There are a variety of tank designs which are of standard use in aquaculture systems. These include round tanks, D-ended tanks, and raceways. General descriptions and benefits of each are listed below.
Round tanks have the advantage of a naturally self cleaning action. As the water swirls around the tank, solids are drawn towards the middle, where the outlet is situated. Due to this property, they are often used in hatcheries, where due to high feed rates, solids loadings (waste feed and faeces) can be very high and also in recirculation systems, to remove the solids as soon as possible, before they break down. Round tanks can be constructed of almost any material, the most common being fiberglass (for tanks 8m diameter and under), steel (lined or unlined) and concrete or concrete block. Other materials can be used as long as it is strong enough to hold the water without distortion and is non-corrosive, non-abrasive and non-toxic. Round tanks generally have a slope of about 1:50 (2%) on the bottom towards the center outlet to increase solids removal efficiency. Other qualities of round tanks include: a good mixing of the water, resulting in easy oxygenation; and less contact of the fish with the tank sides and bottom, due to a higher ratio of tank volume : tank wall and bottom. Many species prefer the consistent current of a round tank to other systems. The disadvantages of round tanks include poor use of land area and difficulties in management (fish removal, screen cleaning), especially in tanks with a diameter larger than 5 meters.
Raceway is a general term given to a straight sided artificial channel in which fish are held. Generally these have a high water turnover rate, occuring in less than one hour. The advantages of raceways are that they can be easily built in series, with the water flowing from one to the other and that they are easy to empty of fish by using a simple crowding screen. To be self cleaning, raceways must be operated at high flow rates and/or high stocking densities, where the movement of the fish keep the faeces and uneaten feed from settling. Disadvantages of raceways are brought about primarily by poor mixing, and include the gradual deterioration of the water quality along the length of the raceway (whereas round tanks tend to be more even). Difficulty in efficient distribution of additional oxygen throughout the raceway can prove to be another disadvantage. Raceways are usually built with a width to depth ratio of between 2:1 and 4:1, with the length limited either by the amount of fish that can be held in a single holding unit or the deterioration of water quality. The even nature of raceways means that they are somewhat flexible, allowing screens to be placed anywhere along the length of the raceway, thus dividing a single unit into 2 or more smaller units. Modifications to the designs include rounded bottoms to concentrate solids for ease of cleaning by vacuuming - especially where small fish are involved. Aeration along the length of the raceway may also be included, which serves to maintain more even oxygen concentrations along the length of the raceway, and also concentrates settled solids into specific areas to make cleaning easier. Other modifications include the addition of barriers in the raceway to create a swirling motion in the water where waste concentrates. Raceways are advantageous in that they can be constructed with basic building materials such as bricks, blocks or poured concrete and require little specialized labour.
Raceways vs Round Tanks
Water exchange is a very important factor to consider when designing an aquaculture system. The water turnover rate will determine how quickly and completely it is filtered and/or sterilized and, what the ammonia and oxygen levels will be. Sedimentation and/or self cleaning characteristics are also very important.In a raceway tank, an almost complete water exchange can be achieved with one equal volume of water. As the water flows into the system, it pushes the old water ahead of it. Self cleaning may be accomplished with high stocking rates and low water levels.In a round tank, a lot of mixing occurs between the new and the old water before it reaches the drain. In a typical 1000L tank, when 1000L of new water is added to the tank, the water exchange will only be about 60%, meaning the tank will now hold 600L of new water. To achieve a 98% water exchange it will take over 9 times the amount of water already in the tank! Self cleaning is typically very good in tanks less than 30 feet in diameter even with flat bottoms partially due to centripetal force.Round tanks may be less expensive but not utilize space as well as raceways. The decision gets even more complicated when you consider deep round tanks (silos), cross flow raceways, "D" ended tanks, and other styles.
D-ended tanks are a type of holding unit which are very economical in terms of space. These tanks can be constructed from most materials, including fiberglass and concrete. They enables a lower tank turnover time, without compromising velocity rates and self cleaning abilities. Inlet pipes and aeration / oxygenation devices are positioned to create the desired water velocity rate. Useful in situations where space and make up water are limited.
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