The aeration process is the process by which water and air come into close contact to remove dissolved gases (such as carbon dioxide) and oxidize dissolved metals such as iron, hydrogen Sulphide and volatile organic compounds (VOCs).
Aeration is frequently the first significant process at a wastewater treatment plant. Aeration removes or modifies elements before they can interfere with treatment processes.
The aeration turbulence scrubbing technique physically removes dissolved gases from the solution and allows them to escape into the surrounding air.
Aeration also aids in the removal of dissolved metals by oxidation, which is the chemical reaction of oxygen from the air with certain unwanted metals in the water.
When these compounds are oxidized, they fall out of the solution and become particles in the water, which can be removed by filtration or flotation.
Aeration efficiency is determined by the amount of surface contact between air and water, which is mostly determined by the size of the water drop or air bubble.
Aeration introduces oxygen into water, which improves palpability by removing the flat flavor. The amount of oxygen that water can hold is mostly determined by its temperature.
Water with an excessive amount of oxygen can become extremely caustic. Excessive oxygen can also cause problems in the treatment plant, such as filter air binding.
Water aeration has long been used in water treatment to remove odor and taste-causing chemicals, oxidize iron and manganese, and improve corrosion control and aesthetics.
However, the method has been used to remove carcinogenic and dangerous substances from water since the mid-1970s.
Volatile organics such as trihalomethanes, radon, trichloroethylene, tetrachloroethylene, 1,1,1-trichloroethane, chloroform and toluene are examples of these compounds.
As a result, water aeration could be the most important water treatment procedure in the twenty-first century.
Types of aeration processes
Aeration may be accomplished in a variety of ways using different types of equipment including surface aeration, submerged aeration, and falling water unit.
The equipment types are listed below:
Falling Water Units (commonly used in water treatment)
1. Spray aerators that splash water into the air. Evaporation and freezing are both a problem.
2. Cascade aerators and hydraulic jumps—these work by directing water over a structure.
3. Fountain aerators or spray—water that cascades or is sprayed over rocks or other materials.
4. Multiple tray aerators with and without coke (commonly used to remove iron and manganese)—water cascaded over a produced tray made of slats and coke.
5. Packed column aeration—air flows up and water is splashed down the column (these are efficient and the most common type).
Surface Aerators (commonly used in the wastewater industry)
1. Mechanical surface aerators physically mix the water surface to increase the water-to-air interaction.
2. Brush: a succession of partially submerged circular brushes are rotated across the water surface to create turbulence. To suspend the brushes over the water, a support structure is required.
3. Floating: A floating aerator pushes water from beneath it to the surface via a draught tube, dispersing it into the air.
Submerged Aerators (commonly used in the wastewater and water industries)
1. Air injection with blowers via static tube or diffuser (fine bubble and coarse bubble).
2. Aeration by jet (the injection of air into pumped water).
Application of the aeration process
Taste and odor removal by aeration
It is sometimes difficult to identify the actual cause of odor and taste problems in water.
Some common odor- and taste-causing compounds include hydrogen sulfide (H2S), methane, algae, oils, phenols, cresols and volatile compounds.
Removal of taste and odor problems is a common application of the water aeration process.
The procedure is ideal for H2S, methane, and volatiles, but not for algae, oils, phenols, or cresols.
Aeration can only be effective if the chemicals are volatile. Many industrial chemicals benefit from aeration.
Nitro, West Virginia, has a traditional installation that uses aeration and granular activated carbon (GAC).
Because of industrial contamination, the raw water had threshold odor numbers (T.O.N.) ranging from 5000 to 6000.
The procedure proved efficient in lowering the taste and odor to 10-12 T.O.N. levels.
Although taste and odor applications are the most common, many other tastes and odors cannot be removed simply by aeration, which may explain why so many early plants were abandoned.
Oxidation of iron and manganese
When the total iron concentration in water is 0.3 mg/L or above, the iron causes the water to have an unpleasant taste and redden in color, which may result in staining of plumbing fixtures and clothing, as well as iron deposits accumulating in the water mains.
The aeration technique is a good and widely used pretreatment method for iron removal.
The process first oxides iron by converting it from a soluble form (Fe2+) to a non-soluble form (Fe3+) that precipitates from water.
When the pH is near 7+, this precipitation process occurs quickly. The iron is removed through sedimentation and filtration of the precipitated iron.
In theory, 1 mg/L of O2 will oxidize approximately 7 mg/L of Fe2+.
Manganese concentrations in water of more than 0.3 mg/L cause dark brown discoloration.
When the pH rises over 9, oxidation will change the manganese from Mn2+ to Mn4+. The reaction is negligibly slow below pH 9.
When tray aerators are used for aeration and the trays become covered with manganese oxides, the adsorption of accumulated oxidation products (Fe2O3 or MnO2) is followed by slow oxidation.
Groundwater Air Injection for Iron Control
Before pumping raw water to the municipal water treatment facility, the air is introduced into the groundwater source to reduce the iron concentration.
The iron in the groundwater is oxidized by the injected air.
This method includes injecting air into groundwater regularly through a network of wells that surround a production well.
Hydrogen Sulfide Removal by aeration
Well, water frequently contains hydrogen sulfide and carbon dioxide. Even low levels of hydrogen sulfide can produce odor and taste issues.
Hydrogen sulfide is a colorless gas with a rotten egg stench that is somewhat heavier than air (SG = 1.192).
The reduction produces molecular hydrogen sulfide, which dissolves and dissociates in water.
Aeration becomes less effective when the pH rises because there are fewer sulfides in the form of H2S to be eliminated by aeration. Municipalities and chemical businesses both use this method.
Ammonia removal
This has limited use in the water sector, although it is more typically employed in wastewater treatment.
The aerated suspended growth method, which uses nitrifying bacteria and aeration to convert ammonia to nitrites and nitrates, is one of the procedures used in the wastewater business.
Reservoir De-stratification and Water Oxygenation
Aeration can mix the water, reduce stratification, and increase the dissolved oxygen level in small reservoirs and ponds that have trouble maintaining dissolved oxygen levels in water near the bottom of the reservoir.
This is achieved by either installing diffusers on the reservoir floor and bubbling air into the water, or by utilizing floating aerators.
Aeration provides oxygen to water, improving its taste, but it also enhances corrosiveness by increasing CO2 levels in the water (resulting from the oxidation of organic matter to CO2).
As a result, there is frequently a trade-off between benefits and disadvantages.
Trihalomethanes Removal
Because trihalomethanes (THM) are relatively volatile, the aeration method is classified as good to outstanding for their removal.
THMs are not efficiently removed by other procedures such as granular activated carbon (GAC), even though GAC is excellent for organic precursors that react with chlorine to create trihalomethanes.
Volatile Organics Removal by aeration
The US EPA has detected a wide range of organic chemicals in our drinking water.
Because some organic chemicals are volatile, aeration would be a useful process choice for eliminating them from water.
Adsorption might be a preferable procedure choice than aeration for removing non-volatile chemicals from water.
Trihalomethanes, which have already been addressed, are among the most frequent volatiles.
These include chlorobenzene, 1,1,1-trichloroethane, tetrachloroethylene, and trichloroethylene.
Aeration can remove up to 95% of these chemicals.
Non-volatile organic compounds are also a source of concern in water sources.
Adsorption is an excellent approach for removing non-volatiles such as styrene, benzene, phthalates, and fluorine.
As a result, it is frequently rational to combine air stripping and carbon adsorption, especially when both volatile and non-volatile chemicals are present.
Because (a) all volatile organics have a propensity for the vapor phase, (b) most organics are hydrophobic (they don’t like water) and (c) air is widely available and affordable, air stripping is particularly ideal for volatile organics.
Nearly 1000 different types of organics have been found in drinking water, and air stripping will be used in additional applications in the future. This will be true, especially in areas of high pollution.
Removal of Radionuclides
Radionuclides are radioactive atoms.
The number of protons and neutrons in an atom’s nucleus, as well as its energy content, are used to characterize its radioactivity.
Radium, uranium, and radon are the most frequent radionuclides found in drinking water.
Nuclear radiation is emitted by radioactive atoms in three forms: alpha, beta, and gamma.
Radionuclide levels in drinking water are measured using a variety of technologies, including counters.
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