SWASecondary Settling Tanks

Univ. Prof. Dr.-Ing F.W. Günthert

Universität der Bundeswehr München

Institut für Wasserwesen

Siedlungswasserwirtschaft und Abfalltechnik

Secondary settling tanks and sludge mass balance

dex summer school 2011

1

SWASecondary Settling Tanks

F. W. GünthertStructure

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Structure:• Glossary of terms

• Tasks of secondary settling tanks

• Settling velocity

• Return activated sludge, settling behavior of biological sludge

• Dimensioning of SST, surface area, depth

• Planning aspects, inlet, outlet, sludge removal systems

• Sludge load balancing

• Conclusion, references

SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

Glossary of terms

Rectangular secondary settling tank (SST)

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SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

Glossary of terms

Centre feed, peripheral take-off, secondary settling tank (SST)

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SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

Abbreviations and units

List of abbreviations and units

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SWASecondary Settling Tanks

F. W. Günthert

• Activated sludge process

SSAT

• Mass balance over the process

RSRS

RSSSATSS+

⋅=1

SSBS

QRS, SSRS

Basic principles - Interactions

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QWW SSEST

SWASecondary Settling Tanks

F. W. Günthert

• Separation of the activated sludge by settling dependent on:– flocculation in the inlet zone– hydraulic conditions in the settling tank– return sludge ratio (RS)– sludge removal procedure

• Thickening and removal of the activated sludge dependent on:– sludge volume index (SVI)– depth of the sludge layer– thickening time (tTh)– type of sludge removal system

Basic principles -Tasks of secondary settling tanks

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SWASecondary Settling Tanks

F. W. Günthert

• Intermediate storage at the beginning of stormwater flow periods dependent on:– hydraulic load (QWW,h)– type of sludge removal system– return sludge facilities

Basic principles -Tasks of secondary settling tanks

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• Avoidance of denitrification and resolution of phosphorous by:– limitation of thickening time– sludge removal performance

SWASecondary Settling Tanks

F. W. GünthertEffluent quality of SST

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The effluent is mainly caused by SS (activated sludge) and dissolved and colloidal matter.

This is dependent on the efficiency of AT and SST.

A suspended solids concentration of 1 mg/l dry solids in the effluent of SST increases the concentration of:

•CBOD by 0,3 to 1,0 mg/l•CCOD by 0,8 to 1,4 mg/l•CN by 0,08 to 0,1 mg/l•CP by 0,02 to over 0,04 mg/l

SWASecondary Settling Tanks

F. W. GünthertClarification

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Settling velocity depends on:• particle volume V [m³]

• particle size [-]

• density of particles [kg/m³]

• density of fluid [kg/m³]

• viscosity of fluid [kg/(m·s)]

• flocculation [-]

• flow conditions (Reynoldsnumber) [-]

• flow stability (Froudenumber) [-]

• Gravitational acceleration [m/s²]

SWASecondary Settling Tanks

F. W. GünthertSettling velocity

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ATV, 1997particle diameter d [cm]

settl

ing

velo

city

v s

[cm

/s]

SWASecondary Settling Tanks

F. W. GünthertSettling velocity

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flocculation zone

hindered settling zone

interim zone

thickening zone

settling behavior (settling zylinder 1l)

SWASecondary Settling Tanks

F. W. Günthert

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Velocity distribution

Distribution of velocities, influenced by density currents

SWASecondary Settling Tanks

F. W. GünthertSettling velocity

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Settling tanks are influenced by:•different settleable particles

•wind, temperature

•inlet and outlet

•density currents

•sludge removal procedure

•tank geometry (L:W; Ø,htot)

SWASecondary Settling Tanks

F. W. GünthertEffluent concentration of SST

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KA 11, 1997specific sludge volume load

efflu

entc

once

ntra

tion

SSES

T

SWASecondary Settling Tanks

F. W. Günthert

• Activated sludge process

SSAT

• Mass balance over the process

RSRS

RSSSATSS+

⋅=1

SSBS

RAS, SSRS

Return activated sludge (RAS)

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SWASecondary Settling Tanks

F. W. GünthertReturn activated sludge (RAS)

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Return activated sludge is determined by:•settling behavior of MLSS (SVI)

•thickening time tth and thickening depth h4 of SST

•short circuiting flow rate between inlet to the sludge hopper (design of inlet, performance of sludge removal system) wind, temperature

•sludge removal system with fitted performance to the thickening time tth•sludge return flow (QRS < 0,75 * QWW,h)

SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

Determinants of settling behaviour of biological sludge

Effect of filamentous organism on floc structure: (a) pinpoint floc, a non-bulking well-settling but poorly flocculating sludge.

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SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

Determinants of settling behaviour of biological sludge

Effect of filamentous organism on floc structure: (b) bulking sludge, a poorly settling but excellently flocculating sludge.

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SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

Determinants of settling behaviour of biological sludge

Effect of filamentous organism on floc structure: (c) ideal sludge, a non-bulking, well-settling and well-flocculating sludge.

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SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

Determinants of settling behavior of biological sludge

Typical particle size distribution of activated

sludge.

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SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

Determinants of settling behaviour of biological sludge

Percentage particle removal plotted against particle size

distribution, showing that it is mainly a significant

proportion of the smaller particles that escape with the effluent and that virtually all of the larger particles are

retained and recirculated in the activated sludge system.

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SWASecondary Settling Tanks

F. W. GünthertShort circuiting flow

(IAWQ Report No. 6, 1997)

The influence of underflow recycle ratio (R) on the short circuiting.

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SWASecondary Settling Tanks

F. W. Günthert

(ATV - DVWK - A 131 E, 2000)

Dimensioning of the secondary settling tank

Suspended solids concentration in the

bottom sludge dependent on the sludge volume

index and the thickening time.

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SWASecondary Settling Tanks

F. W. Günthert

(ATV - DVWK - A 131 E, 2000)

Dimensioning of the secondary settling tank

Recommended thickening time dependent on the degree of wastewater treatment, to avoid denitrification and phosphorus resolution.

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SWASecondary Settling Tanks

F. W. Günthert

Thickening timewithout nitrification: tTH = 1.5 - 2 hwith nitrification: tTH = 1.0 - 1.5 hwith denitrification: tTH = 2 - (2.5 h)

Return sludge concentrationSSRS ≈ 0.7 · SSBS with scrapersSSRS ≈ 0.5 to 0.7 · SSBS with suction facilities

3THBS t

SVI1000SS ⋅=

Bottom sludge concentration

Design procedures - Return sludge concentration

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(ATV - DVWK - A 131 E, 2000)

SWASecondary Settling Tanks

F. W. Günthert

(ATV - DVWK - A 131 E, 2000)

Approximate values of the MLSS

Approximate values for the mixed liquor suspended solids concentration in the biological reactor dependent on the sludge volume index for SSRS = 0,7 * SSBS

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RSRS

RSSSATSS+

⋅=1

SWASecondary Settling Tanks

F. W. Günthert

design criteriae.g. flow rates and loads,

process scheme, sludge age, SVI

design of the secondary settling tank

design of the biological reactor

optimum matchinginteractions

yes

Variation SSATno

Procedure of Dimensioning

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(ATV - DVWK - A 131 E, 2000)

SWASecondary Settling Tanks

F. W. Günthert

(ATV - DVWK - A 131 E, 2000)

Dimensioning of the secondary settling tank

For the design of secondary settling tanks the following are to be determined:

•shape and dimensions of the secondary settling tanks

•permitted sludge storage and thickening time

•return sludge flow rate as well as its control

•type and method of operation of the sludge removal system

•arrangement and design of the inlet and outlet

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SWASecondary Settling Tanks

F. W. Günthert

• Design based on peak wet weather flow QWW,h

• effluent suspended solids concentration XSS,EST ≤ 20 mg/L

• 50 L/kg ≤ SVI ≤ 200 L/kg• Diluted sludge volume DSV ≤ 600 L/m3

• Return sludge rates– QRS ≤ 0.75 · QWW,h for horizontal flow tanks– QRS ≤ 1.0 · QWW,h for vertical flow tanks

• Suspended solids concentration in the influent SSAT > 1.0 kg/m3

Design procedures - application limits

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(ATV - DVWK - A 131 E, 2000)

SWASecondary Settling Tanks

F. W. Günthert

(ATV - DVWK - A 131 E, 2000)

Dimensioning on the Basis of Experience

Standard values for the sludge volume index

The respectively lower values for the sludge volume index (SVI) can be applied, if

•primary settling is dispensed•a selector or an anaerobic mixing tank is placed upstream•the biological reactor is designed as a cascade (plug flow)

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SWASecondary Settling Tanks

F. W. GünthertDesign procedures - Surface area calculation

• Sludge volume loading rate to achieve XSS,EST ≤ 20 mg/L• qSV ≤ 500 l/(m2 · h) for horizontal flow tanks• qSV ≤ 650 l/(m2 · h) for vertical flow tanks

• Optimization between qsv and tank depth should be undertaken

• Differentiating between horizontal and vertical flow by the ratio of vertical to horizontal components

A

h,WWST q

QA =

SVISSqq

EAT

SVA ⋅=

• Surface area calculation

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DSVqq ASV *=

SWASecondary Settling Tanks

F. W. Günthert

(ATV - DVWK - A 131 E, 2000)

Dimensioning of the secondary settling tank

Functional zones and depths of vertical flow (inverse cone) tanks.

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comp. h

com

p. v

SWASecondary Settling Tanks

F. W. Günthert

(ATV - DVWK - A 131 E, 2000)

Design procedures – surface area calculation

Permitted values for the transition area between predominantly horizontal and predominantly vertical flow secondary settling tanks.

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SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

SST design procedures – depth calculation

Typical solids concentration-depth profile in SSTs showing from the top down the clear water zone (h1), the separation zone (h2), the sludge storage zone (h3), and the thickening (or compaction) and sludge removal zone (h4).This profile is accepted as standard for the depth design of SSTs with the ATV design procedures.

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SWASecondary Settling Tanks

F. W. GünthertDesign procedures - Depth calculation

h4: thickening and sludge removal zone

h3: density flow and storage zone

h2: separation and return flow zone

h1: clear water and return flow zone

htot at 2/3 radius

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(ATV - DVWK - A 131 E, 2000)

SWASecondary Settling Tanks

F. W. GünthertDesign procedures - Depth calculation of h1

0.2 m 0.3 m

Clear water zone• safety zone with fixed depth

• with submerged outlet pipes:

m5.0h1 =

37

(Jardin, 2007)

SWASecondary Settling Tanks

F. W. GünthertDesign procedures - Depth calculation of h2

Separation and return flow zone– detention time of 0.5 h for the maximum flow related to the free water

volume– maximum flow:

– free water volume:

– zone depth:

)1( RSqA +⋅

1000/DSV1−

1000/DSV1)RS1(q5.0h A

2 −+⋅⋅

=

DSV30

free water volume

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(ATV - DVWK - A 131 E, 2000)

SWASecondary Settling Tanks

F. W. GünthertDesign procedures - Depth calculation of h3

Density flow and storage zone– storage of sludge expelled from the activated sludge tank in

1.5 h– sludge storage concentration value of 500 L/m3

– decrease of sludge concentration in the activated sludge tank of30% is allowed

500)RS1(q3.05.1h SV

3+⋅⋅⋅

=

39

(ATV - DVWK - A 131 E, 2000)

SWASecondary Settling Tanks

F. W. GünthertDesign procedures - Depth calculation of h4

BS

ThAEAT4 SS

t)RS1(qSSh ⋅+⋅⋅=

Thickening and sludge removal zone

– thickening of the influent sludge load to the bottom sludge concentration

– thickening time depending on the degree of wastewater treatment

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(ATV - DVWK - A 131 E, 2000)

SWASecondary Settling Tanks

F. W. GünthertDesign procedures - Depth calculation of htot

Average design parameters for horizontal flow clarifiers• qSV = 400 to 500 l/(m2 · h)• qA = 1 to 1.2 m/h• htot = 4 to 4.3 m

Influence of qSV on depth and volume (example)• qSV = 320 qA = 0.8 m/h, htot = 3.0 m, D = 28 m, V = 3.600 m3

• qSV = 500 qA = 1.4 m/h, htot = 4.4 m, D = 22 m, V = 3.400 m3

4321tot hhhhh +++=

Total depth at 2/3 radius (htot ≥ 3 m)

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SWASecondary Settling Tanks

F. W. GünthertPlanning aspects - Inlets

Separation performance of clarifiers is influenced substantiallyby the inlet construction

uniform distribution into the tankminimising potential and kinetic energy of the inflow

• entry velocity at maximum flow (QWW,h + QRS) < 10 cm/sdischarge into the

• (separation zone)• density flow and storage zone• (thickening and sludge removal zone)

(beware of short circuiting and re-suspending thickened sludge at high SVI)

flocculation essential for low effluent solids concentration• retention time at maximum flow (QWW,h + QRS) between 3 and 5 minutes• moderate G-values of 50 to 80 1/s

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SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

Circular SSTs - Inlets

Typical unbaffled center feed inlet.

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SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

Typical arrangements of peripheral feed inlets.

Circular SSTs - Inlets

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SWASecondary Settling Tanks

F. W. GünthertPlanning aspects - Inlets

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(Jardin, 2007)

SWASecondary Settling Tanks

F. W. GünthertPlanning aspects - Outlets

• Effluent launders– outboard weirs– inboard weirs– overflow rate < 10 m3/(m · h)– overflow rate with fed on both sides < 6 m3/(m · h)

• Submerged tubes– radially arranged– circularly arranged

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(ATV - DVWK - A 131 E, 2000)

SWASecondary Settling Tanks

F. W. GünthertPlanning aspects - Outlets

47

(Jardin, 2007)

SWASecondary Settling Tanks

F. W. GünthertDesign procedures -Sludge removal design (circular tanks)

Based on a sludge balance consideringreturn sludge flow rateshort-circuit flow rate 0.4 to 0.8 · QRS

Sludge removal in circular tanks

SR

STSRSRSR f

DvahQ⋅

⋅⋅⋅=

4

Typical valuesscraper height hSR: 0.4 to 0.6 mbridge velocity vSR: 72 to 144 m/hremoval factor fSR: 1.5

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(ATV - DVWK - A 131 E, 2000)

SWASecondary Settling Tanks

F. W. GünthertSludge load balancing in SST

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QK * SSAT

QRS * SSRS

QAT * SSAT

QSR * SSBS

BS

ATKRSRSSR

SSSSQSSQQ ** −

>

SWASecondary Settling Tanks

F. W. Günthert

(ATV - DVWK - A 131, 2000)

Dimensioning of the secondary settling tank

Guidance values for the design of sludge scrapers.

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SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

Circular SSTs

Scraper configurations studied in Germany. Type A is the „Nierskratzer“type where α1 > α2 , Type B is a logarithmic spiral with α constant at 45 °, and Types C and D are „windows shade“ type scrapers.

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SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

Rectangular SSTs

Sludge removal systems for rectangular SSTs: (a) blade scraper system, (b) flight scraper system.

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SWASecondary Settling Tanks

F. W. Günthert

(IAWQ Report No. 6, 1997)

Rectangular SSTs

Inlet with flocculation chamber and two paddles with horizontal axes. The sludge is withdrawn near the inlet and after one third of the tank length.

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SWASecondary Settling Tanks

F. W. GünthertPlanning aspects – Sludge Hoppers

• No sludge depositing– slope of the side walls at least 1.7 : 1– walls as smooth as possible

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SWASecondary Settling Tanks

F. W. GünthertConclusions

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• Sludge concentration in activated sludge tank depends on:– sludge characteristics (SVI)– sludge thickening in the clarifier (SSRS)– return sludge flow rate (RS)

• Surface area of SST is determined by:– overflow rate (qA ,SOR)– sludge volume loading (qSV, SLR)

• Tank depth consists of four zones.• Secondary clarifiers designed by the A 131 are relatively

deep compared to other design procedures.

SWASecondary Settling Tanks

F. W. GünthertConclusions

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• Inlet construction and outlet construction influence the performance of SST.

• Proof of sludge removal performance by sludge load balance.

SWASecondary Settling Tanks

F. W. GünthertReferences

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• G.A: Ekama, L. Barnhard, F.W. Günthert, P. Krebs, J.A: McCorquodale, D. Parker, E.J. Wuhlberg:“Secondary settling tanks – Theory, Modelling, Design and Operation”IAWQ Scientific and technical Report No.6, 1997, ISBN 1 900222 03 5

• ATV – DVWK –Standard A 131E : Dimensioning of Single-Stage Activated Sludge PlantsGFA, Hennef 2000, ISBN 3 – 935669-96-8

SWASecondary Settling Tanks

Univ. Prof. Dr.-Ing F.W. Günthert

Universität der Bundeswehr München

Institut für Wasserwesen

Siedlungswasserwirtschaft und Abfalltechnik

wolfgang.guenthert@unibw.de

Secondary settling tanks and sludgemass balance

dex summer school 2011

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