Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environment or is reused for various purposes (called water reclamation).

1. Wastewater Engineering
ACTIVATED SLUDGE PLANT
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2. Activated Sludge Process
In a sewage (or industrial wastewater) treatment plant, the activated sludge process can be used for one or
several of the following purposes:
• oxidizing carbonaceous matter: biological matter.
• oxidizing nitrogenous matter: mainly ammonium and nitrogen in biological materials.
• removing phosphate.
• driving off entrained gases carbon dioxide, ammonia, nitrogen, etc.
• generating a biological floc that is easy to settle.
• generating a liquor(wastewater) that is low in dissolved or suspended material low in dissolved or
suspended material.
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4. Process
In activated sludge process wastewater
containing organic matter is aerated in an
aeration basin in which micro-organisms
metabolize the suspended and soluble organic
matter.
Part of organic matter is synthesized into new
cells and part is oxidized to CO2 and water to
derive energy.
The new cells formed in the reaction are
removed from the liquid stream in the form of a
flocculent sludge in settling tanks.
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5. Process
General arrangement
a) Aeration tank where air (or oxygen) is
injected in the mixed liquor injected in the mixed
liquor.
b) Settling tank (usually referred to as
"final clarifier" or "secondary settling tank") to
allow the biological flocs to settle, thus
separating the biological sludge from the clear
treated water.
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6. Factors that affect the performance of an
activated sludge treatment system.
1. temperature
2. return rates
3. amount of oxygen available
4. amount of organic matter available
5. pH
6. waste flow rate
7. aeration time
8. wastewater toxicity
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7. Design Criteria
HRT(Hydraulic Retention Time):The ratio volume of aeration basin to the flow rate.
HRT in hrs = [V/Q] x 24
HRT in hrs
V= volume in m3
Q= flow rate in m3/day
Food to microorganisms ratio (F/M) • The “food” in the ratio is the BOD entering the process (BOD
load).
F/M = (Q x Y0)/ (V x MLSS)
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8. Design Criteria
Volumetric BOD loading It is ratio of the BOD5 applied per unit volume of aeration tank.
Volumetric Load = [Q x Y0 ] / V
Sludge Age = It may be defined as the average time for which the particles of suspended solids
remain under aeration
Sludge Age =
V= Volume of the tank
XT = Concentration of solids in the influent of the Aeration Tank called MLSS, i.e Mixed Liquor
Suspended Solids in mg/L
Qw= Volume of wasted Sludge per day
XR= Concentration of solids in the returned Sludge or Wasted Sludge in mg/L
Q= Sewage Inflow per day
XE= Concentration of Solids in the effluent in mg/L
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9. Design Criteria
Sludge Volume Index (S.V.I) It is the volume in ml occupied by 1gm of settled suspended solid.
It is used to indicate the degree of concentration of sludge reflecting the physical state of sludge and
also shows the settleability of sewage.
It is found with the help of Imhoff.
One liter of sample collected at the outlet of A.T is placed at rest for about 30 minutes in Imhoff
tank.
The volume of settled sludge (Vs) is found and Sludge Volume Index (SVI) is calculated from
S.V.I = (Vs x 1000)/MLSS (ml/g)
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10. Aeration in ASPAeration units can be
classified as:
1) Diffused Air Units
2) Mechanical Aeration Units
3) Combined Mechanical and diffused air units.
Diffused air aeration:
• compressed air is blown through diffusers.
• Tanks - narrow rectangular channels.
• Air diffusers are provided at the bottom of tank.
• The air before passing through diffusers must be passed through air filter to remove dirt.
• Required pressure is maintained by means of air compressors.
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11. Types of air diffusers
Plate Diffuser
 They are rectangular/square plates made of crystalline alumina or high silica
sand. In this method the compressed air is blown through a perforated plate
diffuser. The air comes out through the holes of the diffuser plate and rises
upward in the form of bubbles. Thus the sewage absorbs oxygen from the air.
Tube Diffuser
 It consists of a perforated tube suspended in the waste water near the bottom
and can be taken out while cleaning. The compressed air is dent through the
tube. The air comes out through the holes with great force and agitates the
sewage.
 10 to 20 % area of the tank is covered with porous tiles.
 The supply of air is done through pipeline laid in the floor of the tank and is
controlled by the valves. Depending upon the size of the air bubbles these can
be classified as fine or medium bubble diffused-air aeration device
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12. Mechanical Aeration Unit
• Objective - bring every time new surface of wastewater in
contact with air.
• Surface aerators - fixed or floating type can be used
• Rectangular aeration tanks are divided into square tank and
each square section is provided with one mixer.
• The impeller are so adjusted that when electric motors starts,
they suck the sewage from the center, with or without tube
support, and throw it in the form of a thin spray over the surface
of the wastewater.
• When the wastewater is sprayed in the air more surface area of
wastewater is brought in contact with the air and hence aeration
will occur at accelerated rate.
• Detention period of the aeration tank treating sewage is usually
5 to 8 hours.
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13. Combined Aerator
Diffused air aeration and mechanical aeration are
combine in a single unit.
The well known type of such combination
is Dorroco aerator.
The aeration of sewage is done by air diffusers as
well as mechanical aerators.
Air diffuser plates are located at the bottom of tank
and the submerged paddles rotate in the direction
opposite to that in which the compressed air rises up
from the air diffusers.
Paddles are rotated by a motor on a horizontal shaft
with a speed of 10-12 rpm.
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14. Advantages of Combine Aerator
1. Aeration is very efficient
2. Detention period is reduced (3-4 hrs)
3. Quantity of compressed air required is less as
compared to the diffused air aeration.
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15. Types of Activated
Sludge Process
.
Conventional aeration:
Both the influent wastewater
and recycled sludge enter at
the head of the tank
Aerated for about 5 to 6 hours
for sewage treatment
Mixed by the action of the
diffusers or mechanical
aerators.
Rate of aeration is constant
throughout the length of the
tank
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16. Tapered Aeration:
BOD load is maximum at the
inlet and it reduces as
wastewater moves towards the
effluent end.
Tapered aeration maximum air
is applied at the beginning and
it is reduced in steps towards
end,
Efficiency of the aeration unit
will be increased and it will
also result in overall economy.
Tapered aeration gives better
performance
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17. Step aeration
Sewage is added at more than
one point along the aeration
channel,
Reduce the load on returned
sludge. The aeration is
uniform throughout the tank.
Because of reduction of
organic load on the return
sludge it gives better
performance
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18. Completely
mixed
Completely mixed flow
regime is used.
The wastewater is distributed
along with return sludge
uniformly from one side of the
tank and effluent is collected
at other end of the tank
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19. Contact
Stabilization
○ The settled wastewater is
mixed with re-aerated activated
sludge and aerated in the contact
tank for 30 to 90 min.
○ During this period the organic
matter is absorbed on the sludge
flocs.
○ The sludge with absorbed
organic matter is separated from
the wastewater in the SST.
○ A portion of the sludge is
wasted to maintain requisite
MLVSS concentration in the
aeration tank.
○ The return sludge is aerated
before sending it to aeration tank
for 3 to 6 h in sludge aeration
tank, where the absorbed organic
matter is oxidized to produce
energy and new cells.
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20. Operational Problems of an ASP
Rising sludge
Sludge sometimes rise or float to the surface
after a relatively short settling period.
Denitrification- in which the nitrites and nitrates
in the sewage are converted to nitrogen gas.
As nitrogen gas is formed in the sludge layers,
much of it is trapped in the sludge mass.
If enough gas is formed, the sludge mass
becomes buoyant and rises or floats to the
surface.
Rising sludge can be differentiated from bulking
sludge by noting the presence of small gas
bubbles attached to the floating solids.
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21. Operational Problems of an ASP
Rising sludge
The problem of rising sludge can be overcome
by:
(i) Increasing the rate of return activated sludge
pumping from the activated sludge settling tank;
(ii) Decreasing the rate of flow of aeration liquor
into the offending tank
(iii) Where possible, increasing the speed of the
sludge collecting mechanism in the settling tank
and;
(iv) Decreasing the mean cell residence time by
increasing the sludge wasting rate.
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22. Operational Problems of an ASP
Bulking Sludge:
A bulked sludge is the one that has poor settling characteristics and
poor compact ability.
Sludge bulking can be caused by
(i) The growth of filamentous organisms (primarily Sphaerotilas)
that will not settle, or
(ii) The growth of micro-organisms that incorporate large volumes
of water into their cell structure, making their density near that of
water and thus causing them not to settle.
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23. Operational Problems of an ASP
Other Factors
(a) Inadequate air supply (i.e., under aeration);
(b) Low pH value;
(c) Prolonged detention of sludge in the settling tanks;
(d) Accumulation of sludge at the bottom of the aeration tank;
(e) Sudden change in the character of sewage due to discharge of
septic solids from the sewage system;
(f) Introduction of certain industrial sewages containing elements
which favour growth of filamentous organisms.
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24. Operational Problems of an ASP
The various remedial measures which may be adopted for
controlling the bulking of sludge are enumerated below:
(i) Reducing the sewage flow to the aeration tank for a short
period.
(ii) Reducing the suspended solids in the sewage.
(iii) Prolonged aeration.
(iv) Reaeration of returned activated sludge.
(v) Chlorination of sewage or return activated sludge, so as to
control the growth of filamentous organisms. A reasonable range
of chlorine requirement is between 0.2 and 1.0 percent by weight.
(vi) Raising of pH value of sewage to about 8 or more by adding
lime.
(vii) Addition of hydrogen peroxide to control the growth of
filamentous organisms.
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25. Advantages and Disadvantages of Activated
Sludge Process
ADVANTAGES OF ACTIVATED SLUDGE
PROCESS
1. Clear sparkling effluent of high quality is
obtained.
2. Process requires small area of land and hence,
the design may be made compact.
3. There is freedom from fly and odour nuisance
due to high degree of treatment given to the
sewage in this process.
4. Process is highly efficient. Removal of SS,
BOD and bacteria are around 90% each.
5. Low cost of installation as compared to that of
trickling filters.
DISADVANTAGES OF ACTIVATED
SLUDGE PROCESS
1. If there is sudden increase in the quantity of
sewage, or if there is sudden change in the
character of sewage, there are adverse effects on
the working of process, and consequently, the
effluent of poor quality is obtained.
2. Operating cost of the process is high.
3. Large quantity of wet sludge obtained at the
end of the process requires suitable method for
its disposal.
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26. Advantages and Disadvantages of Activated
Sludge Process
ADVANTAGES OF ACTIVATED SLUDGE
PROCESS
6. There is comparatively very small loss of head
through the treatment plant.
7. The excess sludge has higher fertilizing values
as compared to sludge obtained from other
treatment methods.
8. Degree of stabilization or nitrification is
controllable within limits, so as to match with
the quantity and character of receiving sewage.
The treatment may be partial or full as desired or
required.
DISADVANTAGES OF ACTIVATED
SLUDGE PROCESS
4. Process is sensitive to certain types of
industrial sewages, particularly in respect of
those which may cause sludge bulking.
5. Process requires skilled supervision for its
efficient working. It becomes necessary to
ascertain that the sludge actually remain active
during the process.
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