A study on the optimal hydraulic loading rate and plant ratios in recirculation aquaponic system
Introduction
Aquaculture probably the fastest growing food-producing sector, now accounts for almost 50% of the world’s food fish and is perceived as having the greatest potential to meet the growing demand for aquatic food. It is estimated that at least an additional 40 million tonnes of aquatic food will be required by 2030 to maintain the current per capita consumption (FAO, 2006).
When fish are cultured, only a small proportion of the feed is converted (25–30%) to useable energy (Rakocy et al., 1993). The balance of nutrients is excreted in solid and dissolved fractions. Dissolved nutrients accumulate in recirculation systems with low water exchange and high feeding rates to levels which approximate hydroponic nutrient solutions.
Recirculation aquaponic system (RAS) is a promising technology in the integration of fish and hydroponic plant production. The fish water, rich in nutrients is used for plant growth, while the plants are used as biofilters for water regeneration. Whilst biofiltration converts the harmful into the harmless, the end point is a buildup of nutrients within recirculation systems, principally consisting of nitrates and phosphates. Nutrient removal by plants improves the quality of effluent and may enhance fish production. The amount of nitrate produced in a fish culture system is directly proportional to two factors: the amount or density of fish in the system and the amount and protein content of the food, as different fish species require different protein content in their respective diets.
Integrated systems use water more efficiently through the interacting activities of fish and plants. The addition of water to a fish tank to satisfy the oxygen requirements depends on the oxygen consumption of the fish, the oxygen concentration in the inlet water and the lowest acceptable concentration in the outlet water (Lekang, 2007). Hence effective HLR can be employed to achieve optimal growth for the fish and plants.
The rate of change in nutrient concentration can be influenced by varying the ratio of plants to fish (Rakocy et al., 2006). However, since the relative proportions of soluble nutrients made available to the hydroponic plants by fish excretion do not mirror the proportions of nutrients assimilated by normally growing plants, the rates of change in concentration for individual nutrients differ. The disparity in accumulation or reduction rates of different nutrients quickly results in suboptimal concentrations and ratios of nutrients, thereby reducing the nutritional adequacy of the solution for plants. Theoretically, the nutrient content of a diet can be manipulated to make the relative proportions of nutrients excreted by fish more similar to the relative proportions of nutrients assimilated by plants. With such a diet, there would be an optimal ration of fish to plants and optimal nutrient supplementation (Seawright et al., 1998).
Several mass balance models have been proposed from previous studies (Pagand et al., 2000, Papatryphon et al., 2005, Schneider et al., 2005, Mongirdas and Kusta, 2006), from which the total nitrogen and phosphorus discharges into receiving waters can be estimated. However, most of these studies were conducted in open systems. Recently, the incorporation of recirculated fish with vegetable hydroponics production has become an interesting model to private sector, aquaculture and environmental scientists (Rakocy et al., 2006, Bakhsh and Shariff, 2007, Endut et al., 2009).
The objectives of this study were to (1) determine the optimum hydraulic loading rate in term of fish production, plant production, and nutrient removal, (2) evaluate the optimum plants ratio in term of daily fish feed input to plant growing area, and (3) study the mass balance of oxygen in achieving sustainable balance between fish and plants.
Section snippets
Experimental design
The recirculation aquaponic system (RAS) utilized is depicted in Fig. 1. The experimental facility was located in a greenhouse of the University of Malaysia Terengganu campus. RAS consisted of a fiberglass rearing tank, hydroponic trough (growing bed), sand filter for solid removal, sump system for denitrification unit, water holding tank and reservoir (pre-aeration). Pipelines made of polyvinyl chloride were installed to connect the culture tank and hydroponic trough to recirculate the water.
Effect of hydraulic loading rates
Specific growth rates (SGRs), feed conversion ratio (FCR) and fish production did not differ significantly between hydraulic loading rates (Table 2). FCR values are in the range of 1.23–1.39. In our study, the same feed is used and the ration is fixed similarly in all culture tanks. Stocking at hydraulic loading rate of 1.28 m/day gives the best production performance (Table 2).
The FCR recorded (1.23–1.39) is not far above the ideal value of 1.0 for culture of African catfish in recirculation
Conclusion
This study demonstrated that the changes in concentrations of different nutrients in aquaponic system differ because of the disparity between the relative proportions of available nutrients generated by fish and nutrients uptake by plants. The optimal HLR in term of fish productions, plant growth and percentage nutrient removal were found to be 1.28 m/day. Reviewing the calculated balances and limitations of intensive integrated aquaculture systems, the perspectives of such integration are very
Acknowledgements
The authors would like to thank the Ministry of Higher Education of Malaysia for financially supporting this research under budget code T-E-210-59020.
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