Exercise Wastewater Treatment

UNIVERSITÄTSTUTTGART

Wastewater Technology – Part 1: Urban Drainage

Exercise - Wastewater Quantity

1

Iosif Mariakakis

(adopted from the lecture of

Prof. Dr.-Ing. Heidrun Steinmetz)

Institute of Sanitary Engineering,

Water Quality and Solid Waste Management

- Sanitary Engineering and Water Recycling -

Wastewater TechnologyPart 2: Wastewater treatment

Exercise: Basic calculations of loads,

wastewater streams and tank dimensions




2

Contents

Calculation of wastewater inflow into a WWTP

Calculation of wastewater load into a WWTP for dry weather conditions

and for design inflow

Dimensioning principles of primary sedimentation

Short description of the Activated Sludge biological treatment

Fundamental parameters of the Activated Sludge biological treatment

Dimensioning principles of the aeration tank of the Activated Sludge

system

Dimensioning principles of the sedimentation tank (secondary

sedimentation) of the Activated Sludge system




3

Basic Flow Scheme of a WWTP

Secondary sludge

= Excess sludge

Dewatering and

-agricultural use

-landfill

-incineration

Primary treatment Biological treatment

Return sludge

Screenings Sand

Influent

Effluent

GreasePrimary

sludge

Aeration

tank

Secondary

sediment.

Raw sludge

Digester

35°C

Thickener and

storage tank

BiogasGas storage

Thickener

Screening Grit

rem.

Grease

rem.

Primary

sediment.

Supernatant

Sludge treatment Guje

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Design inflow of a WWTP

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Definitions

Wastewater (Sewage)

Dry weather flow (DWF)

Combined wastewater flow (CWF)

(about 100 times the dry weather flow)

Qww = Qd + Qi + Qi

QDW = QWW + Qinf

Qcomb = QDW + QR

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Daily Variations of Wastewater Flow

Determination of yearly wastewater flow (sewage flow on all days)

Determination of yearly dry weather flow (dry weather flow on days without rain)

Determination of peak flow during dry weather

iq

C,iA

WW,dwP

WW,aMQ

86400

Inf,aMQ

WW,aMQ

DW,aMQ

Inf,aMQ

Qx

WW,aMQ

DW,Q

max

24

max

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-Arb

eitsbla

tt A

198 (

April 2003)

[ l/s ]

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Design inflow of a WWTP

InfhW Wd QQQ max,,32

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Specific Loads and Concentrations per Inhabitant

Specific Load

Wastewater concentration [mg/l] for a wastewater production of Parameter

g/C·d 150 l/ C·d 200 l/ C·d 250 l/ C·d

BOD5 60 400 300 240

COD 120 800 600 480

N 11 73 55 44

P 1.8 12 9 7

TS 70 470 350 280

< 15 mg/l

< 75 mg/l

< 13 mg/l

< 1 mg/l

< 20 mg/l

Typical effluent

values of a

WWTP with

100,000 EW

e.g. Lmg

dC

LdC

g

CBOD /300200

60000

200

60

5

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Primary sedimentation - dimensioning

Treatment methodqA

[m/h]

t

[min]

T

[m]

PS combined with activated sludge

process (without addition of excess

sludge)6 15 1.5

PS combined with activated sludge

process (with addition of excess

sludge)2 – 3 45 2.0

PS combined with trickling filter or

rotating contactors (with/without

addition of excess sludge)3 30 1.5

(m/h) q

(m³/h) Q (m²) A SurfaceTank

A

dmin

(h)t (m³/h) Q (m³) V VolumeTank dmin




10

Efficiency of Primary Sedimentation

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4

Eli

min

ati

on

[%

]

Retention Time [h]

Settleable Solids

Filterable Solids

BOD5 and COD

Nitrogen

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Activated Sludge Process (scheme)

Influent

Effluent

Excess

sludge

Return sludge

Aeration tank Final sedimentation

Aeration

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Parameters of Activated Sludge System

Sludge Loading (kgBOD/(kgTSS*d)

Sludge Age, Sludge retention time; Mean Cell Residence

Time (d)

Biomass Concentration = Total suspended solids (TSS)

MLSS = Mixed Liquor Suspended Solids (kg/m³)

SV = Sludge Volume (mL/L)

SVI = Sludge Volume Index (mL/g)

Return Sludge Ratio (%)

Excess Sludge Production (kgTSS/d)

Oxygen Concentration and –Consumption (mg/L)

Volumetric Loading (kgBOD/(m³*d))




13

Sludge Volume Index

In this Example

Sludge Volume Index SVI = 110 ml/g or l/kg

(Diluted) Sludge Volume SV=330 ml/l or l/m³

800

600

200

400

1000

800

600

1000

400

200

Start of Test After 30 min

1 liter of Act. Sludge

SSAT =3.0 g/l

Volume: 330 ml/l

SVI=330/3.0

=110 ml/g




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Sludge Loading BX

BSS = Sludge Loading kgBOD5/(kgSS*d)

Bd,BOD = Daily BOD5 load in influent kg/(m3*d)

VAT = Volume of aeration tank m3

SSAT = Dry solids in aeration tank kg SS/m3

M

F

ismsMicroorgan

Food

SSV

BB

ATAT

BOD,d

SS

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Sludge Loading BSS and Treatment Efficiency

100

80

60

40

20

> 13°C

< 11°C

Sludge Loading BSS

BO

D -

Elim

ina

tio

n [%

]

1,0 100,1 0,2

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Sludge Retention Time SRT / Sludge Age tSS

Sludge Retention Time (SRT) = Sludge Age = Mean Cell

Residence Time (MCRT)

The SRT is the average retention time of every sludge floc in the

system

The SRT controls the microbial population in the activated sludge

High SRT – slowly growing organisms can stay in the system (e.g. nitrifying

bacteria)

The SRT is difficult to measure, but it can be calculated as:

Mass of TSS in the aeration tank/daily sludge production

][]/[

³][³]/[, ddsolidskgES

mVmkgXSRT

d

ATATTSS

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Sludge Age tSS (SRT)

Size of the plant

< 20.000 PE > 100.000 PE

Dimensioning temperature Treatment target

10° C 12° C 10° C 12° C

Without nitrification 5,0 4,0

With nitrification 10,0 8,2 8,0 6,6

With nitrogen removal

VD/VBB = 0,2 12,5 10,3 10,0 8,3

VD/VBB = 0,3 14,3 11,7 11,4 9,4

VD/VBB = 0,4 16,7 13,7 13,3 11,0

VD/VBB = 0,5 20,0 16,4 16,0 13,2

Sludge stabilization incl. nitrogen removal

25,0 not recommended

Selection of sludge age according to treatment purpose

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With return sludge, thickened activated sludge is pumped back

into the aeration reactor

RS = Return Sludge Ratio QRS/Q

SSRS = Suspended Solids Concentration in Return Sludge

SSAT = Suspended Solids Concentration in the Aeration Tank

Return Sludge

RS

SSRSSS RS

AT

1

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Sludge Age [days]X/C

4 8 10 15 20 25

0,4 0,79 0,69 0,65 0,59 0,56 0,53

0,6 0,91 0,81 0,77 0,71 0,68 0,65

0,8 1,03 0,93 0,89 0,83 0,80 0,77

1,0 1,15 1,05 1,01 0,95 0,92 0,89

1,2 1,27 1,17 1,13 1,07 1,04 1,01

Specific excess sludge production (kg SS/kg BOD5)

Excess Sludge Production

X/C = Suspended solids / BOD5 in the influent

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Required mass of suspended solids in the biological reactor (kg)

Volume of aeration tank

dDimATSS ESSRTM ,

Volume of the biological reactor (m³)

AT

ATSSAT

SS

MV

,

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Main Parameter of Activated Sludge Processes

Treatment TargetType of

System

Sludge LoadingBiomass Conc.

SSAT

Sludge Age

SRT

kg/(kg•d) kg/m³ d

Part TreatmentHigh

Loaded 1.0 1.5 – 2.0 1

BOD5-RemovalMedium

Loaded0.25 – 0.50 2.0 – 3.0 2 - 4

NitrificationLow

Loaded0.10 – 0.15 3.0 – 5.0 7 - 12

Nitrification and

Denitrification

N-Eli-

mination0.07 – 0.09 3.0 – 5.0 12 - 15

Aerobic

Stabilization

Extended

Aeration0.04 – 0.07 3.0 – 5.0 15 - 30 D

IN E

N 1

22

55

-6 (

4/2

002)




22

Secondary settling tank

Surface loading important

Dependant from the sludge characteristics (SVI)

Calculation of the tank surface

[m²]

][][

)][

3

2

,

kg

L

g

mL

m

kg

L

ghm

L

SVIX

q

SV

qq

ATTSS

SVSVA

A

dSST

q

QA

[m/h]

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Secondary settling tank

Depth of the sedimentation tank

Dependant from the sludge characteristics (SVI)

Dimensioning of the different zones according DWA A- 131

Sludge level

Compression,

Consolidation

Removal of sludge

Outflow

Solids concentrationDSo

DSRS

Water zoneh1

Thickening zone

Removal zoneh4

Separation zone h3

Storage zone h2

Water level

Settling (hindered)

o

h1

h4

h3

h2Inflow Sludge level

Compression,

Consolidation

Removal of sludge

Outflow

Solids concentrationDSo

DSRS

Water zoneh1

Thickening zone

Removal zoneh4

Separation zone h3

Storage zone h2

Water level

Settling (hindered)

o

h1

h4

h3

h2Inflow

h1 = 0.5 m (min. for safety reasons)

h2 = [0.5 qA (1 + RS)] / (1-SV / 1000)

h3 = 0.45 qSV (1 + RS) / 500

h4 = [XTSS,AT qA (1 + RS) tTh] / XTSS,SST

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Exercise Wastewater Treatment

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