AERATION in POTABLE WATER TREATMENT

PURPOSE:
Air is diffused through a water treatment stream to change the concentrations of gases and volatile substances in the water.

BACKGROUND:
Aeration has been employed to improve the quality of the water supply by changing the chemical compounds and gases in the water into more acceptable compounds.
Aeration has two (2) principle process activities:
1) aeration strips gases physically from a liquid solution
2) oxidation - a chemical reaction between oxygen and chemicals or metals in the water

Both are governed to an extent by the ratio of the surface area of the air in contact with the surface area of the liquid solution. (An increase in the surface area of the air results in an increase in the oxygen transfer rate.)
Aeration will reduce the following concentrations:
a) Gases such as carbon dioxide (CO2), hydrogen sulfide (H2S), & methane (CH4)

Carbon dioxide - removed by aeration; corrosive as it creates an acid; occurs more frequently in shallow wells than in deep wells; occurs more in the deeper regions of a lake or raw water reservoir than in the shallow or surface regions.

Hydrogen sulfide - remove by aeration at pH of <6 to 6.5; corrosive as it creates an acid; occurs more in groundwater than in surface waters; reacts with chlorine as part of the chlorine demand value. (Above pH of 8 exists primarily not as a gas but as an ionized form. Solution: lower the pH, then aerate!)

Methane - removed by aeration; reduces the amount available to help create THMs; more prevalent in groundwater sources than in surface water sources.

To highlight: carbon dioxide and hydrogen sulfide are compounds that contribute to the corrosive functions of some waters.
b) Some of the volatile organic compounds that may cause taste and odor problems. Some light industrial chemicals and naturally occurring compounds which are easily oxidized
c) Aeration is sometimes used to reduce iron & manganese concentrations by oxidation. Usually followed by filtration to remove the precipitated particles

Aeration may be used to reduce elevated levels of dissolved oxygen caused by the activity of algae in a nutrient a rich and high level of sunlight energy environment.

Hydrogen sulfide and some organics are examples of taste and odor causing compounds.

Aeration will increase the following concentrations:
a) Oxygen and other atmospheric gases. Also, over-oxygenation of waters produces a corrosive water. Water with little oxygen may have a flat taste to it. Lower water temperatures are able to hold a higher dissolved oxygen concentration level. Generally, 2 to 4 mg/L DO concentrations are acceptable, and not significantly corrosive.
b) Aeration may be used in conjunction with carbon dioxide after a water softening process, to lower the pH of the water, and to "freshen" it prior to the next process, especially if hydrogen sulfides are present in quantities that can be reduced in a cost effective manner.

PROCESS EQUIPMENT DESCRIBED:
Aeration may be undertaken by:
a) Cascades (waterfalls), where water is cascaded over trays, concrete steps, etc to create a thin film of water for oxygen to diffuse into the water, and substances to be released, etc.

b) Spraying systems, where the water is sprayed through the atmosphere, and captured in a vessel. to increase the oxygen content, and substances are released.

c) Diffuser systems where small bubbles of air are passed up through the treatment stream to increase the oxygen content, and substances are released.

d) Packed columns: water is cascaded down over plastic, ceramic, or stainless steel packing pieces with air blown upwards through the media and falling water

OPERATION:
There are several items to be considered in the operation of an aeration process:
a) Algal and bacteriological slime growths, especially if exposed to direct sunlight. Control these by installing a cover to reduce the sunlight; dose with copper sulfate.
b) Freezing temperatures, if it is not an enclosed process. Be cognizant of this fact!

c) Impact on down stream processes as applicable:
1) creation of scum (floating particles) on the surface of the water in the sedimentation basins. Due to the release of the oxygen from the solution which acts like a dissolved air flotation process. (An Alka Selzer tablet!)
2) Release of oxygen in the filters (causes air binding) due to an increase in the water temperature has the water passes through the warm media. (Cold water holds more oxygen, and releases it as it warms) to air bubbles form and avoid spaces in the media. If excessive, it increases the hydraulic head and may disturb the media if the oxygen bubbles increase to the quantity that they rise out of the filter media usually creating a turbidity breakthrough has a result.
3) Corrosion of the equipment and the structure, due to the constant exposure to water and a high oxygen content. This condition maybe reduced by not entering a super saturated oxygen condition

d) Follow the prescribed preventative maintenance program items: which may include maintaining clean air filters; proper lubrication of motor and blower bearings; maintaining clean diffusers, etc.
e) As with all high-energy consuming processes, utilize the minimum amount of air necessary to attain the process goals.
f) As usually occurs, there are usually more process changes for surface waters than groundwater's. Lake, and other surface waters may vary greatly in dissolved oxygen concentration, pH, temperature, and taste and odors from season to season, from day to day, and even diurnally

The following laboratory analysis are performed on the water entering and leaving the process, depending on the specific goal of the aeration process:
a) Dissolved oxygen Primary process control is usually < 4 mg/L dissolved oxygen concentrated treated water
b) pH Best range depends on the process goal:
pH <6 for H2 S scrubbing;
pH 8 to 9 for iron and manganese removal

c) Carbon dioxide. As the amount of carbon dioxide is decreased, the pH rises
d) Iron and manganese
e) Hydrogen sulfide
f) Taste and odor thresholds

Summary of "THE SCIENCE":
Most taste and odor causing compounds are not easily reduced or removed by aeration alone.
a) Remember, aeration may cause a corrosive water to form if over-aeration occurs in some waters.

b) Algal activities are most often impacted very much by aeration, due to the fact that the oils given off by the algae are not easily reduced or eliminated.

c) Some light weight industrial wastes may be reduced by aeration followed by several days storage in a reservoir, which gives time for the oxidation process to proceed.
d) Carbon dioxide and some of the other gas levels are reduced only to the background level of the atmosphere, usually around 5mg/L (depending on the temperature of the water and other factors). The gases are mostly reduced by the scouring action of the air passing through it.

e) Iron and manganese, when placed into an aeration process causes an oxidation reaction to occur, which them allows for the iron and manganese to precipitate out.

f) Hydrogen sulfide is a major contributor to chlorine demand values. Reducing the level of hydrogen sulfides, lowers the cost of disinfection. Reducing its levels also reduces corrosion in piping and in concrete structures. (Combines with water to create sulfuric and sulfurous acids.)

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