This document discusses the activated sludge treatment process for wastewater, emphasizing the biological methods used, including aeration tanks, clarifiers, and various biological reactors such as sequencing batch reactors and trickling filters. It details how microorganisms decompose waste in the presence of oxygen, the role of biochemical oxygen demand (BOD) in wastewater treatment, and design considerations for different systems. Additionally, the document outlines various treatment technologies, including stabilization ponds and their advantages and challenges in managing wastewater effectively.

1. DEPARTMENT OF CHEMICALENGINEERING
CHAPTER- 2
“Activated sludge treatment
for wastewater”
PROF. DEVARSHI P. TADVI
ASSISTANT PROFESSOR
CHEMICAL ENGINEERING
DEPARTMENT
S S AGRAWAL INSTITUTE
OF
ENGINEERING & TECHNOLOGY,
NAVSARI
WASTEWATER ENGINEERING

2. Content
• Physico-chemical and biological treatment strategies and their evaluation
• Theory of activated sludge process (ASP)
• extended aeration systems
• trickling filters (TF)
• aerated lagoons
• stabilization ponds
• oxidation ditches
• sequential batch reactor
• rotating biological contactor, etc.
• Mass balancing in ASP and TF and their design.

3. Activated Sludge
• Activated sludge consists of sludge particles, teeming with living organisms,
produced in either raw or settled wastewater by the growth of organisms
(which include bacteria) in aeration tanks where dissolved oxygen is present.

4. Activated Sludge Process
• The Activated Sludge Process is one of several biological wastewater
treatment alternatives in Secondary Treatment.
• When Activated Sludge is added to wastewater, the organisms in this
mixed liquor quickly decompose the wastes in the wastewater being
treated.
• After a required period of aeration and agitation in the aeration tank, the
mixed liquor usually flows to a separate tank called a CLARIFIER where the
activated sludge is allowed to settle out and the remaining liquid is
discharged as effluent.
• The settled sludge is either disposed of as waste activated sludge or reused
in the aeration tank as return activated sludge.
• Some sludge must always be returned to the aeration tanks to maintain an
adequate population of organisms.

6. Process uses microorganisms to
speed up decomposition of wastes
• Food is known as Biochemical Oxygen Demand (BOD).
• BOD is the measure of oxygen demand in the incoming wastewater.
• A strong wastewater will have a high demand, whereas a weak wastewater
will have a lower demand. BOD is the measure of how much oxygen it will
take to stabilize the waste (or food) that is in the wastewater.
• Organism mass is called Mixed Liquor Volatile Suspended Solids (MLVSS).
• The overall concentration of suspended solids in an aeration tank is called
Mixed Liquor Suspended Solids (MLSS).
• This consists mostly of microorganisms and non-biodegradable suspended
matter.

7. When wastewater is added to activated sludge:
• Microorganisms feed and grow on waste particles in the wastewater.
• As organisms grow and reproduce, waste is removed and wastewater is
partially cleaned.
• The balance of food to organism mass is known as F/M ratio, food to
microorganism ratio.
• An appropriate F/M ratio is necessary to obtain proper performance from
the activated sludge process.

8.  Oxidation and removal of soluble or suspended solids is the result of the activated sludge process
in waste treatment.
 This treatment takes place in a few hours in an aeration tank.
 Stabilized soluble or suspended solids occur when organisms partially oxidize solids.
 Organism activity forms carbon dioxide, water, sulfate, and nitrate compounds.
 Remaining solids are changed to a form that can be settled and removed as sludge during
sedimentation.
 For the activated sludge process to work properly, the operator must control the number of
organisms and the dissolved oxygen level in the aeration tank, and the treatment time in the
aeration tank. When these factors are under control, the process will operate as it should.

9. Technologies for wastewater
Physicochemical
processes
Coagulative
precipitation
Flotation
Flotation process
Membrane
separation
Adsorption
Electrolysis
Chemical
processes
Fe-C Fe-C process
Fenton
Ozonation
Photocatalytic
oxidation
Biological
processes
SBR Sequencing
batch reactor
UASB Upflow
anaerobic sludge
bed process
MBR Membrane
bioreactor
process

10. Suspended growth Attached growth
Aerobic Process • Activated Sludge
Process
• Aerated lagoons
• SBR (Sequencing
batch reactor)
• Oxidation ditch
• Trickling filters
• Rotating
biological
contactors
Anaerobic Process • UASB(Up flow
anaerobic sludge
blanket)
• Anaerobic
digesters
• Anaerobic filters

11. Extended aeration
Extended aeration is a method of sewage treatment using modified activated sludge procedures. It is preferred for
relatively small waste loads, where lower operating efficiency is offset by mechanical simplicity.

12. Stabilization Ponds
• Waste or Wastewater Stabilization Ponds (WSPs) are large, man-made water
bodies in which blackwater, greywater or faecal sludge are treated by natural
occurring processes and the influence of solar light, wind, microorganisms
and algae .
• The ponds can be used individually, or linked in a series for improved
treatment.
• There are three types of ponds,
(1) anaerobic,
(2) facultative and
(3) aerobic (maturation),
each with different treatment and design characteristics.
• WSPs are low-cost for O&M and BOD and pathogenremoval is high. However,
large surface areas and expert design are required.
• The effluent still contains nutrients (e.g. N and P) and is therefore appropriate
for the reuse in agriculture , but not for direct recharge in surface waters.

13. In Out
Blackwater, Faecal
Sludge, Greywater, Brownwater, Faeces, Excreta
Sludge, Fertigation Water, Biogas (if anaerobic pond is
covered)

14. • Advantages
• Resistant to organic and hydraulic
shock loads
• High reducution of solids, BOD and
pathogens
• High nutrient removal if combined
with aquaculture
• Low operating cost
• No electrical energy required
• No real problems with flies or
odours if designed and maintained
correctly
• Can be built and repaired with
locally available materials
• Effluent can be reused in
aquaculture or for irrigation in
agriculture
• Disadvantages
• Requires large land area
• High capital cost depending on the
price of land
• Requires expert design and
construction
• Sludge requires proper removal and
treatment
• De-sludging (normally every few
years)
• Mosquito control required
• If the effluent is reused, salinity
needs to be monitored
• Not always appropriate for colder
climates
https://sswm.info/factsheet/waste-stabilisation-ponds

15. oxidation ditch
• An oxidation ditch is a large
circular basin equipped with
aerators that i s used to
remove organic matter and
pollutants from sewage
through the processes of
adsorption, oxidation, and
decomposition.

17. Sequencing batch reactor (SBR)
• The sequencing batch reactor (SBR) is a fill-and- draw activated sludge
system for wastewater treatment. In this system, wastewater is added
to a single “batch” reactor, treated to remove undesirable
components, and then discharged. Equalization, aeration, and
clarification can all be achieved using a single batch reactor. To
optimize the performance of the system, two or more batch reactors
are used in a predetermined sequence of operations. SBR systems
have been successfully used to treat both municipal and industrial
wastewater. They are uniquely suited for wastewater treatment
applications characterized by low or intermittent flow conditions.

20. PONDS AND LAGOONS
 Suspended culture biological system
 A large shallow earthen basin(lined), which wastewater is retained long
enough for natural purification processes
 Ponds: some oxygen is supplied by diffusion from the air bulk of the
oxygen is provided by photosynthesis
 Lagoons: oxygen is provided by artificial aeration (mechanical aerators)
 Aerobic ponds:
shallow ponds
at all depth dissolved oxygen is available
used as polishing or tertiary pond
 Anaerobic ponds:
deep ponds
dissolved oxygen is absent except for a relatively thin surface
layer
used for partial treatment of strong organic wastewater (as
pretreatment)
 Facultative ponds:
both aerobic and anaerobic zones exist
used as total treatment system for municipal wastewater

21. PONDS AND LAGOONS

22.  Lagoons are classified as Aerobic lagoons and Facultative lagoons
(according to the degree of mechanical mixing)
 Aerobic lagoons:
Sufficient energy is supplied both to meet the oxygen requirement
and to keep the entire tank content mixed and aerated
 Facultative lagoons:
Energy input is only sufficient to transfer the amount of oxygen
required for biological treatment but is not sufficient to maintain the
solids in suspension
A portion of incoming solids will settle along with a portion of
biological solids produced from the conversion of the soluble organic
substrate
In time, settled solids will undergo anaerobic decomposition
Eventually facultative lagoons must be dewatered and the
accumulated solids are removed

23. FACULTATIVE LAGOONS

24.  Two zones- Aerobic, Anaerobic
Upper Zone- Aerobic Zone
Aeration condition is achieved by
by oxygen generated by algae
by penetration of atmospheric oxygen (to a lesser extent)
Symbiotic relationship (mutually beneficial) between algae and
bacteria
Bacteria use O2 as e- acceptor
Oxidize organics in wastewater to end products such as CO2,NO3
and PO4
Algae use these compounds with sunlight as energy source and
produce oxygen as an end product. Produced O2 is used by bacteria

25.  The boundary between the aerobic and anaerobic zones
 Not stationary
aerobic zone can increases downward due to more mixing by
wind and more penetration by sunlight
Anaerobic zone can increases upward due to clam waters and
weak lighting
diurnal fluctuations in aerobic-anaerobic interface
 Lower zone- Anaerobic zone
Sludge along the bottom prevent oxygen transfer to that region
and anaerobic conditions prevail
In anaerobic zone - Organic acids, Gases, Products of
decomposition are foods for organisms in the aerobic zone
Biological solids produced in the aerobic zone ,settle to bottom
where they die and providing food for the anaerobic organisms

26. TRICKLING FILTERS
 Non-submerged fixed film biological reactor
 Attached culture biological system
 A reactor in which randomly packed solid (rock or plastic) provide
surface area for biofilm growth
 Sorption and biological oxidation are the primary means of food
removal
 Ideal filter packing:
High specific surface area per unit volume
Low in cost
High durability
High enough porosity
Chemically resistant surfaces
Hard

27. PACKING MATERIALS
ROCK PLASTIC VERTICAL
FLOW
PLASTIC VERTICAL
FLOW
PLASTIC CROSS
FLOW
REDWOOD
HORIZONTAL RANDOM PACKED

28. DISTRIBUTION SYSTEMS IN TRICKLING FILTERS

29. BIO-TOWERS
Advantages
Avoid plugging – due high
porosity and nature of
packing
Increase ventilation
minimizes odour problem
Less economical
Disadvantages
High pumping cost
Head loss through the deep
bed

30. ROTATING BIOLOGICAL
CONTACTORS (RBCs)
 The cylindrical plastic disks are attached to a horizontal shaft
and are provided at standard unit sizes of approx. 3.5 m in
diameter and 7.5 m in length
 The RBC unit is partially submerged in a tank containing the
wastewater
 As the RBC disks rotate out of the wastewater, aeration is
accomplished by exposure to the atmosphere
 Separate baffled basins are needed to develop the benefits
of a staged biological reactor design

32. Mass balancing in ASP and TF and their design

33. ACTIVATED SLUDGE PROCESS(ASP)
Qo, So, Xo
V, X, S
Qr, Xu
Qu, Xu
Qw, Xu
Qo+Qr
X, S
Q-Qw,
Xe, S

34. Design assumptions
 The influent and effluent biomass concentrations are
negligible compared to biomass at other points in the
system
 The influent food concentration So is immediately diluted to
the reactor concentration S because of the complete-mix
regime
 All reactions occur in the aeration tank

35.  Mass balance for biomass in the system,
Biomass in + Biomass growth = Biomass out(Effluent +Wasted sludge)
0 0
𝑄 𝑋 + 𝑉
𝑚𝑎
𝜇 𝑆𝑋
𝐾𝑠 + 𝑆 𝑑
− 𝑘 𝑋 0
= 𝑄 − 𝑄𝑤 𝑒
𝑋 + 𝑄 𝑋
𝑤 𝑢
𝑉 𝑚 𝑎 𝑆𝑋
𝐾𝑠+𝑆 𝑑
− 𝑘 𝑋 =𝑄 𝑋
𝑤 𝑢
𝐾𝑠+𝑆 𝑉𝑋
𝜇𝑚𝑎𝑥𝑆
= 𝑄𝑤𝑋𝑢
+ 𝑘𝑑

36.  Mass balance for food
Food in – Food consumed = Food out
𝑄 𝑆
0 0 − 𝑉
𝜇𝑚𝑎𝑥𝑆𝑋
𝑌(𝐾𝑠 + 𝑆) 0
= 𝑄 − 𝑄𝑤 𝑤
𝑆 + 𝑄 𝑆
𝑉
𝜇𝑚𝑎𝑥𝑆𝑋
𝑌(𝐾𝑠+𝑆) 0 0
=𝑄 (𝑆 −𝑆)
𝑚 𝑎 0
𝑆 𝑄 𝑌
𝐾𝑠+𝑆 𝑉𝑋 0
= (𝑆 −𝑆)
Hydraulic retention time, = V/Qo
Biological Solid Retention Time, c = VX/QwXu

37. Diffuser Surface Aerators
SST

38. 1) Volumetric Organic Loading Rates(OLR)-(BOD Loading)
VL = (Q*S0)/V
where, Q=Flow rate (m3/day)
S0=Influent BOD5 to aeration tank(mg/L)
V=volume of aeration tank (m3)
The mass of BOD (Kg) in the influent (without including the return sludge)
per unit volume of the reactor(aeration tank) per day
2) Food-to-Mass(Microorganism) Ratio (F/M)
F/M=Q(S0-S)/VX
where, Q=Flow rate (m3/day)
S0=Influent BOD5 to aeration tank(mg/L)
S= BOD5 in the reactor (aeration tank) (mg/L)
V=volume of aeration tank (m3)
X=MLVSS (Mixed liquor (liquid) volatile suspended
solids (mg/L)
DESIGN CRETERIA FOR ASP

39. 3) Biological solid retention time (BSRT)(𝜃𝑐)
𝜃𝑐 =
𝐾𝑔 𝑜𝑓 𝑀𝐿𝑉𝑆𝑆 𝑖𝑛𝑎𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑡𝑎𝑛𝑘
𝐾𝑔 𝑜𝑓 𝑉𝑆𝑆 𝑤𝑎𝑠𝑡𝑒𝑑+𝐾𝑔 𝑜𝑓 𝑉𝑆𝑆 𝑖𝑛 𝑒𝑓𝑓𝑙𝑢𝑒𝑛𝑡
=
𝑉𝑋
𝑄𝑊𝑋𝑈
Assume kg of VSS in effluent=0
where, V=volume of aeration tank
X=Biomass concentration in reactor
𝑄𝑊= flow rate of waste sludge
𝑋𝑈 = biomass concentration in waste
sludge
 BSRT is the duration for microorganism stays in the aeration
tank

40. 4) Sludge Volume Index (SVI)
𝑔
𝑆𝑉𝐼(𝑚𝐿
) = Settled sludge volume(mL/L)∗ 1000
MLSS
 It the volume in mL occupied by 1g of activated sludge after
settling the aerated liquor for 30min,and therefore indicates
the settling characteristics of sludge in the aeration tank
 SVI value – Less- sludge is dense, good settling characteristics
- More-sludge is fluffy, poor settling characteristics
1 L of
Wastewater
Volume of
sludge
settled

41. BASIC DEFINITIONS
• Biofilm layer: The layer of
microorganisms developed on the
filter medium due to aerobic
utilization of substrate, typically 10
micron to 10 mm thick.
• Diffusion layer or stagnant
boundary layer: The layer
separating biofilm and moving
liquid.
• Anaerobic/endogenous zone: The
zone in which oxygen cannot
penetrate and hence anaerobic
conditions prevail.
Trickling Filters
8
TF Mass Balance and Design

42. SUBSTRATE CONCENTRATION (CONTD.)

43. SUBSTRATE CONCENTRATION (CONTD.)
• Rate of substrate flux across boundary layer:
𝑟 = −𝐷 = −𝐷𝑤
𝑑𝑆
𝑑𝑋 𝐿𝑏
𝑆𝑏−𝑆𝑠
( )
𝑠𝑓 𝑤
where:
rsf = rate of substrate surface flux
Dw=molecular diffusion co-efficient for substrate in water
Sb=substrate concentration in bulk liquid
Ss=substrate concentration at biofilm surface
dS/dX=substrate concentration gradient
• Within biofilm, rate of movement
𝑑𝑆
𝑑𝑋
𝑟𝑏𝑓 = −𝐷𝑒
where:
rbf= rate of substrate flux
De=effective molecular diffusion coefficient in biofilm (De< Dw)

45. SUBSTRATE CONCENTRATION (CONTD.)
• Substrate utilization rate:
𝑠𝑢
𝑟 =
𝜇𝑚𝑎𝑥𝑆𝑋
𝑌(𝑆 + 𝑘𝑠)
• Mass balance for biofilm under steady-state:
𝑒
-𝐷 𝐴
𝑑 𝑎𝑡 𝑥
𝑒
𝑑𝑆
𝑑
𝑎𝑡 𝑥+∆𝑥
𝑑𝑋 𝑌 𝑆+𝑘𝑠
+ 𝐷 𝐴 − ∆𝑥𝐴(𝜇𝑚𝑎𝑥𝑆𝑋
)=0
𝑑2
𝑆 𝜇𝑚𝑎𝑥𝑆𝑋
or De𝑑𝑋2 =𝑌
𝑆+𝑘𝑠
……………………………………………………………………..(1)

46. SUBSTRATE CONCENTRATION (CONTD.)
• In equation (1), if we assume S<<ks (i.e. low concentration)
• Then the equation gives the following expression
s
• S=S (
cosh(𝐿𝑓−𝑧
)
𝐿
cosh( 𝑟
)
)………………………………………………………………………(2)
where 𝑟= sqrt(DeksY/µmaxX)
• Based on 𝑟 and Lf we can classify biofilms in two categories
• When Lf> 𝑟, deep biofilm-substrate does not penetrate far
• When Lf< 𝑟, biofilm is a fully penetrated biofilm

47. Youtube Video Links
• https://www.youtube.com/watch?v=hwlirZlFUXA
• https://www.youtube.com/watch?v=FLVWaPUekrs
• https://www.youtube.com/watch?v=cPJADJUhQUs
• https://www.youtube.com/watch?v=-jVqmZXpiNY