Technische Universität Dresden
Peter Krebs
Department of Hydro Science, Institute for Urban Water Management
11 Wastewater treatment
Urban Water Systems
11 Wastewater Treatment
11.1 Boundary conditions 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
Urban Water Systems
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© PK, 2005 – page 2
Effluent standards
Wastewater treatment in Germany
Class
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
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 NO211 Wastewater treatment
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Load variation in WWTP inlet
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11 Wastewater treatment
© PK, 2005 – page 4
Wet-weather load of NH4 and TSS from sewer 4
NH4-load
50
6
Flow rate
5 COD-load
40 30
4 3
Daily average
20
COD and NH4
2
10 0 00:00
NH4 load 3
1
04:00
08:00
12:00
16:00
20:00
NH4 load / (Qd·C0)
COD-load (kg/h)
60
16
7
NH 4-load (kg/h)
70
10 2
8 6
1
4 2
0
0
10:00 12:00 14:00 16:00 18:00 20:00 22:00 11 Wastewater treatment
12
TSS load
0 00:00
Time (hh:mm)
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14
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Urban Water Systems
Q /Q d; TSS load / ( Q d·C 0)
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0:00
2:00 © PK, 2005 – page 6
WWTP load with NH4+ at storm event
Flow rate
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11.2 Layout of a wastewater treatment plant (WWTP)
Time
Conc., load
Load
Concentration Time Urban Water Systems
11 Wastewater treatment
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Urban Water Systems
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Layout of a WWTP Mechanical treatment
11 Wastewater treatment
Biological treatment precipitation chemical
Screen
Grid chamber
Fat chamber
Primary clarifier
Activated sludge tank
Secondary clarifier
River, filtration Sand
11.3 Mechanical treatment
Fat Primary sludge
Return sludge
Thickening Secondary sludge Excess sludge Biogas
disposal, incineration
Use, dewatering, drying, incineration, Disposal
Wash, disposal Digestion
Urban Water Systems
Storage
11 Wastewater treatment
© PK, 2005 – page 9
Typical residence time in reactors Wastewater HRT (h)
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Efficiency of primary clarifier Sludge SRT (d) 100
0.2
Primary clarifier Activated sludge tank
1.5
0.01
90
1
80
10
10
5
2
Secondary clarifier Sludge thickener
2
Digester
20
Storage
100 <1d
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Efficiency (%)
Mechanical pre-treatment
11 Wastewater treatment
settleable solids
70
TSS
60 50 40 30
BOD5
20 10 0
> 100 d © PK, 2005 – page 11
0
1
2
3
4
5
Residence time HRT (h) Urban Water Systems
11 Wastewater treatment
© PK, 2005 – page 12
Effects of primary clarifier on wastewater
TSS
Cin − Cout Cin
Inlet
Outlet*
g TSS / m3
360
180
0.5
BOD5
g O2 /
m3
300
230
0.23
COD
g O2 / m3
450
0.25
TKN
g N / m3
600 60
56
0.067
NH4-N
g N / m3
40
40
0
NO2-N
g N / m3
0
NO3-N g N / m3 g P / m3 Ptot Alkalinity mol HCO3- / m3 *
η =
Unit
Compound
Surface load: qA = Q / A
0 0 1 1 0 10 9 0.1 = f(Drinking water) + NH4-N
L
U U Q
H
VS
U =
Critical case Settling condition
L HRT
VS =
H HRT
L H ≥ U VS VS ≥
U BH Q = LB ASC
Æ VS ≥ qA
Æ Independent of H !
Short residence time
Urban Water Systems
(Hazen, 1904)
11 Wastewater treatment
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Urban Water Systems
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Rectangular sedimentation tank 11 Wastewater treatment Primary / secondary clarifier Flight scraper Effluent weir
11.4 Biological treatment
Inlet Effluent to activated sludge tank or to receiving water Sludge withdrawal
Surface: Primary clarifier
qA = 2 to 6 m/h
Secondary clarifier Urban Water Systems
qA = 0.5 to 1.5 m/h 11 Wastewater treatment
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Biological treatment Suspended biomass
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Important micro-biological processes
Æ Activated sludge system Growth
Implementation of C, N, P in biomass
• sludge flocs with 0.1 – 1 mm diameter • degradation related to biomass
Decay
If external nutrients are rare
Æ Increase suspended biomass concentration
Hydrolysis
Heavily Æ easily degradable substances, through encymes
Aerobic degradation
organic compounds CH2O + O2 Æ CO2 + H2O
Nitrification
NH4+ + 2 O2 Æ NO3- + H2O + 2 H+
Denitrification
5 CH2O + 4 NO3- + 4 H+ Æ 2 N2 + 5 CO2 + 7 H2O
• suspended through turbulence
Sessile biomass
Æ Biofilm system
• biofilm on carrier • little erosion • degradation related to biofilm surface area Æ Increase of specific surface area Urban Water Systems
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Growth of bacteria
Growth of bacteria
Doubling time tD
Æ Growth is limited through nutrients and oxygen
1·tD
0
2·tD
1
2
3·tD
2
2
i·tD
3
2
i
2
2
...
n·tD
...
2n
60
Growth d XB r = = µ ⋅ XB dt
Activated sludge:
50
tD = 6 h
X /X 0
40 30
Sludge age = 10 d
µ (T-1)
0·tD
µmax,2/2 µmax,1/2
Specific growth rate S µ = µmax ⋅ S + KS
KS,1
Substrate, nutrients, O2
KS,2
20
XB µmax S KS
10 0 0
1
2
3
4
5
6
t /t D Urban Water Systems
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Activated sludge system Activated sludge tank Inlet Nutrients
= = = =
Biomass concentration Maximum specific growth rate Substrate concentration Half saturation constant
Urban Water Systems
Activated sludge tank XAST
Sedimentation
Q
Secondary clarifier Q + QWAS
Q
XAST
Xe
Effluent
(QWAS)
QRAS = R·Q
Bacteria
XRAS
Return sludge
Excess sludge
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(XWAS)
Sludge balance in equilibrium
X RAS = X AST Urban Water Systems
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Sludge inventory in activated sludge system Secondary clarifier
Air, O2
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Flow scheme of activated sludge system
1+ R R
Urban Water Systems
mit
Q RAS Q
R =
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Dynamic sludge shift
Hydraulic wash-out of sludge to secondary clarifier Æ sludge must be returned to activated sludge tank
Activated sludge is circled 20 to 50 times Æ sludge concentration in activated sludge tank is maintained
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)
Excess sludge is withdrawn from system Æ equivalent to sludge production Through increased hydraulic loading (wet-weather condition) sludge is shifted to secondary clarifier
Sludge mass (kg COD)
1400 1200
Sludge bed
1000 800 600 400 200 0 0
0,5
1
Time (d) Urban Water Systems
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Aeration in an actvated sludge tank
Dimensioning on sludge loading F/M =
Sludge loading
Q ⋅ BOD5,in
⎛ kg BOD5 ⎞ in ⎜ ⎟ ⎝ kg TSS ⋅ d ⎠
VAST ⋅ X AST
Æ 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)
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© PK, 2005 – page 25
VAST
Volume of activated sludge tank (m3)
XAST
Sludge concentration in activated sludge tank (kg TSS / m3)
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Dimensioning on sludge age
Nutrients demand of micro-organisms
VAST ⋅ X AST 1 Sludge SRT = VAST ⋅ X AST = = SP ESPBOD ⋅ Q ⋅ BOD5,in F / M ⋅ ESPBOD age
Nitrogen Phosphorous
Æ Sludge production is related to sludge mass in activated sludge tank SRT
Æ Partial elimination of nutrients 300 (g BOD5/m3) 60 (g TKN/m3) 12 (g TP/m3)
Wastewater composition in the inlet
Sludge age in (d), 3 – 15 d
Effluent concentrations after 100% BOD5 degradation
ESPBOD Specific excess sludge production per BOD5 converted (kg TSS / kg BOD5) SP
iN = 0.04 – 0.05 (g N / g BOD5) iP = 0.01 – 0.02 (g P / g BOD5)
TKNeff = TKNin – iN·BOD5,in = 60 – 0.045·300 =
Sludge production (kg TSS / d)
TPeff
SP = ESPBOD ⋅ Q ⋅ BOD5,in Urban Water Systems
= 12 – 0.015·300 =
7.5 (g P / m3)
Æ Enhanced nutrients removal is necessary!
11 Wastewater treatment
Nitrification
= TPin – iP·BOD5,in
46.5 (g N / m3)
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NH4+ Æ NO3-
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© PK, 2005 – page 28
Oxygen consumption
Nitrifying organisms („autotrophic biomass“ XA) have a low growth rate µA
Oxygen introduction
α ⋅ SOI ⋅
cs − c = SOC ⋅ f cs
Introduction
Consumption
With production of autotrophic biomass SPA = rA ⋅VAST = µ 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
= SF
1 µA
Æ high sludge age necessary to omit nitrifiers from being washed out of the system Æ Volume of activated sludge tank must be large Urban Water Systems
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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))
α
Reduction factor for wastewater
(0.4) 0.6 – 0.8
Æ The smaller the actual oxygen concentration, the more efficient is the oxygen introduction Urban Water Systems
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Specific oxygen consumption SOC (kg O2 / kg BOD5) T
Design rules
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
1.30
1.25
1.20
1.20
1.15
1.10
-
-
-
2.50
2.00
1.50
> 100‘000 PE -
-
2.00
1.80
1.50
fN < 20‘000 PE
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SRT
11 Wastewater treatment
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
0.8 – 1.1
0.7 – 1.0
1.0
F/M (kg BOD5 / (kg TS · d))
© PK, 2005 – page 31
Trickling filter
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Dimensioning of trickling filter Biofilm grown on internal surface
Trickling filter
Primary clarification
Secondary clarification
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
Sludge withdrawal
Without nitrification
ESPBOD (kg TS / kg BOD5) 0.9 – 1.2
Peak factors for C und N degradation fC
Type of biological reactor
Inflow to trickling filter (m3/d)
Recirculation
BOD5,in BOD5 inlet concentration (kg BOD5 / m3)
Return sludge
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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Degradation in trickling filter 11 Wastewater treatment
Concentration BOD5 Trickling filter
11.5 Final clarification
NH4+
N03-
Æ C-degradation and nitrification are separated in space Urban Water Systems
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© PK, 2005 – page 36
Functions of final clarifiers Separation
Sedimentation
of sludge and cleaned wastewater through Sedimentation
Primary clarifier
Clarification Æ Low effluent concentration
low
of sludge shifted from actibated sludge tank, namely under wet-weather conditions
Thickening Æ High return sludge concentration
Geometry
• Circular, flow from centre to periphery • Rectangular, longitudinal flow • Rectangular, lateral flow • Vertical, upward flow
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Flocculating settling
Hindered settling
Thickening
high none
Final clarifier, sludge bed Final clarifier, bottom
flocculating Particle-Interaction
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Sludge volume index SVI SVI
Free settling
Concentration
Storage
Final clarifier, separation zone
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© PK, 2005 – page 38
Final clarifier, idealised functions
is an indicator for volume of sludge flocs and their settling characteristics 0.5 h
Inlet zone
Effective zone
Clear water layer
Sludge volume h SV = V S (ml/l) H
Separation layer Storage layer
X0
>3m
Thickening layer
H
V
SVI =
SV X0
(ml / g TSS)
hS ATV A131 (2000) Urban Water Systems
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Dimensioning of surface Surface overflow rate
Sludge overflow rate
qA =
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Dimensioning of water depth Q q qSV = SV = AFC SV X AST ⋅ SVI
Clear water layer
h1 = 0.5 m
Separation layer
h2 =
0.5 ⋅ q A ⋅ (1 + R ) 1 − SV 1000
Storage layer
h3 =
1.5 ⋅ 0.3 ⋅ qSV ⋅ (1 + R ) 500
Thickening layer
h4 =
X AST ⋅ q A ⋅ (1 + R ) ⋅ tth X BS
qSV = q A ⋅ X AST ⋅ SVI
Limit values qA
qSV
(m/h)
(l/(m2·h)
Horizontal flow tanks
1.6
500
XBS
Sludge concentration at bottom
Vertical flow tanks
2.0
650
tth
Thickening time
ATV A131 (2000) Urban Water Systems
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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 t SVI th
ATV A131 (2000) © PK, 2005 – page 42
Circular tank
Rectangular tank, longitudinal flow, flight scraper system Chain motor
Scum skimmer
Water level
Effluent launder Effluent
Inlet Sludge
Flocculation chamber Sludge scraper
Sludge hopper
Inlet
With scraper or suction removal device Urban Water Systems
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Rectangular tank, lateral flow, suction system
Inlet channel
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Vertical flow tank Effluent launder
Removal bridge
Scum skimmer Effluent Inlet
Filter
Pump
Return sludge
Thickening
Return sludge Inlet baffle
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Urban Water Systems
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© PK, 2005 – page 46