The habitats of aquatic organisms within the recirculating aquaculture system are tanks with water that is specially prepared for this purpose.. There are 4 kinds of tanks: fiberglass, frame, polypropylene and assembly tanks. Depending on the age of the fish and purpose, the tanks vary in sizes and materials used.
The main objective of the preparatory and the following technological process is recycling of water, as 85 to 95 percent of the water removed from the fish tanks is to be recycled back to the system. Of course, the discharged water needs to be cleared out of waste products resulting from vital function of fish and other contaminating elements in order to be used again.
The first stage of water purification is mechanical filtration. The most efficient device for this purpose is a drum filter, which is a rotating filtering element (microscreen drum) enclosed in the housing. The filter requires periodic rinsing with clean water, which provides solution for two problems: removal of solids (fecal materials, remnants of feed) from the drum, and removal of water with accumulated nitrates and sulfates. The water will be mainly supplied to mechanical filters using gravity flow system. This method of supply does not disrupt the structure of suspended particles, and does not dissolve them in water, thereby positively influencing the quality of mechanical cleaning. Also, it increases efficiency of the system. There is no need to spend money for water supply using additional groups of pumps.
The second stage of water purification is biofiltration. During this stage, dissolved in water nitrogen will be removed from the water. Fish waste and the remains of feed let ammonium nitrogen, which is toxic for aquatic organisms, accumulate in the water. This problem can be solved by converting ammonia nitrogen into nitrates (allowable concentration of nitrates in water could be much higher than the concentration of ammonia). This chemical reaction occurs with involvement of special bioorganisms, i.e. bacteria living on the surface of the biofilter. A biofilter is a container filled with water and "biofilter medium", which provides surface areas on which bacteria colonies, can settle. The biofilter is aerated, which allows to create ‘boiling effect’ inside the filter. The aeration accelerates the filtering process, provides necessary oxygen for the "biofilter medium" and removes carbon dioxide accumulated by breathing fish from the water (degassing).
Further purification is carried out in a stream of water. That is why a pump group is installed after the biofilter. The sump tank, which is used for pumping out of water, is provided with fresh water supply. Thus, the sump tank is refilled with fresh water in a volume equal to the drained volume. Typically, this volume is between 5 to 15 percent of the total water.
Despite high permissible levels of concentration of nitrates in the water, you will need to remove them from the system. Denitrification shall be accomplished by adding clean water daily, or by introduction of a denitrifier. Denitrifier is an oxygenproof biofilter, installed bypassing the main water supply systems. Bacteria in the denitrifier decompose nitrates. The denitrification process takes place with constant infeed from a carbon source.
Filtering of water results in decreasing of alkalinity and pH level. These indicators must be managed by adding normalizing substances to the water periodically. To precisely control the indicators you must use automatic or manual pH level meters.
The disinfection process involves application of ultraviolet together with ozone. Ozone treatment of water will be the first stage. At this stage the ozonizer, a device generating ozone from oxygen and supplying it into the water will be used. Ozone sterilization neutralizes microorganisms, which add odor and color to water. Thereby it makes the water clear and suitable for further sterilization with ultraviolet light. The second stage is cleaning with ultraviolet by exposing the water to UV lamps, which makes the water maximally clean and suitable for the subsequent use in RAS.
Temperature Control of Water
After all the procedures of water cleaning are complete, and after adding fresh water, the water temperature decreases. It is necessary to raise the water temperature until it corresponds to biotechnical standards. For this purpose, you will use the heat exchanger, which like denitrifier is installed by method of bypass. The heat exchanger is connected to a heat source - a hot water with a temperature of 80 to 90 ° C, which warms up the process water in RAS to the required temperature.
For cold water species it is necessary to create conditions for artificial wintering, by reducing the water temperature below the ambient temperature. For this purpose, special installations "chillers" are to be placed outside the fish farming unit. The water cooled in the "chiller" enters the heat exchanger and transfers cold to the process water in RAS.
The final step in preparation of water before it enters the water tank is oxygenation, i.e. saturation the water with oxygen. The water goes through an oxygen cone - oxygenator, which is attached to the source of oxygen (oxygen station or oxygen cylinders). The oxygenator saturates the water with oxygen, the necessary parameters being observed.
Since accuracy in observation of biotechnical standards plays an important role for the quality of fish production, special attention is to be paid to feeding fish population as well. The most effective are automatic feeding systems providing the ability to maximally control the feeding time, types of feed and its amount. A pneumatic feed distribution system is especially effective. It allows provide up to 160 tanks with different age groups of fish. The system is provided with central automatic control, several feed bins for different types of feed, a feeder and conveying hoses. The entire system is controlled by a specially designed software with the possibility to make a detailed program for each tank