Activated Sludge Process

Pakistan Council of Scientific & Industrial Research
Laboratories Complex, Karachi
Off: University Road, Karachi-75280.
ACTIVATED SLUDGE
PROCESSES…

The most common suspended growth process used
for municipal wastewater treatment is the
activated sludge process.

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.

Process
 The process involves air or oxygen being introduced into a
mixture of primary treated or screened sewage or industrial
wastewater combined with organisms to develop a biological floc
which reduces the organic content of the sewage.
 The combination of wastewater and biological mass is commonly
known as mixed liquor.
 In all activated sludge plants, once the wastewater has received
sufficient treatment, excess mixed liquor is discharged into
settling tanks and the treated supernatant is run off to undergo
further treatment before discharge.

 Part of the settled material, the sludge, is returned to the
head of the aeration system to re-seed the new wastewater
entering the tank.
 This fraction of the floc is called return activated
sludge (R.A.S.). Excess sludge is called surplus activated
sludge(S.A.S.) or waste activated sludge(W.A.S).
 S.A.S is removed from the treatment process to keep the
ratio of biomass to food supplied in the wastewater in
balance.
 S.A.S is stored in sludge tanks and is further treated by
digestion, either under anaerobic or aerobic conditions
prior to disposal.

Advantages
 Diverse; can be used for one household up a huge plant
 Removes organics
 Oxidation and Nitrification achieved
 Biological nitrification without adding chemicals
 Biological Phosphorus removal
 Solids/ Liquids separation
 Stabilization of sludge
 Capable of removing ~ 97% of suspended solids
 The most widely used wastewater treatment process

Disadvantages
 Does not remove color from industrial wastes and may
increase the color through formation of highly colored
intermediates through oxidation
 Does not remove nutrients, tertiary treatment is necessary
 Problem of getting well settled sludge
 Recycle biomass keeps high biomass concentration in aeration
tanks

Types of Activated Sludge Processes
Plug Flow
 wastewater is routed through a series of channels constructed
in the aeration basin.
 Wastewater Flows to tank & is treated as it winds its way
through the tank.
 As the wastewater goes through the system, BOD and
organics concentration are greatly reduced.

Variations to this method include:
 adding return sludge and/or in decreasing amounts at
various locations along length of the tank;
 wastewater BOD is reduced as it passes through tank,
 air requirements and number of bacteria required also
decrease accordingly.

Complete Mix
 wastewater may be immediately mixed throughout the
entire contents of the aeration basin (mixed with oxygen
and bacteria).
 This is the most common method used today.
 Since the wastewater is completely mixed with bacteria
and oxygen, the volatile suspended solids concentration
and oxygen demand are the same throughout the tank.

Contact Stabilization
 Microorganisms consume organics in the contact tank.
 Raw wastewater flows into the contact tank where it is
aerated and mixed with bacteria.
 Soluble materials pass through bacterial cell walls, while
insoluble materials stick to the outside.
 Solids settle out later and are wasted from the system or
returned to a stabilization tank.
 Microbes digest organics in the stabilization tank, and are
then recycled back to the contact tank, because they need
more food.

 Detention time is minimized, so the size of the contact
tank can be smaller.
 Volume requirements for the stabilization tank are also
smaller because the basin receives only concentrated
return sludge, there is no incoming raw wastewater.
 Often no primary clarifier before the contact tank due to
the rapid uptake of soluble and insoluble food.

Extended Aeration
 Used to treat industrial wastewater containing soluble
organics that need longer detention times.
 This is the same as complete mix, with just a longer
aeration.
 Advantage - long detention time in the aeration tank;
provides equalization to absorb sudden/temporary
shock loads.
 Less sludge is generally produced because some of the
bacteria are digested in the aeration tank.
 One of the simpler modifications to operate.

Design Consideration
The quality or characteristics of raw waste water to be
treated.
The desired quality or characteristics of effluent or treated
waste water.
The type of reactor that will be used.
Volumetric and organic loading that will be applied to the
reactor.

Amount of O2 required and the aeration system will
provide to supply O2 and to support mixing.
The quantity of sludge that will be generated and
wasted for its further management.
 Besides these nutrient requirements of microbes,
environmental conditions under which plant operated.

Design steps
The design computations require the determination of:
 Volume or dimensions of the aeration tank
 Amount of O2 required and power needed for aeration
 Quantity of sludge that will produced for particular waste
and treatment conditions
 Volume and dimensions of sec. settling tank

Design criteria
 No of aeration tanks, N= min. 2 (small plants)
= 4 or more (large plants)
 Depth of waste water in tank= 3-4.5 m (usually)
= 4.5-7.5 m (diffuse aeration)
= 1-6 m (surface aeration)
 Freeboard= 0.3-6 m (diffuse aeration)
= 1-1.5 m (surface aeration)
 Rectangular aeration tank L:B= 5:1 and B:D=3:1 to 4:1
(depends on the aeration system)

Air requirement:
I. 20-55 m3 of air/Kg of BOD removed for diffuse
aeration when F/M => 0.3
II. 70-115 m3 air/Kg of BOD removed for diffuse
aeration when F/M <= 0.3
 Power required for complete mixing : 10-14
kW/1000 m3 of tank volume for surface aeration
system

Activated Sludge Process

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