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Sea cage air lift aeration and oxygenation systems
Aeration of oxygenation in seawater using Dryden Aqua air diffusers
Oxygenation of sea cages,  1 x diffusers for 30 tonnes of biomass
Dryden aqua air diffusers are perfect for large scale oxygenation with pure oxygen in seawater systems.  When the salinity of the water is over 15ppt, the bubble size from the diffuser decreases to give a very fine mist. Also in oxygenation systems the diffusers may be located 20m or more below the water surface, the transfer efficiency is therefore around 100%.

Dryden aqua air diffusers are tubular, 32mm in diameter, and semi flexible.  The diffusers have their own internal ballast,  but below 6m depth they may require additional ballast. A standard diffusers measures 3m in length and will be supplied with oxygen via a 3/8" or 1/2" hose.  The quantity of oxygen is 5 to 10 cubic metres per hour per diffuser ( approximately 5kg  to 10kg of oxygen dissolved into the water). One diffuser running at 10 cubm/hr will provide sufficient oxygen to give effectively 100% support of 20 to 30 tonnes of fish.  If the diffuser is used to supplement the available oxygen, then one diffuser may be sufficient for up to 50 tonnes of biomass.  Since only one or two diffusers may be required per sea cage,  this makes for a very easy installation.

The location of the diffuser should be such that any current flowing through the cage does not result in a loss of the oxygenated water.  Normally the best location is the centre of large circular cages. In situations where rectangular cages are used either side of a walk-way,  is often the best position.

air diffusers prices click here ( recommend product code 6.2.9)

Aeration
Dryden Aqua have developed an aeration system for use in sea cages to help mix the water in the cage. The following section presents a description of the system.

The system comprise of the following components

    1. Air blower (s)  ( discharge pressure related to depth of diffuser)
    2. Air blower discharge metal pipe
    3. Air ring main
    4. Hose fittings
    5. Air diffusers

Air diffuser generate a mass up-welling of water which creates a substantial current of water from below the cage or through the sides of the cage depending upon where the diffuser(s) are located.  The diffusers are particularly effective in water that suffers from thermal stratification or low dissolved oxygen during slack water.

One air diffusers code 6.2.10 ( 3m long unit) requires an air flow of 10 cubic metres per hour.  If the diffuser is located at 20m or more below the surface of the water,  the air lift effect will generate an upward water circulation of around 1000  cubic metres of water per hour.  If the diffusers are drawing water in from out-side the cage then through the air lift effect the air diffusers will be provide approximately 10kg of oxygen per hour.

Size of the aeration system

The sizing of these systems is not precise, however it would appear that 2 diffuser per 40 m cage should be sufficient.

air diffusers prices click here ( recommend product code 6.2.9)

 
Environmental issues, clean the sea bed
 

Dryden Aqua air diffusers have been used for many years in earth pond raceways, lake and sea cage systems.  One of the applications of the diffusers is to improve water quality and minimise the impact of the waste feed and fish faecal matter on the environment.

Air diffusers located on the seabed or base of a raceway, generate a huge amount of gentle water movement. Fine bubble air diffusers are much more effective at generating a mass movement of water than course bubble diffusers.  Friction between the rising air bubble and the water drags water behind the air bubble,  the finer the bubble, the greater the surface area and water flow.

When air diffusers are used below a  cage system,  the mass up welling of water helps to maintain the feed in the case for a longer period of time thereby maximising the FCR. Also any waste feed or faecal matter passing through the cage will be dispersed over a wider area. 

Waste feed and faecal matter is high digestible by bacteria,  however they can only achieve this task if there is plenty of oxygen in the sediment.  Our air diffusers generate a water motion across the sediment and help to provide oxygen for the bacteria. The conditions are therefore improved for the in-situ  biodigestion of the waste.  A secondary effect is that the redox potential of the water will be increased slightly, and there is less risk from pathogenic reducing bacteria and fungi. The diffusers also appear to reduce the severity of sea lice infestation.