Secondary Settling Tanks SST - Guenthert_Edited
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Transcript of Secondary Settling Tanks SST - Guenthert_Edited
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
2
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)
3
SWASecondary Settling Tanks
F. W. Günthert
(IAWQ Report No. 6, 1997)
Glossary of terms
Centre feed, peripheral take-off, secondary settling tank (SST)
4
SWASecondary Settling Tanks
F. W. Günthert
(IAWQ Report No. 6, 1997)
Abbreviations and units
List of abbreviations and units
5
SWASecondary Settling Tanks
F. W. Günthert
• Activated sludge process
SSAT
• Mass balance over the process
RSRS
RSSSATSS+
⋅=1
SSBS
QRS, SSRS
Basic principles - Interactions
6
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
7
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
8
• Avoidance of denitrification and resolution of phosphorous by:– limitation of thickening time– sludge removal performance
SWASecondary Settling Tanks
F. W. GünthertEffluent quality of SST
9
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
10
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
11
ATV, 1997particle diameter d [cm]
settl
ing
velo
city
v s
[cm
/s]
SWASecondary Settling Tanks
F. W. GünthertSettling velocity
12
flocculation zone
hindered settling zone
interim zone
thickening zone
settling behavior (settling zylinder 1l)
SWASecondary Settling Tanks
F. W. Günthert
13
Velocity distribution
Distribution of velocities, influenced by density currents
SWASecondary Settling Tanks
F. W. GünthertSettling velocity
14
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
15
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)
16
SWASecondary Settling Tanks
F. W. GünthertReturn activated sludge (RAS)
17
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.
18
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.
19
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.
20
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.
21
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.
22
SWASecondary Settling Tanks
F. W. GünthertShort circuiting flow
(IAWQ Report No. 6, 1997)
The influence of underflow recycle ratio (R) on the short circuiting.
23
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.
24
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.
25
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
26
(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
27
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
28
(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
29
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
30
(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)
31
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
32
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.
33
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.
34
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.
35
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
36
(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
38
(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
40
(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)
41
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
42
SWASecondary Settling Tanks
F. W. Günthert
(IAWQ Report No. 6, 1997)
Circular SSTs - Inlets
Typical unbaffled center feed inlet.
43
SWASecondary Settling Tanks
F. W. Günthert
(IAWQ Report No. 6, 1997)
Typical arrangements of peripheral feed inlets.
Circular SSTs - Inlets
44
SWASecondary Settling Tanks
F. W. GünthertPlanning aspects - Inlets
45
(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
46
(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
48
(ATV - DVWK - A 131 E, 2000)
SWASecondary Settling Tanks
F. W. GünthertSludge load balancing in SST
49
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.
50
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.
51
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.
52
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.
53
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
54
SWASecondary Settling Tanks
F. W. GünthertConclusions
55
• 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
56
• 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
57
• 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
Secondary settling tanks and sludgemass balance
dex summer school 2011
58