D20/1WW (Water &

Wastewater Treatment)
Adebayo J. Adeloye & Rabee Rustum
Engineered Systems for Wastewater Treatment (D)
Secondary Sedimentation & Sludge Handling and
Treatment

1
Secondary Biological Treatment
 Usually follows primary settling

2
Secondary Sedimentation Tanks
 Secondary sedimentation is the last stage in
conventional wastewater treatment:
 Receives flow from the biological treatment reactor
 Settles any solids escaping the biological reactor as sludge
at the bottom and releases the resulting effluent to the
environment.
 This function is known as CLARIFICATION
 Concentrates the bottom sludge and returns it to the aerator.
 This concentration function is known as THICKENING

Thus while clarification is common to all (primary &

secondary) sedimentation tanks, thickening is only
required from the secondary tank of the activated sludge
process
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Secondary Sedimentation Tanks
 Because secondary settling tanks for trickling filters
only clarify, whereas AS secondary tanks both clarify
and thicken, design considerations vary:
 Trickling filter secondary tanks- also known as Humus
Tank:
 These behave essentially as primary sedimentation tanks;
 Design uses the same principles based on the surface overflow rate
to determine surface area.
 AS secondary tanks: 2 considerations:
i. Clarification- also uses surface overflow rate to determine surface
area for clarification (AC)
ii. Thickening- uses surface loading rate to determine the surface
area required for thickening (AT)
 Required surface area = max(AC, AT)
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Secondary Sedimentation
AS secondary tanks
 Thickening:
 The purpose of thickening is to concentrate the sludge by
squeezing out the moisture.
 The thickened sludge has a smaller volume for a given
mass, thus making handling much easier and cheaper.
 e.g. recall the relationship between sludge mass and
volume:
i.e. Volume, V, for a given mass M is inversely
M proportional to concentration, s!
V =
10s s = sludge solids concentration (%)
 Hence if sludge is thickened, the volume to be pumped or
tankered is reduced considerably.

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Secondary Sedimentation
AS secondary tanks

M1
Initial Volume V1 =
10s1

M2
Final volume V2 =
10s2

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Secondary Sedimentation
AS secondary tanks
Design Considerations
Area for clarification, AC Q is the design flow (=
3DWF),
Q
Ac = Vo is the surface overflow
V0 rate.
Area for thickening, AT MLSS (kg/m3)is the
biomass concentration
Q × MLSS
AT = entering the secondary
GL tank from the aerator,
GL is the solids loading
rate (kg/m2/hour)
Typical values for Vo and
GL are provided in the
next
8
slide.
Secondary Sedimentation
AS secondary tanks
Design Considerations
Typical Vo and GL for AS secondary settling tanks.

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Secondary Sedimentation
AS secondary tanks
Design Considerations
Example 3.16: An AS aerator operates at an MLSS
concentration of 0.3%. If the design flow rate is 8000 m3/day,
determine the surface area of the secondary tank that will thicken
the MLSS to 1%. Assume surface overflow rate (Vo) for
clarification is 1.21m/hour and GL = 3kg/m2/hour. What is the
actual Vo?
Solution:
 m 3  1  day 
8000  ×
Required area =
 
Q  day  24  hours  max(333.3, 275.5) =333.3m2
Ac = = = 275.5m 2
V0  m 
1.21 
 hour 
 m 3  1  day 
8000  ×  
 m 3  1  day   kg  Q  Day  24  hour 
8000  ×   ×10 × 0.3 3  Actual Vo = = = 1m/hour.
Q × MLSS  day  24  hours  m  A ( )
333.33 m 2
AT = = = 333.33m 3
GL  kg 
3 2 
10  m × hour 
AS Secondary clarifiers-
operational difficulties
In general, sludge will settle and
thicken readily if the process is
operated well.
One way to characterise the
settling/thickening ability is to use the
Sludge Volume Index, SVI:
Sludge Volume Index is the volume in milliliters
occupied by 1 g of a suspension after 30 min settling
Settled sludge volume (mL/L) X 1000
SVI =
suspended solids (mg/L) Vs is the % of a 300 ml tube occupied by sludge after 30
mins of settlement of a completely mixed MLSS
Vs
SVI = However, sometimes the
MLSS(%) performance of the sludge is
SVI<=50- good settling property different from what the SVI
11 suggests- Problems!
150<=SVI<=200- poor settling property
AS secondary settling tanks-
operational difficulties
 2 most common
problems with AS
settling tanks:

 Rising Sludge

 Bulking Sludge

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AS secondary settling tanks-
operational difficulties
Rising sludge
Rising Sludge (RS) is a phenomenon where
occasionally sludge having good settling characteristics
as measured by the SVI will be observed to rise or float
at the surface after relatively short settling period.
 Chief cause of RS is DENITRIFICATION with evolving
nitrogen gas causing the settled sludge to rise.
 Possible controls are:
i. Increasing rate of sludge removal from the settling tank
ii. Increasing aeration rate in the aerator
iii. Decreasing the sludge age, θc.

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AS secondary settling tanks-
operational difficulties
Bulking sludge
Bulking Sludge (BS) is sludge that has poor settling
characteristics and poor compactibility.
 Chief causes of BS are:
1. the presence of FILAMENTOUS BACTERIA when adverse
physical and chemical conditions exist in the clarifier.
2. swelling of bacterial cells in the water that float!

Control:
 Control is by fixing the above problems.
 Alternatively, as an emergency, selectively kill off the filamentous bacteria
using chlorine or hydrogen peroxide (H2O2). The latter is preferred because
it has less effect on normal activated sludge bacteria.
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AS secondary settling tanks-
operational difficulties
Bulking sludge
Causes of adverse conditions are:
i. Physical and chemical characteristics of wastewater:
 Large fluctuations in Q
 Fluctuations in nutrients (e.g. C, N, P and trace elements)
 Fluctuations in pH and temperature
 Nature of wastewater e.g. domestic are unlikely to develop the problem than industrial/trade.

ii. Treatment plant design limitations

 Air supply capacity insufficient
 Return sludge pumping capacity
 Short circuiting or poor mixing in aerator

iii.Operational causes
 Low dissolved oxygen in aerator
 Organic overloading of the aerator
 Bad clarifier operation, e.g. frequency of desludging etc.
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Sludge Handling and Treatment

 Factors affecting sludge handling:

 Type of sludge e.g. primary, secondary (humus or activated
sludge).
 Size of sludge, e.g. activated sludge generates more sludge
than trickling filters.
 Location of the treatment plant, e.g. is it close to the sea,
close to a large city where land is a problem, etc
 Methods of ultimate disposal, e.g. land-fill site, re-use, land
conditioning, etc.
Sludge Handling and Treatment

Available options for sludge handling/treatment are:

1. Storage prior to further processing,
2. Thickening prior to mechanical dewatering or digestion,
3. Conditioning prior to dewatering,
4. Dewatering or mechanical volume reduction,
5. Solids disposal (burial in landfill sites, incineration,
spreading on farmland, production of soil conditioners
Exercise: Only options 1 & 2 will be covered in detail in
the notes; as part of your self study, you should read the
recommended texts and other sources to learn more
about the others!
Sludge Handling and Treatment
Quantities of sludge
 Primary sludge: Computing the mass (Mp) and volume (Vp)
of primary sludge was covered in Part of A of Lesson; so
please refer to it.
Mp
M p = η * Q * SS Vp =
10s
 Secondary sludge:
i. Humus tank: This is similar to primary sludge; so the
same approach should be used.
ii. Activated sludge: refer to the next slide
Sludge Handling and Treatment
Quantities of sludge from AS
process
 Mass: Mass of secondary sludge is the mass of the waste
activated sludge, WAS, which can be determined using

(Vl × X) Vl is the volume of the aerator (m3);

M s = WAS = X is the MLSS concentration (kg/m3) ,
θc θc is sludge age (days),
Ms is mass of secondary sludge (kg/day);
Vs is the volume (m3/day);
s is sludge concentration (%). Typical s values are in
next slide
 Volume:

Ms
Vs =
10 s
Sludge Handling and Treatment
Quantities of sludge from AS
process
Typical sludge concentration for different sludge types

Blending of primary
sludge with either T/F or
AS is often done to
improve overall sludge
thickening. In particular,
AS is very poor
thickening by gravity;
blending it with primary
sludge will improve this!
Sludge Handling and Treatment
Quantities of sludge from AS
process
 Total mass:
Mtotal = Mp + Ms

 Total Volume:
Vtotal = Vp + Vs

Where Mp and Ms are respectively the mass of primary and

secondary sludge; Vp and Vs are the corresponding volumes.
Sludge Handling and Treatment
Quantities of sludge from AS
process
Example:
The influent flow (18,000 m3/day) to the primary sedimentation
tank has SS of 400 mg/l, of which 75% is removed. The AS
aerator (volume = 3000 m3) operates with a sludge age of 6 days
and MLSS of 3000 mg/l. The MLSS is then thickened in the
secondary tank to a concentration of 10,000mg/l. Determine:
i. Mass of primary & secondary sludge generated.
ii. Volume of primary and secondary sludge generated
assuming the primary sludge concentration is 2%.
iii. The total mass and volume of sludge generated/day at the
works
Sludge Handling and Treatment
Quantities of sludge from AS process
Solution
Primary sludge
M p = η × Q × SS
 kg 
 
 m  3
 mg  10
-6
 mg  = 5400 kg/day
Mp = 0.75 × 18000
 day 
 × 400 l  × 10 -3
     m3 
 
 l 

Mp 5400
Vp = Vp = = 270 m 3 / day
10s 10 × 2
Secondary sludge
10 −6  kg 
(V × X)
( ) 3
3000 m × 3000 × −3  3 
10  m   kg 
M s = WAS = l = = 1500 
θc 6(days) )  day 
Ms 1500 Mtotal = 5400 + 1500 = 6900 kg/day
Vs = = = 150m 3 Vtotal = 270 + 150 =420 m3/day
10 s 10 × 1
Sludge Handling
Thickening
Although the AS secondary settling tank thickens, the sludge
still has too much water and further reduction can be achieved
by another thickening of the sludge.

1. Unthickened is the
sludge from a typical
sedimentation tank.
2. Thickened is after
further thickening.
3. In general, further
thickening increases
concentration, s, and
hence reduces
volume
Sludge Handling
Thickening
Thickening can be achieved in one of 2 ways:
1. Gravity thickening:
 Simple and easy to apply but may not be effective for certain
sludge, e.g. activated sludge secondary sludge.
 It relies on sludge settling and compacting under the force of
gravity.

2. Dissolved air flotation:

 Rather than waiting for the sludge to drop to the bottom, they are
aided to float.
 This method suits activated sludge better because unlike primary
sludge, AS is lighter and unlikely to easily settle under gravity.
 Sludge Handling
Thickening
Gravity thickening
 A passive process that takes place in a tank in which the sludge
are just added and allowed a sufficient retention time to settle
and thicken.
 Design consideration follows that used for sizing the
THICKENING area for the AS secondary settling tank, i.e.
M total AT = area required for thickening;
AT = Mtotal is the total mass of sludge
GL
applied (kg/day);
Note that if primary and secondary GL solids loading rate (kg/m2/day).
sludge are applied, to the gravity
thickener, then:
M total = M p + M s
Sludge Handling
Thickening
Dissolved Air Flotation
This method of thickening is suitable for the fine, dispersed nature of the
waste activated sludge, which is more likely to rise than settle.
In this method (see schematic in the next slide),
1. a small quantity of the treated wastewater is aerated under high pressure.
2. The supersaturated liquid is then released near the bottom of a tank through which
the sludge is also introduced at atmospheric pressure.
3. The air bubbles released in solution carry the sludge to the surface.
4. The thickened sludge is then skimmed off.

• Dissolved air flotation may be preferred for separately thickening AS

secondary sludge,
• The primary sludge is thickened separately by gravity thickening.
• Both may be mixed and thickened together by gravity.
Sludge Handling
Thickening
Dissolved Air Flotation
Sludge Handling
Sludge Digestion
Definition: The biological degradation of sludge for the
sole purpose of reducing the bacterial content is known
as DIGESTION.
Digestion:
 Is Important except sludge will be disposed of by
incineration
 takes place in a tank; hence further thickening (and volume
reduction) of the sludge will occur.
 Is normally achieved anaerobically (because of the high
BOD content of sludge and hence difficulty of providing the
oxygen that will be required for aerobic treatment) but
sometimes aerobic digestion of some AS sludge is
practised.
Sludge Handling
Sludge Digestion
 Sludge Digestion
Anaerobic digestion-
main points
 Largely occurs in the absence of oxygen; hence anaerobic.
 Takes a long time, usually with a retention time of the order of
months. Hence the reactor (or digester) is usually large.
 The reaction comprises 2 stages:
1.The conversion of the sludge into organic acids by facultative bacteria
ACID FORMERS
 Robust stage.
 Bacteria are quickly adapting and will handle increased BOD loading leading to
more acid.
2.The conversion of the acids into methane , CO2 and other gases by the
METHANE FORMERS
 Bacteria mostly anaerobic
 Sensitive to the operating conditions, especially pH, temperature, organic shock
loads.
 Sludge Digestion
Anaerobic digestion
 Due to sensitivities of the METHANE FORMERS,
anaerobic digester must be carefully monitored to
ensure the right operating conditions prevail.
 Monitoring involves:
 Monitoring the pH: lower pH is an indication of organic
overload. While this can be tolerated by the ACID
FORMERS leading to production of more acids- IT IS NOT
GOOD FOR THE METHANE FORMERS!
 The amount of methane produced: a well operating
digester will provide almost 70% methane. The methane is a
good source of energy which most works often use for
generating electricity and/or heating the facilities.
 Anaerobic Digesters-
Design Considerations
 Standard rate:
Anaerobic Digesters-
Design Considerations
Main design consideration is to determine the volume of
digester:
Vl = volume of digester

t1 × (V1 + V2 ) V1 = sludge loading rate (m3/day)

Vl = + V2 × t2 V2 = digested sludge accumulation rate

(m3/day)
2 t1 = digestion period (days)
t2 = digested sludge storage period (days)

Typical design variables for single stage digesters

Anaerobic Digesters-
Design Considerations
Example: An anaerobic digester receives 4000kg/day of
thickened sludge at a rate of 300 m3/day from a gravity
thickener. In the digester, 50% of the sludge is
volatilised into gas. If the solids concentration of the
digested sludge is 4%, t1 = 30 days, t2 = 15 days,
determine the volume required in the digester.
Anaerobic Digesters-
Design Considerations
Solution:
t1 × (V1 + V2 )
Vl = + V2 × t 2
2
V1 sludge loading rate (m3/day)= 300 m3/day (given)
V2 digested sludge accumulation rate (m3/day) :
to calculate V2, we need to first find the mass of digested sludge, which is 50%
of the initial mass loading (because 50% is now converted to gas).

M digested = 0.5 × 4000 = 2000(kg/day )

M digested 2000
V2 = Vdigested =
10 s
=
10 × 4
(
= 50 m 3 / day )
30 × (300 + 50 )
Vl =
2
+ 50 × 15 = 6000 m 3 ( )
 Anaerobic Digestion-
2-stage (high rate )digesters
 High rate: low retention time and hence lower combined
volume when compared to standard rate. However, little
or no further thickening occurs.
 Basic Processes in Sludge Treatment
4) Dewatering – Separating the sludge by drying, using
vacuums or pressure
Belt Thickener at Seafield
Output from the centrifuge
https://www.youtube.com/watch?v=M4wBd1_CvNw

https://www.youtube.com/watch?v=2IUZza3ohlo

https://www.youtube.com/watch?v=Iq8HqWtfp_8
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