Study Of An Innovative Isolated Device For Use In Industrialized Turtle-Breeding Tanks

In recent years, turtles have been more and more popular with people in the market for their high medicinal value and nutritional value. Zhejiang province has the largest turtle production, which accounts for about50%of the total production in China. Turtle cultivation has become one of important ways of increasing farmers’income. Industrialized aquaculture in greenhouse is the most important way of turtle cultivation. This farming method is a kind of high density culture mode reared in environment controlled buildings, characterized by high input of artificial feeds. However, for the culture mode, the turtles frequently crawl and swim to the surface of water for a breath, bringing the excrements and residual feeds at the bottom of pond to the living area and leading to a rapid degradation of water quality. The total ammonia nitrogen (TAN) concentration and COD concentration of culture water in the tank may reach as high as100mg/L and300mg/L,respectively, which are very harmful to turtles’health and lead to frequent water exchange that consumes large amount of resources and destroys the environment. This study investigated the effects of an isolated device installed in a turtle-breeding tank on the water quality improvement and wastewater discharge reduction, which separates turtle living area from deposition area containing excrements and residual feeds and then avoids excrements and residual feeds being taken to turtle living area as a result of turtles’activity.In a controlled environment laboratory, a contrast test about the water quality variation and water exchange was carried out in two small size turtle culture tanks<0.87m×0.58m×0.62m>, one with an isolated device, and the other without. During the experiment period, a total of6turtles (about300g in weight) were reared in each tank, which is at a density of12turtles per square meter. Turtles in the culture tank without isolated device respectively weighed286.3g,290.6g,305.3g,296.5g,310.5g and282.8g. Turtles in the culture tank with the isolated device respectively weighed298.6g,311.6g,287.9g,309.3g,303.2g and293.4g. One third of water in tank would be exchanged when the chemical oxygen demand<COD> concentration of culture water exceeded230mg/L. The experiment investigated the actual application effects of the isolated device mainly by analyzing the water quality variation and wastewater discharge. What’s more, the water flow regime in the culture tank with the isolated device was simulated with the help of CFD. The simulation was used to study the water turbulence through changing isolated device’s size, waterflood injection rate and culture water aeration for the further optimization design of the isolated device, which would provide technologies and foundation support for the large-scale applications. Primary results were summarized as follows:1) The total water exchange volume, total nitrogen<TN> and total phosphorus<TP> in the tank without the device were681.21L,68.77g,7.26g respectively, while302.76L,50.80g,5.81g in the tank with the device during the study. The total water exchange volume, TN and TP without the device was obviously higher than that of the tank with the device. Therefore, installing an isolated device in turtle-breeding tank separating turtle living area from deposition area is a simple and effective way to improve water quality and to reduce waste water discharge and the damage to environment.2) In the CFD flow regime simulation, when the gap width among the slats of the isolated device was designed to be0.005m,0.0025m or0.01m, the water flow regime under the isolated plate was not generally affected by turtles’ movement regardless of the isolated plate’s height; when the isolated plate’s height and the gap width was0.16m and0.005m respectively, the water flow regime under the isolated plate was not generally affected by turtles’movement regardless of the isolated plate’s thickness; When fresh water was injected into the turtle culture tank at the speed of lm/s,2m/s and3m/s, the greater waterflood injection rate was, the more violent water turbulence and the larger turbulence range were above the isolated plate. However, the water flow regime under the isolated plate was not affected by the injection rate; Bubbles generated by aeration only moved upwards because of water’s buoyancy. Besides, aeration only caused small-scale water turbulence above the isolated plate and had no effect on water under the isolated plate.3) After the water flow regime simulation to the practical culture tank with the isolated device, we can find that the water flow regime under the isolated plate was not generally affected by turtles’movement when the gap width among the slats of the isolated device was designed to be0.005m,0.0025m or0.01m.