Aeration units are the main units of the activated sludge process, the main aims of which are to supply oxygen to the sewage; to keep the return sludge aerobic and to mix up the return sludge with sewage thoroughly. All the aeration units can be broadly classified as follows: 1. Diffuser Air Units 2. Mechanical Aeration Units 3. Combined Mechanical and Diffused Air Units.
Type # 1. Diffuser Air Units:
These units are suitable for large plants. The compressed air is blown through diffusers in the sewage. The tanks of these units are generally in the form of narrow rectangular channels of 30 × 5 × 3 to 120 × 10 × 5 metres. The bottom of these tanks is provided with ridge and furrows as shown in Fig. 17.2. The diffuser tiles are fixed in the furrow portion by cement or bituminous compounds and are made air-tight by rubber rings.
The air before passing through diffusers must be passed through air filters to remove dirt and other undesirable particles. The required pressure is maintained by means of air compressors.
There are two systems for the distribution of compressed air in the sewage. In the first system, the compressed air is applied evenly on the sewage by diffusing it from the diffuser plates placed along the whole bed of the aeration tank as shown in Fig 17.2.
In the second system, the corners of the tanks are rounded by providing fillets and the diffusers are placed on one side only. By this system spiral flow is obtained in the sewage. Figs. 17.3 and 17.4 shown aeration units which give spiral flow in the sewage.
Air Diffusers:
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Following two types of air diffusers are commonly used:
(i) Jet diffusers.
(ii) Porous diffusers.
Jet diffusers give direct stream of air in the form of jet, downward against a small metal bowl kept just below the nozzle of the jet. The impinging air flashes over the surface of the bowl and escapes in the form of the fine bubbles and stirs the sewage and aerates it.
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The porous diffuser are manufactured in the form of tubes and plates from grains of crushed quartz, aluminium oxide or carbon fused together to form a porous structure having 30 to 40% porosity. The plate diffusers are generally of the shape of tile having size 30 x 30 x 2.5 cm. Sometimes tubular diffusers of about 60 cm length, 4.5 to 8.0 cm internal diameter with 1.0 to 1.5 cm wall thickness are also used.
Porous diffuser plates are fixed in the furrows of the bed of the tank, and allow the air diffusion through them. 10% to 20% floor area is covered with the porous diffuser tiles depending on the porosity and the quantity of the air to be supplied.
The supply of air is done through the pipe line laid in the floor of the tank and is controlled by the valves. Tubular diffusers are held in the pairs between packing glands attached to a central air supply pipe and is controlled by valve.
Standard permeability is defined as the volume of air in m3/m2/min at 21.1°C and 25% relative humidity, which will pass through an area of 1.0 sq.m. of dry porous plate in 1.0 minute under an effective pressure of 5 cm of water. Porous diffusers having high permeability 12 to 25 m3/m2/minute should be used for the aeration units, because they clog less frequently and also require less power and head for diffusion of air through them.
ADVERTISEMENTS:
In common practice porous dome type air diffusers of 10 to 20 cm in diameter are used. These are directly fixed or screwed on the top of C.I. main pipes laid in the bottom of the aeration tanks. They can be easily fixed and removed, and are also cheap in initial as well as maintenance cost. The capacity of these dome type diffusers is to diffuse 0.69 to 1.25 nr’/min of air each unit.
Air Supply:
Air compressors or blowers are used for supplying the air to the aeration units. Normally air is supplied under a pressure of 0.55 to 0.7 kg/cm2. To reduce the dust contents the air before supplying is filtered through viscous filters of hairs covered with oil or glass wool etc. The air velocity in the pipe lines laid under the bed of the aeration units is kept between 60 to 900 m/min. The loss of head is kept from 2.5 to 5 cm of water.
The experiments show that the power required for air supply varies from 20 to 60 B. H. P. per 4500 m3/day of sewage to be treated, depending on the B.O.D. to be removed from the sewage. The quantity of air supplied varies from 1.25 to 9.50 m3/m2 of sewage depending on the strength of the sewage to be treated and degree of treatment to be given.
ADVERTISEMENTS:
Weaker or dilute sewage requires less quantity of air than the strong sewage.
Type # 2. Mechanical Aeration Units:
In diffused air units, the quantity of air which is actually utilized for oxidation is only 5% of the total quantity of air compressed and the remaining 95% serve the purpose of thoroughly mixing only. Due to this reason, that the air does the part of mixing more some mechanical methods were developed for the mixing of sewage. The main object while designing the various mechanical aeration units was kept to bring every time new surface of sewage in contact with air.
The following are some of the mechanical methods which have been developed for the above purpose:
(i) Sheffield or Haworth System:
In this system very long(1000/w) narrow (1.3to 1.7 m) shallow channels were built and the sewage was allowed to flow in them at a velocity of about 0.55 ml sec. Mechanical rotating paddles were installed in these channels which agitate the sewage. This system was very costly, therefore, is not used in modern sewage works.
(ii) Hartly System:
In this system, the narrow channels are provided similar to Sheffield system, but the mixing is done by propeller type device, which is placed at the end of the channel.
(iii) Simplex System:
This is most commonly used in the modern sewage treatment works. The sizes of aeration tanks are from 5 × 5 × 5 to 7 m to 8 × 8 × 5 to 7 m in this system.
The tanks are generally square and their bottoms have the shape of a hopper. In one treatment plant 4 to 6 tanks are provided in series. Fig. 17.5 essentially shows the components of a Simplex aerator. It mainly consists of a vertical uptake tube 50 cm to 70 cm in diameter with its both the ends open, placed in the centre of the tank.
The lower end of this tube is enlarged, whereas an electric motor with impeller-like blades is installed at its top as shown in Fig. 17.5. The impellers are so adjusted that when electric motor starts, they suck the sewage from the tube and throw it in the form of a thin spray over the surface of the sewage.
When the sewage is sucked from the tube more and more of it rushes towards its bottom as shown by arrows in the figure. Thus a continuous circulation is formed and all the sewage is turned on within 20 minutes. When the sewage is sprayed, it also sucks the required quantity of oxygen from the air.
The aspirator mechanism of the spray simplex aeration unit consists of the following:
(a) A rotor at the bottom of the down draft tube, which circulates the tube, aspirates the air and expels it out in the shape of fine bubbles to the bottom of the aeration unit.
(b) A down-draft tube which allows the circulation of the sewage from the top of the square hopper bottom tank to its bottom.
(c) Air tubes, to draw the air into the down draft tube by the action of venturi.
(iv) Kessener Brush System:
Fig. 17.6 shows the essentials of this system. In this system a wheel provided with vanes rotates at the one corner of the surface and gives spiral motion of the sewage of the tank. In place of circular wheel, a brush is also sometimes used.
Following are the main disadvantages of the mechanical aeration units:
1. They require large area for their construction than the diffused air units.
2. They have less operational flexibility.
3. There is every possibility of short-circuiting.
4. These are not suitable for most of the industrial wastes.
5. In hot climate their efficiency is low.
Type # 3. Combined Mechanical and Diffused Air Units:
Aeration units which utilize the mechanical as well as diffused air properties were first tried by Karllmhon. The results were very good. Fig. 17.7 shows such an aeration unit. The mechanical mixing is done by rotating paddles. The diffused air increases the efficiency of this unit.
