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Dryden Aquaculture have been supplying diffusers and aeration systems to the Fish Farming Industry for many years. We have continued to develop our systems, improve the efficiency of our diffusers, and refine the performance of our aeration system under many different fish farm conditions. A list of some of the applications are included in the list below;

• smolt production, fish transportation by lorry/helicopter/ship
• land based salmon farms, lobster holding units
• cage based salmon farms, depuration systems
• trout ponds/raceways, thermal destratification of lakes
• seabass and bream tanks, foam fractionation
• shrimp tanks and ponds turbot
• tilapia, milk fish
• barramundi, crayfish

Dryden Aquaculture diffusers and aeration systems are used with a wide variety of species and types of systems, a more detailed review of some of these applications is detailed below.

In the case of salmonids the oxygen concentration should be maintained above 7mg/l to avoid the fish stress and ensure good feed conversion. If the concentration is between 6 and 7mg/l the fish should all survive, however they will be under a degree of stress which will affect feed conversion and their resistance to disease. Between 4.5 and 6mg/l fish moralities can be expected, the number of moralities will be related to the condition and well being of the fish. Below 4.5mg/l the fish will be under severe stress and mass fish moralities are likely.

For smolt production units our aeration systems are sized to meet 100% of the oxygen requirements of the fish and maintain the dissolved oxygen concentration at approximately 70% saturation with no influent water to the tanks. Some of the benefits which can be gained from our aeration systems are given below.

Security for approximately 5 to 10 hours against complete failure of the influent water. The time limit is dictated by ammonium concentrations.

Support the fish during periods of low water flow rate and or high water temperatures

Support the fish during disease treatment.

Aeration system can be linked to D.A. control system (section9.4.4) for the automatic

control of the blowers.

 Peace of mind that fish will not be lost through low oxygen.

Aeration system on a tank based farm

In the case of large scale trout farms with earth ponds, oxygen concentrations in the water tend to be variable due to different soil types and degree of fouling in the pond. Some rivers, especially slow meandering rivers, tend to exhibit massive fluctuations in dissolved oxygen concentration over a 24 hour period. An aeration system should therefore have the capacity to cope not only with the oxygen demand of the fish but the diurnal change in levels of the influent water, and oxygen requirements due to respiration of the pond.

In comparison with salmon smolts, rainbow trout are more tolerant of low dissolved oxygen levels, however due to the nature of the environment detailed above, an equivalent amount of aeration is often required.

Oxygen concentrators/generators are often proposed for trout farms. In these cases in order to make maximum use of the oxygen, medium to high pressure oxygen injection systems capable of achieving supersaturation levels are sometimes used. However if the output from the concentrator is not 100% pure, then nitrogen gas will also be injected into the water along with the oxygen. Under these conditions gas bubble disease can develop. The oxygen should therefore either be diffused into the water or injected into the water under ambient atmospheric conditions.

We believe Dryden Aquaculture has installed the largest trout farm aeration systems in the UK. The systems are monitored with D.A. dissolved oxygen monitoring systems which are also used to control the air blowers, thus maximizing the efficiency of the system.

By preventing the development of low dissolved oxygen levels the stress to which the fish are exposed is minimized. This has been shown to result in improved feed conversion as well as a greater resistance of the fish to disease such as PKD. In terms of feed conversion only, our aeration systems have been shown to give a pay-back in 1 to 2 years. When improved quality, reduced disease out-breaks and security of the system from pollution incidents are taken into account, the return in capital has often be measured in weeks or months.

The aeration systems increase the level of oxygen discharged from the fish farm, this level can be set so as to maintain levels of oxygen above minimum discharge criteria. Diffused aeration of the water also improves water quality in the ponds reducing ammonium levels, anaerobic conditions, BOD and stabilizing the pH. Aeration therefore greatly improves the environmental conditions for your fish but also improves the quality of the water discharged from the farm.

As a general guide 9 to 18 cubic metres of air are required per tonne of trout per hour. This air flowrate is delivered through either one or two 9cu.m/hr DrydenAqua diffusers.

Our aeration systems are in use for a wide range of species including sea bass & bream, tilapia, catfish and tropical shrimp such as P.monedon and japonicus.

In tank culture systems for sea bass, bream and tilapia, aeration requirements are calculated taking into account the environmental conditions prevailing in the tanks. Aeration is employed in order to achieve production stocking densities. Due to the high water temperature, the air requirements are often approximately twice the level required by salmonids in tank based systems. The situation is however very different for pond culture of marine fish and crustaceans.

The aeration efficiency and water mixing characteristics of the diffusers are enhanced in marine applications, such that a relatively small amount of diffused aeration can have a tremendous impact on the oxygen levels and quality of the water. In a shrimp pond environment it is easily possible to circulate the entire water volume in the pond every 10 minutes, prevent thermal stratification and stabilize water quality.

The diagram below shows the installation of a simple diffused aeration system in a shrimp pond with dimensions approximately 30metres x 30 metres. The aeration system depicted below will turn over the water every 10 minutes and could provide 100% of the oxygen demand of the shrimps (or fish) depending on the stocking density.