Secondary Settling Tanks SST - Guenthert_Edited

Secondary settling tanks and sludge mass balance dex summer school 2011

Univ. Prof. Dr.-Ing F.W. Günthert Universität der Bundeswehr München Institut für Wasserwesen Siedlungswasserwirtschaft Siedlungswa sserwirtschaft und Abfalltechnik Seco Second ndar ary y Sett Settliling ng Tank Tanks s

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Structure

F. W. Günthert

Structure: • Glossa Glossary ry of terms terms • Tasks Tasks of secondary secondary settling settling tanks tanks • Settli Settling ng veloci velocity ty • Return activated sludge, settling behavior of biological sludge sludge • Dimension Dimensioning ing of SST, SST, surface surface area, area, depth depth • Planning Planning aspects, aspects, inlet, outlet, outlet, sludge sludge removal systems systems • Sludge Sludge load load balan balancin cing g • Conclu Conclusio sion, n, refere reference nces s

Seco Second ndar ary y Sett Settliling ng Tank Tanks s

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Structure

F. W. Günthert

Structure: • Glossa Glossary ry of terms terms • Tasks Tasks of secondary secondary settling settling tanks tanks • Settli Settling ng veloci velocity ty • Return activated sludge, settling behavior of biological sludge sludge • Dimension Dimensioning ing of SST, SST, surface surface area, area, depth depth • Planning Planning aspects, aspects, inlet, outlet, outlet, sludge sludge removal systems systems • Sludge Sludge load load balan balancin cing g • Conclu Conclusio sion, n, refere reference nces s

Seco Second ndar ary y Sett Settliling ng Tank Tanks s

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Glossary of terms

F. W. Günthert

Rectangular secondary settling tank (SST)

(IAWQ Report No. 6, 1997)

Seco Second ndar ary y Sett Settliling ng Tank Tanks s

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Glossary of terms

F. W. Günthert

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

(IAWQ Report No. 6, 1997)

Seco Second ndar ary y Sett Settliling ng Tank Tanks s

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 Abbreviations and units

F. W. Günthert

List of abbreviations and units

(IAWQ Report No. 6, 1997)

Seco Second ndar ary y Sett Settliling ng Tank Tanks s

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Basic principles - Interactions

F. W. Günthert

• Activated sludge process SSEST

QWW SSAT SSBS QRS, SSRS

• Mass balance over the process

 RS = SS ⋅ SS  AT   RS 1 + RS Secondary Settling Tanks

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Basic principles -Tasks of secondary settling tanks

•

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

Secondary Settling Tanks

F. W. Günthert

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Basic principles -Tasks of secondary 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

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

Secondary Settling Tanks

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Effluent quality of SST

F. W. Günthert

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 Secondary Settling Tanks

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Clarification

F. W. Günthert

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] Secondary Settling Tanks

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Settling velocity

F. W. Günthert

   ]   s    /   m   c    [   s

  v   y    t    i   c   o    l   e   v   g   n    i    l    t    t   e   s

 ATV, 1997 particle diameter d [cm]

Secondary Settling Tanks

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Settling velocity

F. W. Günthert

flocculation zone hindered settling zone interim zone thickening zone

settling behavior (settling zylinder 1l)

Secondary Settling Tanks

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Velocity distribution

F. W. Günthert

Distribution of velocities, influenced by density currents

Secondary Settling Tanks

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Settling velocity

F. W. Günthert

Settling tanks are influenced by: •different settleable particles •wind, temperature •inlet and outlet •density currents •sludge removal procedure •tank geometry (L:W; Ø,htot)

Secondary Settling Tanks

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Effluent concentration of SST

F. W. Günthert

   T    S    E

   S    S   n   o    i    t   a   r    t   n   e   c   n   o   c    t   n   e   u    l    f    f   e

specific sludge volume load

Secondary Settling Tanks

KA 11, 1997

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Return activated sludge (RAS)

F. W. Günthert

• Activated sludge process

SSAT SSBS RAS, SSRS

• Mass balance over the process

 RS = SS ⋅ SS  AT   RS 1 + RS Secondary Settling Tanks

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Return activated sludge (RAS)

F. W. Günthert

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)

Secondary Settling Tanks

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Determinants of settling behaviour of biological sludge

F. W. Günthert

Effect of filamentous organism on floc structure: (a) pinpoint floc, a non-bulking well-settling but poorly flocculating sludge. (IAWQ Report No. 6, 1997)

Secondary Settling Tanks

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Determinants of settling behaviour of biological sludge

F. W. Günthert

Effect of filamentous organism on floc structure: (b) bulking sludge, a poorly settling but excellently flocculating sludge. (IAWQ Report No. 6, 1997)

Secondary Settling Tanks

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Determinants of settling behaviour of biological sludge

F. W. Günthert

Effect of filamentous organism on floc structure: (c) ideal sludge, a non-bulking, well-settling and well-flocculating sludge. (IAWQ Report No. 6, 1997)

Secondary Settling Tanks

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Determinants of settling behavior of biological sludge

F. W. Günthert

Typical particle size distribution of activated sludge.

(IAWQ Report No. 6, 1997)

Secondary Settling Tanks

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Determinants of settling behaviour of biological sludge

F. W. Günthert

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.

(IAWQ Report No. 6, 1997)

Secondary Settling Tanks

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Short circuiting flow

F. W. Günthert

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

(IAWQ Report No. 6, 1997)

Secondary Settling Tanks

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Dimensioning of the secondary settling tank

F. W. Günthert

Suspended solids concentration in the bottom sludge dependent on the sludge volume index and the thickening time.

(ATV - DVWK - A 131 E, 2000)

Secondary Settling Tanks

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Dimensioning of the secondary settling tank

F. W. Günthert

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

(ATV - DVWK - A 131 E, 2000)

Secondary Settling Tanks

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Design procedures - Return sludge concentration



Bottom sludge concentration

SSBS 



F. W. Günthert

1000 3 = ⋅ t TH SVI

Thickening time  without nitrification:

tTH = 1.5 - 2 h

 with nitrification:

tTH = 1.0 - 1.5 h

 with denitrification:

tTH = 2 - (2.5 h)

Return sludge concentration  SSRS

≈

0.7 · SSBS with scrapers

 SSRS

≈

0.5 to 0.7 · SSBS with suction facilities

Secondary Settling Tanks

(ATV - DVWK - A 131 E, 2000) 26 SWA

 Approximate values of the MLSS

F. W. Günthert

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

 RS SS = SS ⋅  AT   RS 1 + RS

(ATV - DVWK - A 131 E, 2000)

Secondary Settling Tanks

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Procedure of Dimensioning

F. W. Günthert

design criteria e.g. flow rates and loads, process scheme, sludge age, SVI

design of the secondary settling tank design of the biological reactor 

optimum matching interactions yes Secondary Settling Tanks

Variation SS AT no (ATV - DVWK - A 131 E, 2000) 28 SWA

F. W. Günthert

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

(ATV - DVWK - A 131 E, 2000)

Secondary Settling Tanks

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F. W. Günthert

Design procedures - application limits

• 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 • Return sludge rates

≤

600 L/m3

 – QRS ≤ 0.75 · QWW,h for horizontal flow tanks  – QRS ≤ 1.0 · QWW,h for vertical flow tanks • Suspended solids concentration in the influent SS AT > 1.0 kg/m3 (ATV - DVWK - A 131 E, 2000)

Secondary Settling Tanks

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F. W. Günthert

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) (ATV - DVWK - A 131 E, 2000)

Secondary Settling Tanks

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F. W. Günthert

Design procedures - Surface area calculation

•

Surface area calculation

 A ST •

=

Q WW ,h q A

q A

=

qSV SSEAT ⋅ SVI

q SV 

=

q A *  DSV 

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

•

Optimization between q sv and tank depth should be undertaken

•

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

Secondary Settling Tanks

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Dimensioning of the secondary settling tank

F. W. Günthert

comp. h

  v  .   p   m   o   c

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

(ATV - DVWK - A 131 E, 2000)

Secondary Settling Tanks

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Design procedures – surface area calculation

F. W. Günthert

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

(ATV - DVWK - A 131 E, 2000)

Secondary Settling Tanks

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SST design procedures – depth calculation

F. W. Günthert

Typical solids concentration-depth profile in SSTs showing from the top down the clear water zone (h1), the separation zone (h 2), the sludge storage zone (h 3), and the thickening (or compaction) and sludge removal zone (h 4).This profile is accepted as standard for the depth design of SSTs with the ATV design procedures. (IAWQ Report No. 6, 1997)

Secondary Settling Tanks

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F. W. Günthert

Design procedures - Depth calculation

h1: clear water and return flow zone h2: separation and return flow zone h3: density flow and storage zone h4: thickening and sludge removal zone

htot at 2/3 radius (ATV - DVWK - A 131 E, 2000)

Secondary Settling Tanks

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Design procedures - Depth calculation of h1

F. W. Günthert

Clear water zone • safety zone with fixed depth

h1 = 0.5 m • with submerged outlet pipes:

0.2 m

0.3 m

(Jardin, 2007)

Secondary Settling Tanks

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F. W. Günthert

Design 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:

q A ⋅ (1 + RS )  – free water volume:

free water volume

1 − DSV / 1000  – zone depth:

h2

=

0.5 ⋅ q A ⋅ (1 + RS) 1 − DSV / 1000

Secondary Settling Tanks

DSV30 (ATV - DVWK - A 131 E, 2000) 38 SWA

Design procedures - Depth calculation of h3

F. W. Günthert

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 of  30% is allowed

1.5 ⋅ 0.3 ⋅ qSV ⋅ (1 + RS) h3 = 500

(ATV - DVWK - A 131 E, 2000)

Secondary Settling Tanks

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Design procedures - Depth calculation of h4

F. W. Günthert

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

h4

=

SSEAT ⋅ q A ⋅ (1 + RS) ⋅ t Th SSBS

(ATV - DVWK - A 131 E, 2000)

Secondary Settling Tanks

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Design procedures - Depth calculation of htot

 Total depth at 2/3 radius (h tot

h tot 

=

h1 + h2

+

h3

+

≥

F. W. Günthert

3 m)

h4

 Average design parameters for horizontal flow clarifiers • qSV = 400 to 500 l/(m2 · h) • q A

= 1 to 1.2 m/h

• htot = 4 to 4.3 m 

Influence of qSV on depth and volume (example) • qSV = 320  q A = 0.8 m/h, htot = 3.0 m, D = 28 m, V = 3.600 m 3 • qSV = 500  q A = 1.4 m/h, htot = 4.4 m, D = 22 m, V = 3.400 m 3

Secondary Settling Tanks

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Planning aspects - Inlets

F. W. Günthert

Separation performance of clarifiers is influenced substantially by the inlet construction  uniform distribution into the tank  minimising potential and kinetic energy of the inflow • entry velocity at maximum flow (QWW,h + QRS) < 10 cm/s

 discharge 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

Secondary Settling Tanks

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F. W. Günthert

Circular SSTs - Inlets

Typical unbaffled center feed inlet.

(IAWQ Report No. 6, 1997)

Secondary Settling Tanks

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F. W. Günthert

Circular SSTs - Inlets

Typical arrangements of peripheral feed inlets.

(IAWQ Report No. 6, 1997)

Secondary Settling Tanks

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Planning aspects - Inlets

F. W. Günthert

(Jardin, 2007)

Secondary Settling Tanks

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F. W. Günthert

Planning 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

(ATV - DVWK - A 131 E, 2000)

Secondary Settling Tanks

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Planning aspects - Outlets

F. W. Günthert

(Jardin, 2007)

Secondary Settling Tanks

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Design procedures Sludge removal design (circular tanks) 

F. W. Günthert

Based on a sludge balance considering  return sludge flow rate  short-circuit flow rate 0.4 to 0.8 · QRS



Sludge removal in circular tanks

QSR 

=

hSR ⋅ a ⋅ vSR ⋅ DST  4 ⋅  f SR

Typical values  scraper height hSR:

0.4 to 0.6 m

 bridge velocity vSR:

72 to 144 m/h

 removal factor f SR:

1.5 (ATV - DVWK - A 131 E, 2000)

Secondary Settling Tanks

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F. W. Günthert

Sludge load balancing in SST Q AT * SS AT

QK * SS AT

QSR * SSBS

QSR > QRS * SSRS

Secondary Settling Tanks

Q RS * SS RS − QK  * SS AT  SS BS

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F. W. Günthert

Dimensioning of the secondary settling tank

Guidance values for the design of sludge scrapers.

(ATV - DVWK - A 131, 2000)

Secondary Settling Tanks

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Circular SSTs

F. W. Günthert

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.

(IAWQ Report No. 6, 1997)

Secondary Settling Tanks

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Rectangular SSTs

F. W. Günthert

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

(IAWQ Report No. 6, 1997)

Secondary Settling Tanks

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Rectangular SSTs

F. W. Günthert

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.

(IAWQ Report No. 6, 1997)

Secondary Settling Tanks

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Planning aspects – Sludge Hoppers

F. W. Günthert

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

Secondary Settling Tanks

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Conclusions

F. W. Günthert

• 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 (q A ,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.

Secondary Settling Tanks

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Conclusions

F. W. Günthert

• Inlet construction and outlet construction influence the performance of SST. • Proof of sludge removal performance by sludge load balance.

Secondary Settling Tanks

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References •

F. W. Günthert

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 Plants GFA, Hennef 2000, ISBN 3 – 935669-96-8

Secondary Settling Tanks

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