Numerical analysis of the performance of the density current generator for energy-saving shrimp culture

The intensive cultures have expanded rapidly due to their good productivity. However, the sustainability of intensive shrimp culture is increasingly being questioned. Many problems like disease due to virus, excessive nutrient enrichment, and hypoxic condition around the bottom soil are reported in the intensive culture ponds. In order to solve such problems, the water quality of the culture pond is managed every time. A paddle wheel aerator is generally installed per 500 to 1000 m2 in the intensive culture pond. The purposes of the paddle wheel aerator are to supply oxygen in hypoxic water and to transport sludge to the center of the pond. However, a paddle wheel aerator requires much electric power and sometimes occupies the cost more than half of the sales of shrimps. The present study proposes the use of the density current generator, which sucks water from upper and lower layers, and pushes out the mixed water in the middle layer. The purpose of the present study is to analyze numerically the performance of the density current generator for energy-saving shrimp culture. The numerical model is MEC (Marine Environmental Committee) ocean model, which contains the numerical simulation tool of hydrodynamic and ecosystem processes. Aerobic and anaerobic organic matters are considered in the benthic ecosystem model. The vertical profile of water current velocity around a paddle wheel aerator and a density current generator was given from the observations. The effects of shrimps are given as constant values assuming that the density of shrimps is around 40 individuals per square meter, that the feed conversion ratio is 2.5, and that the emission of wastes from shrimps is divided equally into organic and inorganic matters. As a result of numerical simulation, the basic environment in the shrimp culture pond was reproduced when paddle wheel aerators are installed in the pond. Then the performance of paddle wheel aerators and density current generator was compared based on the concentrations of dissolved oxygen and sludge. Sludge can be transported by density current generator using less energy. However, sludge is not eliminated from the pond when the density current generator is used, resulting in requiring more oxygen in water. The present study is the preliminary one to predict the performance of the density current generator. The future study includes the feasibility study in the actual or scale pond, comparing the results of numerical simulation and field investigation.