Design of racks, screens, grit chamber, aeration units, sedimentation tanks, activated sludge and trickling filter processes, rotating biological contactors, sludge digesters and drying beds

1. Design of Facilities for Physical, Chemical
& Biological Treatment of
Waste Water
Bibhabasu Mohanty
Asst. Prof.
Dept. of civil Engineering
SALITER, Ahmedabad

2. Course Content
Design of racks, screens, grit chamber,
aeration units, sedimentation tanks, activated
sludge and trickling filter processes, rotating
biological contactors, sludge digesters and
drying beds

3. SLUDGE TREATMENT…

4. Introduction…
• Sludge refers to the residual, semi-solid material left
from industrial wastewater, or sewage treatment
processes.
• Waste water sludge is the mixture of waste water
and settled solids.
• Depending upon the source it may be primary,
secondary, excess activated sludge.

5. Objectives…
• To reduce the volume of the material to be handled
by removal of liquid portion.
• To decompose the organic matter and inorganic
compounds for reduction in the total solids.

6. GOALS OF SLUDGE TREATMENT…
Volume • Thickening
reduction • Dewatering
Elimination of • If used in agriculture as fertiliser or
pathogenic compost
germs
Stabilisation of • Gas production
organic • Reduction of dry content
substances • Improvement of dewatering
• Reduction of odour
Recycling of • Nutrients, fertiliser
substances • Humus
• Biogas

7. Sludge handling and disposal includes:-
 Collection of sludge
 Transportation of sludge
 Processing of sludge to convert it to a form
suitable for disposal
 Final disposal of the sludge

8. Composition…
• Sludge from plain sedimentation tank- settable solids
(raw sludge)
• This gray in color contain garbage, fecal solids,
debris.
• Bad odor.
• From sec. settling tank following a trickling filter
consists of partially decomposed organic matter.
• Dark brown in color, less odor than raw sludge.

9. Sludge types…
• Primary sludge
 3 to 8 % solids
 About 70% organic material
• Sec. sludge
 Wasted microbes and inert materials
 90% organic material
• Tertiary sludge
 If sec. clarifier is used to remove phosphate, this
sludge contain chemical precipitates.

10. Overview
Wastewater treatment
Primary, secondary, tertiary sludge
Process water
Thickening
Stabilisation Biogas
Thickening Agriculture
Dewatering Disposal site
Drying Construction industry
Incineration Atmosphere

11. Thickening (volume reduction) by Gravity
Gravity separation, similar to settling tank
Additional mechanic stirring to enhance flocculation and
extraction of water and gas
Supernatant is introduced to primary clarifier or – if floatables
and grease contents are high – to grid chamber
Thickened sludge is withdrawn from hopper and introduced to
sludge treatment
For an efficient thickening process the development of gas
bubbles must be prevented

12. Gravity Thickener
Inflow
Scum scimmer
Sludge
liquor
Thickened sludge

13. Thickening by Flotation
Pre treatment: mostly chemical flocculation
Sludge is placed in contact with air-saturated water
(full flow or recycle pressurization)
Air bubbles attach to solid particles
Floating Sludge bubble composite is collected at the
surface
Water is recovered under a scum baffle and removed

14. Thickening by Flotation

15. Sludge stabilization (mass reduction)
• Aerobic digestion
• Anaerobic digestion
Aerobic sludge digestion may be used to treat only
 Waste activated sludge
 Mixtures of waste activated siudge and primary
siudge
 Activated sludge treatment plant without primary
settling

16. Advantages
 Volatile solids reduction is equal that obtained
anaerobically
 Lower BOD concentrations in supernatant liquor
 Production of an odorless, humus-like, biologically
stable end
 Operation is relativeluy easy
 Lower capital cost

17. Disadvantages
 A high power cost is associated with supplying the
required O2
 A digested sludge is produced with poor mechanical
dewatering characteristics
 A useful by-product such as methane is not
recovered

18. Process design
Factors taht must be considered in designing aerobic
digesters include;
 Solid reduction
 Hydraulic retention time
 Oxygen requirements
 Energy requirements for mixing
 environmental condition such as pH, temperature.

19. Anaerobic digestion
 Sludge held without aeration for 10-90 days
 Process can be accelerated by heating to 35-40oC
 These are called High Rate Digesters (10-20 days)
 Advantages
 low solids production
 useable methane gas produced
 Disadvantages
 high capital costs
 susceptibility to shocks and overloads

20. Basic Components of Digester Gas
Anaerobic Digesters
Digested
Sludge
Raw Sludge
Mixing
Heat
Exchanger Circulating
Pump

21. Anaerobic digestion process
Organic acids
Complex CH4 and
and
Organics CO2
H2
Acid producing Methane producing
bacteria bacteria
(acidogens) (methanogenics)

22. Three Mechanisms Occurring:
Hydrolysis Process – conversion of insoluble high
molecular compounds (lignin, carbohydrates, fats)
to lower molecular compounds
Acidogenesis Process – conversion of soluble lower
molecular components of fatty acids, amino acids
and sugars (monosaccharide) to lower molecular
intermediate products (volatile acids, alcohol,
ammonia, H2 and CO2)
Methanogenesis Process – conversion of volatile acids
& intermediate products to final product of methane
and CO2

23. Steps in anaerobic (oxygen-free) digestion:
Particulate and complex organics Hydrolysis Soluble simple
organics
Soluble simple organics Acidogenesis Short organic
acids
Methanogenesis
Short organic acids CH4 & CO2

24. Conventional anaerobic digester High rate anaerobic digester

26. Anaerobic Digester Design
 Mean Cell Residence Time
 Volumetric Loading Factor
 Observed Volume Reduction
 Loading Factors Based on Populations

27. Sludge dewatering
 Dewatering aims to reduce the water content
further.
 The sludge can then be handled like a solid.
 Dewatering can be done mechanically using a filter
press (employing pressure or vacuum), or a
centrifuge.
 Also be done using drying beds.

28. Drying beds
• Most popular methods.
• A drying bed consists of a 30 cm bed of sand with an
under-drainage .
• Sludge is applied on the sand bed and is allowed to
dry by evaporation and drainage of excess water
over a period of several weeks depending on
climatic conditions.

29. • Bacterial decomposition of the sludge takes place
during the drying process while moisture content is
sufficiently high.
• During the rainy season the process may take a
longer time to complete.

30. TRICKLING FILTER
PROCESSES…

31.  Trickling filter is an attached growth process i.e. process
in which microorganisms responsible for treatment are
attached to an inert packing material. Packing material
used in attached growth processes include rock, gravel,
slag, sand, redwood, and a wide range of plastic and
other synthetic materials.

32. Process Description
 The wastewater in trickling filter is distributed over
the top area of a vessel containing non-submerged
packing material.
 Air circulation in the void space, by either natural
draft or blowers, provides
oxygen for the
microorganisms
growing as an attached
biofilm.

33.  The organic material present in the wastewater
metabolised by the biomass attached to the medium.
 The biological slime grows in thickness as the
organic matter abstracted from the flowing
wastewater is synthesized into new cellular
material.

34. Flow Diagram for Trickling Filters
Recirculation= A portion of the TF effluent recycled through the filter
Recirculation ratio (R) = returned flow (Or)/ influent flow (Q)
Or
Recycle
Final
clarifier
Q
Influent
Primary Wast
clarifier sludg
Trickling
filter

35. Advantages
 simplicity of operation
 resistance to shock loads
 low sludge yield
 low power requirements

36. Disadvantages
 relatively low BOD removal (85%)
 high suspended solids in the effluent (20 -30
mg/L)
 little operational control

37. Types of Filters
Trickling filters are classified as high rate or low rate,
based on the organic and hydraulic loading applied to the
unit.
S.No. Design Feature Low Rate Filter High Rate Filter
Hydraulic loading,
1. 1-4 10 - 40
m3/m2.d
Organic loading,kg
2. 0.08 - 0.32 0.32 - 1.0
BOD / m3.d
3. Depth, m. 1.8 - 3.0 0.9 - 2.5
0.5 - 3.0 (domestic
wastewater) up to 8 for
4. Recirculation ratio 0
strong industrial
wastewater.

38.  Hydraulic loading rate is the total flow
including recirculation applied on unit area of
the filter in a day.
 Organic loading rate is the 5 day 20°C
BOD, excluding the BOD of the
recirculant, applied per unit volume in a day.
 Recirculation is generally not adopted in low
rate filters.
 A well operated low rate trickling filter in
combination with secondary settling tank may
remove 75 to 90% BOD and suitable for
treatment of low to medium strength domestic
wastewaters.

39.  The high rate trickling filter, single stage or two
stage are recommended for medium to relatively
high strength domestic and industrial
wastewater.
 The BOD removal efficiency is around 75 to 90%.
 Single stage unit consists of a primary settling
tank, filter, secondary settling tank and facilities
for recirculation of the effluent.
 Two stage filters consist of two filters in series
with a primary settling tank, an intermediate
settling tank which may be omitted in certain
cases and a final settling tank.

40. Process Design
 Generally trickling filter design is based on
empirical relationships to find the required filter
volume for a designed degree of wastewater
treatment.
 NRC equations commonly used.
 NRC (National Research Council of USA) equations
give satisfactory values when there is no re-
circulation, the seasonal variations in temperature
are not large and fluctuations with high organic
loading.

41.  NRC equations: These equations are applicable
to both low rate and high rate filters. The
efficiency of single stage or first stage of two
stage filters, E2 is given by
E2= 100
1+0.44(F1.BOD/V1.Rf1)1/2
 For the second stage filter, the efficiency E3 is
given by
E3= 100
[(1+0.44)/(1- E2)](F2.BOD/V2.Rf2)1/2

42. where E2= % efficiency in BOD removal of single stage or
first stage of two-stage filter
E3=% efficiency of second stage filter
F1.BOD= BOD loading of settled raw sewage in single stage
of the two-stage filter in kg/d
F2.BOD= F1.BOD(1- E2)= BOD loading on second-stage filter in
kg/d
V1= volume of first stage filter, m3
Rf1= 1+R
V2= volume of second stage filter, m3 (1+R/10)2
R=recycle ratio
Rf1= Recirculation factor for first stage, F=recirculation
R1= Recirculation ratio for first stage filter factor
Rf2= Recirculation factor for second stage,
R2= Recirculation ratio for second stage filter.

43. Q. Problem: Design a low rate filter to treat 6.0 Mld of
sewage of 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
 E2= 100
1+0.44(F1.BOD/V1.Rf1)1/2

44.  80 = 100 Rf1= 1, (no recirculation)
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
 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

45. ACTIVATED SLUDGE
PROCESSES…

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

47. 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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

64. ROTATING BIOLOGICAL
CONTRACTORS (RBC)…

65. Rotating Biological Contactors,
commonly called RBC’s, are used in
wastewater treatment plants
(WWTPs). The primary function of
these bio-reactors at WWTPs is the
reduction of organic matter.

66. A fixed growth biological treatment processes
used to consume organic matter (BOD) from
wastewater.
Consists of 2-6 m diameter disks, closely spaced
on a rotating horizontal axis.
Disks are covered with a biofilm.
The disks are only partially submerged in
wastewater.

67.  As the disk rotates the biofilm is exposed to the
wastewater only part of the time.
 The rotation in and out of the wastewater serves to
vary the feeding cycle of the bacteria and
microorganisms that make up the biofilm.
 The shaft rotates about 1-10 rpm (slowly).

68. Advantages/Disadvantages
Advantages Disadvantages
 Short contact periods  Need for covering units
 Handles a wide range of installed in cold climate to
flows protect against freezing
 Easily separates biomass
from waste stream  Shaft bearings and
 Low operating costs mechanical drive units
 Short retention time require frequent
maintenance
 Low sludge production
 Excellent process control

69. Flow Diagram of an RBC

70. Design Criteria
 No of modules = 4-5
 Dia of flat discs = 2-6 m
 Thickness of flat disc = up to 10 mm
 Discs spacing = 30-40 mm
 Speed of rotating shaft = 1-10 rpm
 Disc submergence = 40% of dia
 Thickness of bio-film = 2-4 mm

71.  Organic loading = 3-10 gm BOD/m2 of
disc surface area
 Hydraulic loading = 0.02-0.16 m3/m2-d
 Sludge production = 0.5-0.8 Kg/Kg BOD
removed
 Hydraulic retention time = 0.5 -2.0 h

72. RACKS &
SCREENS...

73. screen is a device with openings for removing bigger
suspended or floating matter in sewage which
would otherwise damage equipment or interfere
with satisfactory operation of treatment units.

74. Figure Definition sketch for types of screens used in wastewater
treatment

75. Design Consideration
Velocity
 The velocity of flow ahead of and through the screen
varies and affects its operation.
 The lower the velocity through the screen, the greater
is the amount of screenings that would be removed
from sewage.
 However, the lower the velocity, the greater would be
the amount of solids deposited in the channel.

76.  Hence, the design velocity should be such as to permit
100% removal of material of certain size without
undue depositions.
 Velocities of 0.6 to 1.2 mps through the open area for
the peak flows have been used satisfactorily.
 Further, the velocity at low flows in the approach
channel should not be less than 0.3 mps to avoid
deposition of solids.

77. Head loss
 Head loss varies with the quantity and nature of
screenings allowed to accumulate between cleanings.
 Head loss through screens mainly depends on:
Size and amount of solids in waste water
Clear openings between bar
 Method of cleaning and its frequency
Velocity of flow through the screens

78. The head loss through clean flat bar screens is
calculated from the following formula:
h = 0.0729 (V2 - v2)
where, h = head loss in m
V = velocity through the screen in mps
v = velocity before the screen in mps

79. Another formula often used to determine the head loss
through a bar rack is Kirschmer's equation:
h = b (W/b)4/3 hv sin q
where h = head loss, m
b = bar shape factor (2.42 for sharp edge rectangular bar, 1.83
for rectangular bar with semicircle upstream, 1.79 for
circular bar and 1.67 for rectangular bar with both u/s and
d/s face as semicircular).
W = maximum width of bar u/s of flow, m
b = minimum clear spacing between bars, m
hv = velocity head of flow approaching rack, m = v2/2g
q = angle of inclination of rack with horizontal

80. The head loss through fine screen is given by
h = (1/2g) (Q/CA)
where, h = head loss, m
Q = discharge, m3/s
C = coefficient of discharge (typical value 0.6)
A = effective submerged open area, m2

81. GRIT CHAMBER...

82. Grit chambers are basin to remove the
inorganic particles to prevent damage to
the pumps, and to prevent their
accumulation in sludge digesters.

83. Types of Grit Chambers
 Mechanically cleaned
 Manually cleaned
 In mechanically cleaned grit chamber, scraper blades
collect the grit settled on the floor of the grit chamber.
 The grit so collected is elevated to the ground level by
several mechanisms such as bucket elevators, jet pump
and air lift.
 Manually cleaned grit chambers should be cleaned at
least once a week.
 The simplest method of cleaning is by means of
shovel.

84. Aerated Grit Chamber
 An aerated grit chamber consists of a standard spiral
flow aeration tank provided with air diffusion tubes
placed on one side of the tank.
 The grit particles tend to settle down to the bottom of
the tank.
 Settling rates dependant upon the particle size and the
bottom velocity of roll of the spiral flow.

85. Design criteria
 Recommended for horizontal flow and aerated grit
chamber.
 Flow= maximum
 Detention time= 30-90 s (usually 60 s)
 Flow through velocity, vh= 0.2-0.4 m/s (usually 0.3 m/s)
 Settling velocity= 0.016-0.021 m/s for 0.2 mm dia particle
= 0.01-0.015 m/s for 0.15 mm dia particles
 Liquid depth= 1-1.5 m
 Length= 3-25 m
 Quantity of grits= 0.022-0.075 m3/1000 m3 of flow

86. Determination of settling velocity
Transition law:
 The design of grit chamber is based on removal of grit
particles with minimum size of 0.15 mm and therefore
Stoke's law is not applicable to determine the settling
velocity of grit particles for design purposes.
v2 = 4g(ρs-ρw)d
3 CDρw

87. Where:
g= acceleration due to gravity (assume 9.81 m/s2)
ρw= density of water (1000 Kg/m3)
ρs= density of solid particles
(normally of specific gravity 2.65=2.65*1000=2650
Kg/m3)
d= dia of particles
CD= coefficient of drag force depends on flow condition

88. AERATION UNITS...

89.  Unit process in which air and water are brought into
intimate contact.
 The contact time and ratio of air to water must be
sufficient for exchange sufficient oxygen.
Advantages
Providing O2 for purification and improving overall
quality.
CO2 reduction-reduces the corrosion.
Raising the pH.
VOC removal
Effective method for bacterial control

90. Methods of aeration
Diffused aeration
Spray aeration
Turbine aeration
Surface aeration

91. Diffused aeration
 Providing maximum water surface per unit volume of
air.
 Air bubbles brought with water in a mixing or contact
chamber.
 A common way to aerate water is via diffused air.
 Air is pumped through some sort of diffuser to
generate small bubbles.

92.  Usually gas is injected into the bottom of the aeration
tank and is allowed to rise to the surface in an open
tank.
 The rising bubbles transfer oxygen to the water, as well
as transport bottom water to the surface.
 The bubbles raising through water create turbulence.
 Untreated water is allowed to enter the tank from top
and exit from bottom.

93. Efficiency of diffused aeration can be improved:
Fine bubbles (0.2 cm dia) as compared to
coarse bubble (2.5 cm dia)
By increasing water depth (9-15 ft)
By improving the basin geometry (width to
depth ratio not exceed 2)
By increasing the retention time (10-30 min)

94. Typical diffused aeration system looks like:

95. There are a large variety of diffuser types. For example ceramic
plates

96. These plates are arranged on manifolds at the bottom of
aeration tanks as shown here.

97. Other types of diffusers include coarse aerators

98. Again, these diffusers would be arranged by a manifold
on the bottom of an aeration tank.

99. To determine the oxygen transfer rate in these diffused
aeration systems, first define the pressure difference
from top to bottom of the tank.
At the surface:
Psurface 14.7(1 0.032 Alt)
Alt = altitude in thousands feet above sea level
Psurface has units of psi

100. 62.4 H
Pbottom Psurface (psi)
144
H = depth of tank (depth of discharge point) in feet.

101. Mechanical Aeration
Basically there are two types of mechanical aeration.
Turbine Aeration:
 In this system coarse bubbles are injected into the
bottom of the tank and then a turbine shears the
bubbles for better oxygen transfer.
 Efficiency of turbine aerators is generally higher than
diffused aeration.

103. Surface Aeration:
In this case a mixing device is used to agitate the
surface so that there is increased interfacial area
between liquid and air.
There are many different proprietary types of
surface aerators .

104. Common surface aerators

105. Design consideration for mechanical aerators is usually
based on Eckenfelder and Ford equation.
C w Cl T 20
N N0 (1.02)
9.17
Notice that there is no depth consideration for
mechanical aeration.

106. Where as:
 N = actual transfer rate (lb-O2/hr)
 N0 = manufacturer specified transfer rate ( lb/hr)
for clean water, 20oC, zero DO.
 Cw = saturation value for oxygen for wastewater
under operating conditions.
 9.17 = saturation DO for clean water, 20oC.
 Cl = the design oxygen concentration in the
aeration basin.
 T = Temp.
 α = oxygen transfer correction factor for waste
water

107. SEDIMENTATION
TANKS...

108. Solid liquid separation process in which a
suspension is separated into two phases –
Clarified supernatant leaving the top of the
sedimentation tank (overflow).
Concentrated sludge leaving the bottom of the
sedimentation tank (underflow).

109. Purpose of Settling
To remove coarse dispersed phase.
To remove coagulated and flocculated
impurities.
To remove precipitated impurities after
chemical treatment.
To settle the sludge (biomass) after activated
sludge process / tricking filters.

110. Principle of Settling
 Suspended solids present in water having specific
gravity greater than that of water tend to settle down by
gravity as soon as the turbulence is retarded by offering
storage.
 Basin in which the flow is retarded is called settling
tank.
 Theoretical average time for which the water is
detained in the settling tank is called the detention
period.

112. Types of Settling
Type I settling (free settling)
Type II settling (settling of flocculated
particles)
Type III settling (zone or hindered
settling)
Type IV settling (compression settling)

113. Design parameters for settling tank
Overflow rate Solids loading Detentio
Types of settling Depth
m3m2/day kg/m2/day n time
Average Peak Average Peak
Primary settling only 25-30 50-60 - - 2.5-3.5 2.0-2.5
Primary settling followed by
35-50 60-120 - - 2.5-3.5
secondary treatment
Primary settling with
25-35 50-60 - - 3.5-4.5 -
activated sludge return
Secondary settling for
15-25 40-50 70-120 190 2.5-3.5 1.5-2.0
trickling filters
Secondary settling for
activated sludge (excluding 15-35 40-50 70-140 210 3.5-4.5 -
extended aeration)
Secondary settling for
8-15 25-35 25-120 170 3.5-4.5 -
extended aeration

114. Design Details
Detention period: for plain sedimentation: 3 to
4 h, and for coagulated sedimentation: 2 to 2.5
h.
Velocity of flow: Not greater than 30 cm/min
(horizontal flow).
Tank dimensions: L:B = 3 to 5:1. Generally L=
30 m (common) maximum 100 m. Breadth= 6
m to 10 m. Circular: Diameter not greater than
60 m. generally 20 to 40 m.

115. Depth 2.5 to 5.0 m (3 m).
Surface Overflow Rate: For plain sedimentation
12000 to 18000 L/d/m2 tank area; for
thoroughly flocculated water 24000 to 30000
L/d/m2 tank area.
Slopes: Rectangular 1% towards inlet and
circular 8%.

116. Problem:
Design a rectangular sedimentation tank to treat
2.4 million litres of raw water per day. The
detention period may be assumed to be 3
hours.

117. Solution: Raw water flow per day is 2.4 x 106 L . Detention
period is 3h.
Volume of tank = Flow x Detention period = 2.4 x 106 x 3/24
= 300 m3
Assume depth of tank = 3.0 m.
Surface area = 300/3 = 100 m2
L/B = 3 (assumed). L = 3B.
3B2 = 100 m2 i.e. B = 5.8 m
L = 3B = 5.8 X 3 = 17.4 m
Hence surface loading (Overflow rate) = 2.4 x 106 =
100
24,000 L/d/m2