1. 12 Sludge Treatment 12.1 Overview 12.2 Thickening
Technische Universität Dresden
Peter Krebs
Department of Hydro Science, Institute for Urban Water Management
Urban Water Systems
12 Sludge Treatment
12.1 Overview
12.2 Thickening
12.3 Biological sludge stabilisation
12.4 Volume reduction
12.5 Sludge disposal
Urban Water Systems
12 Sludge treatment

2. 12.1 Overview 12 Sludge treatment Urban Water Systems

3. Composition of sludge Predominantly water Micro-organisms
Viruses, pathogens, germs in general
Organic particles, heavily bio-degradable
Organic compounds, inert, adsorpted to sludge flocs
Heavy metals
Micro-pollutants, pharmaceuticals, endocrine disrupters
 All non-degraded compounds extracted from wastewater are found in the sludge
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12 Sludge treatment

4. Goals of sludge treatment
Volume reduction
Thickening
Dewatering
Elimination of pathogenic germs
If used in agriculture as fertiliser or compost
Stabilisation of organic substances
Gas production
Reduction of dry content
Improvement of dewatering
Reduction of odour
Recycling of substances
Nutrients, fertiliser
Humus
Biogas
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12 Sludge treatment

5. Overview Wastewater treatment Primary, secondary, tertiary sludge
Thickening
Energy
Process water
Hygienisation
Stabilisation
Biogas
Thickening
Agriculture
Dewatering
Disposal site
Drying
Construction industry
Gujer (1999)
Incineration
Atmosphere
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12 Sludge treatment

6. Sludge Treatment Alternatives
Eckenfelder & Santhanam (1981)
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12 Sludge treatment

7. 12.2 Thickening 12 Sludge treatment Urban Water Systems

8. Thickening by Gravity Gravitative separation, similar to settling tank
Additional mechanic stirring to enhance flocculation and extraction of water and gas
Supernatant is introduced to primary clarifier or – if floatables and grease contents are high – to grid chamber
Thickened sludge is withdrawn from hopper and introduced to sludge treatment
For an efficient thickening process the development of gas bubbles must be prevented
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12 Sludge treatment

9. Gravity Thickener Inflow Scum scimmer Sludge liquor Picket fence
Thickened sludge
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12 Sludge treatment

10. Dimensioning of gravity thickeners surface
Solids overflow rate
qTSS,Th Specific solids overflow rate (kg TSS / (m2 d))
QWAS Inflow to thickener (m3/d)
XTh,in Solids concentration in thickeners inlet (kg TSS / m3)
ATh Surface of thickener (m3)
Typical values for solids overflow rate qTSS,Th and concentration of thickened sludge XTh
qTSS,Th
XTh
Primary sludge
80 – 120
Primary and secondary sludge
Secondary sludge
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12 Sludge treatment

11. Thickening by Flotation
Pre treatment: mostly chemical flocculation
Slude is placed in contact with air-saturated water (full flow or recycle pressurization)
Air bubbles attach to solid particles  lower specific gravity than water
Floating Sludge bubble composite is collected at the surface
Water is recovered under a scum baffle and removed
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12 Sludge treatment

12. Thickening by Flotation
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12 Sludge treatment

13. Flotation unit
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12 Sludge treatment

14. 12.3 Biological sludge stabilisation
12 Sludge treatment
12.3 Biological sludge stabilisation
Urban Water Systems
12 Sludge treatment

15. Anaerobic mesophilic sludge stabilisation
Digester
Heated to 33 – 37°C  process rates are higher
Content of digester is mixed  Sludge and water obtain a similar residence time
Storage unit
Not heated  little biological activity
Not mixed  separation of sludge and process water, which is directed to WWTP
 Control of loading to WWTP, app. 10% of N-loading
Further thickening
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12 Sludge treatment

16. Processes in digester Anaerobic degradation
Degradation of organic substances of app. 50%
Biogas production: 63% CH4 (Methane) 35% CO2 2% other gases (N2, H2, H2S)
 electricity and heating
Organic nitrogen is converged to NH4+
 N-loading of WWTP
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12 Sludge treatment

17. Characteristic values of digester
Mean residence time of sludge
Small units, badly mixed
< 30 d
Medium size units with mixing
20 d
Large plants with mixing
12 – 16 d
Biogas production related to degradation of organic substances
0.9 m3 / kg VSSdegr.
Degradation of organic substances
40 – 55%
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12 Sludge treatment

18. Simultaneous aerobic sludge stabilisation
No primary clarifier  no primary sludge
High sludge age SRT, app. 25 d
Activated sludge tank is larger than that combined with an anaerobic sludge stabilisation
No biogas production
Possibly combined with storage or thickener unit
Stable and simple operation
Urban Water Systems
12 Sludge treatment

19. 12.4 Volume reduction 12 Sludge treatment Urban Water Systems

20. Volume reduction Water content in stabilised sludge > 95% !
 Reduction of water content and volume
Sludge volume
With water content 
 non-linear relation!
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12 Sludge treatment

21. Volume reduction
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12 Sludge treatment

22. Dewatering
Conditioning with flocculation agents (poly-electrolytes) for efficient dewatering
Unit
Operation
Method W
DS
Decanter
Continuous
Centrifuge
> 0.7
< 0.3
Chamber filter press (large plants)
Batch-wise
Hydraulic pressure through plates in water-tight chambers
> 0.6
≤ 0.4
Belt filter press (small plants)
continuous
Pressed between two filter belts around staggered rollers
> 0.7
≤ 0.3
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12 Sludge treatment

23. Drying bed Thin sludge layer (< 20 cm)
Sand layer as drainage and filter layer
Sludge is
first dewatered by drainage
then air-dried through evaporation
Applicable for small plants
Dimensioning  W  0.55 (Imhoff, 1990)
Plant type
Specific surface
Only mechanical treatment
13 PE/m2
Trickling filter
6 PE/m2
Activated sludge plant
4 PE/m2
Urban Water Systems
12 Sludge treatment

24. Drying  Vaporisation of water content Partial drying  W 0.3 – 0.4
Full drying  W down to < 0.1
Contact drying over heated areas
Drying by convection through hot air counter-current inlet app. 600°C, outlet app. 300°C (Imhoff, 1999)
For large plants
Disposal is critical: fire, dust explosion
In granulate form as fertiliser
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12 Sludge treatment

25. 12.5 Sludge disposal 12 Sludge treatment Urban Water Systems

26. Use in agriculture  Recycling of nutrients, from stabilised sludge
Sludge treatment
Fertiliser*
Liquid sludge
P- and N-fertiliser
Dewatered sludge
P-fertiliser, N as storage product
Dried sludge
P-fertiliser
* Limit re. over-fertilisation
Problems
Acceptance
Heavy metals
Micro-pollutants, pharmaceuticals, endocrine disruptors
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12 Sludge treatment

27. Composting  Aerobic biological degradation of organic substances
Prerequisites
Stabilisation
Dewatering
Hygienisation
Approach
Structure means: straw, wood, saw dust, wood chips
Mixture app. 1:1
Water content app. 0,65
 Requirements are more demanding than for sludge use as fertiliser!
Urban Water Systems
12 Sludge treatment

28. Incineration Use of energy content, but not of nutrients
Mono incineration (sludge exclusively)
Calorific value of sludge high enough  no biogas use before, no stabilisation
Water content not minimised (no full drying)
Fluidised bed incinerator, incineration at 800 – 950°C in fluidised sand bed
Expensive!
Co- incineration
In coal power station
In solid waste incinerators
In cement production, ash is bounded to cement
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12 Sludge treatment