumulation of inorganic nitrogen compounds in
bioflocs technology systems could be avoided by maintaining high substrate C/N ratios
within ponds under aerobic condition. Avnimelech (1999) described a complete removal
of 10 mg NH4
+
-N/L within the period of 2 hours following the addition of glucose into
suspension at the concentration 20 times higher than that of total ammonia nitrogen
(TAN). Recent work by Fontenot et al. (2007) used molasses and ammonium salts to
adjust the C/N ratios of shrimp aquaculture wastewater and obtained the result that the
optimal C/N ratio of 10/1 was for the most effective for wastewater treatment. Similar
24
results were reported by Azim et al. (2008) such that the optimal bioflocs development
measured in term of volatile suspended solids (VSS) and BOD5 was observed at the
C/N ratio of 11.6. At this optimal condition, the corresponding bioflocs production rates
ranged from 3 to 5 g C/m3
/day