7 Main Types of Activated Sludge Process | Waste Management

The various types of activated sludge processes are listed below: 1. Conventional Activated Sludge Process 2. Tapered Aeration Process 3. Step Aeration Process 4. Contact Stabilization Process 5. Complete Mix Process 6. Modified Aeration Process 7. Extended Aeration Process.

The characteristics and design parameters for these processes are given in Table 13.3.

These processes are described in details below:

Type # 1. Conventional Activated Sludge Process:

Fig. 13.16 shows a schematic diagram of the conventional activated sludge process which consists of an aeration tank, a secondary settling tank, a sludge return line and excess sludge waste line. The conventional activated sludge process is always preceded by primary settling. Thus in this case the mixture of the settled sewage from the primary settling tank and the returned activated sludge from the secondary settling tank is let in at the head end of the aeration tank and is aerated for a period of about 6 hours.

The influent sewage and returned activated sludge are mixed by the action of diffused or mechanical aeration, which is constant as the mixed liquor moves down the aeration tank. During this period, adsorption, flocculation, and oxidation of the organic matter take place.

The mixed liquor is withdrawn at the tail end of the aeration tank and is settled in the secondary settling tank (or activated sludge settling tank), and from this tank activated sludge is returned at the rate of approximately 25 to 50 percent of the influent sewage flow rate.

The characteristics and design parameters such as MLSS, F/M ratio, HRT, volumetric loading, etc., usually adopted for the conventional activated sludge process are given in Table 13.3. The BOD removal efficiency in this process is 85 to 95 percent.

The conventional activated sludge process employs plug flow regime which is achieved by using the aeration tank of long and narrow configuration with length equal to 5 or more times the width. Because of plug flow regime, the demand of oxygen by the micro-organisms and the F/M ratio are the highest at the head end of the aeration tank and then gradually decreases.

However, air is supplied in the process at a uniform rate along the length of the aeration tank which leads to either oxygen deficiency in the initial zone or wasteful application of air in the subsequent reaches.

The other limitations of the conventional activated sludge process are:

(i) There is a lack of operational stability at times of excessive variations in rate and strength of influent sewage;

(ii) Biological upsets are common; and

(iii) Skilled operation is required.

In spite of these limitations, for historical reasons, the conventional activated sludge process is the most widely used type of the activated sludge process, and plants upto 300 Mid capacity have been built in India. The various shortcomings of the conventional activated sludge process are eliminated in the other activated sludge processes described below.

Type # 2. Tapered Aeration Process:

The objective of tapered aeration is to match the quantity of air supplied to the demand exerted by the micro-organisms, as the mixed liquor traverses the aeration tank. At the inlet of the aeration tank where fresh settled sewage and returned activated sludge first come in contact, the oxygen demand is very high.

The diffusers are spaced close together to achieve a high oxygenation rate and thus satisfy the demand. As the mixed liquor traverses the aeration tank, synthesis of new cells occurs, increasing the number of micro­organisms and decreasing the concentration of available food. This results in a lower food to micro­organism (F/M) ratio and a lowering of the oxygen demand.

The spacing of diffusers is thus increased towards the tank outlet to reduce the oxygenation rate. Thus tapered aeration process involves only a modification in the arrangement of diffusers in the aeration tank and the amount of air consumed as indicated above, and in a strict sense, it is only a modification of the conventional activated sludge process.

The tapered aeration process is widely used because of the following advantages:

(i) There is optimal application of air.

(ii) Due to reduced oxygenation less air is required which results in reducing the size of the compressors as well as the initial and operating costs.

(iii) Avoidance of over-aeration will inhibit the growth of nitrifying organisms, which may cause high oxygen demands.

Type # 3. Step Aeration Process:

The step aeration process is a modification of the conventional activated sludge process in which the settled sewage from the primary settling tank is introduced in the aeration tank at several points along the length of the aeration tank, while the returned activated sludge is introduced at the head end of the aeration tank as shown in Fig. 13.17.

This arrangement equalizes the food to micro-organisms (F/M) ratio, thus lowering the peak oxygen demand. This process was developed by Gould and was first applied at the Tallmans Island plant in New York City in 1939.

The aeration tank is subdivided into four or more parallel channels through the use of baffles. Each channel is a separate step, and the several steps are linked together in series. A typical flow sheet for this process is shown in Fig. 13.17. The returned activated sludge enters the first step of the aeration tank along with a portion of the settled sewage.

The piping is so arranged that an increment of the settled sewage is introduced into the aeration tank at each step. If desired the first step can be used for reaeration of the returned activated sludge alone. One of the important features of the step aeration process is flexibility of operation.

The basic theory of the step aeration process is the same as that of the conventional activated sludge process. However, in step aeration process the oxygen demand is more uniformly spread over the length of the aeration tank, resulting in better utilization of the oxygen supplied.

The multiple-point introduction of settled sewage maintains an activated sludge with high absorptive properties, so that the soluble organics are removed within a relatively short contact period. Higher BOD loadings are therefore possible per unit volume of aeration tank.

Type # 4. Contact Stabilization Process:

The contact stabilization process, also known as biosorption, was developed to take advantage of the absorptive properties of activated sludge. The flow sheet for this process is shown in Fig. 13.18. In some cases, primary settling is eliminated. It has been postulated that BOD removal occurs in two stages in the activated sludge process.

The first is the absorptive phase, which requires 20 to 40 minutes. During this phase most of the colloidal, finely suspended, and dissolved organics are absorbed in the activated sludge. The second phase, oxidation, then occurs, and the absorbed organics are assimilated metabiolically. In the activated sludge processes mentioned so far, these two phases occur in a single tank. In the contact stabilization process, the two phases are separated and occur in different tanks.

The settled sewage from the primary settling tank (or raw sewage) is mixed with reaerated returned activated sludge and the mixture is aerated in a contact aeration tank for a period of 30 to 90 minutes. During this period, the organic matter present in the sewage is absorbed by the sludge floe, and hence this is the absorptive phase.

The sludge is then removed from the mixed liquor by sedimentation in the secondary settling tank, and the returned sludge is aerated for a period of 3 to 6 hours in a sludge aeration tank before it is fed back into the contact aeration tank. During the sludge reaeration, the absorbed organic matter is used for energy and production of new cells, and hence the absorbed organic matter is released and stabilized, thereby absorption capacity of the sludge is restored.

A portion of the returned sludge is wasted prior to recycle, to maintain a constant mixed liquor volatile suspended solids (MLVSS) concentration in the tanks. The characteristics and design parameters for this process are given in Table 13.3. It is to be noted that in determining F/M ratio for this process, both the SS in the contact aeration tank and the SS in the sludge reaeration tank are to be taken into account.

In contact stabilization process the total aeration tank volume (i.e., sludge reaeration tank plus contact aeration tank) is only about 50 percent of those for a conventional- or tapered-aeration plant. It is thus often possible to double the plant capacity of an existing conventional plant by redesigning it to use contact stabilization.

The redesign may require only changes in plant piping or relatively minor changes in the aeration system. Further as compared to the conventional activated sludge process, the contact stabilization process has greater capacity to handle shock organic loadings because of the biological buffering capacity of the sludge reaeration tank.

The contact stabilization process also presents greater resistance to toxic substances present in the sewage as the biological mass is exposed to the main stream containing the toxic constituents only for a short time. However, the air requirements of the contact stabilization process are same as for conventional activated sludge process, the air supply being divided equally between the contact aeration tank and the sludge reaeration tank.

The contact stabilization process is quite effective in the removal of colloidal and suspended organic matter, but it is not very effective in removing soluble organic matter. This process has been found to work very well for the treatment of domestic sewage.

However, before using it for the treatment of industrial sewages or mixtures of domestic and industrial sewages, laboratory tests should be performed. The use of this process for the treatment of industrial sewage is limited largely to sewages in which the organic matter is not predominantly soluble.

Type # 5. Complete Mix Process:

The complete mix process (also known as continuous-flow stirred-tank process) employs a completely mixed flow regime. In a rectangular aeration tank complete mixing is achieved by introducing the mixture of the settled sewage from the primary settling tank and the returned activated sludge from the secondary settling tank, uniformly along the entire length on one side of the tank and withdrawing the aerated mixed liquor uniformly along the entire length on the opposite side, as shown in Fig. 13.19.

In a circular or square aeration tank complete mixing is achieved by mechanical aerators with adequate mixing capacity installed at the centre of the tank. In the complete mix process the organic load on the aeration tank and the oxygen demand are uniform from one end to the other end of the tank. As the mixed liquor passes across the aeration tank from the influent ports to the effluent channel, it is completely mixed by diffused or mechanical aeration.

The complete mix process has the following advantages:

(i) The oxygen demand and the F/M ratio are uniform throughout the aeration tank resulting in the effluent of uniform quality.

(ii) The complete mix process has increased operational stability at shock organic loadings because due to complete mixing there is dilution of sewage over the entire volume of the aeration tank which results in reducing the effect of shock organic loadings.

(iii) Inhibitory materials, which may enter the aeration tank from time to time, are diluted immediately, and as a result, biological upsets are much less frequent with complete mix process than with plug flow process.

(iv) The complete mix process has increased capacity to treat toxic biodegradable wastes like phenols.

(v) The complete mix process may be used for the treatment of many industrial sewages.

(vi) The complete mix plant has the capacity to hold a high MLSS concentration in the aeration tank, thereby enabling the aeration tank volume to be reduced.

Due to the above indicated advantages the complete mix process is the most commonly used activated sludge process.

Type # 6. Modified Aeration Process:

The modified aeration process (also known as minimal solids aeration process) has its flow scheme identical with that of the conventional or tapered aeration process except that primary settling is often omitted. The difference in the processes is that modified aeration uses shorter aeration times, usually 1.5 to 3 hours, and a high food to micro-organism (F/M) ratio. Further as shown in Table 13.3, as compared to other activated sludge processes, modified aeration process has relatively low mixed liquor suspended solids (MLSS) concentration, high volumetric organic loading, low percentage of sludge return, and lesser air requirements.

The resulting BOD removal in the modified aeration process is only 60 to 75 percent, thus the process is not suitable where a high quality effluent is desired. The effluent obtained from the modified aeration process may, however, be used for sewage farming.

Further modified aeration develops dispersed biological growth which does not flocculate and settle quickly. As such some difficulties are experienced with the process because of poor settling characteristics of the sludge and the high suspended solids concentration in the effluent. Alum and polyelectrolytes are, therefore, sometimes used to improve secondary settling.

Type # 7. Extended Aeration Process:

The flow scheme of the extended aeration process and its mixing regime are similar to that of the complete mix process except that primary settling is omitted. As shown in Table 13.3 the extended aeration process employs low volumetric organic loading, long aeration time, high MLSS concentration and low F/M ratio.

The BOD removal efficiency is high. Because of long detention in the aeration tank, the mixed liquor solids undergo considerable endogenous respiration and get well stabilized. The excess sludge does not require separate digestion and can be directly dried on sand beds. Also the excess sludge production is minimum.

The air requirements for the process is higher and the running costs are also therefore high. However, operation is rendered simple due to the elimination of primary settling and separate sludge digestion. The method is, therefore, well suited for small and medium size communities and zones of a larger city, having sewage flow less than 4 Mld.

The process is used extensively for prefabricated package plants that are provided for the treatment of sewage from housing subdivisions, isolated institutions, small communities, schools, etc.

In small plants intermittent operation of extended aeration process may be adopted. Intermittent aeration cycles are:

(i) Closing of inlet and aerating the sewage,

(ii) Stopping aeration and letting the contents settle and

(iii) Letting in fresh sewage which displaces an equal quantity of clarified effluent. Sludge is wasted from the mixed liquor. To handle continuous flows a number of units may be operated in parallel.

The oxidation ditch is essentially an extended aeration process, having certain special features like an endless ditch for the aeration tank and a rotor for the aeration mechanism.