44 MARCH 2022 • WORLD AQUACULTURE • WWW.WA S .ORG shrimp or about one year of continuous water use. Periodic water exchange is the most popular method of nitrate reduction but this is very expensive for inland producers because of the high cost of artificial salts. An alternative solution to prevent nitrate accumulation is denitrification. Denitrification is a microbial process where facultative anaerobes use organic carbon as an electron donor and nitrate as a terminal electron acceptor, resulting in the reduction of nitrate to dinitrogen gas (van Rijn 2006). In addition to reducing nitrate, this process decreases concentrations of phosphate, another nutrient that accumulates in RAS. Denitrification also results in increased alkalinity, which is very useful in buffering against pH swings and may reduce production costs by limiting the need for supplementation with exogenous buffer compounds. To encourage denitrification, it is essential that appropriate concentrations of carbon are maintained. A lack of available carbon often results in incomplete denitrification and nitrite accumulation, while a surplus will often promote ammonification (Yoon et al. 2015, Torno et al. 2018). Most denitrification strategies include the use of substrates such as inert plastic media or settled sludge for the growth of microbes. However, in several recent studies at KSU, some denitrification occurred in systems with no added substrates. In an effort to develop practical, inexpensive denitrification protocols for farmers, this study explored the effects of various carbon-tonitrogen (C:N) ratios on denitrification in tanks with no added substrates. The project examined the potential of denitrifying previously active, aerobic hybrid biofloc-RAS water, by converting the system to an anaerobic environment and adding organic carbon. Trial Methods and Results An experiment with four treatments with three replicate tanks each was designed to test different C:N ratios. The treatments were C:N ratios of 1:1, 3:1, 5:1, and a control treatment. For the C:N ratio treatments, carbon was added daily in the form of Shrimp is the most popular seafood product in Europe and the United States; however, the vast majority is imported from Asia and Latin America, where water is frequently exchanged with natural river or ocean water to remove chemical and physical wastes from ponds. There is growing interest in producing shrimp at inland locations, near consumer markets. This allows farmers to market shrimp as fresh, local, sustainably grown products in regions where fresh shrimp are difficult to acquire and where environmental regulations can be strict. To produce marine shrimp at inland locations, recirculating aquaculture systems (RAS) are used. RAS allow greater physical, biological, and chemical control of the production unit and conserve heat and water (Timmons et al. 2018). This is especially important in temperate climates with no access to seawater, but RAS are also used to produce shrimp in nurseries prior to stocking ponds even in tropical or subtropical coastal regions. This can provide an earlier start to the growing season and ensures that a hardier animal is ready for stocking ponds. In biofloc systems, naturally occurring particles are allowed to accumulate in the water column, providing a secondary food source for some species (e.g., shrimp and tilapia), leading to more efficient feed conversion and reduced nutrient discharge. In most biofloc systems, nitrification is performed by microbes suspended in the water column. However, we have been using hybrid bioflocRAS that have an external moving bed biofilter in addition to suspended particles in the water, as this tends to provide more consistent water quality. Although these systems greatly conserve water and allow higher stocking densities than ponds, nitrate accumulates as a result of nitrification by bacteria associated with suspended particles in the water column and those in biofilms attached to biofilter media. Nitrate is not nearly as toxic as ammonia and nitrite but high concentrations (> 250 mg/L NO3-N) can lead to stress in shrimp and result in mortality (Furtado et al. 2015). In the small-scale shrimp farms that Kentucky State University (KSU) works with (Fig. 1), we normally find worrisome concentrations of nitrate after three complete crop cycles of The Effects of Carbon: Nitrogen Ratio on Suspended-Growth Denitrification in Small-Scale, Closed-System Shrimp Farming Mark E. Johannemann, Leo J. Fleckenstein and Andrew J. Ray FIGURE 1. Indoor shrimp RAS (Photo: Dr. Andrew J. Ray).
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