Water aeration is the process of increasing the oxygen saturation of the water.
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Water aeration is often required in water bodies that suffer from anoxic conditions, usually caused by adjacent human activities such as sewage discharges, agricultural run-off, or over-baiting a fishing lake. Aeration can be achieved through the infusion of air into the bottom of the lake, lagoon or pond or by surface agitation from a fountain or spray-like device to allow for oxygen exchange at the surface and the release of noxious gasses such as carbon dioxide, methane or hydrogen sulfide.
Dissolved oxygen (DO) is a major contributor to water quality. Not only do fish and other aquatic animals need it, but oxygen breathing aerobic bacteria decompose organic matter. When oxygen concentrations become low, anoxic conditions may develop which can decrease the ability of the water body to support life.
Aeration speeds up this process of oxidizing organic and mineral pollution. In fact, if there is sufficient aeration, the fish will be able to survive, where before they suddenly died. By pumping compressed air out to the bottom of a lake, lagoon or pond with the use of a diffuser, the rising air bubbles and the friction caused in the water will bring bottom water to the surface where it is exposed to the atmosphere. Large volumes of water thus release noxious gases to the atmosphere, water picks up oxygen while circulating at the surface.
Natural bacteria are stimulated by aeration and circulation and they will feed on muck, organics and the food that normally feeds algae blooms or aquatic plants growth. Using aeration and bacteria is often a safe and sound form of pollution removal.
Any procedure by which oxygen is added to water can be considered a type of water aeration. This being the only criterion, there are a variety of ways to aerate a pond. There are two categories of pond aeration: surface aeration and subsurface aeration.
Natural aeration is a type of both sub-surface and surface aeration. It can occur through sub-surface aquatic plants. Through the natural process of photosynthesis, pond plants restore oxygen to the water, and thus aerate the pond, providing it with the oxygen necessary for fish to live and aerobic bacteria to break down excess nutrients.[1]
Oxygen can be driven into the water when the wind disturbs the surface of the pond and natural aeration can occur through a movement of water caused by an incoming stream, waterfall, or even a strong flood.
Fountains aerate by pulling water from the surface of the water (usually the first 1–2 feet) and propelling it into the air. Some fountains incorporate the use of a draft tube, which extends deeper and is able to pull water from approximately six feet below the surface, so as to achieve more water circulation.
A fountain consists of a motor that powers a rotating impeller. The impeller pumps water from the first few feet of the water and expels it into the air.[2] This process utilizes air-water contact to transfer oxygen. As the water is propelled into the air, it breaks into small droplets. Collectively, these small droplets have a large surface area through which oxygen can be transferred. Upon return, these droplets mix with the rest of the water and thus transfer their oxygen back to the ecosystem.
Fountains are a popular method of surface aerators because of the aesthetic appearance that they offer. However, most fountains are unable to produce a large area of oxygenated water.[2] Also, running electricity through the water to the fountain can be a safety hazard.
Floating surface aerators work in a similar manner to fountains, but they do not offer the same aesthetic appearance. They extract water from the first 1–2 feet of the water body and utilize air-water contact to transfer oxygen. Instead of propelling water into the air, they disrupt the water at the pond’s surface. Floating surface aerators are also powered by on-shore electricity.[2] Surface aerators are limited to a small area as they are unable to add circulation or oxygen to much more than an 3 metre radius. This circulation and oxygenating is then limited to the first portion of the water column, often leaving the bottom portions unaffected.
Paddlewheel aerators also utilize air-to-water contact to transfer oxygen from the air in the atmosphere to the pond. They are most often used in the aquaculture (rearing aquatic animals or cultivating aquatic plants for food) field. Constructed of a hub with attached paddles, these aerators are usually powered by a tractor power take-off (PTO), a gas engine, or an electric motor. They tend to be mounted on floats. Electricity forces the paddles to turn, churning the water and allowing oxygen transfer through air-water contact.[2] As each new section of water is churned, it absorbs oxygen from the air and then upon its return to the water, restores it to the pond. In this regard paddlewheel aeration works very similarly to floating surface aerators. .
Subsurface aeration seeks to release bubbles at the bottom of the pond and allow them to rise by the force of gravity. Diffused aeration systems utilize bubbles to aerate as well as mix the pond. Water displacement from the expulsion of bubbles can cause a mixing action to occur, and the contact between the water and the bubble will result in an oxygen transfer.[3]
Coarse bubble aeration is a type of subsurface aeration wherein air is pumped from an on-shore air compressor.[4] through a hose to a unit placed at the bottom of the water body. The unit expels coarse bubbles (more than 2mm in diameter),[5] which release oxygen when they come into contact with the water. which also contributes to a mixing of the pond’s stratified layers. With the release of large bubbles from the system, a turbulent displacement of water occurs which results in a mixing of the water.[3] In comparison to other aeration techniques, coarse bubble aeration is very inefficient in the way of transferring oxygen. This is due to the large diameter and relatively small collective surface area of its bubbles[3]
Fine bubble aeration is an efficient way to transfer oxygen to a pond. A compressor on shore pumps air through a hose, which is connected to an underwater aeration unit. Attached to the unit are a number of diffusers. These diffusers come in the shape of discs, plates, tubes or hoses constructed from glass-bonded silica, porous ceramic plastic, or perforated membranes made from EPDM (ethylene propylene diene Monomer) rubber.[2] With EPDM rubber units, there are approximately eight thousand holes per disk through which air is diffused. Air pumped through the diffuser membranes is released into the water at a rate of roughly 30-50 thousand bubbles per second. These bubbles are known as fine bubbles. The EPA defines a fine bubble as anything smaller than 2mm in diameter.[5] This type of aeration has a very high oxygen transfer efficiency (OTE), sometimes as high as 15 pounds of oxygen / (horsepower)hour.[2] On average, diffused pond aeration diffuses approximately 2-4 cfm (cubic feet of air per minute), but some operate at levels as low as 1 cfm or as high as 10 cfm.
By releasing millions of fine bubbles instead of fewer coarse bubbles, fine bubble diffused aeration is able to maximize the surface area of the bubbles and thus transfer more oxygen to the pond per bubble. Additionally, smaller bubbles take more time to reach the surface so not only is the surface area maximized but so are the number of seconds each bubble spends in the water, allowing it more time to transfer oxygen to the water. As a general rule, smaller bubbles and a deeper release point will generate a greater oxygen transfer rate.[6]
One of the drawbacks to fine bubble aeration is that the membranes of ceramic diffusers can sometimes clog and must be cleaned in order to keep them working at their optimum efficiency. Also, they do not possess the ability to mix as well as other aeration techniques, such as coarse bubble aeration.[2]
Windmills[7] may also be used to power fine bubble aerators.
Solar Panels[8] may also be used to power fine bubble aerators and offer many advantages over other aeration methods including; • Solar pond aerators can be used in remote locations, far from any electrical outlets. • Once you have a solar pond aerator installed, you will not have to pay for the energy needed to run it. • As long as they can receive plenty of sunlight, these aerators will work just as well as other aerators.
Circulators are commonly used to mix a pond or lake and thus reduce thermal stratification. Once circulated water reaches the surface, the air-water interface facilitates the transfer of oxygen to the lake water.
Natural resource and environmental managers have long been challenged by problems caused by thermal stratification of lakes.[9] [10] Fish die-offs have been directly associated with thermal gradients, stagnation, and ice cover.[11] Excessive growth of plankon may limit the recreational use of lakes and the commercial use of lake water. [12] With severe thermal stratification in a lake, the quality of drinking water also can be adversely affected.[13][14] For fisheries managers, the spatial distribution of fish within a lake is often adversely affected by thermal stratification and in some cases may indirectly cause large die-offs of recreationally important fish.[15]
One commonly used tool to reduce the severity of these lake management problems is to eliminate or lesson thermal stratification through aeration.[16] Many types of aeration equipment have been used to reduce or eliminate thermal stratification. Aeration has met with some success, although it has rarely proved to be a panacea.[17]
During heavy rain, London's sewage storm pipes overflow into the River Thames, sending dissolved oxygen levels plummeting and threatening the species it supports.[18] Two dedicated McTay Marine vessels, oxygenation barges Thames Bubbler and Thames Vitality are used to replenish oxygen levels, as part of an ongoing battle to clean up the river, which now supports 115 species of fish and hundreds more invertebrates, plants and birds.[18]
The dissolved oxygen concentration within Cardiff Bay are maintained at or above 5 mg/l. Compressed air is pumped, from five sites around the Bay, through a series of steel reinforced rubber pipelines, laid on the beds of the Bay and Rivers Taff and Ely. These are connected to approximately 800 diffusers. At times this is insufficient and the Harbour Authority is using a mobile oxygenation barge built by McTay Marine with liquid oxygen stored in a tank. Liquid oxygen is passed through an electrically heated vapouriser and the gas is injected into a stream of water which is pumped from, and returned to, the bay. The barge is capable of dissolving up to 5 tonnes of oxygen in 24 hours.[19]
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