Water aeration

Fountains aerate the lakes.

Water aeration is the process of increasing the oxygen saturation of the water.

Water quality

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 methods

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 water. These fall into two broad areas – surface aeration and subsurface aeration. There are a number of techniques and technologies available for both approaches

Natural 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, water plants release oxygen into the water 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 water body and natural aeration can occur through a movement of water caused by an incoming stream, waterfall, or even a strong flood.

In large water bodies, autumn turn-over can introduce oxygen rich water into the oxygen poor Hypolimnion.

Surface aeration

Fountains

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

Typical mechanical surface aerator at work. It is often difficult for this type of machine to aerate the entire water column.

A one-horsepower paddlewheel aerator. The splashing may increase the evaporation rate of the water and thus increase the salinity of the water body.

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 water 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 a 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

Paddlewheel aerators also utilize air-to-water contact to transfer oxygen from the air in the atmosphere to the water body. 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 water. In this regard paddlewheel aeration works very similarly to floating surface aerators.

Subsurface aeration

Subsurface aeration seeks to release bubbles at the bottom of the water body and allow them to rise by the force of buoyancy. Diffused aeration systems utilize bubbles to aerate as well as mix the water. 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]

Jet aeration

Subsurface aeration can be accomplished by the use of jet aerators, which aspirate air, by means of the Venturi principle, and inject the air into the liquid.

Coarse bubble aeration

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 lake'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

Fine bubble aeration is an efficient technique of aeration in terms of oxygen transfer due to the large collective surface area of its bubbles.

Fine bubble aeration is an efficient way to transfer oxygen to a water body. 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, PVC or perforated membranes made from EPDM (ethylene propylene diene Monomer) rubber.^[2] Air pumped through the diffuser membranes is released into the water. 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) (9.1 kilograms of oxygen / (kilowatt * hour)).^[2] On average, diffused air aeration diffuses approximately 2–4 cfm (cubic feet of air per minute) (56.6-113.3 liters of air per minute), but some operate at levels as low as 1 cfm (28.3 L/min) or as high as 10 cfm (283 L/min).

Fine bubble diffused aeration is able to maximize the surface area of the bubbles and thus transfer more oxygen to the water 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]

However, almost all of the oxygen dissolved into the water from an air bubble occurs when the bubble is being formed. Only a negligible amount occurs during the bubbles transit to the surface of the water. This is why an aeration process that makes many small bubbles is better than one that makes fewer larger ones. The breaking up of larger bubbles into smaller ones also repeats this formation and transfer process.^[7]

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]

Lake destratification

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.^[8][9] Fish die-offs have been directly associated with thermal gradients, stagnation, and ice cover.^[10] Excessive growth of plankton may limit the recreational use of lakes and the commercial use of lake water.^[11] With severe thermal stratification in a lake, the quality of drinking water also can be adversely affected.^[12][13] 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.^[10]

One commonly used tool to reduce the severity of these lake management problems is to eliminate or lessen thermal stratification through aeration.^[8] 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.^[9]

Oxygenation barges

During heavy rain, London's sewage storm pipes overflow into the River Thames, sending dissolved oxygen levels plummeting and threatening the species it supports.^[14] 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.^[14]

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.^[15]

Similar options have been proposed to help rehabilitate the Chesapeake Bay where the principal problem is lack of filter feeding organisms such as oysters responsible for keeping the water clean. Historically the Bay's oyster population was in the tens of billions they circulated the entire Bay volume in a matter of days. Due to pollution, disease and over-harvesting their population are a fraction of their historic levels. Water that was once clear for meters is now so turbid and sediment ridden that a wader may lose sight of their feet before their knees are wet. Oxygen is normally supplied by "Submerged Aquatic Vegetation" (SAV) via photosynthesis but pollution and sediments have reduced the plant population as well. Resulting in a reduction of dissolved oxygen levels rendering areas of the bay unsuitable for aquatic life. In a symbiotic relation the plants provide the oxygen needed for underwater organisms to proliferate, in exchange the filter feeders keep the water clean and thus clear enough for plants to have sufficient access to sunlight. Researchers have proposed that oxygenation through artificial means as a solution to help improve water quality. Aeration of hypoxic water bodies seems an appealing solution and it has been tried successfully many times on freshwater ponds and small lakes. However no one has undertaken an aeration project as large as an estuary.^[16]

See also

• Aerated lagoon
• Lake ecosystem
• Limnology

References

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