Following are the three methods which are employed for the purpose of aeration in activated sludge process: 1. Diffused Air Aeration 2. Mechanical Aeration 3. Combined Diffused Air and Mechanical Aeration.
Method # 1. Diffused Air Aeration:
In the diffused air aeration method compressed air is introduced into the mixture of sewage and returned activated sludge (or mixed liquor) in the aeration tank through submerged diffusers or air nozzles. A diffused air aeration system consists of diffusers that are submerged in the mixed liquor, header pipes, air distributing pipes, compressors and other appurtenances through which the air passes. By blowing the compressed air through the diffusers air bubbles are produced.
There are various types of diffusers available for this purpose as indicated below:
Types of Diffusers:
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Depending on the size of the air bubbles produced the diffusers are classified as fine bubble diffusers, medium bubble diffusers and coarse (relatively large) bubble diffusers.
The fine bubble diffusers are of three types—plate diffusers, tube diffusers and dome diffusers. The plate and tube diffusers are made of ceramically bonded grains of fused crystalline aluminium oxide, or vitreous silicate-bonded grains of pure silica, or resin-bonded grains of pure silica, and dome diffusers are made of only ceramically bonded grains of fused crystalline aluminium oxide. The diffusers made of these materials are porous in nature having pores of about 0.3 mm diameter, and hence these diffusers are also known as porous diffusers.
The plate diffusers are generally square in shape with dimensions 300 mm x 300 mm x 25 mm. These are installed in concrete or aluminium plate holders, holding six or more plates, which may be set either in recesses or on the bottom of the aeration tank. Groups of plate holders are connected to the air-supply piping at intervals along the tank length, and each group is controlled by a valve.
The plate diffusers, under water, pass 1.2 m3 of air/min/m2 with pressure losses between about 100 mm and 200 mm of water. For the best uniformity of distribution and prevention of clogging problems, a minimum of 0.6 m3 of air/min/m2 under water is advisable and should not exceed 2.5 m3 of air/min/m2 because of higher pressure losses, poor air economy and more rapid clogging due to corrosion. The spacing of the plate diffusers should be 0.6 m and preferably 1 m apart between centres to avoid interference among the rising streams of air bubbles.
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The tube diffusers are 600 mm long with outer diameter 60 mm and thickness of wall equal to about 15 mm. The tube diffusers are screwed into air manifolds, which may run the length of the tank close to the bottom along one side of the tank, or short manifold headers may be mounted on movable drop pipes. With the movable drop pipes, it is possible to raise a header out of the tank without interrupting the process and without emptying the tank. The diffusers can then be removed for cleaning or replacement.
The dome diffuser consists of a dome about 178 mm in diameter. The domes are mounted on a network of polyvinyl chloride (PVC) air piping which runs the length of the aeration tank. Row spacing and diffuser spacing range between 300 and 760 mm. The dome diffusers are installed at a level of 300 to 600 mm above the floor of the aeration tank and these are sometimes preferred to the plate diffusers which are placed at the floor level of the aeration tank.
With porous diffusers, it is essential that the air supplied be clean and free of dust particles that might clog the diffusers. The air supplied to porous diffusers should not contain more than 0.02 mg of dust per m3. Troubles due to clogging from the inside can be reduced by providing air filters ahead of compressors and those due to clogging from outside can be avoided by providing adequate air pressure below the diffusers at all times. However, inspite of such precautions the fine bubble diffusers will require periodical cleaning.
Several types of medium and coarse bubble diffusers are available. The medium bubble diffusers are in the form of plastic-wrapped diffuser tubes or woven-fabric sock or sleeve diffusers. The coarse bubble diffusers are in the form of various orifice devices such as monosparj, or sparger air escapes from periphery of flexible or rigid disc that is displaced when the manifold pressure exceeds the head on the disc, or slot orifice injectors.
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All these diffusers produce larger air bubbles than porous diffusers and consequently have a slightly lower aeration efficiency. However, the medium and coarse bubble diffusers have the advantages of lower cost, less maintenance, and the absence of stringent air purity requirements which offset the disadvantages of slightly lower aeration efficiency.
Diffuser Performance:
The efficiency of oxygen transfer depends on the type and porosity of the diffuser, the size of bubbles produced and the depth of submersion. In general the efficiency of porous fine bubble diffusers varies from 10 to 30 percent or more, depending on tank depth. These diffusers can produce oxygen transfer efficiencies in mixed liquor ranging from about 14 percent at a depth of 3.7 m to 25 percent at 8.0 m. Typically the efficiency for medium bubble diffusers varies from about 6 to 15 percent and for coarse bubble diffusers it is about 6 percent.
Aeration Period:
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The aeration period is the detention time for the mixture of sewage and returned activated sludge flowing into the aeration tank, expressed in hours. A wide variation is found in the period of aeration as it depends on various factors such as strength of sewage and MLSS concentration, desired degree of purification in terms of BOD removal, rate of aeration, proportion of returned activated sludge, etc. Thus a number of empirical formulae and charts have been developed to determine the aeration period.
One such empirical formula applicable for diffused air aeration method is American Public Health Association Formula as given below:
In general for domestic sewage the aeration period varies from 4 to 8 hours, the common value being 4½ hours. A noteworthy feature of the activated sludge process is the rapidity with which organic matter is oxidized when sewage is first brought in contact with activated sludge. About 60% of the organic matter is oxidized during the first hour and the remaining 30 to 35% in the next 5 or 6 hours.
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Quantity of Air Required:
In order to determine the capacity of air compressor, it is necessary to calculate the quantity of air that will be required. On an average, it may be assumed that about 6000 to 9000 m3 of free air will be required per million litres of sewage to be treated. With respect to the BOD removal, the usual rate adopted is 30 m3 of air per million litres for each ppm of BOD to be removed.
However, the actual provision depends on the strength of-sewage and various other factors as indicated below:
The quantity of air to be delivered through the diffuser system depends on the quantity of oxygen to be delivered, which in turn depends on the oxygen demand of the sewage and the efficiency of oxygen transfer of the diffusers with the latter being controlled by the size of the air bubbles produced and the depth of submersion of the diffusers.
The oxygen transfer efficiency at 1 to 2 mg/I of Dissolved Oxygen (DO) in aeration tank varies from 5 to 15 percent for most diffusers with 8 percent being common for fine bubble diffusers and 6 percent for coarse bubble diffusers. The quantity of air to be delivered through the diffuser system can be worked out from the quantity of oxygen to be delivered assuming 23.2 percent oxygen in air and air density of 1.2 kg/m3 under standard conditions. The air requirements for the different activated sludge processes indicated later are given in Table 13.3.
The air delivery systems are designed to deliver 1.5 times the normal air requirements and compressors are installed in multiple units to enable increase or decrease of the air supply. The compressed air is usually introduced under a pressure of 35 to 70 kN/m2 (0.35 to 0.70 kg/cm2).
Air-pipings are designed for velocities of 6 to 30 m/s for pipe diameters of 25 to 1500 mm. Header pipes for compressed air should be located above the sewage level in the aeration tank to avoid back siphonage when the compressors trip.
Volume of Returned Activated Sludge:
The volume of returned activated sludge to be added to the influent sewage depends mainly on the desired removal of BOD. It is generally expressed as percentage of the flow of sewage. Table 13.1 gives the volume of activated sludge to be added to remove the desired BOD. It may be observed that more the desired BOD removal, more is the volume of activated sludge.
Capacity of Aeration Tank:
The capacity of an aeration tank is determined by combining the following three factors:
(i) Aeration period
(ii) Volume of flow of sewage
(iii) volume of returned activated sludge
Fig. 13.3 shows an empirical chart which may be used to determine the aeration period and the volume of returned activated sludge required to determine the capacity of the aeration tank for treating the known volume of flow of sewage. The required aeration period as well as volume of returned activated sludge primarily depend on the desired BOD removal.
Thus knowing the overall 5-day BOD to be removed in the aeration tank the required aeration period in hours and the volume of returned activated sludge in percent may be obtained from the chart shown in Fig. 13.3.
Types of Aeration Tanks:
Following two types of aeration tanks are generally used in the diffused air aeration:
(1) Ridge and furrow type tank
(2) Spiral flow type tank.
(1) Ridge and Furrow Type Tank:
Fig. 13.4 shows a ridge and furrow type aeration tank. The tank is in the form of a narrow channel 30 to 120 m long, 5 to 10 m wide and 3 to 5 m deep. The bottom of the tank is formed into a succession of ridges and furrows or depressions. The diffuser plates are placed in the furrows and compressed air is supplied to the diffusers by header pipe through air distributing pipes.
The diffuser plates are provided at right angles to the direction of flow and the compressed air released from the plates forms air bubbles which cause the required aeration. A free board of about 60 cm is provided at the top.
(2) Spiral Flow Type Tank:
In the spiral flow type aeration tank, diffused air is supplied along only one side of the tank either through plate diffusers placed at the bottom of the tank, or through tube diffusers kept suspended from the top of the tank as shown in Figs. 13.5 (a) and (b). The tube diffusers are more common with spiral flow type aeration tanks.
The corners of the tank are chamfered. Thus as the air bubbles rise, they are deflected by the chamfered corners thereby spiral motion is created. The spiral motion set up in the aeration tank causes the required aeration. Further the spiral motion combined with the longitudinal motion of the sewage flow causes a helical track and therefore a longer travel.
This results in a saving of about 25 percent of diffusers and compressed air. The dimensions of this type of tank are more or less the same as that of ridge and furrow type tank, but its cost of construction is less than that of ridge and furrow type tank.
However, in a spiral flow type tank stagnation pockets may be formed which may cause septicity in part of the activated sludge, unless guarded against. Deposition of solids on the bottom is prevented by maintaining a transverse velocity of about 0.4 to 0.5 m/s across the bottom.
In the diffused air aeration system only about 5 to 10 percent of the air supplied is utilized in oxidation of the organic matter while the remainder is required for the purpose of agitating the mixture of sewage and returned activated sludge (or mixed liquor) to achieve thorough mixing.
Thus a large portion of air simply escapes through the aeration tank without giving oxygen to the sewage and hence there is considerable wastage of compressed air supplied in diffused air aeration system. Further it has been observed that if sewage is exposed to atmosphere, it can absorb sufficient quantity of oxygen for the purpose of oxidation. Both these factors have led to the development of mechanical aeration.
Method # 2. Mechanical Aeration:
In mechanical aeration method the mixture of sewage and returned activated sludge (or mixed liquor) is agitated in the aeration tank by means of some mechanical devices such as paddles, etc., known as mechanical aerators, so as to allow the absorption of oxygen from the atmosphere by the continuously changing surface of the mixed liquor due to agitation, and hence mechanical aeration is also known as surface aeration.
The only requirement in mechanical aeration is to have thorough agitation of the mixed liquor so as to bring it in intimate contact with the atmosphere. If agitation of the mixed liquor is not done, only its surface film gets saturated with oxygen and the oxygen so obtained is not conveyed to the entire body of the mixed liquor.
Earlier mechanical aeration was adopted only for small sewage treatment plants, but with the development of efficient mechanical aerators requiring very little operational attention, mechanical aeration systems are being increasingly adopted for large sewage treatment plants in preference to diffused air aeration systems.
Some of the advantages of mechanical aeration are:
(i) Higher oxygen transfer capacity;
(ii) Absence of air compressors, Air piping and air filters; and
(iii) Simplicity of operation and maintenance.
Aeration Period:
In the case of mechanical aeration method aeration period (or detention time) depends on the system of mechanical aeration adopted and it generally varies from 6 to 8 hours or more.
However, an empirical formula for determining aeration period for mechanical aeration method is M/s Ames Crosta Mills and Co. Ltd. (England) Formula as given below-
Where T = aeration period (hours); and
La = BOD of the influent sewage (mg/1) which is to be removed in aeration tank
Volume of Returned Activated Sludge:
In the case of mechanical aeration method volume of returned activated sludge is usually about 25 to 30 percent of the flow of sewage.
Capacity of Aeration Tank:
Capacity of an aeration tank is determined in the same way as discussed in diffused air aeration except that suitable value of aeration period or detention time is assumed.
Method # 3. Combined Diffused Air and Mechanical Aeration:
In this method, diffused air aeration and mechanical aeration are combined in a single unit, so as to achieve both efficiency of diffused air aeration method and economy of mechanical aeration method simultaneously. This reduces the cost of construction, operation and maintenance.
The aeration of the mixture of sewage and returned activated sludge (or mixed liquor) is carried out by compressed air using plate diffusers as well as by agitation with the help of paddles. An example of such combination is the well-known Dorr aerator or Dorroco aerator shown in Fig. 13.11, which is patented by Dorr Oliver Co. (America).
It consists of an aeration tank about 3.5 to 4.5 m deep and about 7.5 m square, having two rows of plate diffusers placed at the bottom along the centre line of the tank, and completely submerged paddles mounted on horizontal shafts. The air bubbles emerging from the diffusers induce upward flow of the mixed liquor.
The paddles are rotated at a speed of about 10 to 12 r.p.m. in a direction opposite to that of rising air bubbles. The spiral motion set up by the rotating paddles along with the air introduced through diffusers bring about the required aeration of the mixed liquor. For low flows, as at night, and when the sewage is weak only the paddles may be kept working to prevent the sludge deposition and the supply of diffused air may be completely stopped, which will result in saving of power.
Following are the advantages of the combined diffused air and mechanical aeration:
(i) Aeration is very efficient.
(ii) Detention period is reduced. It is about 3 to 4 hours.
(iii) Quantity of compressed air required is less as compared to the diffused air aeration.
Oxygen Transfer Capacity of Aeration Devices:
The aeration devices of the activated sludge plant are designed to provide the calculated oxygen demand of the sewage against a specific level of dissolved oxygen in the sewage. The aeration devices apart from supplying the required oxygen demand should also provide adequate mixing or agitation in order that the entire mixed liquor suspended solids present in the aeration tank will be available for the biological activity.
The recommended dissolved oxygen concentration in the aeration tank is in the range 0.5 to 1 mg/l for conventional activated sludge plants and in the range 1 to 2 mg/l for extended type activated sludge plants and above 2 mg/l when nitrification is required in the activated sludge plant.
Mechanical aerators are rated based on the amount of oxygen they can transfer to tap water under standard conditions of 20°C, 760 mm Hg barometric pressure and zero dissolved oxygen (DO).
The oxygen transfer capacity under field conditions can be calculated from the standard oxygen transfer capacity by the formula-
The oxygen transfer capacities of mechanical aerators, diffused air fine bubble aerators and diffused air coarse bubble aerators under standard condition lie between 1.2 to 2.4 kgO2/kWh, 1.2 to 2 kgO2/kWh, and 0.6 to 1.2 kgO2/kWh respectively.