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
1 SWA
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
2 SWA
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
2 SWA
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
3 SWA
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
6 SWA
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
7 SWA
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
8 SWA
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
9 SWA
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
10 SWA
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
11 SWA
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
15 SWA
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
16 SWA
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
17 SWA
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
18 SWA
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
19 SWA
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
20 SWA
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
21 SWA
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
22 SWA
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
23 SWA
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
24 SWA
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
25 SWA
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
27 SWA
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
29 SWA
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
30 SWA
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
31 SWA
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
32 SWA
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
33 SWA
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
34 SWA
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
35 SWA
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
36 SWA
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
37 SWA
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
39 SWA
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
40 SWA
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
41 SWA
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
42 SWA
F. W. Günthert
Circular SSTs - Inlets
Typical unbaffled center feed inlet.
(IAWQ Report No. 6, 1997)
Secondary Settling Tanks
43 SWA
F. W. Günthert
Circular SSTs - Inlets
Typical arrangements of peripheral feed inlets.
(IAWQ Report No. 6, 1997)
Secondary Settling Tanks
44 SWA
Planning aspects - Inlets
F. W. Günthert
(Jardin, 2007)
Secondary Settling Tanks
45 SWA
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
46 SWA
Planning aspects - Outlets
F. W. Günthert
(Jardin, 2007)
Secondary Settling Tanks
47 SWA
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
48 SWA
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
49 SWA
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
50 SWA
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
51 SWA
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
52 SWA
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
53 SWA
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
54 SWA
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
55 SWA
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
56 SWA
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
57 SWA
