There are several designs and applications for aerators:
Diffused Aerators
Mechanical Surface Aerators Submerged Turbine Aerators
The two most common types of aeration systems are subsurface diffusion and mechanical aeration. Diffused air systems have been around longer than you.
Opened tubes were used or perforated pipes located at the bottom of aeration tanks. But a more efficient process was desired, born to the process, porous plate diffusers. In the diffused air system, compressed air is introduced near the bottom of the tank. Let’s look at the definition for diffused aeration:
“The injection of a gas, air or oxygen, below a liquid surface.”
There are a variety of hybrid air diffusion systems used in the process; we will focus on the basic components.
The following diagram highlights the main parts of the diffused aeration system.
Here is a rare and up-close view of non-porous diffuser heads. Notice the heads that are missing in the bottom photograph.
Aeration
The aerated systems described above need an oxygen supply. Depending on the characteristics of the process, different designs may be used. The oxygen can be supplied to the activated sludge by either diffused aeration, by turbine agitation, by static aerators, or by surface coarse or large bubble diffusers. The last two are also used in lagoon systems.
The diffused aeration systems are also divided into fine bubble, medium and coarse or large bubble diffusers. The fine bubble diffusers are built of porous materials (grains of pure silica or aluminum oxide are bonded ceramically or by resins) which provide very small bubbles of high surface area that favor the oxygen transfer from the air to the wastewater. The medium bubble diffusers are perforated pipes or tubes wrapped with plastic or woven fabric. The coarse or large bubble diffusers can be orifice devices of various types, some of which are designed to be non-clogging.
Fine and medium bubble diffusers
Coarse or Large Bubble Diffusers
With the small or fine bubble diffusers, it is important to use air free of particles that would otherwise clog them. Although somewhat less efficient for oxygen transfer, the coarse bubble diffusers are sometimes preferred because the presence of particles in the air is not a critical problem, and also for their lower cost and maintenance requirements. The diffusers are placed along air manifolds, close to the bottom of the aeration tanks.
The static aerators are vertical tubes placed at the bottom of the aeration tank, with packing material along its length. The compressed air is supplied from the bottom of the tubes, forcing a mixture of air and water through the packing, where most of the oxygen transfer to the wastewater takes place. They have been used mainly in aerated lagoons.
Oxidation ditches are used for nutrient removal.
Blowers
In the diffused aeration system, blowers are used to circulate the tank’s contents by the air-lift effect. The air filter on the blower removes dirt from the air. Therefore, helps prevent diffuser clogging. Before all this begins, we need a power source to drive the blower. Usually, electric motors are used but in remote locations, gas or diesel engines can be used as well. In some states, solar energy is available to provide the power.
As illustrated in the photograph below, the rotation of the motor shaft is transferred to the blower shaft by means of a flexible coupling or through drive belts. The blowers that we will refer to are centrifugal blowers.
The centrifugal blower works like a centrifugal pump or a fan. Rotating impellers or fans cause movement of the air through the blowers. You have an intake side that takes in the air and the discharge side the forces the air out. The number of impellers you have will determine if it is a multi-stage or single-stage blower. The photograph below illustrates the major components of a centrifugal blower.
A lobe blower utilizes positive displacement; it also has an intake and a discharge side. The lobes turn in opposite directions in the casing. As they turn, the air is drawn in through the blower inlet and is trapped. The lobes keep turning, opening the blower
Before we continue let’s review what you just read about the blowers and motors.
1. What are two ways that the motor and the blowers can be attached?
2. When using flexible couplings, what are some maintenance concerns to consider?
Blowers may be provided with additional equipment. For example, safeguards can be installed to protect equipment and operators. Temperature sensors can be used for bearing housing, vibration sensors protect the unit by shutting it down if limits are exceeded. Condensation drains should be provided on the bottom of blowers to drain off any accumulated moisture.
The compressed air from the blowers moves into a system of pipes and valves. The amount of air supplied from the blower is controlled by regulating valves mounted on the intake and/or discharge side of the blower. Usually butterfly valves are used and depending on your budget, you could have manually operated or use automation.
Blowers usually discharge to a common manifold, so check valves are installed at the discharge of each blower. The intake and discharge pipes are called the air mains. They are connected by a flexible connection to allow for vibration and heat expansion in the piping. In the winter months, the best place to be is in the blower room. There is a pressure relief valve on the discharge manifold to protect the blower from excessive back pressure overload. When this occurs the operator will be awakened on the midnight shift. Pressure gauges are used in several areas on the discharge side of the blowers. In some cases you may see them on the intake side for use in calculations of pump efficiency.
On the intake side, where air is supplied, you would have some type of filtering to remove dirt particles that could clog the diffusers. It also protects the blowers from excessive wear.
Replaceable filter units are the simplest for operations. Bag house dust collectors are bulky and expensive, though maintenance may be less. In some cases, electrostatic precipitators may be an advantage, shocking if operators are not careful, in areas of poor air quality. Most systems have utilized pressure drop measuring to indicate when it is time to replace or clean the units.
The above photograph shows air being unevenly distributed.
Diffusers
There are many different design layouts and patterns of diffuser placement. Systems that allow longer and more complete contact between the air and the liquid are preferred. We will focus on fine bubble (porous) diffusers and coarse bubble (nonporous).
Coarse bubble diffusion devices, or large-hole diffusers, produce larger bubbles than porous plates, porous tubes, or synthetic socks. The larger bubbles provide less surface area for air-liquid contact and will result in less oxygen transfer efficiency than that obtained with fine bubble diffusers.
Answer this question:
An air stone like those used in aquariums is a good example of a?
A. Porous material B. Nonporous material Mechanical Aeration
There are several main types of
mechanical aeration devices. The floating and fixed bridge aerators are quite
common. Some use a blade to agitate the tank’s surface and disperse air bubbles into the aeration liquor. Others circulate the mixed liquor by an updraft or downdraft pump or turbine. This action produces surface and subsurface turbulence, while diffusing air through the mixed liquor.
The motor speeds are usually in the 1800 rpm range. This speed is reduced to the 30 to 70 rpm range with gear reducers.
Most vertical motors are mounted on a gear reduction unit as seen in the photograph on the right. The impeller drive shaft can be enclosed in a housing connected directly to the gear box. There is a bearing at the bottom of the shaft that steadies and aligns this shaft. This bearing needs lubrication, always check your manufactures recommendations.
Some plants use an oxidation ditch in which rotating brushes, blades, or disks are rotated partially submerged in the mixed liquor. The
turbulence produced traps the air bubbles and keeps the mixed liquor in motion.
Other systems use both compressed air and a mechanical device to trap the bubbles. In one such system, submerged turbine aeration, air is injected below a rotating turbine blade that shears and disperses the air.
Submerged turbine applications have also used a draft tube operating in a downdraft-pumping mode.
Jet and Aspirator
Aerators provide oxygen transfer by mixing pressurized air and water within a nozzle and then discharging the mixture into the aeration tank. The velocity of the discharged liquid and the rising air plume provide the necessary mixing action.