ACTIVATED SLUDGE &

ITS PROCESS MODIFICATION

Mukesh Kumar Jaga

Barkat Khan Amit Verma

Sushil Kumar Meena

Introduction to the Activated sludge Process

Historical Development

a) Developed around 1913 in Manchester

b) It is a Biological, secondary treatment

Activated sludge: Settled Biomass Activated micro-organism

Function of activated sludge:

1. Biodegradation of the influent organic material

2. To formation of floc particles

Basic principle:

COHNS + O2 + bacteria CO2 + H2O+NH3

+ Energy + other end products Synthesis:

COHNS + O2 + bacteria + energy C5H7NO2

( new cell tissue ) Chemoheterotrophic bacteria Micro-organisms grow and form particles that clamp

together

Activated sludge plant involves:

1. wastewater aeration in the presence of a microbial suspension,

2. solid-liquid separation following aeration,

3. discharge of clarified effluent,

4. wasting of excess biomass, and

5. return of remaining biomass to the aeration tank.

Activated Sludge Process Variables

Mixing Regime Loading Rate

a) hydraulic retention time (HRT)

b)  sludge retention time (SRT)

c) volumetric organic loading

d) Food to Microorganism ratio ( F/ M ) Flow Scheme the pattern of sewage addition the pattern of sludge return to the aeration tank and the pattern of aeration

Activated Sludge Process & Modification :-

CONVENTIONAL ACTIVATED SLUDGE PROCESS

• STEP FEED SYSTEM

• EXTENDED AERATION ASP

• OXIDATION DITCH ACTIVATED SLUDGE PROCESS

• CONTACT STABILIZATION ACTIVATED SLUDGE

• HIGH PURITY OXYGEN ACTIVATED SLUDGE

CONVENTIONAL ACTIVATED SLUDGE PROCESS

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Advantages :

• Allows smaller volume • Flexible operation, zone aeration, accommodates

anoxic and aerobic processes with single biomass for biological nutrient removal

• Less aeration • Better settling characteristics

Disadvantages

• High oxygen demand in inlet zone • May not buffer peak loads as well, although less likely in

activated sludge with high biomass • Less efficiency due to uniform aeration through out the

length of aeration tank but demand is less.

Step Feed System

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Methods to decrease in aeration :-

By decreasing the diffuser’s density (no. of diffusers per unit length) along the length It can be done by changing c/c distance of diffusers

By decreasing the diffuser’s density (no. of diffusers per unit length)

Advantage :

• Suitable for high Bod wastewaters. Eg; food processing industry wastewater

• Lower the peak oxygen demand• Better equalization of waste load ( within aeration tank )• Better distribution of oxygen demand over entire length of

A.T.• Smaller aeration tank volume compared to conventional

activated sludge unit

Extended Aeration System

• External substrate is completely removed.• Auto oxidation (internal substrate is used).• Net growth =0If Q, So, Se & X are the sameVolume required in extended aeration system = 30 times that of conventional ASP

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Advantages :

• Sludge production minimum• Stabilized sludge – no digesters are required• Nutrient requirement minimal

Disadvantages :

• High power requirement• Large volume of aeration tank• Suitable for small communities

Oxidation ditch

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Secondary clarifier

Influent

Sludge return

Effluent

Brush-type aerators

• Working on the principle of extended aeration.

• Compact unit ( sedimentation, aeration & sludge stabilization in one)

• Used for small communities

• Q < 3,800 cum m/day - This unit is preferred

Advantages :

• Typically small systems

• Flexible operation with placement and use of aerators – can be used for nutrient removal

• Very stable process

• No primary clarifier – simpler sludge handling

• Good settling characteristics

Disadvantages : • Long aeration time, larger reactor.

• Higher aeration requirement.

• Mechanical aeration equipment (rotors, large turbines)required to move water around the channel as well as aerate. Can create zones of high oxygen and add maintenance costs. More recent designs use diffusers for aeration and reduce mechanical requirement.

Contact Stabilization Process

Wastewater rich in colloidal organic solids besides soluble BODEg: Dairy industry, meat processing industry

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Contact tank

• Design is based on separation time. Separation of colloidal and soluble substrate based on MLSS.

• No increase in biomass concentration (during separation process no energy generated )

Stabilization tank

• Stabilization of colloidal and soluble BOD by microbes.

• Increase in biomass

• Detention time in stabilization tank is 3 to 4 hrs.

Advantages

• Reduced aeration requirement with short contact tank residence time

• Reportedly better settling sludge

Disadvantages

• Complex operation• Possible reduced treatment for soluble

contaminants in contact tank

HIGH PURITY OXYGEN ACTIVATED SLUDGE

Oxygen added under pressure to keeps dissolved oxygen level high

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Advantages:

• Five times oxygen transfer rate over air • Smaller volume • Higher biomass concentration possible, but limited by

secondary settler capacity

Disadvantages : • Very expensive: covered tanks, high energy cost for O2

generation, gas recirculation equipment • No flexibility (all aerobic processes) • Settling problems have been reported

References :

• www.oakharborcleanwater.org • civil.colorado.edu (activated sludge process

schematics) • www.brighthub.com • httpwww.webapps.cee.vt.edu

• Metcalf & Eddy “Wastewater Engineering, Treatment and Reuse”

• http://nptel.iitm.ac.in