Electro-osmotic Dewatering of Sewage Sludge: Preliminary Results · 2017-11-06 · Act. sludge...

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Electro-osmotic Dewatering of Sewage

Sludge: Preliminary Results

S. Visigalli, P. Gronchi, A. Turolla, A. Brenna, C. Colominas, G. G. Fuentes, R. Canziani

2Roberto CANZIANI 2/29

OBJECTIVES:

Achieving at least 40% dry solids (DS) instead of 20% would

halve wet sludge disposal

allow self-sustained combustion at 850 °C, avoiding thermaldrying (which would consume more energy)

QUESTIONS:

Can we reach at least 40% DS with EDW?

Do the lower disposal costs compensate the energy cost ofEDW?

AIM OF THE STUDY


More QUESTIONS:

How sludge characteristics may affect pressure-driven electro-dewatering?

Is electro-dewatering energy efficient?

AIM OF THE STUDY


source: Mahmoud et al.. 2010. Wat. Res. 44 (8). 2381. modif.

𝑣𝑒𝑝 = 𝛾𝜀0𝜀𝑟𝜁𝐸

𝜂𝑑 ത𝑉

𝑑𝑡=𝜀0𝜀𝑟 𝜁

𝜂𝐸𝐴

electrochemical reactions

pressurepressure

ELECTRO-OSMOSIS DEWATERING OF SLUDGE


STATIC PISTON

– Cylindrical glass vessel (h=176 mm, Ø=80 mm)– Cooling water-jacket– Compressed air system (1-4.5 bar)– Double effect cylinder (200 mm stroke) SMC-CP96

– DC power supply (30 V-5 A)– Anode: DSA – Ti MMO– Cathode: stainless steel mesh (AISI 304)– Cloth: PTT (polytrimethyleneterephthalate)

LAB-SCALE DEVICE


EXPERIMENTAL CONDITIONS

Two steps for EDW tests:

filtration + compression (p = 300 kPa ): tP = 5 min

filtration + compression + electric fieldat 15 V/cm (p = 300 kPa): tE = 15-20 min

________________________________________________

Total duration of the test tT = 20-25 min


SLUDGE SAMPLES – 1

ParameterWWTP 1 - Activated sludge +

+ Aerobic stabilisation

Sample no. 1-A 1-B 1-C

pH 6.9 7.2 7.0

Conductivity (mS/cm) 1.1 1.7 1.7

DSo (%) 2.8 3.0 3.4

VS/DS (%) 69.7 73.1 68.3

CST (s)UC 22.1 34.0 27.0

CT - 11.7 12.0

TTF (min)UC 14.0 25.0 21.0

CT - 3.0 5.0

Zeta potential (mV)UC -13.1 -13.2 -13.2

CT - -9.9 -9.5

Turbidity (NTU) UC 80.4 - -

Polyelectrolyte dose (g/kgDS) - 5.7 5.1

Average DSDW WWTP 2013 – 2014 (%) after belt - press: 17 – 18%




+ Aerobic stabilisation


pH 7.2 7.1 7.3


DSo (%) 0.6 1.9 2.5

VS/DS (%) 65.3 67.9 64.4

CST (s)UC 8.4 12.4 29.6

CT - 23.8 8.7

TTF (min)UC 0.8 4.0 27.0

CT - - 4.0


CT - +32.2 -8.6



Average DSDW WWTP 2013 – 2014 (%) after centrifuge: 16 – 18%



ParameterWWTP 3 - MBR sludge + + Aerobic stabilisation


pH 7.1 7.0 6.9


DSo (%) 1.9 1.4 2.2

VS/DS (%) 74.3 74.0 75.4

CST (s)UC 11.2 11.8 -

CT - 12.0 6.7

TTF (min)UC 4.0 6.0 -

CT - - 2.0

Zeta potential (mV)UC -11.6 -13.3 -

CT - +2.5 -9.1


Polyelectrolyte dose (g/kgDS) n.a. 9.0 n.a.

Average DSDW WWTP 2013 – 2014 (%) after belt - press: 16 – 18%




+ Anaerobic stabilisation


pH 7.1 7.2 7.1


DSo (%) 2.8 3.0 3.2

VS/DS (%) 59.5 66.0 63.9

CST (s)UC 89.6 129.0 101.7

CT - 58.0 27.2

TTF (min)UC 88.0 91.0 96.0

CT - 53.0 16.0


CT - -11.8 -12.1



Average DSDW WWTP 2013 – 2014 (%) after centrifuge: 25 – 26%


RESULTS – Lab-scale tests on Unconditioned Sludge

EDW has a low efficiency without conditioning

UC sludge

sample

DSo

raw sludge

(%)

DSi

centrifuged

(%)

Specific electric

energy consumption

during tE

(Wh/kgH2O)

ΔDS

(%)

DSf

(%)

DSf WWTP

average

(2013-2014)

(%)

P=300 kPa; E=15 V/cm; tE=20 min

1-A-UC 2.8 9.5 65.5 5.5 15.0 17 – 18

2-A-UC 0.6 6.3 47.1 3.1 9.4 16 – 18

3-A-UC 1.9 7.1 49.7 3.8 10.9 16 – 18

4-A-UC 2.8 8.4 51.9 4.5 12.9 25 – 26

polyelectrolyte addition is needed to enhance the process


RESULTS –Lab-scale tests on Conditioned and Thickened sludge

CT sludge

sample

DSo

raw sludge

(%)

DSi

centrifuged

(%)

Specific electric

energy consumption

during tE

(Wh/kgH2O)

ΔDS

(%)

DSf

(%)

DSf WWTP

average

(2013-2014)

(%)


1-B-CT 3.0 8.6 48.3 15.2 23.817 – 18

1-C-CT 3.4 6.6 45.9 17.2 23.8

2-B-CT 1.9 8.0 69.6 12.4 20.416 – 18

2-C-CT 2.5 8.3 82.1 13.4 21.7

3-B-CT 1.4 5.2 58.5 14.6 19.816 – 18

3-C-CT 2.2 5.5 45.7 8.0 13.5

4-B-CT 3.0 8.6 74.7 10.6 19.225 – 26

4-C-CT 3.2 9.0 76.0 14.3 23.3

Polyelectrolyte dosage was too low to have a good EDW efficiency (MBR

sludge)

Best results in terms of ΔDS and specific energy consumption


RESULTS – Lab-scale tests on Conditioned and Thickened sludge

a b

c d


RESULTS – Lab-scale tests on Mechanically Dewatered sludge

DW sludge

sampleTreatment

DSi

(%)

Specific electric

energy

consumption

during tE

(Wh/kgH2O)

ΔDS

(%)

DSf

(%)

DSf WWTP

average

(2013-

2014)

(%)


1-B-DW Act. sludge

Aer. st.

Belt press

16.0 83.2 13.8 29.817 – 18

1-C-DW 16.6 83.7 11.0 27.7

2-B-DW Act. sludge

Aer. st.

Centrifuge

20.5 108.9 6.8 27.316 – 18

2-C-DW 18.1 95.4 9.5 27.6

3-B-DW MBR

Aer. st.

Belt press

16.0 90.1 9.1 25.116 – 18

3-C-DW 15.6 95.2 10.7 26.3

4-C-DW

Act. sludge

Anaer. dig.

Centrifuge21.8 99.7 14.7 36.5 25 – 26

highest DS increaselowest DS increasehighest energy consumption


RESULTS – Lab-scale tests on Mechanically Dewatered sludge

a b

c d

Water removal is still increasing at the end of the tests: longer test would lead to higher final DS


RESULTS – Sludge characterization

Good correlation between CST and TTF values: filterability of the sludge can be defined by any of

the two methods

The higher the absolute value of zeta potential, the greater CST

(and TTF) values



a b

• No correlation exists between CST and DSf or ΔDS (the same applies for TTF).

• A lab-scale EDW device would give better predictions.

• Mechanical dewatering and electro-dewatering efficiency may depend more on VS/DS ratio, conductivity and initial DS content, rather than CST and TTF.



DS

[%]

Data from over 30 plants with different stabilisation and DW techniques:

VS/DS < 65%: DS decreases at increasing VS/DS ratios

VS/DS > 65%: no trend

Anaerobically digested and belt pressed sludge samples

from the same WWTP:DS values decrease at

increasing VS/DS ratios


Lab-scale critical points:– Formation of a dry cake close to the anode

rotating piston to mix the drying cakeLAB-SCALE DEVICE

with ROTATING ANODE

LAB-SCALE DEVICE – Rotating anode


EXPERIMENTAL CONDITIONS

EDW tests with rotating anode

on mechanically dewatered sludge

samples “D” from the same four WWTPs

20 mm-thick sample

compression (300 kPa) + electric field (15 V/cm)

rotation 10 rpm

t = 20 min


SLUDGE SAMPLES – Characterisation of CT sludge (samples «D»)

Parameter WWTP

Sample no. 1-D 2-D 3-D 4-D

pH 6.9 6.8 6.9 7.1

Conductivity (mS/cm) 0.8 0.8 1.5 3.2

DSo (%) 2.5 1.9 1.9 4.3

VS/DS (%) 61.8 64.8 72.6 55.0

CST (s) 9.0 9.1 12.1 42.0

TTF (min) 1.5 2.1 NA 34.0

Zeta potential (mV) -11.0 -10.7 +16.0 -11.8

Polyelectrolyte dose (g/kgDS) 6.9 9.0 9.0 9.0

Overdose of polyelectrolyte: – zeta potential was positive – TTF could not be measured (filter paper broke)


RESULTS – Rotating anode compared with static anode tests

DW sludge

samples D

from 4

WWTPs

DSi

(%)

Specific electric

energy consumption

(Wh/kgH2O)

ΔDS

(%)

DSf

(%)

ΔDSf

(rotating -

static)

(%)

118.4

133.6 7.0 25.4+17.5

1 ROT 141.6 23.7 42.9

221.7

117.2 12.5 34.2+2.9

2 ROT 116.6 15.4 37.1

315.0

119.3 14.3 29.3+2.8

3-ROT 134.9 17.1 32.1

423.8

111.1 19.5 43.3+9.2

4-ROT 171.5 27.3 52.5

– Specific energy consumption of dynamic tests (rotating anode) is generally slightly higher, BUT

– We got a significant DSf increase with the rotating anode


a

RESULTS – Rotating anode

b

c d

1-D-DW

1-D-DW-rot

2-D-DW

2-D-DW-rot

3-D-DW

3-D-DW-rot4-D-DW

4-D-DW-rot


Patent WO 2011/161568 A1(Stradi et al. 2011)

EDW screw press prototype


Tests on conditioned and thickened sludge (CT samples)simulating EDW to replace mechanical dewatering:

EDW applied to aerobically stabilised sludge showed a DS increase of 7 to14% compared with real plant data (2013-2014 average values).

EDW applied to anaerobically digested sludge did not show a significantimprovement if compared with real plant data.

CST, TTF and zeta potential are not good predictors of the electro-dewaterability of a sludge lab-scale electro-dewatering tests on CT sludgemay give useful indications about the applicability of this technique.

CONCLUSIONS – 1/3


Tests on conditioned and dewatered samples (DW)Simulating application of EDW after mechanical dewatering:

IN GENERAL with or without EDW, sludge with VS/DS < 65% can be dewateredbetter than sludge with VS/DS > 65%.

EDW of anaerobically digested DW sludge is more efficient than EDW ofaerobically stabilised DW sludge (typically DS = 12-15% compared to 8-10%)

EDW allows to get DS = 40% with tE = 20 min for anaerobically stabilisedsludge and tE > 30 min for aerobically stabilised sludge.

Electric energy consumption to get DS=10.7% ± 2.7% of DW sludge was 94.0± 9.1 Wh/kgH2O, less than 1/4 of the equivalent primary energy for thermaldrying, (Italian national factor of 0.47 kWhel/kWhth).

CONCLUSIONS – 2/3


Tests with ROTATING ANODE on DW sludge:

Rotating anode

• improves sludge mixing during the tests,

• delays the increase of electric resistance

• prevents the rising of temperature by Joule effect

• increases current densities improving water removal andleading to higher final DS values (3 to 17% higher than staticanode tests).

Specific primary energy consumption is 50 to 70% lower than inconventional thermal treatments.

Future work: improve the lab-scale device and build theprototype

CONCLUSIONS – 3/3


The LIFE14 project no. ENV/IT/000039 “ELECTRO-SLUDGE” has been partly funded with the contribution of the LIFE Programme of the European Unionwww.electrosludge.eu/ (under construction)

Sludgetreat Project no. 611593, co-funded by the European Commission within FP7 (2007–2013) Marie Curie Actions— Industry-Academia Partnerships and Pathways (IAPP)http://cordis.europa.eu/project/rcn/191799_it.htmlwww.sludgetreat.eu (under construction)

Partners:- AST (Modena, I)- Politecnico di Milano (I)- CAP Holding S.p.A. (Milano, I)

Partners:- AST (Modena, I)- Politecnico di Milano (I)- Flubetech (Barcelona, E)- AIN (Pamplona, E)

ACKNOWLEDGEMENTS

http://www.electrosludge.eu/

http://cordis.europa.eu/project/rcn/191799_it.html

http://www.sludgetreat.eu/


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Electro-osmotic Dewatering of Sewage Sludge: Preliminary Results · 2017-11-06 · Act. sludge...

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