CONSOLIDATION AND DRYING PROPERTIES OF SEWAGE SLUDGE Brendan C. - - PowerPoint PPT Presentation

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CONSOLIDATION AND DRYING PROPERTIES OF SEWAGE SLUDGE

Brendan C. O’Kelly

Trinity College Dublin Geotechnical Society of Ireland Seminar

14 March 2005

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OVERVIEW:

• Physical properties of sewage sludge

– effects of level of biodegradation

• Air-drying and material density
• Compressibility and consolidation properties

– liquid slurry and dried compacted sludge material – biologically active and stabilised material

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QUANTIFYING AMOUNT OF PORE WATER Geotechnical literature**: Water content w, mass of pore water to mass of dry solid particles, as % Water treatment literature: Solids content SC, mass of dry solid particles to bulk sludge mass, as %

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(as %)

• TEST MATERIAL

– Tullamore municipal wastewater treatment plant – Anaerobic, activated sludge digestion method – Treated material dewatered to w  720 % using belt filter press

        100 w 1 100 SC

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PHYSICAL PROPERTIES

Liquid limit 315 % Plastic limit 55 % Shinkage limit 10 % Plasticity index 260 % Specific gravity of solids 1.55 Ignition loss 70 % Water content 720 % Void ratio 11 Bulk unit weight 10.2 kN/m3 Dry unit weigth 1.3 kN/m3 pH 8.0

• Typical properties of

slurry direct from wastewater plant

• Properties indicate

sludge material was moderately degraded

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PREPARATION OF MORE STRONGLY DEGRADED MATERIAL i) Further biodegradation of slurry in laboratory at 35 oC

Hydraulic consolidation cell Trapped biogas Overflow container Drying oven Graduated Acrylic cylinder, 25 l capacity Sludge specimen

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STABILIZATION (optional): slurry pasteurized by

heating at 80oC for 3 hours ii) Sludge also stored outdoors in drums and allowed degrade naturally over ten-year period

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PHYSICAL PROPERTIES AND STATE OF BIODEGRADATION

Ignition loss

– (% dry mass) – indirect measure of

• rganic content

– assess state of degradation

A B C D F E 1.5 1.6 1.7 1.8 1.9 45 50 55 60 65 70 75

Ignition loss (%) Specific gravity of solids (A) Slurry direct from wastewater plant, and after (B) three, (C) four and (D) 30 months, and (E) ten years. (F) Slurry degraded in the lab at 35oC

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AIR-DRYING AND DENSITY

• Sludge material dried slowly outdoors

0.0 0.4 0.8 1.2 1.6 200 400 600

Water content, (%)

Bulk density, Dry density, (tonne/m3) 0 % air voids curve Bulk density Dry density

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COMPRESSIBILITY AND CONSOLIDATION PROPERTIES

• TEST APPARATUS:

i) Oedometer consolidation cell

Porous platen Loading cap Porous stone

Test specimen

Confining ring

• Test specimen dimensions:

– 76mm dia. x 19mm high, to 100mm dia. x 40mm high – Consolidation pressures increasing from 3 kPa for slurry, up to 400 kPa for dried compacted sludge material

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ii) Hydraulic consolidation cell

• Test specimen dimensions:

250mm diameter x 50mm high Effective confining pressures of 100 to 300 kPa Specimen

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CONVENTIONAL ONE-DIMENSIONAL CONSOLIDATION CURVE

Logarithm time, (minutes) Initial compression Primary consolidation Secondary compression Specimen compression

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TEST RESULTS

Moderately degraded sludge material:

• biologically active

i) Slurry direct from treatment plant (w  720 %)

20 40 60 80 1 10 100 1000 10000

Time, (minutes) Cumulative strain, (%)

3 6 12 25 50 100 50 12 3 Vertical stress (kPa)

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Dried, standard Proctor compacted sludge material ii) Wet of optimum water content, w130%

5 10 15 20 25 30 1 10 100 1000 10000

Time, (minutes) Cumulative strain, (%)

25 50 100 200 400 200 100 50 25 Vertical stress (kPa)

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iii) Compacted at optimum water content, w  100%

2 4 6 8 10 12 14 1 10 100 1,000 10,000

Time, (minutes) Cumulative strain, (%)

25 50 100 200 400 200 100 50 37 Vertical stress (kPa)

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Comparison: Moderately and strongly degraded sludge material, biologically stabilised

iv) Slurry direct from treatment plant (w  720 %)

20 40 60 80 1 10 100 1,000 10,000 100,000

Time, (minutes) Axial strain, (%)

25 50 75 100

Degree of consolidation, (%)

Strain ( 70 % ignition loss) Strain ( 55 % ignition loss) Dissipation excess p.w.p. (55 % ignition loss) sc = 100 kPa, sc = 300 kPa,

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SUMMARY OF COMPRESSIBILITY DATA

2 4 6 8 10 12 1 10 100 1000

Effective Stress, (kN/m2) Void Ratio

Optimum sludge compaction Liquid sludge

Cc Cs Cc = Compression index Cs = Swell index

Void ratio, volume of void space to volume of solid particles Calculate amount of consolidation settlement in sludge monofill

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CONSOLIDATION

• Primary consolidation:

– compression associated with dissipation of excess porewater pressure

• Secondary compression:

– indefinite creep, and in case of sludge, significant contribution due to ongoing degradation

• Primary consolidation occurred fairly rapidly but

constituted only minor part of overall strain response

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• coefficient of permeability k, for moderately degraded

slurry of the order of 10-9 m/s

– greater permeability for higher states of biodegradation – but permeability decreases significantly with increasing applied stress

• Secondary compression by far dominant mechanism

causing settlement

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SUMMARY

• Physical and mechanical properties affected

by level of degradation

– Specific gravity, density and permeability increase with increasing level of biodegradation – Strongly biodegraded, stabilised sludge material consolidates more readily – bioegradation continues many years after treatment at waterwater plant

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• Sludge material dried slowly outdoors
• Sludge material practically impermeable,

k<10-9 m/s

• Settlement of landfilled material occurs very slowly,

and occurs mainly due to creep and ongoing biodegradation

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