Aeration for Aquaculture

Fish, and the microorganisms that they live with, all require oxygen for their respiration.  This oxygen, referred to as dissolved oxygen (DO), is present dissolved in the water and the fish access it across their gills via osmosis.  Because the concentration of oxygen is limited it can be utilised by these organisms to the point where fish are unable to take up oxygen via their gills due to the lack of osmatic gradient between DO in the water and their blood.  At this point the fish will suffer and could even die; this situation can arise fairly rapidly if the DO is not being replaced at the rate at which it is being consumed.  It is thus important to ensure that the fish population, and the associated microorganisms, have adequate oxygen available to them at all times.  As we feed the fish they consume and digest their feed, converting it into energy and growth.  This process requires oxygen as a fuel and it is known that each 1kg of feed requires approximately 0.5kg of DO, enabling us to calculate the amount of DO the system requires.

Aeration can be supplied in several different ways including air blowers, air pumps and paddle wheel aerators.  Side channel blowers are most commonly used as they provide large volumes of air at a convenient pressure for use in aquaculture systems at a water depth of around 1.0 – 1.5m.  The blower is situated centrally, and aeration is piped to the various points across the farm where it is required, meaning that a single power point is required, and that theft of the machine can be avoided by placing the blower inside a secure cage or room.  At the release points the pipes feed air into diffusers which efficiently break up the air into bubbles, which are released into the water and rise through the water.  The rising bubbles create friction against the water molecules, dragging the water upwards to the surface.  Interaction between the rising bubbles and water does result in some gaseous exchange but the bulk of the transfer occurs at the surface where the deep water now contacts the air, releasing carbon dioxide and replenishing oxygen.  Tiny bubbles rise slowly and thus create less vertical water movement, and additional pressure is required to get the blower to break the air stream into such tiny particles.  Larger bubbles demand less energy to create and are better at stirring the water; these are thus preferred.

There are occasions where aeration is required in smaller quantities but at a deeper water depth, and in this instance an air pump may be used.  In reality, however, a blower may still be the most effective method of stirring the water by using the blower to provide aeration for airlift pumps.  An airlift pump is essentially an open ended pipe with an elbow at the top end.  The elbow is ½ submerged and air from the blower is injected into the vertical pipe about 1.0m below the surface.  The rising bubbles inside the pipe create friction, dragging the water in to the pipe up and out via the elbow into the direction the elbow is pointing in, thereby creating vertical and horizontal water movement.  Depending on the depth of the water the vertical length of piping can be extended to just off the floor to optimise circulation.  Paddle wheel aerators may also be used for the aeration of earth ponds, but they require a power point at each pond, they are expensive and blowers with diffusers are better at converting kilowatts of energy into aeration.

A second function of air driven stirring in aquaculture is to keep the moving bed biofilter active.  This is achieved by running a network of piping or diffusers across the floor of the biofilter tank.  Bubbles rising vigorously from the piping cause the entire water volume to be stirred continuously, keeping the biomedia in constant motion for effective bacterial conversion of ammonia to nitrate.  Once again, the air/water interface sees oxygen being replenished and carbon dioxide given off into the atmosphere.

There are primarily two options that can be used to create the bubbles and stir the water in order to achieve gaseous exchange in fish farming, these being airstones and diffusers.  Airstones are made for coarse sand particles that are shaped into a block or cylinder.  Air from the blower is forced through this structure and breaks up due to the fine pathways between the sand grains, and is released into the water as fine bubbles.  Resistance is high, so airstones produce fine bubbles but fewer than for a diffuser.  Because airstones are inexpensive they are typically used in hatcheries where a large number of small tanks is housed, making them a cost effective solution.  Diffusers are made from rubber particles which are fused into a hollow tube.  Air is forced into the tube and disperses through the walls of the diffuser as mid-sized bubbles.  Diffusers are more expensive than airstones but produce significantly more aeration, and thus stirring, due to the lower resistance they create.  Diffusers are used in larger tanks and ponds where significant volumes of water need to be stirred.

Due to the high need for oxygen in an aquaculture setting, we need an efficient method of oxygen replacement, and a blower with diffusers is the first choice system.