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Activated Sludge Process - Activated Sludge Wastewater Treatment Process


It is a:

  • Unit process
  • Biological treatment process
  • Suspended growth process
  • Aerobic process

Activated Sludge:


Is defined as a ‘Suspension’ of microorganisms, both living and dead’ in a wastewater. The microorganisms are active by an input of air (oxygen) thus known as activated-sludge. Activate-sludge is that sludge which settle down in a secondary sedimentation tank  after the sewage has been freely aerated and agitated for a certain time in an Aeration tank.

Working Mechanism

The activated-sludge contains numerous bacteria and other microorganisms, when it is mixed with raw sewage saturated with oxygen, the bacteria perform the following function.

  1. Oxidize the organic solids.

  2. Promote coagulation and flocculation and convert dissolved, colloid and suspended solids into settle able solids. In practice the following operations are carried out in an activated - sludge process.

  3. The sewage is given treatment in the primary sedimentation tank. The detention time is kept as short as 1-1/2 hours.

  4. The settled sewage form the Primary Sedimentation Tank is the mixed with the required quantity of activated-sludge in the aeration tank. The mixture of activated-sludge and wastewater in the aeration tank is called ‘mixed liquor or mixed liquor suspended solids MLSS or MLVSS mixed liquor volatile suspended solids’.

  5. The Mixed Liquor Suspended Solids is aerated for 6-8 hours in the aeration tank, called the hydraulic detention time according to the degree of purification. About 8 m3 of air is provided from each m3 of wastewater treated. The volumes of sludge returned to the aeration basin is typically 20 to 30% of wastewater flow air supply 8-10 m3 of sewage

  6. The aerated Mixed Liquor Suspended Solids resulting in the formation of floc particles, ranging in size from 50 to 200pm.which is then removed in the secondary sedimentation tank by gravity settling, leeching a relatively clear liquid as the treated effluent. Typically greater than 99% of suspend solids can be removed in the clarification step.

  7. Most of the settled sludge is returned to the aeration tank (and is called return sludge) to maintain the high population of microbes that permits rapid breakdown of the organic compounds. Because more activated-sludge is produced tan is desirable in the process, some of the return sludge is diverted or wasted to the sludge handling system for treatment and disposal.

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Activated Sludge Process

Derivation of F/M Ratio:

Q = Flow of Sewage ( m3/day)
BOD =  organic matter (mg/l)
FOOD = Q ( m3/day) x BOD (mg/l)
FOOD = Q x BOD / 1000 (Kg of BOD/ day)
V = Volume of Aeration ( m3)
MLSS = Mixed liquor suspended solids (mg/l)
Micro-organisms = V ( m3) x MLSS (log/l) / 1000 = V x MLSS / 1000 (kg of MLSS in aeration tank)

F/M ratio:

A parameter of organic loading rate in the design aerated sludge parameter in the design of Trickling Filter in organic loading   rate = kg of BOD / m3-d

F/M ratio = Activated Sludge Process

F/M ratio = BOD / MLSS x t kg of BOD / Kg of MLSS/day
FM ratio varies between 0.2 -0.5 day-1

  • F/M ratio -0.5 day-1 has a good settleabilty of a sludge. ( even in some cases it can go to 1)
  • F/M ratio -<0.2 Food is very limited so the bacteria will die.
  • F/M ratio 70.5 day-1 Food is more so the bacteria will move the effluent (failure of the system)
  • If high F/M ratio, filamentous bacteria will also grow. They not settle easily because of long tails, get entangled with each other. Food to micro organism ratio(F/M) is a common used parameter in the activated-sludge process which is defined as

Activated Sludge Process Design

Design of Activated Sludge Systems:

Design of activated-sludge process involves details of sizing and operation of the following main elements.

  1. Aeration tank (reactor)-capacity and dimensions.
  2. Aeration system-oxygen requirements and oxygen transfer system.
  3. Final sedimentation tank – (deifier)
  4. Return activated sludge system.SV1
  5. Excess activated sludge withdrawal system and subsequent treatment and disposal of waste sludge. Since the whole process takes place in al liquid medium the hydraulic regime essentially in the aeration tank and final sedimentation tank.
  6. MLSS – a mixture of settled sewage + activated sludge dissolved oxygen < 2mg/l

Design Criteria

  1. F/M ratio = 0.2 – 0.5 day -1 or 0.2 – 0.5 kg BOD's / kg MLSS – d
  2. Detention time (aeration time) of sewage = 6 to 6 hours
  3. MLVSS or MLSS = 1500 -3000 mg/l
  4. Air supply = 10 m3/ m3 sewage treated
  5. return sludge = 0.25 -10 of Q (influent sewage flow) Qr / Q = 0.20-0.30 = Vs/100Vs (Volume of sludge)
  6. Depth = 3-5m
  7. L=W ratio =5:1
  8. SVI 50-150 ml/gm

Sludge Volume Index (SVI-TEST)

It is the measure of the settleability and compatibility of sludge and is made from a laboratory column setting test.


The sludge volume index is defined as ‘the volume in mm occupied by 1 gm of sludge after it has settled for a specified period of time’ generally ranging from 20 min to 1 or 2 hr in a 1 – or 2-l cylinder. One-half hour is most common setting time allowed to the mixed liquor to settle for 30 min. ( larger cylinder is desirable to minimize bridging of sludge floe and war effects). Take the reading let Vs is the settled volume of sludge (ml/l) in 30 min.

* If SVI is 50 - 150 ml/mg, the sludge settle ability is Good.

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Return Activated Sludge System:

  1. The activated sludge form the underflow of the final setting tanks should be returned to the inlet of the aeration tanks at a rote sufficient to maintain the MLSS concentration at the design value.
  2. The flow are needed for return-sludge is determined form the incoming sewage flow rate and the concentration at which the sludge is with drawn form the final setting tanks.

Hence a simple measure of the underflow concentration form the setting tanks is required. The parameter conventionally employed for this purpose the sludge volume index, SVI which is defined as 4 the volume occupied by sludge containing 1.0g of sludge soiled (dry weight) after 30 min setting and thus it has ht units ml/g. Some time represented as SDI i.e sludge density index. Once the SVI and operating MLSS concentration (x) is known, the required rate of activated sludge return can be determined

R = 100 / [ 106/ (x) (SVI) -1] where r = return sludge flow rate as a % age of incoming sewage flow.


It is the removal of solid particles form a suspension by settling under gravity.


It is a similar termn which refers specifically to the function of a sedimentation removal.


It means the separation of water from Suspended Solids

where R = return sludge flow rate (ML/D) for Q in ML/D)



  1. Activated sludge was introduced in 1941 and has undergone many variations and adaptations.
  2. The main objective of many modifications has been to increase the loading capacity of the basic plug flow activated sludge plant by provision of optimum condition design parameters for different variations are summarized in table. It is worthy of note that 5 modifications tapered aeration step aeration the CMAS process, the pure oxygen system and the deep shaft process all aim at either the improvement of oxygen transfer efficiency t the efficient distribution of available oxygen to match demand. A flow sheet of most of the commonly used variations is similar to that of CAS (Conventional Activated Sludge).


Volumetric loading = kg of BOD
Aerial loading rate = gm of BOD
Td = V/Q in days and grater than 5 days.


The combined and mutually- been facial action of algae and bacteria in this process is called algal-bacterial symbioses.

  • Shock loading (CSTR)
  • BODu

Aerated Lagoons:

Aerate lagoons are activated sludge units operated without sludge return. Historically they were developed from waste stabilization ponds in temperate climate where mechanical aeration was used to supplement the algal oxygen supply in winter. It was found, however that soon after the aerations were put into operation the algal disappeared and the microbial flora resembled that of activated sludge. Aerated lagoons were now usually design as completely mixed not-return activated sludge units. Floating aerates are most commonly used to supply the necessary oxygen and mixing power.

Sludge Treatment:

Anaerobic sludge treatment cell Primary Sedimentation Tank and Secondary Sedimentation Tank are basically organic these can treated to aerobic.

  • Anaerobic ponds and septic tank are for waste water treatment .
  • Sludge treatment = Anaerobic sludge treatment.

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  • Two stage digestion up
  • High rate digestion up
  • Fixed film processes. A swm zone


SLUDGE: the concentrated impurities settled at the bottom of the flower bed of sedimentation tanks.


To decompose or breakdown by heat and moisture or chemical action. (to invent food equable forms)

Sludge treatment:

Aerobic digestion it is defined as ‘it is the use of microbial organisms in the absence of oxygen I for the stabilization of oxygen materials by conversion to mean and inure produce including CO2.

Organic matter + H2O amoebas CH4+ CO2 + NH3+ H2S + heat

Benefices of anaerobic digestion. Types of anabolic detectors. It’s of two types:

  • Conventional (stranded) or low-rate digester or cold digester.
  • High rate digesters / two stage digester are characterized by continuous miring except at time of sludge with draw.
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