Comparative analysis of nitrogen and phosphorus budgets in a bioflocs aquaculture system and recirculation aquaculture system during over-wintering of tilapia (GIFT, Oreochromis niloticus)
Introduction
To obtain higher quantities and scalable aquaculture products efficiently, the highest possible stocking densities and highly nutritious aquaculture feeds are usually employed in intensive aquaculture systems (Kutty, 2005). This process generates a certain amount of waste in the form of nitrogen (N) and phosphorus (P), which may stimulate primary productivity and eutrophication processes in the receiving water body (Bouwman et al., 2011, 2013). Consequently, the rapid growth in aquaculture production over the past decades has been met with increasing concern over potential environmental impacts (Mente et al., 2006). An understanding of N and P budgets, obtained by quantifying their inputs and outputs, and hence the efficiency of their utilization in the culture system is critical to evaluate the sustainability of the target aquaculture system (Dien et al., 2018). N and P budgets in aquaculture systems have been widely studied (Zhang et al., 2018). However, relevant information on the comparative analysis of N and P budgets in bioflocs technology (BFT) aquaculture systems and recirculation aquaculture systems (RASs) during over-wintering of tilapia is lacking in the literature.
RASs and BFT aquaculture systems are widely used today as viable and environmentally friendly substitutes for traditional aquaculture systems (Ray et al., 2017; Vinatea et al., 2018). RASs typically have one or more solids filter(s) to remove approximately 80 % of solids from the water and an external biofilter to provide attached surfaces for nitrifying bacteria (Timmons and Ebeling, 2007; Rijn, 2013). Less than 10 % of the total water volume is exchanged due to the backwashing of the solids filter, the biofilter or reducing the nitrate nitrogen (NO3−-N) converted from ammonia nitrogen (NH4+-N) and nitrite nitrogen (NO2−-N) (Vinatea et al., 2018). BFT aquaculture systems contain substantial amounts of bioflocs, which is composed of a dense microbial community (mainly heterotrophic bacteria in theory) and produced using feces (Ray and Lotz, 2014). The only potential external unit for BFT aquaculture systems is a settling tank to control the bioflocs level (Azim and Little, 2008; Ray et al., 2010). Therefore, in BFT aquaculture systems, minor water is exchanged because of evaporation and surplus bioflocs removal (Vinatea et al., 2018). Compared to other aquaculture systems, the additional benefit of BFT aquaculture systems is the reuse of waste nitrogen through the microbial biomass (Hargreaves, 2006, 2013; Vinatea et al., 2018). By externalizing biofiltration, RASs may allow for greater control and stability in water quality parameters (Hargreaves, 2013). Significantly, BFT aquaculture systems are biologically complex, and spikes in toxic ammonia and nitrite concentrations in the system generally become more difficult to control (Prangnell et al., 2016).
Nile tilapia (Oreochromis niloticus) is one of the most widely used aquaculture species and represents approximately 80 % of cultured tilapia worldwide (FAO, 2018). Tilapia is a temperate water species and sensitive to low water temperatures (Crab et al., 2009). To reach market size production, tilapia fingerlings are always cultured in the winter season to achieve a normal weight (> 50 g) before the grow out season (Crab et al., 2009). Larger fish also need be reserved over the winter months for market demand (Charo-Karisa et al., 2005). Heated facilities, geothermal water or greenhouses have been used successfully for the over-wintering of tilapia (Dan and Little, 2000a,b) and may be required to justify the investment (Crab et al., 2009). Additionally, water exchange should be kept to a minimum in order to lose as little heat as possible when cold water is used (Crab et al., 2009). Both RASs and BFT aquaculture systems can minimize water exchange and, correspondingly, minimize losses in energy when the replaced or added water is colder. Culturing tilapia in RASs and BFT aquaculture systems has been carried out extensively (Brazil, 2006; Crab et al., 2009; Losordo and Westerman, 2010). Temperature is the key factor that influences the bacterial activity in the culture system (Zhang et al., 2018), and the success of both RAS and BFT aquaculture system operation depends on the activity of bacteria growing in the culture system. This results in the following question: which of the two systems is better and can serve as an efficient and viable alternative for over-wintering of tilapia?
Our previous study investigated the growth performance, welfare, and primary cost of growing out Nile tilapia in RASs and BFT aquaculture systems (Luo et al., 2014). Detailed information about fish growth performance, water quality dynamics and primary production cost in RASs and BFT aquaculture systems for over-wintering of tilapia has been described in our previous paper (Cao et al., 2019). The present article is focused on evaluation of the N and P budgets in RASs and BFT aquaculture systems during the over-wintering of tilapia.
Section snippets
Cultured system
The current experiment was carried out from Jan. 16 to Mar. 20, 2018 in Shanghai, China. Most of the time, the air temperature was less than 10 ℃ during the experimental period. The experimental-scale RASs used here were composed of six tanks, one drum filter, and two biological filters, located in a greenhouse. The operational volume of each tank was 2 m3. Three tanks were connected to the water treatment system and designated as the RAS group. The water in the RASs was renewed 24 times per
Performance of RASs and BFT systems
No significant differences were observed for pH, alkalinity, DO and NH4+-N between the RAS and BFT groups (p > 0.05). The concentrations of NH4+-N in the RAS and BFT groups were less than 2.0 mg/L. The concentration of NO2−-N in RAS was less than 1.12 mg/L, which was significantly higher than that in the BFT systems (0.19 mg/L) (p < 0.05). The maximum concentration of NO3−-N in the BFT systems was 336.91 mg/L, which was significantly higher than that of RAS (118.4 mg/L) (p < 0.05). The average
The performance of RASs and BFT systems in the current experiment
The present work focused on the evaluation of N and P of RASs and BFT systems during the over-wintering of tilapia. The biomass gained by tilapia and the water consumption per kg of feed were used primarily to describe the performance of the experimental systems. More specific information about the survival and growth of the tilapia and the water quality is given in our previous paper (Table 4) (Cao et al., 2019). The water quality parameters had values within the ranges suitable for tilapia
Conclusion
Nitrogen and phosphorus budgets in BFT aquaculture systems and RASs for over-wintering of tilapia (GIFT Oreochromis niloticus) for 64 d were compared in the current study. The recovery rate of nitrogen in the BFT systems was estimated at 48 ± 5 % of the input nitrogen, which was highly significant compared to that of RAS (37 ± 4 %). No significant difference was observed between RASs and BFT systems in terms of phosphorus recovery rates. Much higher amounts of nitrogen and phosphorus were
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was funded by the Shanghai Science and Technology Commission Project (19DZ2284300).
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