Two organic carbon application rates to control inorganic nitrogen in minimal water exchange, biofloc, shallow water, shrimp nursery systems

The objectives of this study were to develop and test a quantitative method for reactive carbon application to control inorganic nitrogen, and to compare the effect of carbon application using 40% and 60% microbial conversion efficiency (MCE) while leaving a residual 11.3 mg/l nitrate nitrogen (NO₃-N) level. The organic carbon requirement was based on the carbon to nitrogen ratio of the elemental composition of microbial cells. The source of supplemental organic carbon was short-chained fructooligosaccharide (scFOS). Correction for moisture was duplicated on the first 2 days of scFOS application, so the actual efficiency rates were 35.1% and 58.3%. The proposed carbon quantitative method was effective in predicting the amount of carbon required to control inorganic nitrogen. Both 35.1% MCE and 58.3% MCE maintained total ammonia nitrogen (TAN) and nitrite nitrogen (NO₂-N) at desired levels of equal to or less than 2.3 mg/l and 3.1 mg/l, respectively. The amount of carbon applied using 35.1% MCE was higher than with 58.3% MCE. The 58.3% MCE treatment resulted in slightly higher NO₃-N levels than 35.1 % MCE. The most toxic species of inorganic nitrogen, TAN and NO₂-N, are assimilated by heterotrophic bacteria before NO3-N, permitting decreased reactive carbon input and water quality improvement. The benefits of 58.3% MCE vs. 35.1% MCE were lower organic loading, reduced water replacement, and decreased costs. The total water replacement associated with biofloc control was 0.24% using 35.1% MCE and 0% using 58.3% MCE. After a culture period of 14 days the mean weight was 65.5 mg and 61.9 mg for 31.5% MCE and 58.1% MCE, respectively, and a survival of 79.5% for both MCE’s.