Environmental and Energy Requirements for Different Production Densities of Nile Tilapia (Oreochromis niloticus) in Recirculating Aquaculture Systems: Laboratory and Computer Simulation Studies

Abstract

A Recirculating Aquaculture System (RAS) attempts to provide sustainable utilization of the available water resources by reducing water pollution and water acquisition costs. Improper matching of RAS components yields inflated cost of production and consequently leads to system failure. The significant challenges in RAS are to maintain favourable water quality for the fish and create conducive conditions that minimize the cost of energy required. In Kenya, many Recirculating Aquaculture Systems have not been able to strike a balance between the optimal levels of water parameters and the cost of energy required to run the system. This study, therefore, aimed at evaluating environmental and energy requirements for different production densities of Nile tilapia (Oreochromis niloticus) in a RAS. In this study, both production density and water flow rates were varied, and water quality parameters namely Dissolved oxygen, ammonia, pH, EC, and temperature monitored. Tilapia stocking densities were varied between 2.3 kg/m3 and 10 kg/m3 while flow rate was varied from 2.0 L/min and increased at intervals of 1 L/min to a flow rate of 10 L/min. The energy consumed for the different stocking densities and flow rates was also monitored using installed electricity meters. Crushed pumice rock packed in a 1000L tank was used as the biofilter. A RAS prediction model, model based on physical, chemical, or biological laws and theories, was developed using the Matrix laboratory (MATLAB) app-designer programming environment. Purification efficiency (PE) was computed as a proportion of the amount of ammonia removed from the RAS water by the biofilter. The study showed that ammonia removal was reduced with an increasing flow rate. The Purification Efficiencies (PE) of the pumice rock biofilter ranged from 79.18% at 2.0 L/min to 9.79 % at 10.0 L/min. Both pH and Electrical conductivity increased with increasing flow rate at all stocking densities. Dissolved oxygen increased with flow rate. The energy demand by the pump and the aerators increased progressively with flow rate from 0.5 kWh at 2.0 L/min to 2.3 kWh at 10.0 L/min. The developed RAS model made predictions of energy and water quality for different stocking densities and flow rates. An evaluation of the model prediction accuracy by comparing the observed data and the model predicted data gave R2 values for ammonia, pH, dissolved oxygen, electrical conductivity, and energy as, 0.95, 0.89, 0.23, 0.87 and 0.85 respectively. The study showed that environmental parameters of a RAS are greatly affected by variations in stocking densities and flow rates (P<0.05). Energy consumption increased from as low as 0.4 kWh at 2.0 L/min to as high as 2.3 kWh at 10.0 L/min for each stocking density. The developed RAS model demonstrated sufficient capability to predict environmental requirements for different stocking densities. From the study, we recommended that to maintain good RAS water quality and increased production and profits among farmers using RAS in Kenya, the right combination of stocking density, energy, and water flowrate should be utilized in RAS practices. More similar studies on RAS should be carried out for other fish species such as African catfish as well as with other biofilter media other than pumice to develop suitable biofilter materials for use in RAS for increased fish production.