Aeration of water removes gases or oxidizes impurities, such as iron and manganese, so that they can be removed later in the treatment process.
The constituents that are commonly affected by aeration are:
Volatile organic chemicals, such as benzene, found in gasoline, or ichloroethylene, dichloroethylene, and perchloroethylene, examples of solvents which are used in dry-cleaning or industrial processes.
Hydrogen sulfide (rotten-egg odor)
Iron (will stain clothes and fixtures)
Manganese (black stains)
Various chemicals causing taste and odor
Carbon dioxide is a common gas produced by animal respiration.
Apart from being naturally present in the air, it is produced by the combustion of fossil fuels.
It is used by plants in the photosynthesis process.
Surface waters have a low carbon dioxide content, generally in the range of 0 to 2 mg/l.
Water from a deep lake or reservoir can have high carbon dioxide content due to the respiration of microscopic animals and lack of abundant plant growth at the lake bottom.
Concentration of carbon dioxide varies widely in groundwater, but the levels are usually higher than in surface water.
Water from a deep well normally contains less than 50 mg/l, but a shallow well can have a much higher level, up to 50 to 300 mg/l.
Excessive amounts of carbon dioxide above a range of 5 to 15 mg/l in raw water can cause three operating problems:
It increases the acidity of the water, making it corrosive. Carbon dioxide forms a“weak” acid, H2C03 (carbonic acid).
It tends to keep iron in solution, thus making iron removal more difficult.
It reacts with lime added to soften water, causing an increase in the amount of lime needed for the softening reaction.
Most aerators can remove carbon dioxide by the physical scrubbing or sweeping action caused by turbulence.
At normal water temperatures, aeration can reduce the carbon dioxide content of the water to as little as 4.5 mg carbon dioxide per liter.
A poisonous gas, hydrogen sulfide can present dangerous problems in water treatment.
Brief exposures--less than 30 minutes--to hydrogen sulfide can be fatal if the gas is breathed in concentrations as low as 0.03 percent by volume in the air.
The Immediate Dangerous to Life and Health (IDLH) level for hydrogen sulfide is 300ppm.
Hydrogen sulfide occurs mainly in groundwater supplies.
It may be caused by the action of iron or sulphur reducing bacteria in the well.
The rotten-egg odor often noticed in well waters is caused by hydrogen sulfide.
Hydrogen sulfide in a water supply will disagreeably alter the taste of coffee, tea, and ice.
Hydrogen sulfide gas is corrosive to piping, tanks, water heaters, and copper alloys that it contacts.
Occasional disinfection of the well can reduce the bacteria producing the hydrogen sulfide.
Serious operational problems occur when the water contains even small amounts of hydrogen sulfide.
Disinfection of the water can become less effective because of the chlorine demand exerted by the hydrogen sulfide.
There could be corrosion of the piping systems and the water tanks.
Aeration is the process of choice for the removal of hydrogen sulfide from the water.
The turbulence from the aerator will easily displace the gas from the water.
The designer of the system needs to consider how the gas is discharged from the aerator.
If the gas accumulates directly above the water, the process will be slowed and corrosive conditions can be created.
Methane gas can be found in groundwater.
It may be formed by the decomposition of organic matter.
It can be found in water from aquifers that are near natural-gas deposits.
Methane is a colorless gas that is highly flammable and explosive.
When mixed with water, methane will make the water taste like garlic.
The gas is only slightly soluble in water and therefore is easily removed by the aeration of the water.
IRON AND MANGANESE
Iron and manganese minerals are commonly found in soil and rock.
Iron and manganese compounds can dissolve into groundwater as it percolates through the soil and rock.
Iron in the ferrous form and manganese in the manganous form are objectionable for several reasons.
Water containing more than 0.3 mg/l of iron will cause yellow to reddish-brown stains of plumbing fixtures or almost anything that it contacts.
If the concentration exceeds 1 mg/l, the taste of the water will be metallic and the water may be turbid.
Manganese in water, even at levels as low as 0.1 mg/l, will cause blackish staining of fixtures and anything else it contacts.
Manganese concentration levels that can cause problems are 0.1 mg/l and above.
If the water contains both iron and manganese, staining could vary from dark brown to black.
Typical consumer complaints are that laundry is stained and that the water is red or dirty.
Water containing iron and manganese should not be aerated unless filtration is provided.
TASTE AND ODOUR
Aeration is effective in removing only those tastes and odors that are caused by volatile materials, those that have a low boiling point and will vaporize very easily.
Methane and hydrogen sulfide are examples of this type of material.
Many taste and odor problems in surface water could be caused by oils and by-products that algae produce.
Since oils are much less volatile than gases, aeration is only partially effective in removing them.
Oxygen is injected into water through aeration.
This is, in most cases, beneficial. It increases the palpability of the water by removing the flat taste.
The amount of oxygen that the water can hold is dependent on the temperature of the water.
The colder the water, the more oxygen the water can hold.
However, water that contains excessive amounts of oxygen can become very corrosive.
Excessive oxygen can cause additional problems in the treatment plant by, for example, causing air binding of filters.
If operated properly, a process called air stripping can be quite effective in removing volatile organic chemicals (VOCs) from water.
The presence of VOCs, many of which are man-made or formed during industrial processes, is increasingly becoming a problem for public water suppliers.
US EPA has set Maximum Contaminant Levels for many VOCs (see the Public Water Supply Regulation chapter).
A major concern is that VOCs may be carcinogens. Example of VOCs are benzene from gasoline and trichloroethylene from dry cleaning establishments.
Air stripping has been shown to be capable of removing up to 90 percent of the most highly volatile VOCs.
It can be accomplished by letting the water flow over cascade aerators or in specially designed air-stripping towers.
In these, water is allowed to flow down over a support medium or packing contained in the tower, while air is being pumped into the bottom of the tower.
Discussion About Chemical Substances Affected By Aeration