Technische
Universität
Dresden
Peter Krebs
Department of Hydro Science, Institute for Urban Water Management
Urban Water Systems
11 Wastewater Treatment
11.1 Boundary conditions
11.2 Layout of a wastewater treatment plant (WWTP)
11.3 Mechanical treatment
11.4 Biological treatment
11.5 Final clarification
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 1
11 Wastewater treatment
11.1 Boundary conditions
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 2
Wastewater treatment in Germany
In 2000 more than 10‘000 communal WWTPs
Class
Number
Total capacity in mio PE
> 100’000
272
83.1
10’000 – 100’000
1’817
56.1
2’000 – 10’000
2’617
12.3
50 – 2’000
5’677
3.2
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 3
Effluent standards
Class
COD
(mg/l)
BOD5
(mg/l)
NH4-N
(mg/l)
N*
(mg/l)
Ptot
(mg/l)
1
< 1000 PE
60 kg BOD5 / d
150
40
-
-
-
2
< 5000 PE
300 kg BOD5 / d
110
25
-
-
-
3
< 10000 PE
600 kg BOD5 / d
90
20
10
-
-
4
< 100000 PE
6000 kg BOD5 / d
90
20
10
18
2
5
> 100000 PE
6000 kg BOD5 / d
75
15
10
13
1
* N = Sum of NH4+, NO3-, und NO2Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 4
Load variation in WWTP inlet
70
6
NH4-load
50
5
COD-load
40
30
3
Daily average
20
COD and NH4
2
10
0
00:00
4
NH 4-load (kg/h)
COD-load (kg/h)
60
7
1
04:00
08:00
12:00
16:00
20:00
0
00:00
Time (hh:mm)
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 5
Wet-weather load of NH4 and TSS from sewer
4
16
NH4 load
NH4 load / (Qd·C0)
3
12
TSS load
10
2
8
6
1
4
2
0
0
10:00 12:00 14:00 16:00 18:00 20:00 22:00
Urban Water Systems
Q /Q d; TSS load / ( Q d·C 0)
14
Flow rate
11 Wastewater treatment
0:00
2:00
© PK, 2005 – page 6
Flow rate
WWTP load with NH4+ at storm event
Time
Conc., load
Load
Concentration
Time
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 7
11 Wastewater treatment
11.2 Layout of a wastewater
treatment plant (WWTP)
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 8
Layout of a WWTP
Mechanical treatment
Biological treatment
precipitation
chemical
Screen
Grid
chamber
Fat
chamber
Primary
clarifier
Activated
sludge tank
Secondary
clarifier
River,
filtration
Sand
Fat
Primary
sludge
Return sludge
Thickening
Secondary sludge
Excess sludge
Biogas
disposal,
incineration
Use,
dewatering,
drying,
incineration,
Disposal
Wash,
disposal
Digestion
Urban Water Systems
11 Wastewater treatment
Storage
© PK, 2005 – page 9
Example: WWTP Chemnitz-Heinersdorf
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 10
Typical residence time in reactors
Wastewater
HRT (h)
Mechanical pre-treatment
0.2
Primary clarifier
1.5
Activated sludge tank
Sludge
SRT (d)
0.01
1
10
10
5
2
Secondary clarifier
Sludge thickener
2
Digester
20
Storage
100
<1d
Urban Water Systems
11 Wastewater treatment
> 100 d
© PK, 2005 – page 11
11 Wastewater treatment
11.3 Mechanical treatment
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 12
Automatically cleaned trash rack
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 13
Sieve
Slotted sieve
≥ 0.5 mm
Perforated tin
≥ 3 mm
Hans Huber AG, Typ Ro9
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 14
Basics for rack design
• Flow velocity: 0,6  v  2,5 m/s
• Widen flume to compensate for racks cross section
• Consider backwater effect
– Hydraulics:
 d
Δh  β   
e
4
3
v2

 sinα
2g
– + due to clogging
 lower bottom to compensate for backwater effect
• Safety for maintenance (redundant)
• Building around racks (Frost, smell)
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 15
Surface load:
qA = Q / A
(Hazen, 1904)
L
U
U
Q
Critical case
Settling condition
H
VS
L
U 
HRT
VS 
H
HRT
L
H

U
VS
VS 
U BH
Q

LB
ASC
 VS  qA
 Independent of H !
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 16
Grid chamber
• In connection with combined systems
• Effects of non-organic particles:
– Abrasion of steel (e.g. pumps)
– Clogging (Sludge hopper, pipes, Pumps)
– Sedimentation (activated sludge tank, digester)
 High maintenance requirements
Sludge in sand is hindering
Sand in sludge is detrimental for operation !
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 17
Empirical sedimentation velocities
Kalbskopf, 1966
Particle
diameter
Sedimentation velocity
[mm]
= 100%
[cm/s]
= 90%
[cm/s]
= 85%
[cm/s]
0,125
0,160
0,200
0,250
0,315
0,17
0,29
0,46
0,74
1,23
0,26
0,44
0,78
1,25
2,00
0,31
0,56
0,99
1,60
2,35
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 18
Design rules
• Flow velocity:  0,3 m/s
• Width extending with height
b
5Q
h
l  h
• Length:
u
vs
• Sand collector: appr. 0,2 x 0,3 m (not too large!)
• Venturi flow measurement after grid chamber
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 19
Aerated grid chamber
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 20
Efficiency of primary clarifier
100
90
settleable
solids
Efficiency (%)
80
70
TSS
60
50
40
30
BOD5
20
10
0
0
1
2
3
4
5
Residence time HRT (h)
Urban Water Systems
11 Wastewater treatment
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Effects of primary clarifier on wastewater
Unit
Inlet
TSS
g TSS / m3
360
180
0.5
BOD5
COD
TKN
g O 2 / m3
g O 2 / m3
g N / m3
300
600
60
230
450
56
0.23
0.25
0.067
NH4-N
NO2-N
NO3-N
g N / m3
g N / m3
g N / m3
40
0
1
40
0
1
Compound
Ptot
g P / m3
Alkalinity mol HCO3- / m3
*
Cin  Cout
 
Cin
Outlet*
0
0
0
10
9
0.1
= f(Drinking water) + NH4-N
Short residence time
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 22
Rectangular sedimentation tank
Primary / secondary clarifier
Flight scraper
Effluent weir
Inlet
Effluent to
activated sludge
tank or to
receiving water
Sludge
withdrawal
Surface:
Urban Water Systems
Primary clarifier
qA = 2 to 6 m/h
Secondary clarifier
qA = 0.5 to 1.5 m/h
11 Wastewater treatment
© PK, 2005 – page 23
11 Wastewater treatment
11.4 Biological treatment
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 24
Biological treatment
Suspended biomass
 Activated sludge system
• suspended through turbulence
• sludge flocs with 0.1 – 1 mm diameter
• degradation related to biomass
 Increase suspended biomass concentration
Sessile biomass
 Biofilm system
• biofilm on carrier
• little erosion
• degradation related to biofilm surface area
 Increase of specific surface area
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 25
Important micro-biological processes
Growth
Implementation of C, N, P in biomass
Decay
If external nutrients are rare
Hydrolysis
Heavily  easily degradable substances, through
encymes
Aerobic
degradation
organic compounds CH2O + O2  CO2 + H2O
Nitrification
NH4+ + 2 O2  NO3- + H2O + 2 H+
Denitrification
5 CH2O + 4 NO3- + 4 H+  2 N2 + 5 CO2 + 7 H2O
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 26
Growth of bacteria
Doubling time tD
0·tD
1·tD
2·tD
3·tD
i·tD
...
n·tD
20
21
22
23
2i
...
2n
60
Activated sludge:
50
tD = 6 h
X /X 0
40
30
Sludge age = 10 d
20
10
0
0
1
2
3
4
5
6
t /t D
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 27
Growth of bacteria
Growth
d XB
r 
   XB
dt
Specific growth rate
S
  max 
S  KS
XB
max
S
KS
=
=
=
=
 (T-1)
 Growth is limited through nutrients and oxygen
max,2/2
max,1/2
KS,1
KS,2
Substrate, nutrients, O2
Biomass concentration
Maximum specific growth rate
Substrate concentration
Half saturation constant
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 28
Activated sludge system
Activated
sludge tank
Secondary
clarifier
Air, O2
Sedimentation
Inlet
Nutrients
Effluent
Bacteria
Return sludge
Urban Water Systems
11 Wastewater treatment
Excess
sludge
© PK, 2005 – page 29
Sludge inventory in activated sludge system
Activated
sludge tank
XAST
Q
Secondary
clarifier
Q + QRAS
Q
XAST
Xe
(QWAS)
QRAS = R·Q
XRAS
(XWAS)
Sludge balance in equilibrium
X RAS  X AST
Urban Water Systems
1 R
R
mit
11 Wastewater treatment
QRAS
R 
Q
© PK, 2005 – page 30
Flow scheme of activated sludge system
Hydraulic wash-out of sludge to secondary clarifier
 sludge must be returned to activated sludge tank
Activated sludge is circled 20 to 50 times
 sludge concentration in activated sludge tank is maintained
Excess sludge is withdrawn from system
 equivalent to sludge production
Through increased hydraulic loading (wet-weather condition)
sludge is shifted to secondary clarifier
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 31
Dynamic sludge shift
Sludge mass (kg COD)
3000
2500
2000
1500
1000
500
Activated sludge tank
0
0
0,5
1
1,5
2
1,5
2
Time (d)
Sludge mass (kg COD)
1400
1200
Sludge bed
1000
800
600
400
200
0
0
0,5
1
Time (d)
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 32
Aeration in an actvated sludge tank
Oxygen
Bacteria
Sludge
Organic compounds
Effluent,
water and
sludge
Air
Inflow from mechanical treatment
Return sludge
Dimension:
communal wastewater treatment 2 m3 wastewater per m3 reactor volume and day
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 33
Dimensioning on sludge loading
Sludge loading
F/M 
Q  BOD5,in
VAST  X AST
 kg BOD5 
in 

 kg TSS  d 
 BOD5 inlet load is related to sludge mass in
activated sludge tank (AST)
F/M
BOD5 loading related to dry sludge mass (Food/Microorganisms)
Q
Inflow to WWTP (m3/d)
BOD5,in BOD5 concentration in inlet (kg BOD5 / m3)
VAST
Volume of activated sludge tank (m3)
XAST
Sludge concentration in activated sludge tank (kg TSS / m3)
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 34
Dimensioning on sludge age
VAST  X AST
1
Sludge SRT  VAST  X AST 

SP
ESPBOD  Q  BOD5,in
F / M  ESPBOD
age
 Sludge production is related to sludge mass in
activated sludge tank
SRT
Sludge age in (d), 3 – 15 d
ESPBOD Specific excess sludge production per BOD5 converted
(kg TSS / kg BOD5)
SP
Sludge production (kg TSS / d)
SP  ESPBOD  Q  BOD5,in
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 35
Nutrients demand of micro-organisms
Nitrogen
Phosphorous
iN = 0.04 – 0.05 (g N / g BOD5)
iP = 0.01 – 0.02 (g P / g BOD5)
 Partial elimination of nutrients
Wastewater composition in the inlet
300 (g BOD5/m3)
60 (g TKN/m3)
12 (g TP/m3)
Effluent concentrations after 100% BOD5 degradation
TKNeff = TKNin – iN·BOD5,in = 60 – 0.045·300 =
TPeff
= TPin – iP·BOD5,in
= 12 – 0.015·300 =
46.5 (g N / m3)
7.5 (g P / m3)
 Enhanced nutrients removal is necessary!
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 36
Nitrification
NH4+  NO3-
Nitrifying organisms („autotrophic biomass“ XA) have a low
growth rate A
With production of autotrophic biomass
SPA  rA VAST   A  X A,AST VAST
and a safety factor SF the necessary sludge age yields
SRT  SF
VAST  X A,AST
SPA
 SF
VAST  X A,AST
μ A  X A,AST  VAST
1
 SF
μA
 high sludge age necessary to omit nitrifiers from being
washed out of the system
 Volume of activated sludge tank must be large
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 37
Development of activated sludge system
C-Elimination
aerobic
Nitrification
aerobic
De-nitrification
anoxic
aerobic
„Bio-P“
anoxic anaerobic
Urban Water Systems
anoxic
aerobic
11 Wastewater treatment
© PK, 2005 – page 38
Oxygen consumption
Oxygen introduction
cs  c
  SOI 
 SOC  f
cs
Introduction
Consumption
SOC
Specific O2-consumption
(kg O2 / kg BOD5)
cs
O2 saturation concentration
(g O2 / m3)
c
O2 concentration
(g O2 / m3)
f
Peak factor for variations
(-)
SOI
Spec. O2 introduction rate to clean water
(kg O2 / (m3·h))

Reduction factor for wastewater
(0.4) 0.6 – 0.8
 The smaller the actual oxygen concentration, the more
efficient is the oxygen introduction
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 39
Specific oxygen consumption SOC (kg O2 / kg BOD5)
T
Sludge age in d
(°C)
4
8
10
15
20
25
10
0.85
0.99
1.04
1.13
1.18
1.22
12
0.87
1.02
1.07
1.15
1.21
1.24
15
0.92
1.07
1.12
1.19
1.24
1.27
18
0.96
1.11
1.16
1.23
1.27
1.30
20
0.99
1.14
1.18
1.25
1.29
1.32
1.25
1.20
1.20
1.15
1.10
-
2.00
2.50
1.80
2.00
1.50
1.50
-
Peak factors for C und N degradation
fC
1.30
fN < 20‘000 PE
> 100‘000 PE -
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 40
Design rules
Type of
biological reactor
SRT
F/M
Without
nitrification
Nitrification
>10°C
Denitrification
Simultaneous
aerobic sludge
stabilisation
< 20‘000 PE
5
10
12 – 18
25
> 100‘000 PE
4
8
10 – 16
–
0.30
0.15
0.12
0.05
0.9 – 1.2
0.8 – 1.1
0.7 – 1.0
1.0
(kg BOD5 / (kg TS · d))
ESPBOD
(kg TS / kg BOD5)
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 41
Concentrations in a plug flow areation tank
Qin
QRS
QES
O2
BSB5
NH4+
NO32Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 42
Trickling filter
Trickling filter
Primary
clarification
Biofilm grown on
internal surface
Secondary
clarification
Recirculation
Sludge
withdrawal
Urban Water Systems
Return sludge
11 Wastewater treatment
© PK, 2005 – page 43
Dimensioning of trickling filter
Surface loading
BSu
BSu
Q  BOD5,in

a VTF
Surface loading (g BOD5 / (m2·d))
without nitrification 4 (g BOD5 / (m2·d)), with nitrifi. 2 (g BOD5 / (m2·d))
Q
Inflow to trickling filter (m3/d)
BOD5,in BOD5 inlet concentration (kg BOD5 / m3)
VTF
Volume of trickling filter (m3)
a
Specific biofilm surface per volume of trickling filter (m2 / m3 TF)
100 – 140 – 180 (m2 / m3 TF)
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 44
Degradation in trickling filter
Concentration
BOD5
Trickling
filter
NH4+
N03-
 C-degradation and nitrification are separated in space
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 45
11 Wastewater treatment
11.5 Final clarification
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 46
Functions of final clarifiers
Separation
of sludge and cleaned wastewater through
Sedimentation
Clarification  Low effluent concentration
Storage
of sludge shifted from actibated sludge tank,
namely under wet-weather conditions
Thickening  High return sludge concentration
Geometry
• Circular, flow from centre to periphery
• Rectangular, longitudinal flow
• Rectangular, lateral flow
• Vertical, upward flow
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 47
Sedimentation
Primary clarifier
Concentration
low
Free
settling
Final clarifier,
separation zone
Flocculating
settling
Hindered
settling
Thickening
high
none
Final clarifier, sludge
bed
Final clarifier,
bottom
flocculating
Particle-Interaction
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 48
Sludge volume index SVI
SVI
is an indicator for volume of sludge flocs and their
settling characteristics
0.5 h
Sludge volume
hS
(ml/l)
SV  V
H
X0
H
V
SVI 
SV
X0
(ml / g TSS)
hS
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 49
Final clarifier, idealised functions
Inlet zone
Effective zone
Clear water layer
Separation layer
Storage layer
>3m
Thickening layer
ATV A131 (2000)
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 50
Dimensioning of surface
Q
qSV
qSV



AFC
SV
X AST  SVI
Surface overflow rate
qA
Sludge overflow rate
qSV  q A  X AST  SVI
Limit values
qA
qSV
(m/h)
(l/(m2·h)
Horizontal flow tanks
1.6
500
Vertical flow tanks
2.0
650
ATV A131 (2000)
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 51
Dimensioning of water depth
Clear water layer
h1  0.5 m
Separation layer
0.5  q A  1  R 
h2 
1  SV 1000
Storage layer
1.5  0.3  qSV  1  R 
h3 
500
Thickening layer
X AST  q A  1  R   tth
h4 
X BS
XBS
Sludge concentration at bottom
tth
Thickening time
Urban Water Systems
1.5 – 2.0 h without nitrification
1.0 – 1.5 h with nitrification
2.0 – (2.5) h with denitrification
11 Wastewater treatment
X BS 
1000 1 3
tth
SVI
ATV A131 (2000)
© PK, 2005 – page 52
Calculation procedure (plant without nitrogen
elimination)
•
•
•
•
•
•
•
•
•
•
•
•
estimate or measure inflow load
choose system
estimate sludge volume index
choose thickening time
calculate concentration of bottom sludge
calculate return sludge concentration
choose recycle ratio  estimate XAST
calculate surface of secondary clarifier
calculate h1, h2, h3, h4 of secondary clarifier
check thickening time
calculate volume of activated sludge tank
is balance between AST and SC optimised?
Urban Water Systems
11 Wastewater treatment
Variation
XAST
© PK, 2005 – page 53
Circular tank
Scum skimmer
Flocculation
chamber
Sludge
scraper
Inlet
With scraper or suction removal device
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 54
Rectangular tank, longitudinal flow, flight
scraper system
Chain motor
Water level
Effluent launder
Effluent
Inlet
Sludge
Sludge
hopper
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 55
Rectangular tank, lateral flow, suction system
Inlet
channel
Removal bridge
Scum skimmer
Inlet baffle
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 56
Vertical flow tank
Effluent
launder
Effluent
Inlet
Filter
Pump
Return
sludge
Thickening
Return sludge
Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 57