Dynamic loading tests for final settling tanks

~

Pergamon

Waf. Sci. Tech. Vol. 34, No. 3-4, pp. 267-274,1996. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved. 0273-1223/96 $15'00 + 0'00

PH: 50273-1223(96)00582-3

DYNAMIC LOADING TESTS FOR FINAL SETTLING TANKS Patrik Baumer*, Peter Volkart* and Peter Krebs ** * Laboratory of Hydraulics, Hydrology and Glaciology (VAW), Swiss Federal Institute of Technology, ETH Zentrum. CH-8092 Zurich, Switzerland ** Swiss Federal Institute for Environmental Science and Technology (EAWAG), Ueberlandstrasse 133, CH-8600 Dubendorf, Switzerland

ABSTRACT Dynamic experiments conducted in a pilot scale settling tank at Zurich's wastewater treatment plant (WWTP) are presented. The experiments were carried out with real activated sludge. They showed that vertical perforated walls transverse to the main flow direction are very beneficial particularly for wet-weather flow. The increase of the effluent suspended solids concentration is significantly dampened with dynamic hydraulic loading. Installing several perforated walls enables uniform velocity profiles to be attained, enhancing flocculation. The total sludge mass stored in the final settling tank may be increased by this system and requires special attention when the removal system is designed. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd

KEYWORDS Dynamic loading; effluent quality; final settling tanks; flocculation; sedimentation; sedimentation efficiency; sludge blanket behaviour; wet-weather flow. INTRODUCTION Except for purification plants with final filtration, final settling tanks are usually the last stage of a WWTP. The separation of activated sludge from purified water is very important because the recipient directly receives the effluent of the final settling tank. It is well known that the flow field in activated sludge clarifiers is not uniformly distributed as assumed by Hazen (1904). Typically a strong density current along the bottom with high forward velocities is observed with a reverse flow in the upper tank region. This phenomenon was identified in field measurements conducted by Larsen (1977), Bretscher et ai. (1984) and Baumer et ai. (1995) as well as in hydraulic model tests and numerical investigations by Krebs (1991). Even though Anderson (1945) already demonstrated the above mentioned flow field, this problem was never explicitly taken into acount in dimensioning guidelines. Common design approaches are either based on the solid flux method (Metcalf and Eddy, 1991) or on a fixed maximum sludge volume loading rate qsv [l/m 21l] in order to detennine the required surface area and the total depth of the basin (ATV, 1991). It is thus not surprising, due to the effects caused by density currents, such as short circuiting, high effluent suspended solids concentrations, overloading at wet-weather flow, disadvantageous residence time distribution etc., that the design depths of final settling tanks continually increased. According to the German Association of 267

268

P. BAUMER et af.

Wastewater Engineering the increase in the required depths for final settling tanks has been considerable over the past twenty years (ATV, 1975, 1991). In Switzerland, as in numerous other countries, most WWTPs were constructed before 1975. Most final settling tanks are therefore insufficiently deep. This paper demonstrates one possible measure to improve t~e sedimentation efficiency of final settling tanks not only for steady state conditions but also under dynarruc loading. PILOT PLANT In order to investigate the flow field and the sludge settling characteristics a pilot plant was constructed at Zurich's WWTP. The pilot settler is located between the aeration basins and the final settling tanks of the plant. It is made of concrete and glass elements and has a total length of 15 m, a width of 1 m and a maximum water depth of 3 m. The thickened sludge is transported to the sludge hopper at the inlet by a scraper mechanism with a blade height of 15 cm and a distance from blade to blade of 4 m. The scraper velocity for the investigation presented in this paper was held constant at 2 cmls. On one side of the flume every alternate metre is made of glass (over the entire water depth) to enable the observation of the hydraulic and settling processes. The configuration of the pilot plant is shown in Fig. 1.

D II1II

glass concrete

15.00

o

5m

Figure 1. Longitudinal section of the I m wide pilot settler at Zurich's WWTP.

MEASURING EQUIPMENT The inflow Qo is fed from a connecting channel between aeration basins and final settling tanks of the real WWTP and is measured with an inductivity flow meter and recorded online on a Macintosh I1x computer. The inflow regulation valve is controlled by a computer program. Both the effluent Q and the return sludge flow rate QRS are measured with a Poncelet gauging weir. Their sum corresponds to the total influent to the basin QO=Q+QRS' The total suspended solid sludge concentration of the inflow TSS in and the concentration of the return sludge TSS RS were analysed in the laboratory. For each test the sludge volume index SVI was determined. The effluent solid concentration TSS E is measured with a turbidity meter (BTGIMET-3000) in formazin turbidity units ([FTU] = [TE/F]) and is recorded online. The sludge blanket height h s is detennined visually. SETTLING BEHAVIOUR OF ACTIVATED SLUDGE Column settling tests were carried out to obtain the correlation between settling velocity vS and the TSS values of Zurich's activated sludge. The column diameter was 28 cm and the column height was 1.5 m or 0.75 m. As column diameters larger than 90 cm are required for any wall effect to be negligible, a correction factor to the single settling velocities was used according to Boller (1992). The dependence of the sludge blanket settling velocity upon the sludge concentration is an exponential function (Vesilind, 1968):

Dynamic loading tests for final settling tasks

Vs

=vo·e- C-TSS [m/h].

269

(1)

For the tests in ~he 1.5 ~ high column the values for Vo and Care 13.9 [m/h] and 0.514 [m 3/kg] respectively, and for the settl10g tests 10 the 0.75 m high column 11.0 [m/h] and 0.469 [m 3/kg]. ~e correlation between settling velocity and sludge concentration is shown in Fig. 2. It is evident that the

difference between the values for the two column heights is almost negligible.

10 n:-1~r==7=I=:::::::=::::=:=F=:====F=====i=====!:===:::::!======:i:-.. j colup1n hei9h~ H=1.5 ~ ""'-;y = 13.9i- e"(-O.5~4x) R2=

• :

8

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:

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I-I:~i-Ll-II-

2

2

345

TSS [kg/m3 ]

Figure 2. Correlation between sludge settling velocity

8

Vs [m/h] and sludge concentration TSS [kg/m~.

Table 1. Input data and tank configurations for dynamic tests Test No.

TSSln [kglnr']

SVI [I/kg]

I

2.8

89

rIlm 2'hl 249 - 498

n

2.0

73

146 - 292

ill

2.9

90

261·522

N

2.6

85

221·442

V

2.2

75

165 - 330

qsv=OR·TSSln·SVI

Inlet Configuration

Perrorated Walls

Inlet baffle: xo=O.5 m. ho=O.5 m Inlet baffle: xo=O.5 m. ho=O.35 m; horizontal slab above sludl!:e hoooer Inlet baffle: xo=O.5 m. ho=O.5 m Inlet baffle: xo=O.5 m. ho--o.5 m Inlet baffle: xo=O.5 m. ho=O.35 m; horizontal slab above sludl!:e hoooer

3 at x=3.75. x=7.5 and x-ll.25 m (~=O.17) 3 at x=3.75. (,,7.5 and x=1l.25 m ( =0.21) 1 atx=7.5 m (~=0.21)

DYNAMIC LOADING TESTS Five configurations of final settling tanks were studied under dynamic loading conditions. It was examined whether perforated walls would be beneficial with respect to hydraulic, settling and flocculation efficiency of a basin, as found by Krebs et ai. (1992) based on laboratory experiments and a theoretical approach. An overview of the test conditions is given in Table 1 and the hydrograph for the hydraulic loading is shown in Fig. 3. The overflow rate OR during the first three hours of each test was constant at 1 m1h. For the following 30 minutes it was increased to 1.5 mIh and thereafter reduced to 1 mIh again for another 30 Minutes. For the fifth hour of each test the overflow rate was increased to 2 m1h. Finally during the last two hours of each test the overflow rate was maintained at 1 mIh again. The return sludge flow rate was held

270

P. BAUMER et at.

constant at 4.17 lis, and thus the return sludge ratios R=QRS/Q were 1.0 for OR= 1 m/h, 0.67 for OR= 1.5 mJh and 0.5 for OR=2 m/h, respectively.

OR (m/h)

2.0 1.5 1.0

0.5

o

--+---+-I~

o

1

2

3

4

5

7

6

8

time (h)

9

Figure 3. Hydrograph for dynamic loading.

Figure 4 shows the different inlet structures. Both the distance Xo of the inlet baffle from the inlet wall and the distance ho from the tank bottom to the lower end of the inlet baffle were constant at 0.5 m for tests No. I, III and IV. For tests No. II and V a horizontal slab with a length of 1 m was installed 15 cm above the sludge hopper in order to prevent short circuiting from the inlet into the sludge hopper.

ITests No. I,III,IV I

01 t=O. 17

® It=O.21 I

1

E

0 0

C\i E

IX)

'"

ITests No. II and V I

C\i E

IX)

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ci

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C\I C\I

N N

I... Figure 4. Inlet configurations.

E

E

Xo=O.5m .......... _---,.

1m

ci

ci

I...

1m

Figure 5. Perforated wall with a porosity fraction of 0.17 (a) and 0.21 (b).

In Fig. 5 the different perforated wall configurations are shown. The porosity fraction of a perforated wall is

z =FJ!FST'

(2)

where Fh is the total area of the holes and F ST the cross-sectional area of the settling tank. For z = 0.17 the configuration of the perforated walls is given in Fig. 5a and for z = 0.21 in Fig. 5b. The hole diameters Dh were 10 em.

Dynamic loading tests for final settling tasks

ffi'1oo.oo 6O.CO

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20000

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30000

time [sec]

a)

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r

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r-

10000

5000

I

15000

20000

30000

25000

time [sec]

b)

Figure 6. Inflow and effluent quality over time for tests No. I (a) and II (b).

w

UJ

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30.00 •.'"'- J ...

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0.00 "'-_ _---l~--_+_.&.......&...--L+--...&...--+----+-----1 30000 20000 25000 5000 10000 15000 o time [sec]

b)

....

~ 30.00 20.00

i-... 10.00 8 0.00

c)

1.5 mIh mIh

OR.' mlh

~ 40.00 w iaI

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al• .a.1 'Ill

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20000

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time [sec]

Figure 7. Inflow and effluent quality over time for tests No. III (a). IV (b) and V (c).

30000

272

P. BAUMER et al.

RESULTS In Figs 6 and 7 the effluent quality over time is shown for the hydrograph of Fig. 3 and the test cases defined in Table 1. After about two hours of the initial loading at OR= 1 m/h the effluent concentration reached an equilibrium of roughly 20 to 35 [TEIF], which remained constant for about one hour before the loading was increased. The reaction time of test No. V is somewhat slower, a steady state was only reached after 2.5 hours (Fig. 7c). For tests No. I and II, where no walls were installed, the effluent quality deteriorates rapidly after an increase of the influent. This deterioration takes place with a retardation of only 15 to 30 minutes after the increase of the inflow (Fig. 6). After the decrease of the hydraulic loading back to dry-weather conditions (OR= 1 m/h), a reduction of suspended solids in the effluent could be observed but the level of the effluent quality at the end of the test run is still poorer than before. For test No. I the sludge volume loading rate qsv was in the range of 249 to 498 lIm 2-h whereas for test No. II qsv it was between 146 and 292 11m2-h. In tests No. III, IV and V the deterioration of the effluent quality caused by higher inflow is dampened by the in-tank baffles (Fig. 7). The mass of suspended solids in the effluent hardly increases even after a rise from OR=l m/h to OR=2 m/h. The level of the effluent quality is a lot more stable than in the conventional final clarifiers of tests No. I and II. For test No. V, where only one perforated wall at mid length was installed, there is a slight increase of the effluent suspended solids concentration after the increase of hydraulic loading. A reduction of suspended solids after a diminution of the inflow could hardly be measured even one hour after the decrease of the inflow. A final settling tank partitioned by more than one perforated wall reacts more stable and is less sensitive to any changes. The positive influence of perforated walls on the effluent quality may be explained by the equalizing effect on the velocity field and the enhanced flocculation (Krebs et ai., 1992). The main difference between test runs shown in Fig. 7 is in the amount of perforated walls installed. For tests No. III and IV the basin was divided into 4 equal chambers with lengths of 3.75 m each while for test No. V the basin was only halved with one perforated wall. It seems that the more walls are used for baffling the tank the more stable the system becomes. However, baffling a final settling tank induces a completely different sludge regime compared to conventional tanks. Fig. 8 shows the average sludge blanket height h s [cm] over time for tests No. I, III and IV. The gradient of hs over the length of the basin for the original tank (test No. I) is very different from those in the basins with perforated walls (tests No. III and IV), where the total sludge mass stored in the tank is significantly increased. Particularly for the wall configuration of test No. III the difference of the sludge blanket height from one chamber to the next is significant. This difference causes a further density current in the following part of the basin and is responsible for the disturbance of the settling process in the next chamber. For practical applications it is more difficult and takes a lot more time to remove the settled sludge from the chambers at the rear of the tank. This disadvantage could be reduced by adding holes in the perforated walls close to the bottom (test No. IV), by a sludge suction mechanism (Baumer et ai., 1995) and partly by increasing the return sludge ratio during the period of higher inflow. The used sludge scraper mechanism is definitely less efficient in removing return sludge than a suction system. Former experiments have shown that the energy input by the scraper blades detrimentally affects the flow field as well as the settling efficiency. The samples of the return sludge concentrations represent a mean value over a time period of 200 sec (= 4 [m]12 [cm/s]). Six samples were taken at 0", 40 ", 80 ", 120 ", 160 ", 200 " because preliminary tests showed that the return sludge concentration is dependent on the current position of the scraper blades (distance from blade to blade = 4 m; scraper velocity v r = 2 cm/s). The horizontal slab above the sludge hopper installed in tests No. II and V was intended to avoid short circuit currents from the inlet to the hopper. Comparing the return sludge concentration normalized by the inflow concentration TSSRSffSS in no distinction between the tests with (No. II and V) and without the slab (No. I, III and IV) could be made (Fig. 9). Although Fig. 9 shows a certain coherence between return sludge concentration TSS RS [kg/m 3] and overflow rate OR, a steady state situation is not reached with respect to return sludge when the mass balance is considered.

Dynamic loading tests for final settling tasks

L

~

250

~

273

t..................

I::J\_it: ~'"

100 ..._ _. . 1._ _...

overflow 'ale

2.5

2.0 ~

~

1.5 ~

i

CD

1.0

~

50

100

200

300

400

tlme[mln)

a)

3.0

300

2.5

250

2.0

~ S

I!!

1.5 ~

1.0

i

2.5

200

I

II)

150

~

100

,0.5 ,

0.5

~~ed=::I::::i:!:i::~,--,-,-......l--,--,-.................Ll-..................J 0.0 100

300

200

400

J:--l...............-L..........=4-...l.-~~......l..........::::........::~~==,o.o 200

100

500

~~

300

400

500

~~

~

~

Figure 8. Sludge blanket height (average) h s [em] over time for tests No. I (a), III (b) and IV (c). 5

r;::::::::::I::::::::::::::c:::::;-;-""""-""""-r""""-j • • OR [rrlhJ _TSSRSlTSSin [-J Tnl No.1 - II" TSSRSlTSSin [-J Test No. II _TSSRSlTSSin [-J Tnl No. III _TSSRSlTSSin [-J Tes. No. IV ---& TSSRSfTSSin [-J Tesl No. V

4

1.

...................................::i",::·.·.·.·.·.·. .·.·.·

., It'

r

\i j;

i.. !...

i

I

I:

.

··~·lWM'".;:.·· ..·~···················1···················

.

•

i . . . lII· . . •••··

•

· • · • • . . •· . .

I

o.. . . . . . . . ""-'.......L-'-~ o

.-r,

··t···········,····r··················

100

~..

i

i ··i..-..···········•··..·..·•·•··•·· ·

I

.

..........!.....o--'-.-.-o-.J.-'-.....................l-
200

300

400

500

tlme[mln)

Figure 9. Normalized return sludge concentration TSSRSffSS in over time.

SUMMARY AND CONCLUSIONS Dynamic loading tests in a pilot final settling tank. showed that perforated walls transverse to the main flow direction are beneficial with regard to the settling efficiency. By reducing the well known bottom current caused by density effects the hydraulic residence time distribution is improved. Perforated walls generally have a braking effect on this bottom current. The flow field in a basin equipped with perforated walls

274

P. BAUMER et al.

becomes more stable. Furthermore, flocculation is enhanced by such in-tank baffles. Consequently the settling efficiency is increased. Particularly for combined sewer systems, where high inflow variations occur, it is very important that the WWTP operates reliably under dynamic loading. The presented experiments have proven that dividing a final settling tank with perforated walls has a stabilizing effect on the effluent quality even under increased hydraulic loading conditions. Because the total sludge mass retained in a baffled tank is greater than in a conventional tank, improved sludge removal is required. The sludge blanket regime can also be managed by an increased return sludge ratio during wet-weather inflow. ACKNOWLEDGEMENT The pilot settler was built at Zurich's main WWTP. The support of Zurich's sewage treatment department is kindly acknowledged. This work was financially supported by the Swiss Federal Office of Environment, Forest and Landscape (BUWAL). Their kind support is gratefully appreciated. Many thanks go to Andrew Faeh for reviewing the English. REFERENCES Anderson, N. E. (1945). Design of Final Settling Tanks for Activated Sludge. Sewage Works Journal, 17(1),50-65, Lancaster, PA. ATV (1975). Lehr- und Handbuch der Abwassertechnik. Bd. 11,2. Auflage, Verlag Von Wilhelm Ernst & Sohn, Berlin, Miinchen, Dusseldorf (in German). ATV (1991). Arbeitsblatt A 131 - Bemessung von einstufigen Belebungsanlagen ab 5000 Einwohnergleichwerten. Gesellschaft zur Forderung der Abwassertechnik e. V. (GFA), SI. Augustin (in German). Baumer, P., Volkart, P., Bretscher, U. and Krebs P. (1995). Untersuchungen zur Verbesserung der Stromung in den NachkHirbecken der KHiranlage Einsiedeln. KorrespondenzAbwasser, 42(4),598-612 (in German). Boller, M. (1992). Sedimentation. Vorlesungsskript, ETH Zurich (in German). Bretscher, U., Hager, W. and Hager, W. H. (1984). Untersuchungen uber die Stromungs- und Feststoffverteilungen In NachkHirbecken. GWF-wasser/abwasser, 125(2), 81-90 (in German). Hazen, A. (1904). On Sedimentation. Trans. ASCE, Paper No. 980, 45-71. Krebs, P. (1991). The Hydraulics of Final Settling Tanks. Wat. Sci. Tech., 23(4-6),1037-1046. Krebs, P., Gujer, W. and Vischer, D. (1992). Improvement of Secondary Clarifiers Efficiency by Porous Walls. IAWPRC, Wat. Sci. Tech., 26(5-6), 1147-1156. Larsen, P. (1977). On the Hydraulics of Rectangular Settling Basins, Department of Water Resources Engineering, Lund Institute of Technology, Report No. 1001. Metcalf and Eddy (1991). Wastewater Engineering: Treatment, Disposal and Reuse. Metcalf and Eddy Inc. 3rd edn, 1334 pp. Vesilind, A. P. (1968). The Influence of Stirring in the Thickening of Biological Sludge. Dissertation, University of North Carolina, 214 pp.