The following are the various types of activated sludge process: 1. Tapered Aeration 2. Step Aeration  3. High Rate Treatment or Modified Aeration 4. Two Stage Aeration 5. Activated Aeration 6. Reaeration 7. Contact Stabilization 8. Complete Mix 9. Extended Aeration Method.

Type # 1. Tapered Aeration:

This is also called controlled aeration. If samples are collected at various points in the aeration units, it will be found that the B.O.D. of the mixed liquor is not uniform throughout; it is maximum near the inlet and minimum near the outlet. Due to this reason if the diffused air is applied at uniform rate, it cannot be utilized properly.

If the distribution of compressed air is done in proportion to B.O.D. it, should be 40% near the inlet, 25% in the central portion and 20% near the outlet and the remaining 15% in the sludge reaction tank (if provided). If the diffused air is applied at such a changing rate, it will be called tapered aeration. By tapered aeration the efficiency of the aeration unit will be increased and it will also result in the overall economy.

Type # 2. Step Aeration:

If the sewage is added to the returned sludge at more than two points along the aeration channel, the load coming on the returned sludge is changing. In the previous article, it has been stated that the B.O.D. of the mixed liquor is different at different points.


This can be fulfilled by tapered aeration (controlling the diffused air) or by controlling the load on oxidizing returned sludge. The latter method is known as step-aeration. Fig. 17.9 shows this method diagrammatically.

Step Aeration

Step aeration is used in the following conditions:

(i) When the dissolved oxygen in the final effluent is dropping off steadily.


(ii) When there is persistent increase in the volatile matter contents present in the activated sludge.

(iii) When the process is overloaded or activated sludge is of poor quality and has sphaerotilus growth.

The step-aeration requires smaller area, it can take shocks easily, and the volume of aeration units can be reduced to about half that of conventional units.

Type # 3. High Rate Treatment or Modified Aeration:

This is also known as ‘Modified aeration’. In this method less quantity of returned sludge is used, therefore, it is called high rate treatment. In high rate treatment shorter detention period (2 hours), lesser amount of compressed air and small quantity of returned sludge is used.


Following are the main characteristics of this treatment:

(i) Less quantity of return sludge about 10% to 25% of the quantity of sewage is used in this method.

(ii) Detention period of 2-3 hours is provided.

(iii) Quantity of air is about 3.15 m3/cu.m of sewage quantity is used.


(iv) Smaller sludge age 0.2 to 0.5 days or smaller concentration of suspended solids 500 to 1000 mg/litre is provided in the mixed liquor.

(v) This process removes 70 to 80% of suspended solids and 80 to 85% of B.O.D. from the raw sewage.

The activated sludge produced in the process is much thicker and dense than the activated sludge produced in the ordinary process. Due to denseness the sludge settles at faster rate. Due to increase in sludge settling rate, the load as the secondary settling tanks can be raised up to 50 m3/m2/day and the size of the settling tanks is reduced.

Following are the advantages of high rate treatment:


(i) Smaller volume of aeration units, secondary settling tanks and lesser area of the land is required. Therefore, it gives overall economy.

(ii) Due to lesser amount of air for aeration and lesser amount of return sludge, the operational cost is reduced.

(iii) Larger volume of the gas is obtained in the digester.

The main disadvantage is that the sludge becomes septic soon and requires early removal and disposal from the settling tanks.

Type # 4. Two-Stage Aeration:

In two-stage aeration the sewage is allowed to flow in a pair of aeration and sedimentation units. The activated sludge is either returned for seeding purposes or excess sludge from the second stage is again sent to the first cycle from which both of it and excess of first stage are mixed with influent. Fig. 17.10 shows the various steps in this aeration.

Two Stage Aeration

Secondary sludge is either returned and wasted within each stage, or sometimes the excess sludge from the second stage is recycled to the first stage and from there, both the sludge are disposed off. This process is most suited for treating the strong industrial sewage which has high value of B.O.D.

Type # 5. Activated Aeration:

It is a combination of two tapered activated sludge plants in parallel using the same settled sewage as influent. The excessive activated sludge of one plant is used as seeding for another plant, which is called activated aeration.

The activated sludge of the second plant is disposed off after suitable treatment. The main advantage in this aeration is the saving in diffused air consumption which is reduced by nearly 60% and gives great saving in operational cost.

Type # 6. Reaeration:

In this method, the minimum aeration of sewage is done after adding returned sludge to it. To satisfy the B.O.D. reaeration of returned sludge is done before mixing it with the sewage. If the reaeration is done in advance of trickling filter, it is called bioaeration. Figs. 17.11 illustrates the steps of reaeration.

Type # 7. Contact Stabilisation:


This process is also known as biosorption. In this process, sewage may be treated either after primary settling or without primary settling. The sewage is aerated along with return sludge for a comparatively short period of 0.5 to 1.5 hrs, when the sludge absorbs the organic matter in the sewage.

The mixed liquor is then settled in a secondary settling tank. The return fraction of the sludge withdrawn from the settling tank is reaerated in a separate sludge reaeration tank for a period of 3-6 hrs, before it is fed back into the contact aeration tank. During the reaeration of the sludge, the absorbed organics are stabilised restoring the adsorptive capacity of the sludge.

This process is quite effective in the removal of colloidal and suspended organic matter, but it is not very effective in removing soluble organics. This method is most suitable for the treatment of fresh domestic sewage containing only a low percentage of soluble B.O.D. The process has greater capacity to handle shock organic loadings than conventional method, because of the biological buffering capacity of the sludge reaeration tank.

It has greater resistance to toxic substances in the sewage as the biological mass is exposed to the main stream of sewage containing the toxic constituents only for a short time. The air requirements of the process are the same, as for the conventional system, but the total aeration tank volume required (sludge reaeration tank plus contact aeration tank) is only about half. Therefore this process presents an effective method of uprating existing conventional type plants when sewage characteristics are satisfactory.

Contact Stabilisation

Type # 8. Complete Mix:

The complete mix activated sludge process employs a completely mixed flow regime. In a rectangular tank, complete mixing is achieved by distributing the sewage and the return sludge uniformly along one side of the tank and withdrawing the aerated sewage, uniformly along the opposite side. In case of circular or square tank, complete mixing is achieved by mechanical aerator with adequate mixing capacity installed at the centre of the tank.

This process has the capacity to hold a high MLSS level in the aeration tank enabling the aeration tank volume to be reduced. The plant has increased operational stability at shock organic loading and also increased capacity to treat toxic biodegradable wastes like phenols. The complete mix plant is less liable to upset by slugs of flows of toxic wastes.

Complete Mix Plant

Type # 9. Extended Aeration Method:

The flow scheme of the extended aeration process and its mixing regime are similar to that of the complete mix process. The oxidation ditch also conforms to the extended aeration principle and relies on aerating the mixed liquor in an endless ditch. Primary settling is omitted in the extended aeration method, but commination is often provided for screenings.

This process employs low organic loading, long aeration time, high MLSS concentration and low F/M. It has high efficiency of B.O.D. removal. Due to long detention in the aeration tank, the mixed liquor solids undergo considerable endogenous respiration and get will stabilised. The excess sludge does not require separate digestion and can be directly dried on sand beds.

Oxidation Ditch

The air requirements for the process are high and the running costs are also quite high. The operation is simple, due to the elimination of the primary settling and separate sludge digestion. This method is most suitable for the small communities having sewage flow less than 4 mLd.

Table 17.1 gives the characteristics and design parameters of different activated sludge system.

Characteristics and Design Parameters of Different Activated Systems