ABSTRACT
In this study, three different culture systems, recirculating aquaculture system (RAS), biofloc-RAS (BRAS), and biofloc system (BS) were investigated in terms of water quality characteristics and removal of inorganic nitrogen compounds. All systems had a 250-liter rearing tank. Each RAS had a sedimentation tank, an airlift pump, and a scoria biofilter tank. In the biofloc system, aeration was done by a perforated pipe in the bottom. In the BRAS, a biofloc production tank, an airlift pump, and a sedimentation tank were designed. After preparing the systems, common carp juveniles with a density of 6.25 kg/m3 were stored in each system and farmed for three months. The results showed that the RAS, BRAS, and BS reached a steady state condition in terms of ammonia on days 24, 30, and 36, respectively, and in terms of nitrite, on days 36, 42, and 60, respectively. The average amount of ammonia in the RAS and BS was significantly lower and higher than in other systems, respectively. The mean nitrite in the RAS was significantly lower than in the BRAS and BS. Nitrate values in all systems increased during the study period. In the biofloc system compared with other systems, the mean weight, weight gain, and SGR were significantly lower, whereas, FCR was significantly higher. In general, the RAS and biofloc-RAS performed better than the biofloc system under conditions of negligible water exchange.
 1. INTRODUCTION
Recent droughts forced us to use new technologies in the aquaculture industry to reduce our dependence on water resources. During the past four decades, the recirculating system has been the most common ultra-intensive culture system worldwide (Hisano et al., 2021). In these systems, by using physical and biological treatment units, it is possible to reuse water and produce it with high density (Irani and Agh, 2019). Despite the many advantages of these systems, the high initial investment and operational cost have limited their use, especially in less developed areas.
In recent years, a new technology called biofloc has been developed, which allows the production of fish and shrimp with little water consumption (Avnimelech, 2007). However, the most important problems of these systems are the unsustainability of removing nitrogen compounds and controlling flocs and excess solids. For this reason, other innovations emerged in these systems, and by integrating recirculating systems with biofloc, systems were developed in which the culture units are separate from the biofloc units (De Schryver and Verstraete, 2009). In this study, to use the advantages of both systems and eliminate their disadvantages, recirculating and biofloc systems were combined and a hybrid system was designed in which the biofilter was removed and biofloc was produced outside the rearing tank.
2. MATERIALS AND METHODS
Nine systems were designed and implemented in three treatments: 1- recirculating system, 2- recirculating-biofloc system and 3- biofloc system. All systems had a 250-liter fish-rearing tank. Each recirculating system had a sedimentation tank, an airlift pump, and a scoria biofilter tank. In the biofloc system, aeration was done by a perforated pipe in the bottom. Each recirculating-biofloc system had a biofloc production tank, an airlift pump, and a sedimentation tank.
After preparing the systems, common carp juveniles were stocked with a density of 6.25 kg/m3 in each rearing tank and were kept for three months under experimental conditions.
3. RESULTS
Ammonia values in recirculating and recirculating-biofloc systems reached a peak of 1.83 and 2.29 mg/liter, respectively, on day 15 and then decreased. In the biofloc system, ammonia levels peaked (2.21 mg/L) three days earlier, but the decline slope was much slower than in the other systems. The recirculating and recirculating-biofloc and biofloc systems reached a steady state on the 24th, 30th, and 36th days, respectively. The average amount of ammonia in the recirculating system (0.52 mg/L) was significantly lower and in the biofloc system (1.05 mg/L) it was significantly higher than in other systems (Table 1).
Nitrite values in recirculating, recirculating-biofloc and biofloc systems peaked on days 24 (0.66 mg/L), 21 (0.85 mg/L), and 30 (0.86 mg/L), respectively, and then decreased. The nitrite reduction, like ammonia, was faster in the recirculating system than in other systems. So that the recirculating system reached a steady state on the 36th day, whereas this condition occurred in the recirculating-biofloc and biofloc systems on the 42nd and 60th days, respectively.
In the biofloc system compared with other systems, the mean weight, weight gain, and SGR were significantly lower, whereas, FCR was significantly higher.
4. DISCUSSION AND CONCLUSION
In all systems, ammonia values increased, reached a peak, and then decreased. At the same time as ammonia decreased, nitrite levels increased and decreased after a certain time. Nitrate values had an increasing trend until the end of the research period. Researchers believe that a biological treatment unit is ready to use in recirculating systems when it has reached a steady state (Delong and Losordo, 2012), and a steady state is a condition in which the amounts of ammonia and nitrite in the outlet water are relatively constant (Colt et al., 2006). In general, in the present study, in terms of ammonia, the recirculating, biofloc-recirculating, and biofloc systems reached a steady state on days 24, 30, and 36, respectively, and in terms of nitrite, these systems reached a steady state on days 36, 42, and 60, respectively. Therefore, it can be concluded that the recirculating and recirculating-biofloc systems performed better than the biofloc system in conditions of negligible water exchange. On the other hand, the biofloc-recirculating system can be more economical and easier to manage because it does not need a biofilter system.
ACKNOWLEDGEMENT
We would like to thank the staff of Artemia and Aquaculture Research Institute of Urmia University, especially Mr. Saeed Hajinejad, who helped in the implementation of this research.
REFERENCES
Hisano, H., Barbosa, P.T.L., Hayd, L.A. and Mattioli, C.C. 2021. Comparative study of growth, feed efficiency, and hematological profile of Nile tilapia fingerlings in biofloc technology and recirculating aquaculture system. Tropical Animal Health and Production, 53(1), pp.60. doi: 10.1007/s11250-020-02523-z.
Irani, A., Agh, N., 2019. Design and Management Principles of Recirculating Aquaculture Systems, Urmia University, 230 p.
Avnimelech, Y. 2006. Bio-filters: the need for a new comprehensive approach. Aquacultural Engineering, 34, pp. 172–178. doi: 10.1016/j.aquaeng.2005.04.0.
De Schryver, P. and Verstraete, W. 2009. Nitrogen removal from aquaculture pond water by heterotrophic nitrogen assimilation in lab-scale sequencing batch reactors. Bioresource Technology, 100, pp. 1162–1167. doi: 10.1016/j.biortech.2008.08.043.
Colt, J., Lamoureux, J., Patterson, R. and Rogers, G. 2006. Reporting standards for biofilter performance studies. Aquacultural Engineering, 34, pp. 377-388. doi: 10.1016/j.aquaeng.2005.09.002.
DeLong, D.P. and Losordo, T.M., 2012. How to Start a Biofilter. SRAC Publication, 4502, pp. 4.