1. 11 Wastewater Treatment 11.1 Boundary conditions
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
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2. 11.1 Boundary conditions 11 Wastewater treatment Urban Water Systems

3. Total capacity in mio PE
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
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4. Effluent standards COD (mg/l) BOD5 (mg/l) NH4-N (mg/l) N* (mg/l)
Ptot (mg/l)
Class
1 < 1000 PE kg BOD5 / d
150 40 - - -
2 < 5000 PE kg BOD5 / d
110 25 - - -
3 < PE kg BOD5 / d 90 20 10 - -
4 < PE kg BOD5 / d 90 20 10 18 2
5 > PE kg BOD5 / d 75 15 10 13 1
* N = Sum of NH4+, NO3-, und NO2-
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5. Load variation in WWTP inlet10 20 30 40 50 60 70
00:00
04:00
08:00
12:00
16:00
20:00
Time (hh:mm)
COD-load (kg/h) 1 2 3 4 5 6 7 NH
-load (kg/h)
Daily average
COD
and
-load
COD-load
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6. Wet-weather load of NH4 and TSS from sewer16
NH4 load 14
Flow rate ) C 3
TSS load 12 d Q 10
load / (Qd·C0) 2 8
TSS load / ( 6 4 ; NH 1 4 d Q / Q 2
10:00
12:00
14:00
16:00
18:00
20:00
22:00
0:00
2:00
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7. WWTP load with NH4+ at storm event
Time
Flow rate
Time
Conc., load
Load
Concentration
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8. 11.2 Layout of a wastewater treatment plant (WWTP)
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9. precipitation chemical
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
Use,
dewatering, drying,
incineration,
Disposal
disposal,
incineration
Wash,
disposal
Digestion
Storage
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10. Example: WWTP Chemnitz-Heinersdorf
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11. Wastewater HRT (h) Sludge SRT (d)
Typical residence time in reactors
Wastewater HRT (h)
Sludge SRT (d)
Mechanical pre-treatment
0.2
0.01
Primary clarifier
1.5 1
Activated sludge tank 10 10
Secondary clarifier 5 2
Sludge thickener 2
Digester 20
Storage
100
< 1 d
> 100 d
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12. 11.3 Mechanical treatment 11 Wastewater treatment Urban Water Systems

13. Automatically cleaned trash rack
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14. Sieve Slotted sieve ≥ 0.5 mm Perforated tin ≥ 3 mm
Hans Huber AG, Typ Ro9
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15. 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:
+ due to clogging
lower bottom to compensate for backwater effect
Safety for maintenance (redundant)
Building around racks (Frost, smell)
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16. Surface load: qA = Q / A (Hazen, 1904)VS H
Critical case
Settling condition
 VS  qA
 Independent of H !
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17. 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 !
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18. Empirical sedimentation velocities
Kalbskopf, 1966
Particle diameter
Sedimentation velocity
[mm]
= 100%
[cm/s]
= 90%
= 85%
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
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19. Design rules Flow velocity:  0,3 m/s Width extending with height
Length:
Sand collector: appr. 0,2 x 0,3 m (not too large!)
Venturi flow measurement after grid chamber
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20. Aerated grid chamber
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21. Efficiency of primary clarifier
100 90
settleable
solids 80 70
TSS 60 50
Efficiency (%) 40 30
BOD 5 20 10 1 2 3 4 5
Residence time HRT
(h)
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22. Effects of primary clarifier on wastewater
Compound
Unit
Inlet
Outlet*
TSS
g TSS / m3
360
180
0.5
BOD5
g O2 / m3
300
230
0.23
COD
g O2 / m3
600
450
0.25
TKN
g N / m3 60 56
0.067
NH4-N
g N / m3 40 40
NO2-N
g N / m3
NO3-N
g N / m3 1 1
Ptot
g P / m3 10 9
0.1
Alkalinity
mol HCO3- / m3
= f(Drinking water) + NH4-N
* Short residence time
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23. Rectangular sedimentation tank
Primary / secondary clarifier
Flight scraper
Effluent weir
Inlet
Effluent to activated sludge tank or to receiving water
Sludge withdrawal
Surface:
Primary clarifier qA = 2 to 6 m/h
Secondary clarifier qA = 0.5 to 1.5 m/h
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24. 11.4 Biological treatment 11 Wastewater treatment Urban Water Systems

25. 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
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26. 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
NH O2  NO3- + H2O + 2 H+
Denitrification
5 CH2O + 4 NO H+  2 N2 + 5 CO2 + 7 H2O
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27. Growth of bacteria Doubling time tD 0·tD 1·tD 2·tD 3·tD i·tD ... n·tD20 21 22 23 2i
... 2n
Activated sludge:
tD = 6 h
Sludge age = 10 d
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28. Growth of bacteria  Growth is limited through nutrients and oxygen
Substrate, nutrients, O 2 m (T -1 )
Growth
max,2/2
max,1/2
Specific growth rate
KS,1
KS,2
XB = Biomass concentration
max = Maximum specific growth rate
S = Substrate concentration
KS = Half saturation constant
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29. Activated sludge system
Activated sludge tank
Secondary clarifier
Air, O2
Sedimentation
Effluent
Inlet
Nutrients
Bacteria
Return sludge
Excess
sludge
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30. Sludge inventory in activated sludge system
Activated sludge tank
Secondary clarifier
XAST Q
Q + QRAS Q
XAST Xe
QRAS = R·Q
(QWAS)
XRAS
(XWAS)
Sludge balance in equilibrium
mit
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31. 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
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32. Dynamic sludge shift Sludge bed Activated sludge tank
500
1000
1500
2000
2500
3000
0,5 1
1,5 2
Time (d)
Sludge mass (kg COD)
Activated sludge tank
200
400
600
800
1000
1200
1400
0,5 1
1,5 2
Time (d)
Sludge mass (kg COD)
Sludge bed
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33. Aeration in an actvated sludge tank
Oxygen
Bacteria
Organic compounds
Sludge
Effluent, water and sludge
Air
Inflow from mecha-nical treatment
Return sludge
Dimension:
communal wastewater treatment 2 m3 wastewater per m3 reactor volume and day
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34. Dimensioning on sludge loading
 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)
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35. Dimensioning on sludge 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)
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36. Nutrients demand of micro-organisms
Nitrogen iN = – (g N / g BOD5)
Phosphorous iP = – (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 = (g N / m3)
TPeff = TPin – iP·BOD5,in = 12 – 0.015·300 = (g P / m3)
 Enhanced nutrients removal is necessary!
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37. Nitrification NH4+  NO3-
Nitrifying organisms („autotrophic biomass“ XA) have a low growth rate A
With production of autotrophic biomass
and a safety factor SF the necessary sludge age yields
 high sludge age necessary to omit nitrifiers from being washed out of the system
 Volume of activated sludge tank must be large
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38. Development of activated sludge system
aerobic
C-Elimination
aerobic
Nitrification
anoxic
aerobic
De-nitrification
anaerobic
anoxic
aerobic
„Bio-P“
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39. Oxygen consumption Oxygen introduction 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
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40. Specific oxygen consumption SOC (kg O2 / kg BOD5)
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
Peak factors for C und N degradation fC
1.30
1.25
1.20
1.20
1.15
1.10 fN
< 20‘000 PE - - -
2.50
2.00
1.50
> 100‘000 PE - -
2.00
1.80
1.50 -
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41. Design rules Type of biological reactor Without nitrification
Nitrification >10°C
Denitrifi-cation
Simultaneous aerobic sludge stabilisation
SRT
< 20‘000 PE 5 10
12 – 18 25
> 100‘000 PE 4 8
10 – 16 –
F/M
(kg BOD5 / (kg TS · d))
0.30
0.15
0.12
0.05
ESPBOD
(kg TS / kg BOD5)
0.9 – 1.2
0.8 – 1.1
0.7 – 1.0
1.0
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42. Concentrations in a plug flow areation tank
Qin
QRS
QES O2
BSB5
NH4+
NO32-
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43. Primary clarification Secondary clarification
Trickling filter
Biofilm grown on internal surface
Trickling filter
Primary clarification
Secondary clarification
Recirculation
Return sludge
Sludge withdrawal
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44. Dimensioning of trickling filter
Surface loading
BSu 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)
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45. Degradation in trickling filter
Concentration
BOD5
Trickling filter
NH4+
N03-
 C-degradation and nitrification are separated in space
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46. 11.5 Final clarification 11 Wastewater treatment Urban Water Systems

47. 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
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48. Particle-Interaction
Sedimentation
Primary clarifier
Final clarifier, separation zone
low
Free settling
Flocculating settling
Final clarifier, sludge bed
Concentration
Hindered settling
Final clarifier, bottom
Thickening
high
none
flocculating
Particle-Interaction
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49. Sludge volume index SVI
SVI is an indicator for volume of sludge flocs and their settling characteristics
0.5 h H hS X0 V
Sludge volume
(ml/l)
(ml / g TSS)
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50. Final clarifier, idealised functions
Inlet zone
Effective zone
Clear water layer
Separation layer
> 3 m
Storage layer
Thickening layer
ATV A131 (2000)
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51. Dimensioning of surface
Surface overflow rate
Sludge overflow rate
Limit values qA
qSV
(m/h)
(l/(m2·h)
Horizontal flow tanks
1.6
500
Vertical flow tanks
2.0
650
ATV A131 (2000)
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52. Dimensioning of water depth
Clear water layer
Separation layer
Storage layer
Thickening layer
XBS Sludge concentration at bottom
tth Thickening time
1.5 – 2.0 h without nitrification
1.0 – 1.5 h with nitrification
2.0 – (2.5) h with denitrification
ATV A131 (2000)
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53. 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?
Variation XAST
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54. Circular tank With scraper or suction removal device Scum skimmer
Flocculation chamber
Sludge scraper
Inlet
With scraper or suction removal device
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55. Rectangular tank, longitudinal flow, flight scraper system
Chain motor
Effluent launder
Water level
Effluent
Inlet
Sludge
Sludge hopper
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56. Rectangular tank, lateral flow, suction system
Inlet channel
Removal bridge
Scum skimmer
Inlet baffle
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57. Vertical flow tank Effluent launder Effluent Pump Return sludge Inlet
Filter
Thickening
Return sludge
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