As we all know, "recirculating aquaculture" is the model with the greatest development prospects in modern aquaculture industry, and it has distinct advantages, such as: ① recycling of aquaculture water to save water resources ② closed and controllable environment, not affected by climate ③ not affected by foreign bacteria and viruses ④ saving temperature control costs ⑤ wastewater concentration and integrated treatment, reducing costs, etc.
However, if a good "circulating aquaculture system" wants to achieve continuous output of production benefits, it requires relevant practitioners to take the core key points into consideration when designing and using the entire system to prevent the designed system from being "good-looking but not practical". Below, we will briefly analyze the following key points that are easily overlooked by practitioners, and then elaborate on them according to the actual situation.
1. Water quality standards for system design
The referenced literature for the aquaculture water quality standards of common species is based on standard toxicological experiments. It is usually aimed at short-term exposure of young fish to a constant concentration, while maintaining other water quality parameters within an acceptable range. It is difficult to extrapolate these experiments to the entire production cycle, and to analyze them in combination with environmental factors (for example, high ammonia nitrogen and high carbon dioxide under supersaturated dissolved oxygen conditions). In addition, the aquaculture density is very low, and the data fluctuations are very small. These are all experimental conclusions from scientific research institutes that are not practical, and data from large commercial aquaculture companies are missing. This requires us to: ① not be superstitious about the data in some literature ② it is important to archive and record this part of the data during commercial aquaculture ③ real large-scale commercial aquaculture experience is very important.
2. Challenges of heavy metal accumulation to limited water exchange
Copper is present in low levels in most feeds and if blowdown is inadequate, copper levels can accumulate to levels that are hazardous to the health of the fish; similarly, components used to build the system will continually leach “materials” into the system, potentially making the water toxic. Our fish diets contain a number of nutrients (elements such as calcium, sodium, potassium, copper sulfate, and hydrocarbons) that are either absorbed by the fish or released into the water as dissolved or solid waste.
In limited water exchange systems, heavy metals can gradually accumulate to toxic levels, such as copper, zinc and chromium from corrosion of pipes and equipment or vitamin premixes. The toxicity of heavy metals depends largely on water chemistry. Ultimately, there are only a few data on the effects of pheromones, endocrine disruptors and permeates from coatings, PVC components and linings. These chemicals can have profound adverse effects on aquatic animals, such as reduced fertility and growth rates by interfering with the endocrine system of aquatic animals. The environmental chemistry of different endocrine disruptors is complex and the cost of detection and analysis is expensive. Therefore, their monitoring and removal pose a major challenge to our safe production; therefore, they are also the main reason why the zero water exchange model cannot currently achieve the high yield of the large water exchange model (the other is the consumption and supplementation of trace elements).
3. How to reasonably arrange the water inlet and sewage outlet
The operation mode of circular culture pond is to flow water into the culture pond along the tangential direction of the culture pond wall, so that the water flow rotates around the center of the culture pond to form a rotating flow. The non-slip state of water flow between the bottom and the side wall of the fish pond will produce secondary flow, which has a clear tendency to flow inwards in the diameter at the bottom of the pond and outwards in the diameter at the surface of the pond. This radial flow inward along the bottom of the culture pond will bring settleable solids to the central drain, thereby generating a self-purifying force in the circular culture pond. However, in such a circular culture pond, the area near the central drain will become a vortex-free zone with low velocity and poor mixing, and particles will settle to the bottom of the pond. Because of the existence of this vortex-free zone, small circulations will occur in certain locations, resulting in poor gradients in water quality (especially dissolved oxygen) and relatively static areas for particle sedimentation.
In addition, the degree of particulate matter also depends on the extent to which the fish stir up the sediment to resuspend it through their own movement. This explains to some extent why the lower density culture ponds cannot be cleaned as well as the higher density culture ponds. In addition, because the solids produced by aquaculture have a specific density relatively close to that of water (usually 1.05-2.20), the bottom of the pond sloping towards the central drain does not improve the self-cleaning ability of circular culture ponds. The sloping bottom is only useful when the pond needs to be drained for maintenance. The use of a combination of vertical and horizontal branching inlet pipes can achieve the most uniform mixing of the water in the culture pond. The inlet pipe must be placed slightly away from the wall so that the fish can swim between the pipe and the wall. The benefits of this are: uniform mixing; prevention of short backflow of water flow; uniform velocity along the depth and radius of the culture pond; effective transportation of solid waste to the bottom of the pond and discharge to the central drain. For large circular culture ponds with a diameter greater than 6m, placing multiple inlet pipes at different locations in the culture pond can improve the particle removal rate and water quality uniformity, and is more convenient for naked eye observation. Recent experience has shown that when using a vertical inlet pipe, it is best to orient the outlet at 45° to the pool wall. This produces a flow pattern that is adequate to replace a horizontal inlet pipe.
Experience shows that the diameter of the bottom drain should be about 10% of the diameter of the circular pool. The distance from the capture point to the external discharge pipe should be as short as possible to prevent solids from settling in the bottom drain pipe. A small cone bottom in the center can achieve this purpose, but it also requires another plate to cover the cone bottom and lift a certain gap to generate sufficient suction. It is recommended to use horizontal rectangular holes instead of circular holes at the drain because they are easier to clean, have a larger open area, and are less likely to clog.