CE3040

Environmental Engineering
(Wastewater)

Prof. Indumathi M. Nambi

 INDEX
Sl.No Topic Slide no: Peavy
Rowe
Pg no:
1. Sewage collection system 5-7 348-354
2. Sewer design 8 354-355
3. Wastewater characteristics 12-32 14-49
4. Self-purification of natural streams, DO sag 33-37 64-94
curve
5 Wastewater treatment process 38-39 212-217
6 Unit operations & Unit process 41-46
7 Preliminary treatment: Screens, Grit chamber 47-60 217-224
Equalization Tanks
8 Primary treatment: Clarifiers, types of settling 61-68 224-229
9 Biological Treatment: Fundamentals, 69-101 229-234
Microbial growth curve, Nature-based
systems
10 Types of reactors 102-109
11 Secondary treatment: Suspended & Attached 110-116
growth system
12 Activated sludge plants: Modifications, Mass 117-138 230-242
balance
13 Moving Bed biofim reactor (MBBR), 139-154
Membrane biofilm reactor (MBR)
14 Trickling Filters, Rotating Biological contactors, 155-184 257-268
Sequential batch reactors
15 Secondary settling tank 185-200 268-277
16 Tertiary treatment : Nutrient removal & 201-229 277-
Disinfection 278,295-
301
17 Treatment for Reuse purpose: Membrane 230-237
processes
18 Anaerobic Treatment systems: Septic tanks, 238-270
Anaerobic baffled reactor, Upflow sludge
blanket reactor
19 Sludge treatment & Disposal 271-287 278-294
20 Numericals on ASP & Trickling Filter
Wastewater characteristics
& Sewage collection system
Prof. Indumathi M.Nambi

1
 Need for sanitation
• Untreated sewage causes -

Allow decomposition of
organic matter- nuisance and
malodorous gases

Pathogens- dwell in human

intestine tract

Sewage nutrient – aquatic

plants( toxic compounds-
accumulation)

2
 Households by type of toilet facility as per 2011 census

Household toilet 81.4 % No household toilet 18.6%

Insanitary (Dry Sewerage

Septic tanks Community Open defection
Pit latrine 8.8% and Baho) connection
38.2% Toilets 6% 12.6%
latrines 1.7% 32.7%

3
As per CPCB 2009
• 498 Class-I cities and 410 Class-II towns- together generates 38524 MLD
• Out of which 11787 MLD (31%)- treated

As per world bank 1993

Diseases Female Male Total

Diarrheal Diseases 14.39 13.64 28.03

Intestinal Helminths 1.00 1.06 2.06

Trachoma 0.07 0.04 0.11

Hepatitis 0.17 0.14 0.31

Total water borne and 15.63 14.88 30.51

water related diseases

4
Definition of terms
• Wastewater- outlet of used water from domestic,
commercial, industrial etc., waste water is characterized
by physical, chemical and biological
• Sullage – WW from kitchen and bathrooms
• Sewage- human extra ww
• Sewer- Pipe carrying sewage/WW
• Soil pipe- pipe carrying sewage from WC
• Wastepipe- pipe carrying sullage from bathrooms,
kitchens, sink wash basin
• sewerage system- sewer of different pipes and size
collecting ww from town into ww treatment plant

5
Components of Sewage collection system
Sewer mains
Pumps
Pumping stations
Manholes
Drop Manholes
Lampholes
Clean-outs
Gullies
Catch Basins
Flushing Tanks
Grease & Oil Traps
Inverted siphons
Storm Regulators

6
Types of Sewerage System

● Combined sewerage
System:mixture of indoor sewage
and storm sewage
● Separate sewerage System:
carrying household sewage or
storm sewage but not both

7
Hydraulic design of sewers
Hydraulic formulas for determining flow velocity in sewers: Manning’s Formula-
Flow velocities in the sewer should be such that neither the suspended materials get
silted up nor the pipe material got scoured up
First limitation limits the minimum velocity & second limitation limits the maximum
velocity
Self cleansing velocity: 0.45m/sec
Maximum velocity:0.8 m/s

8
Contaminant Concentration (mg/l)
Weak Medium Strong
Total Solids (TS) 390 720 1220
Total Dissolved Solids 270 500 860
(TDS)
Fixed 160 300 520
Dissolved 110 200 340
Total Suspended solids 120 210 400
(TSS)
Fixed 25 50 85
Dissolved 95 160 315
Settleable solids 5 10 20
Oil and Grease 50 90 100
Nitrogen as N 20 40 70
Organic 8 15 25
Free Ammonia 12 25 45
Nitrites 0 0 0
Nitrates 0 0 0
9
Contaminant Concentration (mg/l)
Weak Medium Strong
Phosphorus as P 4 7 12
Organic 1 2 4
Inorganic 3 5 10
Chlorides 30 50 90
Sulfate 20 30 50
5 day Biological oxygen demand 110 190 350
(BOD5)
Total organic carbon (TOC) 80 140 260
Chemical oxygen demand (COD) 250 430 800
Volatile organic compounds (VOCs) <100 100-400 >400
Total coliform (No./100 mL) 106-108 107-109 107-1010
Fecal coliform (No./100 mL) 103-105 104-106 105-108
Cryptosporidum oocysts (No./100 mL) 10-1-100 10-1-101 10-1-102
Giardia lamblia cysts (No./100 mL) 10-1-101 10-1-102 10-1-103
Source: W Metcalf, L., Eddy, H. P., & Tchobanoglous, G. (1979). Wastewater engineering: treatment, disposal, and reuse. New
York: McGraw-Hill. 10
 Sl. Parameter Concentration in mg/l
No
Treatment Plant 1 pH 5.5-9
Effluent Standards 2 Temperature oC 45
3 Oil & Grease 20
4 Phenolic compounds 5
5 Ammonical nitrogen 50
6 Cynade 2
7 Chromium hexavalent 2
8 Chromium 2
9 Copper 3
10 Lead 1
11 Nickel 3
12 Zinc 15
13 Arsenic 0.2
14 Mercury 0.01
15 Cadmium 1
16 Selenium 0.05
17 Fluoride 15
18 Boron 2
19 Radioactive materials 10-7
(i)Alpha emitters(µc/ml) 10-8 11
(ii)Beta emitters(µc/ml)
 Physical characteristics
• Colour – yellowish (fresh)
Grey brownish- partially decomposed
Black – fully decomposed
Other colour – presence of industrial waste
Odour – fresh sewage odourless
as decomposition starts- offensive odour

12
Temperature

Biological processes Chemical processes

13
 The use of freeze-thaw techniques also
leads to a marked decrease in the copy
number of SARS-CoV-2 RNA. This led to
a drastic reduction in the PCR signal for
the viral gene from the sample.

Reference-News Medical life science

N-gene copy numbers and RNA concentration detected in wastewater

from WWTP in Salzburg after 0, 2, 7, and 9 days of storage at 4 °C as
well as after freezing. (n = 4, Median, Box: min-max).

14
Turbidity
• Measured by Formazin Turbidity unit (FTU)
• Formazin Nephelometric Units (FNU)
• Jackson Turbidity Unit (JTU)
• Nephelometric Turbidity unit (NTU)

Turbidity determination in lab:

https://www.youtube.com/watch?v=sj3b2nfuJmU

15
Chemical characteristics
• Chemical characteristics indicates stage of sewage decomposition, its strength, extent and type of treatment
required
Solids- 99.9% water and 0.1 total solids
Total solids present in four forms- suspended solids (SS), dissolved solids(DS), Colloidal Solids and settleable
solids

Noyyal and its lake system, though still in use, is

overwhelmed by the urban, agricultural and
industrial pollution it receives. Studies suggest that
the Total Dissolved Solids (TDS) in some of these
lakes are as high as 8,000 ppm, far exceeding
the World Health Organization (WHO) drinking
water guideline of <1,000 ppm. Encroachments for
public and private use, solid waste dumping and, in
some cases, reduced water flows and connectivity of
canals and lakes are slowly eroding the ability of the
system to self-cleanse and revive.

Reference : The Print

16
Determination of Total solids Determination of settleable solids

Total solids determination in lab:

https://www.youtube.com/watch?v=NlAqT7dWng4 17
pH
• pH determined by pH meter (Potentiometer)
• Less than 7 – acidity and greater than 7 – alkaline
• Fresh sewage is alkaline with time pH falls due to acid production by bacteria

pH determination in lab:
https://www.youtube.com/watch?v=vwY-xWMam7o

18
19
Nitrogen Content 430 manatees have perished in 2021.
News : National Geography
More than 430 manatees have perished in 2021.

Native grasses which once lined the bottoms of Florida’s

springs—such as Fanning Springs, pictured here—are being
choked out by algae fed by fertilizer and nutrient runoff. This
lack of vegetation is causing many manatees to starve to death
in the winter.

Nutrient runoff, which feeds algal growth and

deoxygenation of water, can be deadly for manatees

Researchers turn wastewater nutrients into fertilizer

NPHarvest is a process developed by Aalto University
researchers that allows for the recovery of nutrients from
the wastewater treatment process.- Reference water World

Kjeldahl nitrogen determination in lab:

https://www.youtube.com/watch?v=TWaPfYqAt4c 20
Chlorides Fats, oil and grease

Climate change: Jet fuel from waste 'dramatically lowers'

emissions- BBC NEWS

It normally requires the use of virgin vegetable oils as

well as waste fats, oil and grease to make the synthetic
fuel.
At present, it is more economical to convert these oils
and wastes into diesel as opposed to jet fuel - which
requires an extra step in the process, driving up costs.

21
Toxic
Reference-Down to Earth Heavy metals emerging Oxygen demand
as
potential threat to public health on urban beaches:
Study

22
Maharashtra Pollution Control Board underlines high pollutio ..

Read more at:

http://timesofindia.indiatimes.com/articleshow/81249268.cms?utm_source=contentofinterest&utm_medium=text&utm_campaign=cppst

Maharastra Pollution Control Board underlines high

pollution in Ramala Lake
The water samples collected twice from the lake – first
and second week of February – has high BOD (190-112
mg/L) due to sewage pollution- Reference- THE
TIMES OF INDIA

23
• BOD = Consumed DO x(volume of diluted sample/ volume of sample)

24
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Chemical oxygen demand-

Total organic carbon

TOC analyser
29
Theoretical oxygen demand
• Oxygen required to oxidize the organic matter can be theoretically calculated, if the organic matter present in
wastewater is known.

Biological characteristics
of wastewater
• Bacteria, fungi, algae, protozoa etc
• Bacteria- predominant
• Most are harmless and non pathogenic and helps in degradation

30
• New study shows microplastics turn into 'hubs' for pathogens, antibiotic-resistant bacteria
• researchers found certain strains of bacteria elevated antibiotic resistance by up to 30 times while living
on microplastic biofilms

Microscopy images –biofilm on polyethylene

microplastics

Provided by New Jersey Institute of Technology

31
 Population equivalent
• Industrial wastewater compared with per capita normal domestic waste water
• Standard BOD5 of industrial sewage = [ Standard BOD5 of domestic sewage per person per day] x [ population
equivalent]

32
Self purification of Natural streams
Dilution, sedimentation and oxidation-reduction in sunlight

33
Oxygen deficit of a polluted river stream
Oxygen deficit: Saturation DO- Actual DO

34
Streeter-Phelps Equation

35
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Wastewater treatment processes

38
Wastewater treatment processes

39
 Wastewater treatment
processes: Primary, Secondary,
Tertiary
-Prof. Indumati M. Nambi
IIT Madras
40
 UNIT OPERATIONS AND UNIT PROCESSES
• Methods of treatment in which application of physical forces
predominate, are known as unit operations.
• Methods of treatment in which chemical or biological activities are
involved , known as unit processes.

41
Physical unit operations

42
Chemical unit processes

43
 Biological unit processes
• To stabilize the organic matter (Soluble and non settleable)
• To reduce the amount of dissolved phosphorus and nitrogen in the final effluent

44
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46
Preliminary treatment

47
SCREENS
Screens are used in wastewater treatment for
the removal of coarse solids. Screens are either
manually or mechanical cleaned.
Thus retained materialat screens is called as
screenings

Manual screens Mechanical screens

48
49
Design Criteria for Bar Screens

50
51
 Grit chamber
• Grit chambers are long narrow
tanks that are designed to slow
down the flow so that solids such
as sand, coffee grounds, and
eggshells will settle out of
the water.
• Grit causes excessive wear and
tear on pumps and other plant
equipment.
Peavy Rowe: Pg no: 221-224

52
There are two general types of grit chambers
1. Horizontal flow grit chamber
2. Aerated grit chamber

Design considerations

• Length – 10 to 18 m
• Depth of liquid – 1 to 1.3 m
• Velocity is controlled by means of
velocity control device.

53
Horizontal flow grit chamber
Aerated grit chamber

54
Collection of grit
• In some cases steep bottom slope is provided which will
collect the grit at Central Point of Removal.
• Grit Removal is achieved by air pumps for small aerated grit
chambers.
• Grit can also be removed by tubular conveyors, buckets type
collectors, elevators screws conveyors, grit pumps and clam
shell buckets.

55
Disposal of grit
• Filling Low lying area.
• Incineration.
• Well-washed grit has been used
on sludge drying beds, as a cover
for screenings, and as a surfacing
material for walks and roadways.

56
Equalization tanks
• Flow equalization is damping of flow rate variation so that a constant or nearly
constant flow rate is achieved.
• In some cases Equalization may be provided after primary treatment & before
biological treatment.
• The design must provide for sufficient mixing to prevent solid deposition &
concentration variation.

57
Types of Equalization tanks

58
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Volume determination of Equalization tanks
It is determined by using an inflow mass diagram in which cumulative
inflow volume is plotted versus the time of day.

60
Primary treatment process
• The objective of primary treatment is the removal of settleable organic and inorganic solids by
sedimentation
• Approximately 25 to 50% of the incoming biochemical oxygen demand (BOD5), 50 to 70% of the total
suspended solids (SS), and 65% of the oil and grease are removed during primary treatment.
• Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are also removed
during primary sedimentation but colloidal and dissolved constituents are not affected.

61
Clarifier
• Clarification is the process
to remove suspended
solids from water which is
done by Clarifier.
• Clarifiers work on the
principle of gravity settling.
• The expected range for
percent removal in a
primary clarifier is 90% -
95% settle able solids , 40%
- 60% suspended solids.

62
63
Zones in the tank

There are four important zones in the tank:

(a) Inlet zone – at the central well, which
has a round baffle plate, the flow is
established in a uniform radial direction so
that short-circuiting does not take place.
(b) Settling zone – where settling is
assumed to occur as the water flows
towards the outlet.
(c) Outlet zone – in which the flow
converges up and over the decanting
weirs.
(d) Sludge zone – where settled material
collects and is pumped out.

64
Factors Affecting Sedimentation Process
The efficiency of sedimentation depends on:
• The size and the form of the particles.
• The density of the particles. The composition of the suspension.
• The concentration of the suspension
• The temperature
• The depth and form of the sedimentation reservoir.
• The distance that the liquid travels through reservoir.
• The velocity of the liquid.
• The way in which the liquid flows into the reservoir and is removed.

65
Design parameters for Sedimentation tank

66
 Types of sedimentation tanks

(a)rectangular horizontal flow tank

(b)circular, radial-flow tank;
(c)hopper-bottomed, upward flow tank

Primary Treatment : Peavy & Rowe, pg no:224-229

67
68
Biological Treatment
Prof. Indumathi M.Nambi

69
Contents
• Fundamentals & Terms Involved
• Role of microorganisms and Microbial Growth curve
• Different types of biological Treatment systems
• Nature-based treatment systems: Constructed wetlands, Algal
ponds, Waste stabilization ponds

70
Fundamentals & Terms
Involved

71
Objective of Biological treatment
• dissolved and particulate biodegradable - acceptable end products.

• capture and incorporate suspended and non- settleable colloidal solids

into a biological flocs or biofilms.

• transform or remove nutrients such as nitrogen and phosphorus

• specific trace organic constituents and compounds.

72
 Definition of terms
• Aerobic process – presence of oxygen
• Anaerobic process- absence of oxygen
• Anoxic Process- in absence of denitrification
• Facultative process- in presence or absence of
oxygen
• Combined aerobic/anoxic/anaerobic processes-
various combination to achieve specific treatment
objectives

73
Different Types of Biological treatment

74
Nitrification process Biological Phosphorus removal

75
Denitrification Stabilization

76
 Nitrification is a biological process where bacteria change ammonia (NH)
Biological Phosphorus Removal – Simple Explanation into nitrate (NO) in two steps.
This is a process that uses special bacteria to remove phosphorus from wastewater. Steps in Nitrification:
In the first tank (without oxygen): Ammonia Nitrite (NO)
Bacteria called PAOs (Polyphosphate Accumulating Organisms) take in organic matter and release phosphorus into water. Done by bacteria called Nitrosomonas
In the second tank (with oxygen) This is the first step
The same bacteria take back the phosphorus, but store more than before inside their cells. Nitrite Nitrate (NO)
Waste sludge (with phosphorus-rich bacteria) is removed from the system. Done by bacteria called Nitrobacter
Result: This is the second step
Phosphorus is taken out of the water along with the sludge.
Too much phosphorus in rivers causes algae to grow, which harms fish and water quality. Why It’s Important:
This method is eco-friendly and low-cost, using natural bacteria. Ammonia is toxic to aquatic life.
Nitrification removes ammonia from wastewater.
It's part of biological treatment in sewage plants.
Nitrification = Ammonia Nitrite Nitrate (using bacteria)

Role of Microorganisms &

Microbial Growth curve
Denitrification Stabilization – Simple Explanation

Denitrification is a biological process where bacteria remove nitrogen from wastewater by changing nitrates (NO) into nitrogen gas (N).
How It Happens:
After nitrification, water has nitrate (NO).
n a tank without oxygen (anoxic tank), special bacteria use nitrate instead of oxygen to survive.
These bacteria convert nitrate nitrogen gas, which leaves into the air.

Stabilization part:
This process stabilizes the nitrogen in wastewater by turning harmful forms (like nitrate) into harmless nitrogen gas, which is safe for the environment.
Why It’s Important: Removes nitrogen, which can pollute rivers and cause algae growth.
Makes treated water safe to release into nature.

77
MLSS means the solid stuff (in mg/L) present in the aeration tank of a wastewater treatment plant.
It includes microorganisms (tiny living things) and non-biodegradable solids (waste that doesn’t break down easily).
MLSS is very important in the activated sludge process, which is a method to treat wastewater.
Role of Microorganism in wastewater
It makes sure there are enough active microorganisms in the tank to eat and clean the organic waste in the water.

Mixed Liquor Suspended Solids treatment

The right amount of MLSS helps in good cleaning of dirty water.
If MLSS is too low – not enough bacteria to clean waste.
If MLSS is too high – tank may get overloaded or not work properly.
• MLSS in mg/L
• Contains microorganism and non biodegradable
suspended matter
• MLSS important part in activated sludge process to
ensure sufficient amount of active biomass
available to consume applied organic pollutant at
any time

78
 Microbial growth curve
• Cell growth directly proportional to food utilization
• If S represents the quantity of soluble food (in milligrams per litre) and X represents the quantity of biomass
(in milligrams per litre), the rate of utilisation of food dS/dt and the rate of biomass growth dX/dt can be
represented graphically
Why Bacteria Decrease in the Endogenous Phase – Simple Explanation In the endogenous phase (last stage of microbial growth):
Food is mostly gone – bacteria don’t have enough to eat.
Bacteria start to use their own stored energy (inside their cells) to stay alive.
This leads to cell weakening and death.
Some bacteria may even be eaten by other microbes (cell decay).
As a result, the number of bacteria decreases.

79
Microbial growth

80
Nature-based systems

81
 What is Wetland?
• A wetland is a land area that is saturated with water with distinct ecosystem
• The characteristics that distinguish wetland from other land forms or water
bodies are vegetation of aquatic plants adapted to the unique hydric soil.

Natural

Wetland

constructed
82
 Natural wetland
• Natural wetland systems have often been described as the earth’s kidney because
they filter pollutants from water that flows through on its own to receiving lakes,
streams and oceans

RAMSAR wetland site India

83
 Constructed wetlands
• Constructed wetlands are artificial wastewater
treatment systems consisting of shallow (usually
less than 1m deep) ponds or channels which have
been planted with aquatic plants and which rely
upon natural microbial, biological, physical and
chemical processes to treat wastewater.
• Provided with impervious clay or synthetic liners
and engineered structures to control the flow
direction, liquid detention time and water level
• By various process chemicals are considerably
removed or settled and clean water is drawn. These
chemicals include nitrogen, ammonia, phosphorus
and pathogens
• Constructed wetlands are most economical as
compared to conventional treatment units which
needs more energy for its process and this method
requires cheaper materials

84
 Types

Up flow
Surface flow
Constructed
Vertical flow
wetland
Subsurface flow Down flow

Horizontal flow

85
 Surface flow
Free water surface Subsurface Wetlands
• Water surface exposed to the atmosphere and • The water surface is below ground level
water flows over soil media.
• In this water flows below media
• A channel is dug and lined with an impermeable
• No water on soil surface but subsurface is
barrier such as clay or geo textile. The flow bed
saturated
is then covered with rocks, gravel and soil.
Vegetation is also planted. The usual depth of the
wastewater is 10 to 45cm above the ground level.
Vertical subsurface flow
• As the water flows slowly through the wetland,
simultaneous processes clean the wastewater and
cleaned water is released through the outlet pipe

86
 Horizontal flow Wetland Advantage of Constructed
Consists of wetlands
• Liner • Less expensive to build and operate than
mechanical systems
• Inlet structure
• No energy required to operate
• Bed(including media and plants)
• Little maintenance
• Outlet structure
• Provides wildlife habitat
• Produces no biosolids or sludge that requires
disposal

87
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 Objectives
• Generally the wastewater generated in the houses is released to the wastewater carriage system. However its is
very difficult to treat the wastewater generated in a centralized way in Bangalore city.
• Hence, there is a necessity of decentralized wastewater treatment facilities at household level.
• Necessity of constructed wetlands is to achieve the following
• To treat domestic waste water generated in the houses at household level using constructed wetland
• To test the quality of treated waste by using constructed wetland for other beneficial uses (recycling of treated
wastewater)
Removal Mechanism
Organic matter Aerobic microbial degradation
Anaerobic microbial degradation
Suspended solids Sedimentation
Filtration
Pathogens Sedimentation
Filtration
Natural die-off
Predation
UV radiation

89
Removal Mechanism

Nitrogen Nitrification
Plant uptake
Matrix adsorption
Phosphorus Matrix absorption
Plant uptake
Metals Adsorption and cation exchange
Complexation
Precipitation
Plant uptake
Microbial oxidation/ reduction

90
Stabilization ponds (also called wastewater lagoons) are large, shallow open ponds used to treat sewage naturally using sunlight, air, and microorganisMS
Used to treat sewage and wastewater in a low-cost, natural way.
Ponds are filled with wastewater, and microorganisms clean it over time.Sunlight helps algae grow, and algae produce oxygen for bacteria.
Bacteria break down the waste in water using that oxygen
. Waste stabilization ponds
Water stays in the pond for days or weeks, giving enough time for treatment. Types of Stabilization Ponds:
Anaerobic Pond – no oxygen, breaks down heavy organic waste.
Facultative Pond – both with and without oxygen (middle layer), does most of the treatment.
Maturation Pond – final pond, helps remove harmful germs and makes water cleaner.

91
 Stabilization ponds
Pretreatment classification
Lagoons or oxidation ponds • None – receives raw untreated WW
• Facultative • Screening – receives screened raw WW
• Tertiary • Primary – Pond acts as a form of secondary treatment
• Aerated • Secondary _ pond acts as a tertiary (polishing)
treatment
• Anaerobic’
Discharge classification
• Secondary treatment in rural areas
• Complete retention – water remove via evaporation/
• Polishing ponds percolation
• Serve 7% of population (1000’s)
• Controlled discharge
• 90% of ponds serve population < 10,00 • Discharge is regulated
• Long detention times
• Continuous discharge
• Discharge is not regulated
• Qout = Qin 92
 Facultative Ponds
Design
• Most common • Water depth 2 – 5ft (3ft freeboard)
• Anaerobic( bottom layer) and aerobic (upper) • <2’ encourages weeds
• Bacteria break down organics • >5’ encourages anaerobic
• Nitrogen/phosphorus/CO2 conditions
• Algae and reaeration (wind) provides O2 • Usually enclosed by earth dikes
into cells
• BOD < 30mg/L because of algae (50 – 100 mg/L)
• Cells designed for flexibility to
• Don’t operate weel in cold weather operate in parallel or series
• Cant handle Industrial ww’s • BOD loading 20# per acre per day
(north)
• BOD loading upto 50# per acre per
day (south)
• Typical retention times of 3- 6
months

93
 Tertiary Ponds Aerated Lagoons
• Maturation/polishing ponds • Completely mixed
• Can reduce SS/BOD/fecal coliform/ Ammonia • First stage treatment of municipal WW
• Used after trickling filters/activated sludge • Pre-treatment of industrial WW
• Water depth 2 – 3’ (mixing, sunlight) • Basins 10- 12’ deep
• BOD load < 15# per acre-day • Pier- mounted floating mechanical aerators
• Detention times 10 – 15 days • No algae
• Odour free if highly aerated

94
 An anaerobic lagoon is a type of wastewater treatment pond where waste is broken down without oxygen.
It is a deep pond filled with wastewater or sludge.
Inside the lagoon, no oxygen is present (anaerobic conditions).
Anaerobic bacteria (bacteria that don’t need oxygen) break down organic waste.
Anaerobic Lagoons The process creates gases like methane and carbon dioxide.
It is used to treat strong waste, like animal waste, sewage sludge, or industrial waste.
The treatment is slow, but low-cost and energy-saving.

• May be covered
• High strength WW
• Meat processing
• Dairy waste
• Temperature must be high- 75 to 82 F
• BOD loading 20# per thousand ft3 per day
• Gaseous end products of CO2 and CH4

95
 Algae Based Wastewater Treatment
Algae-based treatment uses algae (tiny green water plants) to clean wastewater. It is a natural and eco-friendly method.
Algae use sunlight to make their own food, so no extra energy is needed.
Algae turn sunlight and wastewater nutrients into useful biomass (can be used for biofuel, fertilizer, etc.).
Algae absorb nitrogen (N) and phosphorus (P), which helps prevent pollution in rivers and lakes.
Algae help lower the amount of organic waste, making water cleaner.Algae can reduce harmful bacteria like coliforms in the water.
Algae can take out toxic metals like lead, cadmium, mercury, arsenic, etc., from the water.
Wastewater already contains N & P, so it becomes a cheap food source for algae growth.

Why algal technology is sustainable and effective?

• Ability to photosynthesize
• Conversion of solar energy into useful biomasses
• Effective removal of nutrients (N & P)
• Reduction of BOD
• Inhibition of coliforms
• Removal of heavy metals and toxic minerals (lead, cadmium, mercury,
scandium, tin, arsenic and bromine)
• Wastewater containing N&P could be used as a cheap nutrient source for
algal biomass production
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Wastewater treatment Process: Traditional Vs Algae based
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 Engineered System
• Hypo/ Hyper concentrated cultures
• Immobilized system
• Dialysis cultures
• Tubular photo bioreactors
• Waste stabilization pond

Hypo/Hyper Concentrated Cultures:

Horizontal Tube
Hypo-concentrated: Low concentration of microorganisms in culture.Hyper-concentrated: High concentration of microorganisms, boosting their activity to treat more waste quickly.
Immobilized System: Photobioreactor
Microorganisms are attached to a surface (like beads or a solid support), so they stay in place to break down waste This system improves the efficiency of wastewater treatment.
Dialysis Cultures:
A type of semi-permeable membrane is used to separate waste components while allowing nutrients and microorganisms to pass through for efficient treatment.
Tubular Photobioreactors:
Tubular reactors are long, tube-like structures where algae or other microorganisms grow and use sunlight to treat wastewater.This system enhances the photosynthesis process for cleaning.
Waste Stabilization Pond: 98
Large shallow ponds used to treat wastewater naturally by using sunlight, microorganisms, and algae.
Plate Photobioreactor

99
High Rate Algal Pond

100
 Uses of Algal Biomass
• Methane production
• Composting
• Production of liquid fuels
(pseudo-vegetable fuels)
• As animal feed or in
aquaculture
• Production of fine chemicals

101
Types of reactors used in
treatment systems
Prof. Indumathi M.Nambi

102
Reactors (Treatment Units)
• The units or vessels that hold wastewater for
treatment by chemical or biological processes are
normally called as reactors
• The units that are used for separation of solids from
liquid by settling or flotation are termed as basins or
tanks

Factors influencing the design

• size (capacity or volume) of a reactor
• treatment system selected
• order of reaction rate assumed
• flow conditions
103
Types of Reactors
Depending upon the flow and operating conditions and the
method of mixing, the reactors have been classified as under :

• Continuous - flow Stirred Tank Reactor (CFSTR)

• Plug - Flow Reactor (PFR)
• Completely Mixed Batch Reactor (CMBR)
• Fluidized Bed Reactor (FBR)
• Packed Bed Reactor (PBR)

104
Continuous - flow Stirred Tank Reactor (CFSTR)

105
Plug - Flow Reactor (PFR)

106
Completely Mixed Batch Reactor (CMBR)

107
Fluidized Bed Reactor (FBR)
• A reactor in which the filled packing
material expands and gets fluidized
when the wastewater to be treated
moves upward in the reactor is called
a FBR.
• Normally, air is also introduced along
with the influent flow from the inlet.
• Such reactors are becoming popular to
treat wastewaters biologically either in
aerobic or anaerobic conditions.
108
Packed Bed Reactor (PBR)
• A reactor in which the
filled inert packing
material for the growth of
biomass is kept packed
(or fixed) is called a PBR.
• The flow of wastewater
through the reactor may
be upward or downward.
• The packing material
commonly used is slag,
rock or ceramic.

109
Secondary treatment
Secondary water treatment is a specialized process that helps in removing
the microbial unicellular life or the large amounts of microbes.

There are two phases to

biological treatment

◦ “Mineralization” of the waste

organics producing CO2 + H2O +
microbes
◦ Separation of the microbes
and water

110
111
Suspended & Attached
growth processes

112
 Suspended growth processes
• Microorganism are maintained in suspension by appropriate
mixing methods

• Many of the processes are operated aerobically

• Anaerobic processes are also used for treatment of industrial wastewater having high organic content and
organic sludge

• The most common process used in domestic wastewater

• Activated sludge process

• Stabilization ponds

• Upflow anaerobic sludge blanket reactors

113
Suspended growth system

114
 Attached Growth processes
Microorganisms stick to a solid, inert material that doesn’t break down in water.

• Microorganism are attached to an inert packing material

• Packing materials include:
• Rock, Gravel, Sand
• Slag
• Redwood
• Wide range of plastic and other synthetic materials
• Operate as aerobic and anaerobic processes
• The packing can be submerged completely in liquid or not submerged
• The most common process
• Trickling filter
• Rotating biological contactors
Trickling Filter: Wastewater flows over a layer of packing material with microorganisms attached, where they break down pollutants.
Rotating Biological Contactors (RBC): Large discs rotate in wastewater, allowing microorganisms on the discs to treat the water as it flows over them.

115
 Attached Growth System
The activated sludge process is a commonly used method to treat wastewater using microorganisms to break down organic pollutants.
Steps Involved in the Activated Sludge Process:
Wastewater Aeration with Microbes:
Wastewater is mixed with microorganisms in a large tank, and air is bubbled in (aeration).
The microbes use oxygen to break down the waste in the water.
Solid-Liquid Separation:
After aeration, the solid particles (microorganisms and waste)
need to be separated from the cleaned water.
This is done in a settling tank where solids settle to the bottom.
Discharge of Clarified Effluent:
The clean water (effluent) is then discharged, as it is now free
from most pollutants.
Wasting of Excess Biomass:
Any extra microorganisms (biomass) that aren't needed are
removed from the system (wasted).
Return of Remaining Biomass: The remaining usefu
l microorganisms are returned to the aeration tank to keep
the process going

116
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.

117
118
 Activated Sludge Processes
• Activated sludge – sludge particles with growth of microorganism in aeration tank
• Activated Sludge Process- biological treatment process that uses microbes to degrade organic matter as their
substrate to produce high quality effluent under aerobic condition

119
Mechanism of activated sludge processes

120
Factors influencing ASP

121
 Activated Sludge Processes

Economical to produce
high quality effluent

Only require small Advantage Low Construction cost

land

Reasonable maintenance
cost

122
 ASP

Hydraulic
overload
Poor
primary nitrification
clarification

limits

Nutrient Organic
shortage underload

Organic
overload

123
Reactors types in ASP

124
 Modified ASP
• Conventional ASP Tapered aeration ASP
• Tapered aeration
• Step aeration
• Contact stabilization
• Extended aeration
• Completely mixed process
Conventional ASP

125
Step aeration Contact stabilization tank

126
Extended aeration Completely mixed aeration

127
Advantages of ASP
• 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

128
Disadvantages of ASP

• 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

129
Mass balance

130
Design Parameters
•

131
Mass balance on biomass
•
Effluent water & sludge

132
Mass balance on substrate

133
•

134
•

135
Mass balance on biomass around settling tank

136
•

137
References
Peavy & rowe, 230-242

138
 Moving Bed Biofilm Reactor

• Moving bed biofilm reactor (MBBR) is a type

of wastewater treatment process that was first
invented by Prof. Hallvard
Ødegaard at Norwegian University of Science
and Technology in the late 1980s.
• It was commercialized by Kaldnes Miljöteknologi
(owned by Veolia Water Technologies).
• The MBBR system is a biofilm process.

139
 MBBR - Process
• MBBR is an aerobic attached biological growth process
• It does not require primary clarifier and sludge recirculation.
• Raw sewage, after screening and de-gritting, is fed to the biological
reactor.
• Floating plastic media is provided which remains in suspension. Biological
mass is generated on the surface of the media.
• Excess biological mass leaves the surface of media and is settled in
clarifier.
• Usually a detention time of 5 to 12 hours are provided in the reactors.
• Animation

140
MBBR

Screen Grit FAB FAB2

Chamber 1
Influent

Blow
er Treated
Sewage

Filter press/ Lam Chlorinator

Sludge drying ella (Cl2/Hypo
beds Solution
Settl
er
Thickener

141
 Design details
• Surface area of the media is very
important
• Media usually 20- 40% of the tank volume.
• The sp gravity of the media and geometry
of the media are very important
• The media should have a sp. gravity
almost similar to water
• It should be always moving.

142
 Advantages
• Very easy system to operate and maintain. Can be used for
retrofitting existing STPs, if the capacity is not sufficient.
• The advantage is that spare parts are cheaper and long
lasting, so operational expenses are lower than with MBR
(Membrane Bio Reactor).
• Biofilm processes in general require less space than activated
sludge systems because the biomass is more concentrated,
and the efficiency of the system is less dependent on the final
sludge separation.
• MBBR systems don't need a recycling of the sludge, which is
the case with activated sludge systems.

143
 Membrane Bioreactors (MBRs) in
wastewater treatment and reclamation

144
Membrane Bioreactors (MBRs)
• combination of a suspended growth biological treatment method, with membrane filtration equipment

• The membranes are used to perform the critical solid-liquid separation function.

• reduced footprint, usually 30-50% smaller than an equivalent conventional active sludge facility with secondary
clarifiers and media tertiary filtration.

• https://youtu.be/GC2u4qdWTJI?t=53

145
Flow Schematic for MBR System

146
Process Scheme of MBRs

147
 Different MBR Configurations: Side-stream
(external) and submerged (internal)

148
A Typical Membrane Casette

149
 Typical MBR Effluent Quality
• BOD < 2.0 mg/L
• TSS < 2.0 mg/L
• NH3-N < 1.0 mg/L (with nitrifying MBRs)
• Total Phosphorus < 0.1 mg/L (with inclusion of
anaerobic zone)
• Total Nitrogen < 3-10 mg/L (with inclusion of
anoxic zone: denitrification)
• SDI < 3.0
• Turbidity < 0.5 NTU
• Total Coliforms < 100 cfu/100 mL
• Fecal Coliforms < 10 cfu/100 mL
• Coliform Reduction > 5-6 log removal
• Virus Reduction < 4 log removal
150
Operation and Maintenance
Fouling- Biggest Problem

• occurs due to interaction

membrane and activated
sludge liquor

• Increases Hydraulic
resistance

• Increases operating
costs

151
Fouling control
Many other anti-fouling strategies have been proposed for
MBR applications.
1.intermittent permeation,
2.Membrane
3. Adding of cleaning agents (like hypochloride)

152
Merits and Demerits of MBR system
• Low hydraulic retention time and hence low foot print (area) requirement
• Less sludge production
• High quality effluent in terms of low turbidity, TSS, BOD and bacteria
• Nutrient Removal is possible
• Stabilized sludge
• Ability to absorb shock loads
Demerits
• High construction cost
• Very high operation cost
• Periodic replacement of membranes is required
• High membrane cost
• Fouling of membrane

153
MBR Plants

MBR process animation

https://youtu.be/GC2u4qdWTJI?t=49

MBR plant in operation

https://youtu.be/5JL6uuF5zio

MBR cleaning

https://youtu.be/Rs7Obe3DtxU

154
Trickling filters

155
156
157
158
159
160
Design Criteria for Trickling filters

161
162
 ROTATING
BIOLOGICAL
CONTACTORS
Contactors
Primary Secondary
Treatment Clarifier

Influent Effluent

Solids Removal 163

Rotating biological contactors
• RBC is a fixed film, aerobic, biological wastewater treatment system.
• The primary function of these bio-reactors is the reduction organic
matter.

164
165
RBC Secondary Treatment
Rotating Plastic Media
1.6 rpm Provides Large Surface Area

40 %
Submerged

166
ADVANTAGES OF RBC PROCESS
Simple Operation
Low Energy Requirements
Nitrification
Few Nuisances
Wide Flow Range
Large Biological Population
Handles Shock Loads
Low Head Loss
167
 RBC Flow Scheme
INFLUENT
Primary
Treatment
Rotating
Biological
Pretreatment
Contactors

Disinfection

Secondary
Clarifiers EFFLUENT
Solids Handling

168
RBC COMPONENTS

CONTACTOR

TANK

CLARIFIER
169
RBC COMPONENTS
CONTACTOR

Discs

Shaft
Individual Disc
170
 Oxygen
Organics

Liquid Film

Biomass

Media
(disc) Oxygen

171
 Train
Baffles

5
Stages
(Zones of Treatment)
172
 2 Trains
Influent
5 Stages

1st Stage

Effluent

173
STAGING

174
 DISADVANTAGES
OF
RBC PROCESS
Limited Controls
Enclosures
Limited Experience and
Training
175
 References
RBC: Peavy Rowe: 264-268

176
Sequential batch reactor
• SBR technology is a method of wastewater treatment in which all phases of the
treatment process occur sequentially within the same tank.
• The sequencing batch reactor is a fill and draw activated sludge system. In this system,
wastewater is added to a single “batch” reactor, treated to remove undesirable
components, and then discharged.

177
The various stages in the sequence
are as follows:
Stage 1: Filling

Stage 2: Reaction

Stage 3: Settling

Stage 4: Decanting

Stage 5: Idling

Stage 6: Excess sludge wasting

178
 Sequential Batch reactors

• It is a completely mixed reactor

• Working in the principle of ASP
• Mostly designed as an Extended aeration Process
• A batch process:
• As it works in sequence, multiple reactors can treat a continuous flow of
wastewater.

179
 SBRs

• All SBRSs are having five steps in common

– Fill
– React (aeration)
– Settle (sedimentation/clarification)
– Decant (draw)
– Idle
• For continuous operation, at least two tanks should be provided: while one tank
receives the flow other tank completes the treatment cycle

180
SBR Operating cycle

181
SBR tank in reaction mode

182
Air Diffusers

183
 Advantages and disadvantages of SBR
• A single reactor vessel thus requires • A higher level of sophistication system
small space. • Potential plugging of aeration devices
• Minimal footprint. during operation
• High nutrient removal capabilities.
• The BOD removal efficiency is generally
85 to 90%
• Filamentous growth elimination

184
SECONDARY SETTLING TANK
-Prof. Indumati M. Nambi
IIT Madras

185
Secondary Clarification/Settling
• Secondary treatment represents a
substantial organic load and must be
removed to meet acceptable effluent
standards
• Removal is accomplished by
❑ In ponds and lagoons - settling within the
reactor
❑ In activated sludge and attached-culture
systems - Secondary Clarifier

186
Activated-Sludge Clarifiers
Objectives
• They must produce an effluent sufficiently clarified to meet discharge standard.
• They must concentrate the biological solids to minimize the quantity of sludge that
must be handled
Concentrated suspension
• Suspension in which particles are close enough together so that their velocity fields
overlap with those of neighboring particles
• A significant upward displacement of water occurs as particles settle.
• These factors act to-prevent independent settling
• Groups of particles settle at the same rate regardless of size difference
• The collective velocity of particles depends on several variables
• Velocity is inversely proportional to the concentration
187
Zone settling
• In secondary clarifiers, the solid concentration must be increased from
the concentration of the reactor X to the concentration of the clarifier
underflow Xu
• Settling velocities change correspondingly resulting in zones with
different settling characteristics
• Zone settling can be illustrated by a simple batch analysis in a column

188
Batch analysis
If a column is filled with a concentrated suspension and allowed to settle quiescently, the
contents will soon divide into zones as shown in fig

189
Different zones
• In zone B, the initial concentration Co is preserved and settles at a uniform velocity
characteristic of that concentration. The resulting clarified zone, zone A, is lengthened at
same velocity
• Zone D, in which particles are mechanically supported from below is called compression
zone and particles in this zone have only a slight velocity resulting from consolidation
• Zone C is called thickening zone, contains a concentration gradient ranging from slightly
greater than Co just below zone B to slightly less than the concentration at the top of the
compression zone
• As time progresses, the interfaces between the zones move relative to each other. The C-
D interface moves upward as particles from Zone C drop into zone D
190
Relationship between initial concentration and settling curves

191
Continuous-flow analysis
• The zone settling principles for batch analysis are
also applicable, within limits, to continuous-flow
secondary clarifier
• Because the A-B phase is stationary water in the
clarified zone rises toward the overflow at a rate
equal to collective velocity, thus satisfying the
clarification function
• The thickening function is accomplished via the
concentration gradient in the thickening and
compression zones and is more difficult to
determine.
• The thickening function can be found out by
solids flux method 192
Solids flux
Mass of solids per unit time passing through a unit area perpendicular to the
direction of flow. In secondary clarifiers, it is the product of the solids
concentration times the velocity
Solids flux for a clarifier due to underflow transport
Gu = vu Xi = (Qu/A)Xi Where,
Solids flux due to gravity settling Vu = transport velocity
Qu = underflow rate
Gg = vg Xi A = area of the tank
Total solids flux, Xi = solids concentration
Vg = Settling velocity
Gt = Gu + Gg

193
Solids flux as a function of solids concentration and
underflow velocity.

194
Secondary clarifier design
• Designed for effluent clarification and solid thickening, both relate directly to surface area
• To determine the required surface area, an underflow concentration is selected and the
overflow rate and limiting solids flux established
• Data needed is obtained from batch analysis
• The straight line portion of the interface vs. time graph establishes the settling velocity
and thus establishes overflow rate
• Obtaining appropriate sludge samples for batch analysis is often difficult
• Where analytical data are not available, the engineer must rely on literature values for
design data

195
Designing a secondary clarifier for activated sludge. The results of the column analysis are
shown in the table below

Conc MLSS, 1400 2200 3000 3700 4500 5200 6500 8200
mg/l
Velocity. m/h 3.0 1.85 1.21 0.76 0.45 0.28 0.13 0.089
The influent concentration of MLSS is 3000 mg/L, and the flow rate is 8000 m3/d. Determine the
size of the clarifier that will thicken the solids to 10,000 mg/L.
Solution
1. Calculate the solids flux
G = MLSS(kg/m3) x velocity(m/h)
Conc MLSS, 1400 2200 3000 3700 4500 5200 6500 8200
mg/l
G, kg/m2.h 4.20 4.07 3.63 2.8 2.03 1.46 0.9 0.73

196
2. Plot solids flux vs. MLSS concentration. Draw a line from the desired underflow
concentration, 10000 mg/L. The value of G at the intersection, 2.4 kg/m2

3. Determine total solids loading to the clarifier

197
4.Determine the surface area of the clarifier.

198
Attached-Culture Systems Clarifier
• Similar to that for primary clarifiers
• The clarification function is the important parameter
because sludge thickening is not a factor.
• Commonly used overflow rates from 25 to
33 m3 /m2.d with a maximum of 50 m3/m2 .d.
• No sludge is recycled to the reactor, so the
underflow is negligible compared to the overflow
• The total quantity of solids generated by attached-
culture systems is generally less than that generated
by suspended-culture processes
199
References
• Secondary clarification: Peavy & Rowe: pg no 268 to 277

200
Tertiary Treatment

201
Need of Tertiary Treatment of Wastewater:
• Continued increase in population
• Limited water resources
• Contamination of both surface and groundwater
• Uneven distribution of water resources and
• periodic droughts

202
Nutrients removal – Nitrogen & Phosphorus
Nitrogen removal:
• Air stripping
• Nitrification-Denitrification
• Breakpoint chlorination

Phosphorus removal:
• Chemical precipitation
• Biological phosphorus removal

203
 Nitrogen
Common forms:

• Organic bound Nitrogen – Nitrogen bound in organic compounds

• Most common form to release ammonia in water streams by deamination. Proteins are compounds consists of
amino acids joined by peptide bonds. When these proteins are hydrolyzed, then ammonia is released.

• Outside our body, urine is released which contains urea. This urea gets hydrolyzed to give ammonium carbonate
in high quantity.

• Ammonia (NH3) or ammonium ions (NH4+): Comes from human or animal excreta or industrial processes.

• Nitrite (NO2-) – Not stable form and its presence in wastewater denotes that oxidation of nitrogen is
incomplete.

• Nitrate (NO3-) – Most stable and oxidized form of Nitrogen and chemically unreactive in dilute aqueous solution.

• Total Kjeldah nitrogen = Organic nitrogen + NH4+

• Total nitrogen = TKN + nitrite + nitrate 204

Protein formation by peptide bond
formation

205
Nitrogen Cycle

206
 Problems by excess Nitrogen in water
bodies
1. Excess N may cause eutrophication (growth of small plants on ponds/lakes top surface) and so hindering
sunlight, oxygen penetration in water.

2. Ammonia consumes dissolved oxygen in water streams more than carbonaceous biomass (4.57g O2/gm of
NH3-N compared to 1.4 g O2/g biomass) and so DO of stream decreases.

3. Ammonia reacts with Cl2 to form chloramines, which are less effective disinfectants, so efficiency of chlorine
disinfection decreases.

4. Ammonia gas is a toxic gas and is toxic to fishes (Ammonium ion is not toxic!).

5. Nitrate causes blue baby disease to infants.

• Nitrogen concentration limit:

• Inorganic N should be < 0.3mg/L to prevent algal growth

• Total nitrogen should be < 10mg/L as NH3-N

207
 Nitrogen
• Conversion of ammonium to nitrite - partial oxidation by Nitrosomonas bacteria

NH4+→ NH2 → NH2OH → NHOH → NOH → NO → NO2-

• Overall reaction

2NH4+ + 3O2 → 2NO2- + 4H+ + 2H2O

• Complete oxidation by Nitrobacter bacteria

2NO2- + O2 → 2NO3-

• Overall reaction

NH4+ + 2O2 → NO3- + 2H+ + H2O 208

Nitrification and Denitrification

209
210
Nitrification and Denitrification – Flow diagram

211
 Nitrification - Denitrification
• Conversion of ammonium to nitrate (Nitrification) and then nitrate to nitrogen gas (Denitrification).

• Nitrification:

NH4+ + 2O2 → NO3- + 2H+ + H2O

• Denitrification:

6NO3- + 2CH3OH → 6NO2- + 2CO2 + 4H2O

6NO3- + 3CH3OH → 3N2 + 3CO2 + 6OH-

• Overall Denitrification reaction:

6NO3- + 5CH3OH → 3N2 + 5CO2 + 6H2O + 6OH-

212
 Nitrification - Denitrification

• Most of the nitrifiers are - Autotrophs (i.e. they need inorganic carbon as food
source) (e.g. Nitrosomonas and Nitrobacter bacteria). They need oxygen
(aerobic environment).

• Most of the denitrifiers are - Heterotrophs (i.e. they need organic carbon as
food source). So we need to externally add methanol CH3OH as carbon source if
water does not have sufficient organic carbon source. Oxygen shouldn’t be
there (anaerobic/anoxic environment).
213
 Nitrogen removal in
wastewater treatment
plant

(a) Combined carbon & nitrogen removal (a) Anoxic tank is before oxic/aerobic tank

(b) Separate carbon & nitrogen removal (b) Anoxic tank is after oxic/aerobic tank 214
 Phosphorus - P
• Common forms:
1. Organic Phosphorus – P bound in organic compounds (e.g. ATP, ADP, RNA, DNA)
2. Orthophosphate – Most reactive phosphate and plants use this form only (PO43-). It
consists of negative radicals like PO43-, HPO42- and H2PO4-. (HnPO4(3-n)-)
3. Condensed phosphate – It is inactive form and needs to be converted to
Orthophosphate (by hydrolyzing) before being consumed by plants (e.g.
pyrophosphate, tripolyphosphate). It comes from detergents.
• Problems:
• Even trace concentration (lesser than nitrogen concentration) causes eutrophication.
215
216
 Chemical precipitation
• At slightly acidic pH, phosphorus combine with trivalent aluminium or iron cations to form
precipitate.

• Al3+ + HnPO4(3-n)- → AlPO4 + nH+

• Fe3+ + HnPO4(3-n)- → FePO4 + nH+

• At high pH, calcium also forms insoluble complex with phosphorus. So, lime can also be
used.

• 5Ca(OH)2 + HnPO4(3-n)- → Ca5(OH)(PO4)3 + nH2O + (9-n)OH-

• This reaction requires minimum pH of 9. So, lime addition would give both pH increament
and phosphorus precipitation. 217
 Chemical precipitation
• Chemical requirement usually exceed the stoichiometric requirement may be
upto a factor of 3 because of flocs formation (e.g. iron and aluminium would
form hydroxide flocks). However this extra addition of chemicals assist in
flocullation & precipitation of other colloidal solids along with phosphorous.

• Phosphorous removal may be done at either primary or secondary or tertiary

treatment unit. Selection of point of application depends on efficiency
requirement, wastewater characteristics and type of secondary treatment
employed.
218
 Biological phosphorus removal
• Some bacterias like phosphorus accumulating organisms (PAO) have capacity to accumulate
phosphorus in their cells (as polyphosphate) more than ordinary bacterias. So this extra uptake is
called luxury uptake.

1. In anaerobic conditions, these PAO would assimilate fermentation products (e.g. volatile fatty
acids) as intracellular polyhydroxybutyrate (PHB) into their storage cells with the constant release
of orthophosphate outside from their storage cells. This causes an increase in phosphorus in the
reactor.

2. In subsequent aerobic condition, these PAO utilise the stored PHB to form new cells and these
new cells would accumulate orthophosphate released in the anaerobic reactor.

3. As the biomass is removed continuously from reactor, the stored phosphorus would also get
removed.
219
Biological phosphorus removal

220
Biological phosphorus removal

221
References
Peavy & Rowe: pg no: 295 to 301

222
 Disinfection
• Disinfection is a process where a significant
percentage of pathogenic organisms are killed or
controlled.
• Chlorination/dechlorination has been the most
widely used disinfection technology; ozonation and
UV light are emerging technologies

223
224
Disinfection - mechanism

Ref: Peavy Rowe : pg no:

225
Breakpoint chlorination

226
 Breakpoint chlorination
• Breakpoint chlorination is a term which gives an idea of the extent of chlorine added to water.
• It represents, that much dose of chlorination, beyond which any further addition of chlorine will appear as free
residual chlorine.
• When chlorine is added to water, it first of all, generally reacts with NH3 present in water so as to form
chloramines.
• If chlorine is kept adding to water, then the residual chlorine will go on increasing with respect to addition of
chlorine.
• However some chlorine is consumed for killing bacteria and thus amount of residual chlorine shall be slightly less
than added, as shown by the curve AB.
• If the addition of chlorine continued beyond this point B, the organic matter present in water gets oxidized, and,
therefore, the residual chlorine contain suddenly falls down as shown by curve BC.
• The point C is the point beyond which any further addition of chlorine will appear equally as free chlorine, since
nothing of it shall be utilized.
• This point C is called Breakpoint, as any chlorine that is added to water beyond this point, breaks through the
water and appears as residual chlorine.
• The addition of chlorine beyond breakpoint is called as break point chlorination.
• This residual of free chlorine (generally kept as 0.2-0.3mg/L), appearing after breakpoint, is not easily removed
except by sunlight, and, therefore, it takes care of future contamination of water in the distribution system.227
a. NH4+ + HOCl → NH2Cl (monochloramine) + H2O + H+

b. NH2Cl + HOCl → NHCl2 (dichloramine) + H2O + H+

c. NHCl2 + HOCl → NHCl3 (trichloramine) + H2O + H+

d. NHCl3 + HOCl → N2 + H2O + H+

• HOCl and OCl- are called free chlorine

• Chloramnes are called combined chlorine

• Overall reaction:

2NH4+ + 3HOCl → N2 + 3H2O + 3HCl + 2H+

• In other words:

2NH4+ + 3Cl2 → N2 + 6HCl + 2H+

Clearly if sufficient Chlorine is reacted with ammonia in water, nitrogen in the form of N2 gas will be released from solution.
Theoretically, the atomic ratio of Cl:N to achieve breakpoint is 3:1, which by mass is 7.6:1 (Reference: Benefield) 228
 Breakpoint chlorination
Advantage:
• Practical and economical for low ammonia concentration.

Dis-advantage:
• Cost of chlorine.
• Introduction of TDS in water.

229
Removal of dissolved solids
for reuse purpose

230
Membrane process
Membrane process is any method that relies on a membrane barrier to filter or
remove particles from water. Fluid is passed through the membrane because of
the pressure difference between one side of the membrane and the other.
Contaminants remain on one side.

231
232
233
Electrodialysis

234
235
236
Ion exchange treatment
• Ion Exchange can be used in wastewater
treatment plants to swap one ion for another
for the purpose of demineralization.
• There are basically two types of ion exchange
systems, one which is using the anion resins
and another is the cation exchange resins.

237
ANAEROBIC TREATMENT
SYSTEMS
-Prof. Indumati M. Nambi
IIT Madras

238
Anaerobic wastewater treatment

• Biological process (fermentation process) where microorganisms degrade organic

contaminants present in waste water in the absence of oxygen.
Anaerobic
microorganis
CH4 + CO2
Organic Pollution ms
(Biogas)

• It can be used as pretreatment prior to aerobic municipal wastewater

treatment.
• Often used to treat industrial wastewater that contains high organic matter
content in warm temperatures.

239
 Anaerobic digestion and
Anaerobic wastewater treatment regenerative thermal oxidiser
component of Lübeck mechanical
biological treatment plant in
Germany, 2007 240
Microbiology of Anaerobic Conversions
• Hydrolysis: Extracellular enzymes
produced by hydrolytic
microorganisms decompose complex
organic polymers into simple soluble
monomers.
• Acidogenesis: Hydrolyzed
compounds are converted by
acidogens to a mixture of volatile
fatty acids and other minor products
such as H2, CO2 and acetic acid.
• Acetogenesis: Acetogenic bacteria
further convert the VFAs to acetate,
CO2 and hydrogen.
• Methanogenesis: Bacterial
Source: https://www.e-education.psu.edu/egee439/node/727 conversion of hydrogen and acetate
241
into final end products, methane and
Operating factors affecting anaerobic degradation

• pH value range:6.5-8
• Operating temperature:Bacteria have two optimum ranges of temperature, termed as
mesophilic (25-40 degree Celsius) and thermophilic (50-65 degree Celsius)
temperature optimum.
• Loading rate:loading rate of a system should never be high.
• Retention time:The longer is the retention time period, the better is the degradation
of the organic matter.
• Composition of waste water:bio-methanization potential is dependent upon four
major concentrations which are- lipids, proteins, carbohydrates and cellulose.
• Toxics: Inhibitory effects of certain materials on degradation if their concentration
becomes too high

242
Advantages and Disadvantages

Advantages Disadvantages
Less energy required Long start-up period
Less biological sludge produced Alkalinity addition
Lower nutrient demand Post treatment
Methane production, a potential Biological nitrogen and phosphorous
energy source removal is not possible
Smaller reactor volume required More sensitive system
Elimination of off-gas air pollution Susceptible to toxic compounds
Rapid response to substrate addition Production of odors and corrosive
after long periods without feeding gases
243
Types of anaerobic reactors

Low rate anaerobic reactors High rate anaerobic reactors

Anaerobic Pond Anaerobic Contact process
Septic tank Anaerobic Filter (AF)
Standard rate anaerobic digester Upflow Anaerobic Sludge Blanket
(UASB)
Imhoff Tank Fluidized Bed Reactor
Hybrid Reactor: UASB/AF
Anaerobic Sequencing Batch
Reactor(ASBR)

244
Septic tank

• Onsite wastewater disposal system

• Septic tank is sludge digestion cum sedimentation tank with large detention
time(12 -36 hrs)
• It is suitable for population up to 300.
• The septic tank is a buried, water-tight container usually made of concrete,
brick masonry, fiberglass or polyethylene.
• It hold the wastewater long enough to allow solids to settle down to the
bottom (forming sludge), while the oil and grease floats to the top (as scum)
• Compartments and a T-shaped outlet prevent the sludge and scum from
leaving the tank and traveling into the drainfield area.

245
• The liquid wastewater (effluent) then exits the tank into the drainfield
• Finally, the wastewater percolates into the soil, naturally removing harmful
coliform bacteria, viruses, and nutrients.

Example for Septic Tank

246
Septic tank

247
Criteria:
• Hydraulic detention time plus solids storage
• 1 to 2 days detention of design flow
• Add solids storage volume equal to 1/3 – 1/2 of the above hydraulic detention
• Rate of accumulation of sludge:30 litres/person/year
Advantages Disadvantages
It can be easily constructed Its size should be very large
to serve many people
No maintenance problem Odour problem
It excellently remove BOD It needs periodic cleaning (6
months-3 years)
Very less amount of solids Working of septic tank is
are produced unpredictable
Low Cost 248
? Design the dimensions of a septic tank for a small colony of 150 persons
provided with an assured water supply from municipal head works at a rate of
120 litres/person/day. Assume any data
Solution:
Quantity of water supplied = per capita rate X population
=120 X 150 litres/ day
=18000 l/day
Assuming 80% of water supplied becomes sewage
Quantity of sewage produced = 18000 X 0.8 = 14400 l/day
Assuming detention time to be 24 hrs
24
Quantity of sewage produced during detention period =14400 x
24
=14400 litres

249
Assuming the rate of deposited sludge as 30 litres/capita/year and period of
cleaning as 1 year
• Volume of sludge deposited = 30x 150 x 1 =4500 litres
• Total required capacity of the tank = capacity of (sewage + sludge)
=14400+4500= 18900 l= 18.9 cu.m
Assuming 1.5 m as depth of tank
18.9
Surface area of tank = =12.6 𝑚2
1.5
If ratio of length to width is 3:1
3: 𝐵2 = 12.6
B= 2.05 say 2.1 m, provide width=2.1m
length =6m
Area of cross section =6 x 2.1 = 12.6 𝑚2
Dimension : 6 m x 2.1 m x (1.5+0.3) m {0.3 m free board}

250
Anaerobic baffled wall reactors
• Improved Septic Tank with a series of baffles under which the grey-,
black- or the industrial wastewater is forced to flow under and offer the
baffles from the inlet to the outlet .
• In anaerobic baffled reactor, the wastewater passes over and under the
baffles.
The biomass accumulates in
✓ Between the baffles which may in fact form granules with time.
✓ The baffles present the horizontal movement of biomass in the reactor.
✓ Hence a high concentration of biomass can be maintained within the
reactor.
• The compartmentalized design separates the solids retention time from
the hydraulic retention time, making it possible to anaerobically treat
wastewater at short retention times of only some hours.
251
252
253
Advantages & Disadvantages
Advantages Disadvantages
Resistant to organic and hydraulic shock Long start-up phase
loads
No electrical energy is required Requires expert design and construction
Low operating costs Low reduction of pathogens and nutrients
Long service life Effluent and sludge require further treatment
and/or appropriate discharge
High reduction of BOD Needs strategy for feacal sludge
management (effluent quality rapidly
deteriorates if sludge is not removed
regularly)
Low sludge production; the sludge is Needs water to flush
stabilized
Moderate area requirement (can be built Clear design guidelines are not available yet
underground) 254
Up Flow - Anaerobic Sludge Blanket Reactor (UASB)
• High rate anaerobic treatment
• Developed in 1970s by Prof. G. Lettinga in Netherlands
• It is a single tank process for anaerobic treatment of
wastewater.
• It is the most popular anaerobic wastewater
treatment system across the world
• It is extensively used for industrial waste water
treatment and for municipal sewage as well.
Source:
https://www.iwapublishing.com/news/flow-
anaerobic-sludge-blanket-reactor-uasb

255
Zones and Components

▪ Inlet zone: uniform distribution of influent

▪ Sludge banket zone/ Reaction zone: holds
granular biomass and allows biochemical
reactions to take place
▪ Settling / clarifier zone: for solids settling
▪ Gas-liquid-solid separator: separates the three
phases; gas to a collection pipe, solids pushed to
settling zone and water flows to effluent pipe /
channel
Source:
https://microbewiki.kenyon.edu/index.php/Upflow_An
anerobic_Sludge_Blanket

256
Working Principle:
• Wastewater enters from bottom and
flow upwards
• WW flows through a suspended sludge
blanket transforming it into biogas and
biomass(<5%)
• Solids are retained by a filtration effect
of blanket and settling in clarifier zone
• Upflow regime and motion of gas
bubbles allow mixing without
mechanical assistance
• Gas-liquid-solid separator at the top of
the reactor allow gases to escape and
prevent the outflow of solids Large-scale UASB reactor followed by a post-treatment in trickling filters

257
258
Advantages & Disadvantages

Advantages Disadvantages
High reduction of BOD Unstable treatment with variable hydraulic
and organic loads
Can withstand high organic and hydraulic Requires skilled personnel for operation and
loading rates maintenance
Low sludge production Long start-up time to work at full capacity
Biogas can be used for energy A constant source of electricity is required
No aeration system required Effluent and sludge require further treatment
Nutrient rich effluent can be used for Not adapted for cold regions
agricultural irrigation
Low land demand Requires expert design and construction

259
Sludge Digesters
• Stabilization of sludge withdrawn from the
sedimentation basin.
• By decomposing the organic matter under
controlled anaerobic conditions.
• 40% - 60% of organic solids converted into CO2
and CH4 gas.
• Remaining organic matter will be chemically
stable and odourless with 90% - 95% of moisture
content.
• This process reduces the sludge into three forms:
➢ Digested sludge
➢ Supernatant liquor
➢ Gases of decomposition 260
• Standard rate digestion process: the digester contents are usually unheated
and unmixed. The digestion period may vary from 30 to 60 d.
• High rate digestion process: the digester contents are heated and completely
mixed. The required detention period is 10 to 20 d.

261
Stages in sludge digestion

• Acid fermentation/Acid production: Acid formers stabilize organic

solids through hydrolysis. Soluble products get fermented to volatile
acids
• Acid regression : volatile acids acted upon by bacteria. Form acid
carbonates and ammonia compounds with evolution of small
amount of H2S and CO2
• Alkaline fermentation :Protein and organic acids attacked by
methane formers. Forms ammonia, organic acids and gases. Liquid
separates out from solids and the digetsive sludge is formed

262
Factors affecting sludge digestion process

• Temperature : Rate of digestion increases with temperature.

Zone of thermophilic digestion: 40-600C
Zone of mesophilic digestion:24-400C
• pH value: Optimum value (7.2-7.4)

• Seeding with digested sludge: speeds up the digestion process

• Mixing and stirring of raw sludge with digested sludge :helps in

better decomposition

263
Design considerations:
• Cylindrical in shape, circular in plan –
dia 3 to 12m
• Slope of bottom hopper floor – 1:1
to 1:3
• Depth of digestion tank – 6m
• Except in large plants not more than
2 units are provided.
• The capacity provided ranges from
21 to 61 lpcd.

264
Advantages Disadvantages
Low Operating Long start-up
costs time
Proven Affected by
Effectiveness changes in
loading
conditions

Burnable gas Explosive gas

produced produced

265
Design:
• If the process of sludge digestion is assumed to be linear, then
capacity of digester (in cu.m) is
𝑉1 + 𝑉2
𝑉= ( )t
2
Where V1= raw sludge added per day, cu.m/d
V2= equivalent digested sludge produced per day, on completion of
digestion V2 ≈ V1/3
t=digestion periods in day
• When the daily digested sludge could not be removed, even though
digestion gets completed, then consider separate capacity
(Monsoon Storage)
𝑉1 + 𝑉2
Thus total capacity, 𝑉 = ( )t +V2T
2
266
Contd…
• When the change during digestion is assumed to be parabolic then
the average volume of digesting sludge
2
[ 𝑉1 - (𝑉 - 𝑉2 )]
3 1
• Total capacity without monsoon storage
2
𝑉 =[ 𝑉1 - (𝑉 - 𝑉2 )]t
3 1
• Total capacity with monsoon storage
2
𝑉 =[ 𝑉1 - (𝑉1 - 𝑉2 )]t +𝑉2 T
3
T is the number of days for which the digested sludge is stored and is
called monsoon storage

267
? Design a sludge digester for the primary sludge with the help of
following data:
Average flow=20 MLD
Total suspended solids in raw sewage = 300 mg/L
Moisture content of Digested sludge =85%
Assume any other suitable data required
Solution: Mass of suspended solids in 20 ML of sewage flowing / day
300×20×106
= kg/day
106
= 6000 kg/d
Assuming 65% solids are removed in primary settling tank

268
• Mass of solids removed in the primary settling tank
= 65% X 6000 kg/d =3900 kg/d
Assuming that the fresh sludge has a moisture content of 95%, we have
5 kg of dry solids will make=100 kg of wet sludge
100
3900 kg of dry solids will make = X 3900
5
= 78,000 kg of wet sludge per day
Assuming spec gravity of wet sludge as 1.02
78000 3
Volume of raw sludge produced =V1 = 𝑚 / day =76.47 𝑚3 / day
1020
Volume of digested sludge = V2 at 85% moisture content
100−𝑝1
𝑉2 = 𝑉1 [ ]
100−𝑝2

269
 100−95
𝑉2 = 𝑉1 [ ] = 25.49 𝑚3 / day
100−85
Assuming digestion period as 30 days
2
Capacity =[ 𝑉1 - (𝑉1 - 𝑉2 )]t
3
2
=[ 76.47- (76.47-25.49)]30
3
= 1274.5 cu.m
Now, providing 6m depth of the cylindrical digestion tank,
We have cross sectional area of the tank= (1275/6)=212.5 𝑚3
212.5
Dia of tank= √ Τ m = 16.45 m say 16.5 m
𝜋 4
Hence, provide a cylindrical sludge digestion tank 6m deep and 16.5 m diameter
with an additional hoppered bottom of 1:1 slope for collection of digested sludge

270
Sludge treatment &
Disposal

271
 SLUDGE
PROCESSING
-Prof. Indumati M. Nambi
IIT Madras

272
Sludge disposal and treatment
• Sludge refers to the residual, semi solid
material left from industrial wastewater or
sewage treatment processes
• Contains many objectionable materials and
must be disposed of properly
• Sludge disposal facility usually represents 40-
60 % of the construction cost
• 50 % of operating cost

273
Sludge characteristics
Primary sludge
• Primary settling removes usually 40-60 % of influent solids
• Contains inorganic and coarser fraction of the organic colloids
• Contains a sizable fraction of the influent BOD, will become aerobic within a few hours
• Mp = c x SS x Q , where c = efficiency of primary clarifier
SS = Total suspended solids in effluent, kg/m3
Q = flow rate, m3/d
Secondary sludge
• Primarily composed of biological solids
• Ms = Y’ x BOD5 x Q, where Y’ = biomass conversion factor
274
Sludge characteristics
The volume wet sludge can be approximated by
V = M/ 1000.S
Where, V = volume of sludge produced, m3/d
M = Mass of dry solids, kg/d
S = solid content expressed as decimal fraction
1000 = density of water, kg/m3

275
Sludge thickening

Several techniques are available for volume reduction

• Vacuum filtration
Semi solid state incineration
• Centrifugation

• Gravity thickening
Liquid state Further biological treatment
• Floatation

276
Gravity thickeners
• Similar to secondary clarification • Vertical pickets on the scraper cause
• Deeper than secondary clarifier to provide horizontal agitation
greater thickening capacity • Design based on pilot plant analysis
• Eliminating half the volume

277
 Flotation
• Secondary effluent is subjected to • Air bubbles attach to solid particles
aeration under a pressure of 400 kPa • Thickened sludge is skimmed off at the top
• Sludge is passed in contact with this • Liquid is removed near the bottom and
supersaturated liquid returned to aerator

278
Sludge Digestion
Anaerobic and Aerobic Complex waste
Anaerobic
• Most common for dealing with
Acid formers
primary sludge
Advantages
Organic acids
• Less biomass production and other
• Usable methane gas produced intermediates
Disadvantages
• High capital cost Methane formers
• Susceptibility to shock and
overloads CH4
CO2

279
Hydrolysis
Anaerobic bacteria breakdown complex organic molecules (proteins,
cellulose, lignin and lipids) into soluble monomer molecules such as
amino acids, glucose, fatty acids and glycerol.

Acidogenesis
Conversion of sugar, aminoacids and fatty acids to organic acids (acetic,
propionic, formic, lactic, butyric acids), alcohols and ketones (ethanol,
methanol, glycerol and acetone), acetate, CO2 and H2

Methenogenesis
Conversion of volatile acids and other intermediates to final product of
methane and CO2
280
Standard rate anaerobic digester

281
High rate
•digesters
More efficient and require less volume • Optimum temperate – 35oC (95oF)
• Contents are mechanically mixed to ensure • Alternative dewatering system – Second stage
better contact digester
• Unit is heated to increase the metabolic rate • Second stage reactor is usually covered and
of microorganisms equipped for gas recovery

282
Design parameters of anaerobic digesters

Souring of anaerobic reactor

• Methane formers function within a pH range of 6.5 to 7.5
• During shock loading acid formers respond quickly and produce increased amount
of acids
• The rate of acid removal by methanogenesis falls behind the rate of acid
production, reactor pH falls which further reduce methanogenic activity.
• Buffering capacity of digester is therefore very important
283
Aerobic digestion
• Restricted to biological sludges in the
absence of primary sludge
• Common application – Stabilizing sludge
wasted from extended aeration system
• Endogenous respiration process
• Less sensitive to environmental factors
• Energy consumptive
• Digested sludge is relatively inert but
dewaters poorly

284
Sludge disposal
Incineration
• Water content should be sufficiently reduced
• Supplemental fuel is required and municipal solid waste may be used for this purpose

285
Sludge disposal
Placement in a sanitary Landfill
• Measures should be taken to contain leachate and isolate the sludge
Incorporation into soils as a fertilizer or soil conditioner
• Nutrient value of the sludge is beneficial to vegetation
• Granular nature may serve as a soil conditioner

286
 References
Sludge treatment & Disposal: Peavy & Rowe, Pg no: 278 to 294

287
Activated Sludge • Q – water flow rate
• yi – influent BOD (Si)
ASP notations
Process Design • V – volume of aeration tank (LBH)
• X – MLSS (Mixed Liquid Suspended Solids) or MLVSS (Mixed Liquid Volatile
Suspended Solids)
• Qe – effluent flow rate
• Xe – MLSS in effluent (Xe = 0 if not given)
• ye – effluent BOD (ye = 0 if not given)
• Qu – underflow rate (flow rate of sludge)
• Xu – MLSS in underflow (sludge)
• Qw – flow rate of waste sludge (taken for treatment)
• Qr – flow rate of recycling sludge

Q + Qr
Aeration tank Secondary
Q, yi V, X Qe, ye, Xe
clarifier
Xu

Qu , X u

Qr , X u Qw , X u 11
CE3040
Design Parameters
1. F/M ratio
F Mass of food applied per day
=
M Mass of microorganisms
F Q yi
=
M VX
F/M = 0.2 to 0.4 per day
Sometimes F/M is considered as the ratio of mass of food consumed to the mass of microorganisms
F Q (yi − ye )
=
M VX
2. Mean Cell Residence Time (MCRT - θc )
• It is the average life of microorganisms (sludge age)
• It is defined as mass of cells (microorganisms or MLSS) in the reactor (aeration tank) to
mass of cells wasted per day
• θc = 4 to 15 days
Mass of MLSS in aeration tank
θc =
Mass of MLSS wasted per day

VX VX
θc = =
Q w Xu + Qe X e Q w Xu + Q − Qw Xe 12
CE3040
Design Parameters
3. BOD removal efficiency (𝜂 = 85% to 95%)
yi − ye
η= × 100
yi

4. Volumetric Loading Rate (or) Organic Loading Rate (or) BOD Loading Rate (0.3 to
0.6 kg of BOD/day/m3)
Total BOD applied per day
OLR=
Unit volume reactor
Q yi
OLR =
V

13
CE3040
Design Parameters
5. Aeration period (or) Detention time (4 TO 8 hours)
V LBH
DT = =
Q Q
6. Sludge Volume Index (SVI)
Space occupied in ‘mL’ by 1 gram MLSS after settling for 30mins from 1 L sewage sample is known as
Sludge Volume Index (SVI – 50 to 150 mL/gm)
106
SVI ≈
Xu
7. Recycling ratio (0.25 to 0.5)
𝐐𝐫 𝐗
=
𝐐 𝟏𝟎𝟔
−𝐗
𝐒𝐕𝐈
X – 1500 to 3000 mg/L
Xu – 9000 to 12000 mg/L
14
CE3040
Problem

The following data pertains to ASP

Wastewater flow rate = 10000 m3/day

Influent BOD = 150 mg/L

Effluent BOD = 20 mg/L

MLSS = 3000 mg/L

MLSS in underflow = 10000 mg/L

F/M = 0.25 / day

θc= 10 days

Find the following:

(i) Volume of aeration tank

(ii) Aeration period

(iii) BOD removal efficiency

(iv) Volumetric Loading Rate

(v) Mass of solids wasted per day

(vi) Volume of solids wasted per day

(vii) SVI

(viii) Recycling ratio

 Solution
F Q yi
(i) Volume of aeration tank =
M VX
10000 × 150
0.25 =
V × 3000

V = 2000 m3

V 2000
(ii) Aeration period DT = = = 0.2 day = 4.8 hours
Q 10000

yi − ye 150 −20
(iii) BOD removal efficiency η= × 100 = × 100 = 86.67 %
yi 150

16
CE3040
 Q yi F
(iv) Volumetric loading rate OLR = = X = 0.25 × 3000 = 0.75 kg of BOD/d/m3
V M
Mass of MLSS in aeration tank
(v) Mass of solids wasted per day θc =
Mass of MLSS wasted per day
2 × 3000
Mass of solids wasted per day = = 600 kg/day
10

(vi) Volume of solids wasted per day

Mass of solids wasted 600 kg/d L
Volume of solids wasted per day = = = 60000 = 60 m3 /d
Xu 10000 × 10−6 d
OR
Qw Xu + Q − Qw Xe = 600 kg/day
Since Xe is not given; Xe = 0
Therefore, Qw Xu = 600
600
Volume of solids wasted per day = = 0.06 MLD = 60 m3 /d
10000

17
CE3040
 106 106
(vii) Sludge Volume Index SVI ≈ = =100 mL/gm
Xu 10000
(viii) Recycling ratio
Qr X 3000
= 6 = 6 = 𝟎. 𝟒𝟐
Q 10 10
−X − 3000
SVI 100
Q
To find exact values of r write mass balance equation around secondary clarifier
Q

Q + Qr
Aeration tank Secondary
Q, yi V, X Qe, ye, Xe
clarifier
Xu

Qu , X u

Qr , X u Qw , X u
18
CE3040
 Q + Qr
Aeration tank Secondary
Q, yi V, X Qe, ye, Xe
clarifier
Xu

Qu, X u

Qr , X u Qw , X u

Mass in = Mass out

Q + Qr X = Qu Xu + Q e Xe = Qr + Qw Xu + (Q − Q w )Xe
Since Xe is not given, Xe = 0
10000 + Qr 3000 = Qr + 60 10000
Q r = 4277 m3 /day
Qr 4277
= = 𝟎. 𝟒𝟑
Q 10000

19
CE3040
 Trickling filter
• Design criteria
 National Research Council
(NRC) Equation
Low rate or single High rate or two stage
stage TF TF

Recirculation factor
Where

• E1- BOD removal efficiency of the filter in single stage TF or first filter of two stage
TF (%)

• E2- BOD removal efficiency of the second filter in two stage TF (%)

• W1- BOD loading to the filter in single stage TF or first filter of two stage TF (kg
BOD per day)

• W2 - BOD loading to the second filter of two stage TF (kg BOD per day)

• V1 - Volume of the filter in single stage TF (m3)

• V2 - Volume of the second filter of two stage TF (m3)

• F1- recirculation factor for first stage filter

• F2 - recirculation factor for second stage filter

• f- treatability factor (0.9 for domestic wastewater)

TRICKLING FILTER
Problem 1: Design a low rate filter to treat 6.0 MLD of sewage
having BOD of 210 mg/l. The final effluent should be 30 mg/l
and organic loading rate is 320 g/m3/d.

Solution: Assume 30% of BOD load removed in primary

sedimentation i.e., = 210 x 0.30 = 63 mg/l.

Remaining BOD = 210 - 63 = 147 mg/l.

Percent of BOD removal required = (147-30) x 100/147 = 80%
BOD load applied to the filter = flow x conc. of sewage (kg/d)
= 6 x 106 x 147/106 = 882 kg/d
To find out filter volume, using NRC equation
E1= 100
1+0.44(w/V1.f1)1/2
80 = 100 f1= 1, because no circulation.
1+0.44(882/V1)1/2
V1= 2704 m3
Depth of filter = 1.5 m, Filter area = 2704/1.5 = 1802.66 m2, and
Diameter = 48 m < 60 m
Hydraulic loading rate = 6 x 106/103 x 1/1802.66 = 3.33m3/d/m2 < 4 hence o.k.

Organic loading rate = 882 x 1000 / 2704 = 326.18 g/d/m3 which is approx. equal to 320.