Aeration Diffusion

Air Diffusion Aerators

ADVANTAGES OF DIFFUSED AIR SYSTEMS

• Oxygen transfer is controlled by varying the
  air supply rate.
• Introduction of compressed air to the digester generally adds heat to the system, thus minimizing temperature loss during cold weather.
• Overall heat loss from the system is minimized because of the relatively small degree of surface turbulence.

The equipment used to deliver oxygen to an aeration system is typically provided by surface
mechanical type aerators or diffused aeration systems. Common types of mechanical
surface aeration equipment include low speed mechanical aerators, direct drive surface aerators
and brush type surface aerators.
Diffused aeration systems include a low pressure, high volume air compressor (blower), air piping system and diffusers that break up the air by the dispersement of bubbles throughout the aeration tank.

The diffusion of air can be accomplished with several types of diffusers.

• Coarse Bubble Diffusers
• Fine Bubble Diffusers
• Jet Aerators
• Submerged Turbine Aerators
• Surface Mechanical Aerators

Diffusers are normally located near the tank bottom. Optionally, they may also be placed along one tank side to produce a spiral or cross-roll pattern or may be installed as a floor-mounted grid system. The installation must support air flow rates of 0.33 to 0.67 L/m3 x (20-40 cfm/1000 cu ft), which are usually required to ensure adequate mixing. The air flow rates that are necessary to meet oxygen transfer requirements could also depend on digester loading.

Both fine bubble and coarse bubble diffusers have been used in aerobic digesters. Plugging of diffusers is a potential problem in aerobic digesters, especially in digesters whose operation includes periodic settling and supernatant removal. When air is turned off, slude and sediments can enter the air piping and adhere to the inner walls of piping or diffusers. Nonclog and porous media fype devices are more resistant than the large bubble orifice type diffusers to this type of plugging. However, surface fouling of porous diffusers may occur.

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Function of Aeration

OXYGEN TRANSFER VIA AERATION

Oxygen transfer, which is a vital part of numerous wastewater treatment processes, is the process by which oxygen is transferred from the gaseous to the liquid phase. The functioning of the aerobic process (such as activated sludge) depends on the availability of sufficient oxygen. Because of oxygen’s low solubility and its consequential low rate of oxygen transfer, the amount of oxygen required to sufficiently meet the needs of aerobic waste treatments is not introduced through normal surface air-water interfaces, but through additional interfaces.

Oxygen can be supplied by means of air or pure oxygen bubbles introduced to the water, which create gas-water interfaces. In wastewater treatment facilities, submerged bubble aeration is most frequently accomplished by dispersing air bubbles into the liquid through diffusion aerators, or “bubble diffusers”
.

The diffuser or bubble aeration process consists of contacting gas bubbles with water for the purpose of transferring gas into the water. The most commonly used diffuser system consists of a matrix of perforated tubes (or membranes) or porous plates arranged near the bottom of the tank to provide maximum gas to water contact.

For GOOD PERFORMANCE, the rate of supply of dissolved oxygen should be equal to the rate of oxygen consumption exerted by the mixed liquor under any given set of circumstances.

While a number of equipment and operational parameters interact to influence the efficiency and rate of transfer of oxygen for a given volume of water being aerated, aeration devices are evaluated on the basis of the “quantity of oxygen tansferred per unit of air introduced to the water for equivalent conditions”.

AERATION TO MAINTAIN AN AEROBIC CONDITION

The function of aeration in wastewater treatment is to maintain an aerobic condition. Its basic purpose is to improve water quality for subsequent reuse.

Aeration for wastewater or water treatment (or oxygen transfer) is well-studied. Methods developed to estimate the oxygen demand are biochemical oxygen demand (BOD), chemical oxygen demand (COD), total oxygen demand (TOD), total organic carbon (TOC), and theoretical oxygen demand (ThOD).

Aeration can bring about the physical removal of taste- and odor-producing substances, such as hydrogen sulfide (H₂S) and other volatiles and the chemical removal of metals (iron, manganese), gases (hydrogen sulfide), and other compounds (organics and inorganics) through oxidation and settling. Aeration is also used extensively for the biological oxidation of both domestic and industrial organic wastes.